Water tracers in oilfield applications: Guidelines

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Water tracers in oilfield applications: Guidelines

C. Serres-Piole a,b, H. Preud’homme b,n, N. Moradi-Tehrani a, C. Allanic a, H. Jullia a, R. Lobinski b

a TOTAL, CSTJF, Avenue Larribau, 64018 Pau Cedex, Franceb CNRS/UPPA, Laboratory of Analytical Bio-Inorganic and Environmental Chemistry (LCABIE), UMR 5254—IPREM, 2 Avenue Angot, 64053 Pau Cedex9, France

a r t i c l e i n f o

Article history:

Received 23 May 2011

Accepted 13 August 2012Available online 30 August 2012

Keywords:

water tracers

oil reservoir

passive

partitioning

detection

a b s t r a c t

A key parameter in tracing tests is the selection of the molecules used as water tracers. Many previous

tests have failed because of improper selection of these molecules. To address this issue, the first part of

this paper provides guidelines, offering data and advice for choosing the best possible tracers for a

tracing campaign. This part of the paper presents the different types of water tracers proposed and used

in oilfield applications, from the first qualitative tracer study in the 1960s to tracer studies in the 2000s,

with their respective advantages and drawbacks. The oil industry began to conduct interwell tracer

tests with molecules already successfully used in hydrology. These compounds included radioactive

species and stable isotopes, chemicals such as fluorescent dyes, and inorganic ions. Some of the early

chemical tracers have been rejected because of issues with adsorption onto the rock. Radioactive

species, with a low detection limit, a low reactivity, and a low presence in the environment, have been

widely used. However, their use has become more restricted throughout the world in response to the

radioactive hazards associated with their use. Therefore, new types of non-radioactive tracers were

developed and tested in the 1990s. Currently, however, few chemical molecules possess characteristics

matching the selection criteria of an effective water tracer (with regard to environmental and economic

aspects, etc.). The most effective molecules currently used as water tracers are the fluorinated benzoic

acids (FBA); these molecules can be detected with very low limits of detection (LOD) using analytical

techniques such as gas chromatography or ultra-high-performance liquid chromatography coupled

with mass spectrometers (GC/MS and UHPLC/MS-MS, respectively).

The second part of the paper deals with the analytical aspect of a tracing test. An alternative

technique to the fluorescence methods currently used for the naphthalene sulfonic acids (NSA) is

proposed: UHPLC/MS-MS. With this original tool, FBA and NSA could be simultaneously detected in

water samples in only one 5-min analysis. Other molecules, halogenated boronic acids, were also tested

analytically for their potential application as tracers. However, these molecules were not retained

because of their overly high LOD, requiring the injection of large quantities into oil reservoirs.

& 2012 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2. Water tracers and tracing tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1. Tracer types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.2. Types of tracing tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.3. The selection criteria for water tracers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.3.1. Artificial tracers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.3.2. Natural tracers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Contents lists available at SciVerse ScienceDirect

journal homepage: www.elsevier.com/locate/petrol

Journal of Petroleum Science and Engineering

0920-4105/$ - see front matter & 2012 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.petrol.2012.08.009

Abbreviations: EDTA, ethylene diamine tetra acetic acid; FBA, fluorinated benzoic acid; FID, flame ionization detector; FLD, fluorescence detector;

GC, gas chromatography; HPLC, high-performance liquid chromatography; HTO, tritiated water; IC, ionic chromatography; ICP-MS, inductively coupled plasma-mass

spectrometry; IR-MS, isotopic ratio-mass spectrometry; IWTT, interwell tracer test; MS, mass spectrometer; MS-MS, tandem mass spectrometry; NMR, nuclear magnetic

resonance; NSA, naphthalene sulfonic acid; PITT, partitioning interwell tracer test; SWTT, single-well tracer test; UHPLC, ultra-high-performance liquid

chromatography; UV, UV detectorn Correspondence to: LCABIE, UMR 5254 CNRS, IPREM, 2 Avenue Angot, 64053 PAU Cedex9, France. Tel.: þ33 559407738; fax: þ33 559407781.

E-mail address: [email protected] (H. Preud’homme).

Journal of Petroleum Science and Engineering 98–99 (2012) 22–39


3. An overview of water tracers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.1. Passive tracers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.1.1. Natural tracers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.1.2. Artificial tracers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3.1.3. Other types of tracers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2. The partitioning tracers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2.1. For SWTT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2.2. For PITT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.2.3. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.3. Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.4. Discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4. Molecules analytically studied at the laboratory scale. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.1. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.1.1. Reagents and chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.1.2. Standards and samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.2. Example applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.2.1. NSA, alternative analytical techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.2.2. Halogenated boronic acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Appendix A. Supporting information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1. Introduction

Tracer techniques are a powerful diagnostic tool in numerousscientific disciplines (Bjornstad et al., 1990; Divine and McDonnell,2005) and for technologies in many industrial sectors. Tracer testswere first used in the early 1900s in hydrology (Du and Guan, 2005;Guan et al., 2005). Today, these tests are used with increasingfrequency in oilfield applications (Bingyu et al., 2002; Bjornstad,1991; Jin et al., 1997) when no other investigation technique isapplicable. Tracer tests provide a better understanding of thestudied oil reservoir (including interwell connections, connectionsbetween layers and heterogeneities) (Coronado and Ramirez-Sabag,2008; Coronado et al., 2009; Guan and Du, 2004; Manichand et al.,2010; Nugroho and Ardianto, 2010; Sinha et al., 2004).

Petroleum companies began to conduct interwell tracer testsin the 1960s (Asadi and Shook, 2010; Du and Guan, 2005; Guanand Du, 2004; Shook et al., 2009) by selecting molecules alreadysuccessfully used in hydrology (Ahmad et al., 2003). Althoughseveral methods exist to obtain precise information for reservoirdescription purposes, such as production rates of reservoir fluidsand 4D seismic and pressure testing (Barree et al., 2010;Bjornstad, 1998; Bjornstad et al., 1997; Bragg et al., 1978; Myaland Wesson, 1981; Huseby et al., 2008, 2010; Hutchins et al.,1991; Prakash Das et al., 2009; Sathyamoorthy et al., 2009),tracers have proven a very useful and efficient experimental toolin complex reservoirs where data are difficult to obtain with othertechniques (Asadi and Shook, 2010; Bennion et al., 1995; Cubilloset al., 2006; Shook et al., 2009). With the development of chemicaltracers in the 1990s, the water tracer test has become increasinglyimportant for the petroleum industry (Guan et al., 2005). Thesetests are increasingly used in the design of enhanced oil recoverymethods to follow the flow though the reservoir, control break-throughs (Manichand et al., 2010; Manrique et al., 2010), andselect the most appropriate wells for enhanced oil recoveryprojects (Nugroho and Ardianto, 2010).

According to Du and Guan (2005) (Shook et al., 2009), of 43tracer tests reported in the literature in the 2000s, 60% wereaqueous phase tracers and 70% were interpreted qualitatively(Nava et al., 2009).

A key parameter in tracer tests is the appropriate selection ofthe molecules used as water tracers. The success of a tracer testdepends on this parameter. One of the objectives of the present

paper is to provide guidance for the appropriate selection oftracers according to the study and the desired objectives.

2. Water tracers and tracing tests

The selection of the tracer type and tracing test (Gardien et al.,1996) for use in a tracing campaign design depends on theapplication, the specific conditions of the reservoir, the studyobjectives, and the nature of the data under investigation. Twomain types of water tracers are referenced in the literature:natural and artificial, with either passive or partitioning proper-ties. Three types of tracer tests are referenced: non-partitioningand partitioning interwell tracer tests and the single-well tracertest (Anisimov et al., 2009).

2.1. Tracer types

Natural and artificial tracers have been used for many years(Chrysikopoulos, 1993). Natural tracers are defined as elementsalready naturally present in the studied environment (Asakawa,2005). Artificial tracers are defined as non-naturally existingcompounds that are introduced into the reservoir.

A natural tracer is used for information about the distributionof the production in time, the water origin with the identificationof the source of the water produced and the contribution of theinjected water to the producing well (Huseby et al., 2009; Khanet al., 2006; Nava et al., 2009). An artificial tracer is used for bothqualitative (interwell connections, flow direction, the pathway ofthe water, heterogeneity) and quantitative information (velocitiesof the fluids, permeability, residual oil saturation (SOR))(Bjornstad, 1991; Du and Guan, 2005).

The differences between a passive and partitioning tracerresult from their chemical natures and their particular physico-chemical behaviors in the oil reservoir (Devegowda et al., 2009;Oyerinde, 2004).

The passive tracer has the same physico-chemical propertiesas the fluid in which it is injected (water). Also called a ‘‘non-partitioning’’ or ‘‘aqueous’’ tracer, it moves at the approximatevelocity of the water phase without chemical reaction or inter-action (sorption, ion-exchange) with reservoir phases (formation

C. Serres-Piole et al. / Journal of Petroleum Science and Engineering 98–99 (2012) 22–39 23


water, oil) or with the rock as it moves through the oil reservoir(Asadi and Shook, 2010; Guan and Du, 2004; Huseby et al., 2009;Oyerinde, 2004; Shook et al., 2009). In fact, a tracer classified aspassive will never be totally passive because of the small amountof adsorption and desorption that occurs.

The partitioning tracer is soluble in both the oil and the waterphases (Knaepen et al., 1990). This partitioning phenomenon withthe oil phase (physical and chemical interactions with the rock orthe fluids) causes a ‘‘chromatographic delay’’ in the response ofthe partitioning tracer in comparison with the passive tracer (Jinet al., 1997; Oyerinde, 2004; Tang, 1995). Four mechanisms canaffect this delay: a partitioning effect with the other fluids (oil),adsorption to the rock, ion exchange, and size exclusion. The firstmechanism is the most recognized.

A passive tracer can be selected in a tracing campaign whenthe reservoir engineer requires both qualitative (a better under-standing of the water pathway, interwell connections, etc.) andquantitative information (sweep volumes, flow velocities, perme-ability estimates, etc.) (Gardien et al., 1996; Huseby et al., 2009;Khan et al., 2006; Lliassov et al., 2002; Meza et al., 2007; Navaet al., 2009; Somaruga et al., 2001).

When the reservoir engineer requires quantitative informationabout the oil in place (SOR), the simultaneous injection of apassive tracer (used as reference tracer) and a partitioning traceris required (Devegowda et al., 2009; Lliassov et al., 2002; Tang,1995; Tang and Zhang, 2001).

2.2. Types of tracing tests

To obtain qualitative and quantitative information, interwelltracer tests (IWTTs) with passive tracers are used.

To estimate the residual oil in place, two types of tests, usingboth passive and partitioning tracers, are possible (Oyerinde,2004): the partitioning interwell tracer test (PITT) (Asakaka,2005; Jin et al., 1997), described by Cooke in 1971 (Cooke,1971; Deans and Mut, 1997; Tang, 2002, 2003a; Tang andZhang, 2000), and the single-well tracer test (SWTT), patentedby Exxon in 1971 (Deans, 1971; Deans and Mut, 1997). These twotests represent useful tools for evaluating the efficiency ofenhanced oil recovery methods (Buijse et al., 2010; Deans,1978; McGuire et al., 2005).

In the SWTT test, a partitioning tracer is injected and reactswith the reservoir water to form a second tracer in situ (Anisimovet al., 2009). The difference between the relative breakthroughtimes of the partitioning and non-partitioning tracers is relativeto the SOR (Tang and Harker, 1991).

Since 1971, this SWTT technique has been used in more than200 sandstone and carbonate reservoirs. When the oil fractionalflow is negligible, the SOR can be estimated by the followingformula, proposed by Deans (Chatzichristos et al.):

SOR¼ðDt=t0Þ

½ðDt=t0ÞþKow�

where t0 is the breakthrough time of the passive tracer, Dt is thetime delay on the arrival between the partitioning and ideal tracers,and Kow is the partitioning coefficient, defined as the ratio of theconcentration of the partitioning tracer of the immobile phase to itsconcentration of the mobile phase (Kow¼Ctracer

oil /Ctracerwater).

Whereas the PITT is used to quantify the volume of bothmobile and residual oil over the entire reservoir volume sweptby the tracers, the SWTT is used to quantify oil near the wellregion only (Deans and Carlisle, 1986; Sinha et al., 2004;Valestrand et al., 2010). However, the SWTT generally yieldsmore rapid results. Thus, the SWTT was the most frequentlyused application of partitioning tracers for measuring the SORin the 1990s (O’Brien et al., 1978; Tang, 1995) and is still

considered more practical than the PITT by the oil industry(because of difficulties in interpreting PITT results) (Somarugaet al., 2001).

2.3. The selection criteria for water tracers

2.3.1. Artificial tracers

Before the 1990s, many IWTT tests using artificial tracers wereunsuccessful in hydrological and oilfield applications (Davis et al.,1980; Reeves et al., 1996). One of the reasons for this failure wasimproper selection of the artificial tracers. Some compounds wereused without sufficient knowledge of their behavior under reser-voir conditions. Some chemical compounds were adsorbed to therock, were broken down (Bjornstad et al., 2001) or progressed tooslowly (Reeves et al., 1996). These problems led to an incorrectinterpretation of the data.

One of the most important parameters in a tracing test is theappropriate selection of tracers (Hutchins et al., 1991). Manyparameters (temperature, pressure, pH, rock type and properties,ionic force, water salinity, biological activity) have an influence ontracer behavior (Bjornstad et al., 1994). Thus, the examination of anartificial tracer before injection, including its physicochemical prop-erties, is a required step in the tracer test (Bjornstad et al., 2001).

To correctly use artificial tracers and to conduct a quantitativeanalysis of tracer data, the selection criteria are as follows (Beckerand Coplen, 2001; Behrens et al., 2001; Bjornstad, 1991, 1998; Carret al., 1997; Cubillos et al., 2006; Dombrowski and Stetzenbach,1993; Gozalpour et al., 2005; Jin et al., 1997; Shook et al., 2009;Wood et al., 1990):

(1) A tracer must be inert under injection, reservoir and produc-tion conditions; it must be highly stable thermally, chemi-cally, physically, and biologically for several years.

(2) A passive tracer must passively follow the phase in which it isinjected (water), i.e., not be adsorbed to the rock or parti-tioned with oil and be water soluble.

(3) A partitioning tracer must have a constant and reversiblepartitioning (Shook et al., 2009) with oil.

(4) A tracer must not influence the physical properties of thereservoir fluids.

(5) A tracer must not be present in the fluids of the reservoir toavoid background noise.

(6) A tracer must be detectable in very low concentrations withsimple and fast analytical methods to reduce the costs and theenvironmental impact via small injected quantities.

(7) A tracer must be at an acceptably low level of toxicity for thepopulation and the environment.

(8) A tracer must not be capable of bio-accumulation.(9) A tracer must be commercially available in large quantities at

an acceptable cost.

2.3.2. Natural tracers

To apply natural tracers in oilfield applications, three condi-tions must be met. First, a careful characterization (chemical andisotopic analysis) of the fluids is required before, during and afterthe water breakthrough (containing the natural tracer). Second,the production water and the injection water must differ sig-nificantly in composition (sea water or aquifer water) (Manichandet al., 2010). Third, the passive natural tracer must be inert (nochemical reactions or ion exchanges).

If these conditions are met, the breakthrough of injected watercan be naturally detected due to a significant change of thecomposition of the produced water from the formation water in

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relation to the composition of the injected water (Bjornstad et al.,1990; Huseby et al., 2009).

3. An overview of water tracers

Few publications about the use of tracers in the oil industryprovide details about the choice of tracers or the design of theinterwell tracer tests (Du and Guan, 2005; Guan et al., 2005). Priorto the 1990s, the oil industry literature contained fewer docu-ments accessible to the public on tracer technology than did thehydrological industry literature. This lack of public literatureslowed the development of analytical methods for tracers. How-ever, this situation changed substantially in the 1990s (Kleven,2008). Field studies with water tracers are available in Dugstadet al. (1999), de Melo et al. (2001), Gieles and Beuthan (2004), andMeza et al. (2007).

Researchers have proposed different tracer classifications.Three tracer types are referenced by Zemel (1995): natural,radioactive, and chemical tracers. Other authors describe fourmain types of tracers: radioisotopes, fluorescent dyes, water-soluble alcohols, and water-soluble salts (Jaripatke andDalrymple, 2010) or chemical tracers (applied in the 1950s),radioactive isotopes (applied in the 1970s), stable isotopes(applied in the 1980s), and microelement tracers (applied afterthe 1990s) (Bingyu et al., 2002).

Because the passive water tracers must be in a form thatfollows the water pathway, they are often anionic or neutralatoms or molecules; indeed, the anions tend to be more con-servative than the cations in natural systems (Chrysikopoulos,1993).

3.1. Passive tracers

3.1.1. Natural tracers

Every compound present in a given environment, such asreservoir water, can be a natural tracer (Huseby et al., 2009).The main advantage of the natural tracers is their presence in thereservoir water, resulting in a lower cost than artificial tracers,which must be bought and injected into the reservoir (Manichandet al., 2010; Wei, 2010). The use of natural tracers preventspossible problems associated with the addition of a foreignsubstance (Raheim and Smalley, 1986). In addition, naturaltracers are more environmentally friendly than chemical orradioactive tracers (Asakaka, 2005). The efficiency of naturaltracers, described by Huseby et al. (2005, 2009), is also explainedby the fact that every water body is characterized by uniqueisotopic ratios (Skilbrei et al., 1990; Smalley et al., 1988).

Several elements and molecules have been proposed as tracersin oilfield applications: Cl� , Br� , SO4

2� , Ca2þ , Mg2þ , Naþ , Kþ ,Sr2þ , Ba2þ , HCO3

� (Bjornstad, 1991; Huseby et al., 2005, 2009;Manichand et al., 2010; Munz et al., 2010; Wei, 2010; Ziegleret al., 2001). However, some of these elements can be affected bychemical reactions (ion exchange) with the rock matrix or thewater of the reservoir formation. Only SO4

2� , Cl� , Kþ , and Mg2þ

can be considered passive seawater tracers.Stable isotopic elements, such as 18O, 2H, and 13C (Ahmad

et al., 2003; Birkle et al., 2006; Khan et al., 2006) have also beenused as natural tracers, as have the following ratios (Bjornstad,1991; Huseby et al., 2005; Zemel, 1995; Ziegler et al., 2001): d18O(18O/16O), dD (2H/1H), d13C, d37Cl, 87Sr/86Sr (Birkle et al., 2006;Bjornstad et al., 1990; Munz et al., 2010; Raheim and Smalley,1986), Cl/Br (Alcala and Custodio, 2008), and 34S/32S (Husebyet al., 2005).

Among these isotopic elements, 18O and 2H are consideredexcellent passive water tracers because these elements belong to

the water molecules themselves. These elements have often beenused to determine the origin of water or to characterize theefficiency of injected water (Ahmad et al., 2003; Huseby et al.,2005, 2009). The strontium ratio (87Sr/86Sr), proposed by Raheimand Smalley (1986), has been widely and successfully applied as anatural tracer for the injection of seawater into reservoirs(Huseby et al., 2009). The stable isotopic ratio of chloride is alsouseful in tracing the origin and transport mechanisms of water(Ziegler et al., 2001).

Some ratios, such as 2H/1H and 87Sr/86Sr, are only appropriatefor sandstone and limestone reservoirs (Ziegler et al., 2001). Theseratios can be modified by water–rock interactions. Other ele-ments are not always appropriate for use as water tracers; forinstance, Ba2þ can be neutralized by precipitation or by theformation of undissociated molecules of the form BaS04

(Bjornstad, 1991), and SO42� can precipitate with barium or

strontium ions (Huseby et al., 2005).The natural elements, such as dD, d18O, and 87Sr/86Sr, can be

detected by mass spectroscopy (MS) techniques (Birkle et al.,2006; Bjornstad, 1991; Khan et al., 2006; Zemel, 1995), such asICP-MS (Birkle et al., 2006). In the case of a reservoir in the NorthSea, the sensitivity limit was of 0.5–1% content of injection waterinto formation water.

Several studies have confirmed the utility of natural ele-ments for IWTT tests by providing information about thebreakthrough of the injected water containing a natural ele-ment as a tracer (Ahmad et al., 2003). However, the use of thesetracers has not been frequently reported in the literature, likelybecause of the need for sophisticated and expensive analyticalequipment for very sensitive mass spectrometry techniques(Ahmad et al., 2003; Bjornstad, 1991). In addition, the accuracyand the sensitivity of a tracer test using natural tracers dependson the degree to which the three conditions described inSection 2.3.2 are fulfilled (Bjornstad, 1991; Zemel, 1995).A significant difference in composition between productionwater and injection water is not always detected. The analyticaltechniques and the data provided by this type of test arerespectively more complex and difficult to interpret than thoseof a tracer test with artificial tracers. As a result, artificialtracers are often preferred in many cases of interwell tracertests because they allow for a higher degree of accuracy of thebreakthrough detection, and their ease of interpretation pro-vides a better characterization and understanding of the reser-voir (Ahmad et al., 2003; Manichand et al., 2010).

3.1.2. Artificial tracers

3.1.2.1. Radioactive elements. For oilfield applications between the1960s and the 1990s, tracer tests were generally conducted withradioactive elements (Bingyu et al., 2002; de Melo et al., 2001; Duand Guan, 2005; Fjerstad et al., 1993).

3.1.2.1.1. Neutral species. The neutral element with the bestcharacteristics for a use as a passive tracer is tritium (3H) in theform of tritiated water (HTO) (Wheelet et al., 1985). This elementis an optimal water tracer because it has the same physico-chemical properties as water molecules (Samuelsen et al., 2010).

HTO is a powerful tool in hydrology (Bouchaou et al., 2008)and geothermal applications (Dennis et al., 1981). It is evenconsidered the best passive water tracer available for vapor-dominated geothermal fields. In addition, HTO has been largelyused in oil reservoirs, as reported in the literature (Abernathyet al., 1994; Agca et al., 1990; Ahmad et al., 2003; Ali et al., 2000;Allison et al., 1991; Anisimov et al., 2009; Bennion et al., 1995;Bingyu et al., 2002; Bjornstad, 1991; Bjornstad et al., 1990; Chenget al., 2005; Coronado et al., 2009; de Melo et al., 2001, 2005; Du



and Guan, 2005; Huseby et al., 2009; Khan et al., 2006; Khisamovet al., 2009; Lliassov et al., 2002; Maure et al., 2001; Nitzberg andBroman, 1992; Oyerinde, 2004; Quang and Loder, 2006; Riedel,1975; Samuelsen et al., 2010; Skilbrei et al., 1990; Tang, 1995,2003a; Tang, 1992; Valestrand et al., 2010; Vilela et al., 1999;Zemel, 1995).

HTO has a low reactivity with the fluids and the reservoir rock(Bjornstad et al., 1990). It has been successfully tested in thelaboratory and under various field conditions (sandstone, lime-stone, and carbonate reservoirs with a recovery percentage of85.2%) (Khisamov et al., 2009). However, HTO is not applicable insome cases because of interference caused by a high presence ofnatural tritium (Adams et al., 1992; Chrysikopoulos, 1993).

HTO guarantees an easy and safe manipulation because of itslow radiation emissions, and the cost is relatively low (de Meloet al., 2001; Hutchins et al., 1991).

HTO can be analyzed on site, and it is easily detectable at lowconcentrations by the liquid scintillation counting method(Chatzichristos et al.; de Melo et al., 2001; Khan et al., 2006).Different limits of detection (LODs) are referenced in the litera-ture, from 2–4 Bq/L (Bjornstad, 1991; Wheelet et al., 1985; Zemel,1995) to 37–74 Bq/L (Lichtenberger, 1991). These LODs could beenhanced by an order of magnitude using specialized sample-enrichment techniques (Birkle et al., 2006). However, whenhundreds of samples must be analyzed, these methods areeconomically unattractive (Lichtenberger, 1991).

HTO is still largely used as a reference tracer (Huseby et al.,2010; Valestrand et al., 2010) in many oilfield tests to evaluateother compounds as tracers (Bjornstad et al., 2001) or as amaterial balance tracer in SWTT methods (Sheely and Baldwin,1982).

Other neutral radioactive molecules were also proposed astracers, including benzoic acids and light alcohols (methanol,ethanol, isopropanol), ketones and aldehydes, labeled with 3Hand 14C (Bingyu et al., 2002; Bjornstad et al., 1994; Quang andLoder, 2006; Tang, 1995). The advantages of these molecules aretheir reasonable cost (with the exception of 14C) and low hazardlevels. The labeled benzoic acids and light alcohols were testedsuccessfully in reservoirs at 150 1C in Vietnam. These compoundsshowed the same behavior as HTO, having similar breakthroughcurves (Quang and Loder, 2006). With half-lives of 12.4 (Khanet al., 2006) and 5730 years for 3H and 14C, respectively, they areconsidered useful for long-term tracer tests. Among these mole-cules, tritiated methanol is an effective water tracer with highstability in the reservoir (Wood et al., 1990) and minimal lossesby adsorption or chemical, radiochemical or biological degrada-tion. However, some alcohols exhibited a partitioning behavior athigh temperature (100 1C) or a biodegradation below 70–80 1C inoil reservoirs, in the case of tritiated ethanol. Tritiated alcoholscan also be analyzed by liquid scintillation counting (Bingyu et al.,2002) at the ng/mL level.

3.1.2.1.2. Anionic and cationic species. Several anionic and cationicelements have been tested as passive water tracers (Abernathy et al.,1994; Allison et al., 1991; Bjornstad, 1991; Bjornstad et al., 1994;Tester et al., 1979; Wheelet et al., 1985; Zemel, 1995), including 36Cl�

(Naþ , 36Cl�), 129I� (Birkle et al., 2006), 125I� (Skilbrei et al., 1990),131I� , 35SCN� , S14CN� (Ali et al., 2000; Huseby et al., 2008), (Kþ ,SCN� , NH4

þ , SCN�), 35SO42� , 82Br� (NH4

þ , Br�), H14CO3� , 57, 58, 59,

60Co3þ ([Co(CN)6]3�) (Bennion et al., 1995; Cayias et al., 1990; Chenget al., 2005; Hutchins et al., 1991; Lichtenberger, 1991; Lliassov et al.,2002; Nitzberg and Broman, 1992; Oyerinde, 2004; Tang, 1995), and60Co2þ ([60Co2þ-EDTA], denoted [60CoY]2� (ethylene diamine tetra-acetic acid or YH4)).

These elements present a low reactivity with the fluids andwith the rock. They are stable at high temperatures (Bjornstadet al., 1994): up to 200 1C for 131I� and 82Br� and up to 90–100 1C

for 35SCN� , S14CN� and 60Co2þ (Tester et al., 1979). Among them,36Cl� , 125I� , 131I� and Co2þ have been successfully used insandstone and limestone reservoirs, with a similar passive beha-vior as HTO. In addition, 36Cl� , 57, 60Co2þ , and 129I� are suitablefor long-term tracer tests, as opposed to 125I� , 131I� , 35SCN� ,35SO4

2� , 82Br� , 58Co2þ , which have short half-lives, on the order ofdays (Wheelet et al., 1985).

However, some ions have drawbacks. For example, 36Cl� and[57, 58CO(CN)6]3� are costly. In addition, some tests have shownthat 36Cl� , 125I� , 131I� , 35SO4

2� could be affected by the negativesurface of the rock (ion exclusion) and by possible chemicalreactions (131I� , 35SO4

2�) (Bjornstad, 1991; Bjornstad et al.,1994). H14CO3

� may have a strong interaction with the matrix inlimestone or carbonate reservoirs (sorption or precipitation asCaCO3). [57CO(CN)6]3� and [57, 60CoY]2� exhibit low recoveries asa result of adsorption in the reservoir (Bjornstad et al., 1994).35SCN� and 35SO4

2� have a proven application only in sandstone(and limestone for 35SCN�) reservoirs. Finally, during a field test,S14CN� exhibited a restitution curve that differed from HTO butwas similar to the other anionic passive tracers. Since the 1990s,radioactive cobalts have not been used widely as a result of safetyconcerns (absorption or complexation on the well tubing)(Lichtenberger, 1991).

They are easily detectable at low concentrations using anaccelerator mass spectrometer for 36Cl� and 129I� (Bjornstad,1991) or liquid scintillation counting for S14CN� , 35SCN�

(LOD�0.005 Bq/L) and 35SO42� (LODo0.05 Bq/L) (Bjornstad,

1991; Zemel, 1995). The liquid scintillation counting and gammaspectroscopy methods are also used for 125I� (LODo0.05 Bq/L)and [56; 57; 58; 60Co(CN)6]3� (LOD�0.01 Bq/L) (Bjornstad, 1991).

Radioactive cationic molecules have rarely been used in IWTTtests because of adsorption issues. In addition to cobalt, otherelements have been tested (Bennion et al., 1995; Bjornstad, 1991;Bjornstad et al., 1994; Hutchins et al., 1991; Wheelet et al., 1985),including 22Naþ (Naþ , Cl�) (Bjornstad, 1998; Vilela et al., 1999;Zemel, 1995), 134, 137Csþ (Du and Guan, 2005; Wood et al., 1993),63Ni2þ , 59Fe3þ , 85,90Sr2þ ([90SrY]2�), Zn2þ ([65ZnY]2�), and Ir3þ

([192IrY]� or [192Ir(Cl)6]3�). Other radioactive molecules, such asradon, have also been proposed to follow the injected fluids (Navaet al., 2009).

Among these radioactive cationic molecules, only 22Naþ , 135,

137Csþ , 63Ni2þ (Bennion et al., 1995) and [65ZnY]2� are used inlong-term tracer tests because of their half-lives of several years(2.6 years for 22Naþ). 22Naþ (LOD between 0.06 and 0.10 Bq/Lusing liquid scintillation counting (Bjornstad, 1991; Zemel, 1995))has been described as a useful tracer in sandstone reservoirs athigh temperatures in saline waters (according to laboratory tests).134, 137Csþ yielded excellent results during tests (Bjornstad et al.,1994) in a carbonate reservoir. 90Sr2þ and [90SrY]2� have shownan acceptable recovery during a field test (Bjornstad et al., 1994).

However, 22Naþ and 134, 137Csþ can also be affected by thenegative surface of the rock (low reversible sorption) and aredelayed in comparison with HTO. 63Ni2þ showed a strong sorptionin a sandstone reservoir (Bjornstad, 1991) during field tests. 85Sr2þ

has never been detected in a production well during a field test.In 2000, 22Naþ was successfully used as passive water tracer

in the development of a low-cost, two-dimensional positronemission tomography system. This tool, commonly used innuclear medicine, is an imaging technique that can measuretwo- or three-dimensional tracer distributions and, thus, canvisualize tracer flow inside porous media (Haugan, 2000).

Radioactive species such as scandium, iridium, and antimony,analyzed by gamma ray spectrometry (Scott et al., 2010), wererecently proposed as tracer tools (Sanford et al., 2010; Scott et al.,2010) to better understand fracture geometry or to evaluate thedistribution of fluid across a long perforated interval.



3.1.2.1.3. Conclusion. Despite their potential radiation hazards,toxicity (radioactive halides), and short half-lives (with theexception of HTO), radioactive tracers have advantages thatmotivated their use until the 1990s (de Melo et al., 2001; Duand Guan, 2005; Fjerstad et al., 1993): ease of measurement(Bingyu et al., 2002); detection at very low concentrations (Adamset al., 1992; Bjornstad, 1991; Bjornstad et al., 1990, Zemel, 1995),requiring very small injection quantities (Bennion et al., 1995); avery low natural background in the environment (Du and Guan,2005); and few reactions with the fluids and with the rock.

Radioactive tracers were long considered more reliable thanchemical tracers (Bennion et al., 1995). However, in the 1990s,chemical species have been preferred to radioactive species as aresult of radiation hazards (Adams et al., 1992; Bjornstad et al., 2001).

3.1.2.2. Chemicals. Prior to the 1980s, only a few chemical tracers hadproven their efficiency or reliability (Adams et al., 1986, 1992;Hutchins and Saunders, 1992): the orthophosphate anion (H2PO4

� ,HPO4

2� , PO43�), detected by colorimetry (mg/mL detection level)

(Brown, 1959); various water-soluble iodide salts (NaI, KI)(Sandiford, 1967) (mg/mL level) and water-soluble thiocyanates andsalicylates patented by Sandiford (1967) (mg/mL level); water-solublesalicylate and thiocyanate ions (Sandiford, 1967); and PBTS(polyhydroxyalkyl-bis-triazinylamino-stilbene), detected by UV(Saniford and Knight, 1973). Salicylic acid and EDTA were alsoproposed by Riedel (1975), but their efficiency was not proven.Finally, these molecules were largely abandoned as a result ofproblems such as adsorption onto the rock; destruction (oxidization,precipitation, etc.); natural interference and contamination, whichmake analysis difficult (fluorescent compounds); large requiredvolumes and expense required for accurate detection, contrary toradioactive elements (for example, an alcohol requires severalthousand liters compared to only 5 mL of HTO) (Bennion et al., 1995).

Since the 1990s, interest in chemical tracers has greatly increased(Adams et al., 1992; Bjornstad et al., 2001) because they presentadvantages over radioactive tracers: a lack of radiation hazards andeasier detection as a result of simpler analytical techniques (gaschromatography (GC), high performance liquid chromatography(HPLC), nuclear magnetic resonance (NMR), ionic chromatography(IC), fluorimetry, colorimetry, UV spectroscopy, etc.) (Bennion et al.,1995; Bjornstad, 1991; Zemel, 1995), reaching the pg/mL level(Bennion et al., 1995).

3.1.2.2.1. Inorganic elements. Inorganic elements are interest-ing because they are cheap to purchase in large quantities (withthe exception of I�) (Hutchins et al., 1991) and to analyze(Cheung et al., 1999; Dalla Costa and Gaudet, 2006). They arealso easier to use than water isotopes (Dalla Costa and Gaudet,2006) and are typically available in water-soluble sodium orammonium salts (Lichtenberger, 1991).

Among the inorganic elements, halides, Cl� (Carter and Sides,1979; Ohno et al., 1987), Br� (Hetterschijt et al., 2000), and I�

(Axelsson et al., 2001; Bjornstad et al., 1990; Riedel, 1975;Wagner, 1976) have often been applied as conservative tracersin the oil industry (Bingyu et al., 2002; Shook et al., 2009) andgeothermal applications (Axelsson et al., 2001; Kumagai et al.,2004; Yanagisawa et al., 2002). These applications are welldocumented (Hutchins et al., 1991).

Cl� and Br� have proven their conservative behavior (Bensonand Bowman, 1994; Dalla Costa and Gaudet, 2006; Munz et al.,2010; Wienhofer et al., 2009) with low adsorption and lowthermal, chemical, and biological reactivity at low temperatures.However, a weak delay in porous systems (anion exchange) hasbeen observed for Cl� (Carter and Sides, 1979) and Br�. Inaddition, these elements can only be used in cases of low aquifersalinity or where their concentration in the injected water is

higher than in the formation water (to avoid the need to injectlarge quantities) (Wagner, 1976). I� is not recommended in oilreservoirs because of its natural presence in formation waters(Wagner, 1976) and its less conservative behavior than Cl� andBr� (Bjornstad et al., 1990; Chrysikopoulos, 1993).

Other molecules have also been proposed as tracers(Bjornstad, 1991; Bjornstad et al., 1997; Cheung et al., 1999;de Melo et al., 2001; Hutchins et al., 1991; Zemel, 1995), includingNO3� (NaNO3, LiNO3, NH4N03, KNO3) (Bjornstad et al., 1994;

Carter and Sides, 1979; Song et al., 1995; Yang et al., 2000) andNO2� (KNO2) (Ohno et al., 1987), SCN� (NaSCN, NH4SCN, KSCN)

(Agca et al., 1990; Ali et al., 2000; Allison et al., 1991; Cayias et al.,1990; Cheng et al., 2005; Gregory and Kocsis, 1984; Hutchinset al., 1991; Lichtenberger, 1991; Lliassov et al., 2002; Oyerinde,2004; Quang and Loder, 2006; Shook et al., 2009; Song et al.,1995; Tang, 2003a; Vilela et al., 1999; Yang et al., 2000),[CO(CN)6]3� (Quang and Loder, 2006), SO4

2� , BO33� , HBO3

2� , andB407

2� (Bjornstad, 1991, 1998).Among these molecules, KNO2 was successfully tested in a

56-d laboratory test (Ohno et al., 1987). Liþ (LiCl) and I� alsodemonstrated their usefulness as tracers able to follow the fluidsduring core tests in the laboratory (Shields et al., 2008).

The thiocyanate ion SCN� and the nitrate ion NO3� have often

been used as tracers in the oil industry to study fluid distribution(Song et al., 1995). However, researchers have found mixedresults concerning their stability. SCN� , characterized by a verylow adsorption and presence in oil reservoirs, has been used withgood results in a sandstone reservoir (with a recovery of 90%)(Agca et al., 1990; Lichtenberger, 1991; Stetzenbach andFarnhalm). However, it has also been shown that SCN� is notbiologically and chemically stable, with possible degradation attemperatures above 100 1C (Quang and Loder, 2006) and adsorp-tion to rock (laboratory test for NH4SCN over 56 days, withC/C0¼0.4 after 56 days) (Ohno et al., 1987).

Nitrate salts (NO3�) are not always applicable because they are

subject to chemical and bacterial degradation in aerobic andanaerobic conditions in oilfield brines (Bjornstad et al., 1990;Carter and Sides, 1979; Cheung et al., 1999; de Melo et al., 2001).

Among the other tested elements, [Co(CN)6]3� has beendecomposed after three months in a reservoir at 150 1C (biologicalreactions) (Quang and Loder, 2006), and BO3

3� (lost by adsorption(Bjornstad et al., 1994)), HBO3

2� , B4O72� (Bjornstad, 1991, 1998)

were not detected in produced sampled waters of the tested field(Zemel, 1995).

These anions are generally detectable by IC or HPLC(Lichtenberger, 1991; Yanagisawa et al., 2002). The IC LOD isapproximately 1 mg/mL for I� and Br� and approximately 0.1 mg/mL for NO3

� and SCN� (Hutchins et al., 1991). SCN� was alsoanalyzed by spectrophotometry, with an LOD of approximately1 mg/mL (Gregory and Kocsis, 1984). I� is also detectable by UVadsorption and Liþ by ICP-AES (Shields et al., 2008), but thesensitivity depends on the water salinity and overall composition.

3.1.2.2.2. Alcohols. The alcohols and ketones are chemical pro-ducts with a low cost (Cheung et al., 1999). As a result, thesecompounds have greatly interested the oil industry for more than50 years for use as tracers.

Only ketones and alcohols from C1 to C3 with Kow near zero areapplicable as passive water tracers, including methanol (Kow¼

0.0001), ethanol (Kow¼0.0004) (Cockin et al., 2000), propanol(n-propanol or NPA for normal propyl alcohol), isopropanol (IPA forisopropyl alcohol) (Kow¼0.03) (Bingyu et al., 2002; Bjornstad, 1991;Bjornstad et al., 1997; Carter and Sides, 1979; Cayias et al., 1990;Cheung et al., 1999; Du and Guan, 2005; Gregory and Kocsis, 1984;Hutchins et al., 1991; Jin et al., 1997; Lichtenberger, 1991; Shooket al., 2009; Vilela et al., 1999; Zemel, 1995) and acetone (Cayias et al.,1990).



However, despite their stability at very high temperatures (above300 1C for methanol) (Adams et al., 2004), these products havedrawbacks. For example, CH3OH is toxic (Cheung et al., 1999). Inaddition, these compounds can be subject to bacterial degradation(Carter and Sides, 1979; Hutchins et al., 1991; Zemel, 1995) at lowtemperatures. Thus, they are only suitable for moderate tempera-tures, according to Lichtenberger (1991). Finally, an initial highbackground noise for alcohols is possible in oil reservoirs because ofthe presence of chemical reagents containing many alcohols, leadingto a false interpretation of the tracer test results.

Among these alcohols, a field test of propanol showed adsorp-tion on the rock, with a recovery of 50% (Hutchins et al., 1991).Isopropanol is less susceptible to biological attack than eithermethanol or ethanol (Zemel, 1995). Acetone, also less affected bybacteria, was considered an effective tracer in 1990 (Cayias et al.,1990).

Alcohols, such as methanol, ethanol, isopropanol, and acetone,can be analyzed by GC/FID (mg/mL LOD for alcohols) (Cayias et al.,1990; Lichtenberger, 1991) or spectrophotometry (mg/mL LOD)(Bingyu et al., 2002). LODs of 100 ng/mL were also reached(Adams et al., 2004).

Methanol can also be analyzed by a GC coupled with a thermalconductivity detector (Wood et al., 1990). After 2004, ethanolanalyzed by GC (with a solid-phase microextraction) could bedetected with an LOD of 1 ng/mL (Adams et al., 2004; Mella et al.,2006).

3.1.2.2.3. Fluorescent molecules. Fluorescent dyes have severaladvantages: they are easy to use (Cheung et al., 1999; de Meloet al., 2001) and inexpensive to buy and analyze. The moleculestested in oilfield applications include fluorescein (Coronado et al.,2009; de Melo et al., 2001, 2005; Khalil and Oliveira, 1999; Ohmset al., 2009; Stray et al., 2005), eosin, rhodamine, (Du and Guan,2005; Lichtenberger, 1991; Ohms et al., 2009), and rhodamine-B(Bjornstad, 1991, 1998; Bjornstad et al., 1997; Cheung et al., 1999;Hutchins et al., 1991; Riedel, 1975; Zemel, 1995).

Fluorescein is thermally stable and is generally not adsorbed tothe geothermal reservoir rock (Axelsson et al., 2001; Rose et al.,2001b). With a highly stable fluorescence, fluorescein has beensuccessfully used in a carbonate reservoir with an 82.1% recoverypercentage (Khisamov et al., 2009). However, according to theseauthors, fluorescein, eosin, and rhodamine may sometimes adsorbonto the reservoir rock, partition with the oil phase (Cheung et al.,1999; de Melo et al., 2001; Lichtenberger, 1991; Wienhofer et al.,2009) and be biologically degraded (Stetzenbach and Farnhalm).Rhodamine-B has been lost during field tests, according toBjornstad (1991). Laboratory tests showed that fluorescein wasquickly degraded above 260 1C (Adams et al., 1992) and wasunstable at 200 1C in the presence of oxygen.

As a result of these shortcomings, the application of such dyes(fluorescein, rhodamine) is limited to fractured reservoirs withquick breakthroughs (up to 5 days) (Cheung et al., 1999;Lichtenberger, 1991).

These chemicals are relatively non-toxic at low concentrations(ultra-traces) (Becker and Coplen, 2001; Behrens et al., 2001),with the exception of rhodamine-B, which exhibits genotoxicproperties and is not recommended for water tracing (Behrenset al., 2001).

These dyes are detectable on site by fluorimetry (LOD at lowng/mL levels (0.3 ng/mL for fluorescein) assuming a negligiblebackground fluorescence) (Chrysikopoulos, 1993; Hutchins et al.,1991; Kleimeyer et al., 2001; Wienhofer et al., 2009; Yanagisawaet al., 2002). However, interference is possible during analysis as aresult of the natural fluorescence in the reservoir (oil and water)(Bennion et al., 1995; Cheung et al., 1999). Another method,spectrofluorimetry, enables the achievement of an LOD of 10 pg/mL for fluorescein.

The dyes are also easily detectable by colorimetry (mg/mLlevel) (Khalil and Oliveira, 1999) and UV (fluorescein detected at0.1–10 mg/mL) (Khalil and Oliveira, 1999).

As an alternative to these techniques and to reach LOD at thesub-pg/mL level, Kleimeyer et al. (2001) proposed, for hydrologi-cal applications, high-performance liquid chromatography (HPLC)coupled with a high-sensitivity laser-induced, multi-wavelengthfluorescence detector. A fluorescein concentration of 40 fg/mLcould be detected.

A fiber optic probe can also be coupled to a light source, andboth the fluorescence and reflectance signal can be detected(Ramos and Stephenson, 1999).

Other commercially available fluorescent molecules, includingnaphthalene sulfonic acid (NSA), naphthalene disulfonic acid(NdSA) and naphthalene trisulfonic acid (NtSA), have also beenproposed as conservative water tracers in oil reservoirs (Hutchinsand Saunders, 1992; Stray et al., 2005), but their use in oilreservoirs has occasionally been controversial in the 2000s(Hirtz et al., 2001).

Polycyclic aromatic sulfonic acids were first evaluated forgeothermal applications. Among them, the un-substituted NSAwere the most promising based on the selection criteria (thermalstability up to 300 1C, good detectability) (Rose and McPherson,1997).

NSA, NdSA, and NtSA were successfully tested in lab and ingeothermal applications (Adams et al., 1992; Hirtz et al., 2001;Mella et al., 2006; Rose et al., 2001b, 2003; Sanjuan et al., 2006;Stray, 2006; Stray et al., 2005; Yanagisawa et al., 2002, 2009).These compounds have a conservative behavior, are very ther-mally stable (until 250–330 1C) in liquid-dominated geothermalreservoirs and are resistant to adsorption to the rock (strongelectronic charge in the sulfonic acid group) and do not posehazards to the environment for long times (up to 4 years for 1,5-NdSA (Sanjuan et al., 2006)) according to Rose et al. (2001a).

2-NSA and 2,7-NdSA are the most stable compounds amongthe tested polyaromatic sulfonic acids (Rose et al., 2001a). How-ever, 2-NSA is not always recommended in oilfield applicationsbecause of its possible presence in the wells as an additive (Stray,2006).

The polyaromatic sulfonic acids substituted with hydroxy andamino groups were also successfully tested up to 250 1C (Roseet al., 2001b). However, their thermal stability is lower than NSA,and the compounds substituted with amino groups can react toform stable and fluorescent compounds.

The biphenyl, p-terphenyl and fluorene sulfonic acids, pro-posed by Stray (2006), have a high thermal stability similar toFBA. Among them, 4,40-biphenyl-disulfonic acid has a very lowadsorption to the rock (tested at 195 1C over 60 days).

NSA are detectable by HPLC/FLD, with an LOD of 200 pg/mL(Rose et al., 2001a, 2001b, 2003), or at the ng/mL level by HPLC/UVusing a pre-concentration procedure (Hutchins and Saunders, 1992).The choice of fluorescence detector can be determined by thecompounds’ fluorescence properties and financial considerations.

The biphenyl, p-terphenyl and fluorene sulfonic acids are alsodetectable by HPLC/FLD (10–100 pg/mL) (Stray, 2006). The poly-aromatic sulfonic acids substituted with hydroxy and aminogroups present a better detectability (pg/mL level) by fluorimetrythan NSA (Rose and McPherson, 1997; Stray, 2006).

3.1.2.2.4. Substituted benzoic acids and fluorinated benzoic acids. Inthe 1990s, other types of tracers were developed for hydrothermal,geothermal and oilfield applications (Hutchins and Saunders, 1992),including sodium benzoate (Sanjuan et al., 2006) and variouslysubstituted benzoic acids (Chrysikopoulos, 1993; Hirtz et al., 2001;Hutchins and Saunders, 1992; Rose and McPherson, 1997; Roseet al., 2001b), such as trifluoro-p-toluic acid (Ghaddhab et al., 2010)and fluorinated benzoic acids (FBA) (Ali et al., 2000; Bjornstad,



1994b, 1998; Du and Guan, 2005; Dugstad et al., 1999; Galdiga,1998; Galdiga and Greibrokk, 1998a, 1998b, 1998c; Gieles andBeuthan, 2004; Hampton et al., 2001; Hernandez et al., 2002a;Huseby et al., 2009; Meza et al., 2007; Quang and Loder, 2006; Strayet al., 2005).

These aromatic compounds are not adsorbed to negativelycharged reservoir rocks because of electrostatic repulsion forces(Rose and McPherson, 1997). The substituted carboxylic andbenzoic acids are take an anionic form in the basic pH of naturalgroundwater and, as a result, are highly soluble in water.

Initial laboratory tests showed conservative behavior for thesecompounds (150 1C, 172 h, in nitrogen and oxygen atmospheres)(Adams et al., 1986).

Some of these compounds, such as tetra benzenic carboxylicacids and methyled benzoic acids (Adams et al., 1992; Hutchinsand Saunders, 1992; Rose et al., 2001b), are characterized by ahigh thermal stability (250–300 1C) and a low presence in thefluids in the reservoir.

However, the methyled benzoic acids can be subject tobiodegradation. However, in geothermal studies, bacteria arenot active at temperatures over 100 1C (the use of a preservative(NaN3) between sampling and analysis avoids this problem(Adams et al., 1992)).

According to laboratory tests, fluorinated phenyl acetic acidsare less stable than the FBA, and perfluorinated benzoic acids arethe least stable compounds, being subject to chemical decom-position (Adams et al., 1992).

The use of sodium benzoate in geothermal applicationsshowed that the tracer behaved conservatively over a long period(up to 4 years) (Sanjuan et al., 2006), but its use in oil reservoirswas not advised according to Hirtz et al. (2001).

Commercially available, these acids can be detected at very lowconcentrations (ng/mL level) by HPLC/UV methods (Adams et al.,1992; Hutchins and Saunders, 1992; Rose and McPherson, 1997).

Finally, of all these aromatic acids, the oil industry haspreferred to focus on fluorinated organic compounds with a highlevel of stability and detectability (Adams et al., 2004): FBAcompounds (19 molecules are commercially available). Even ifthey are less thermally stable than their methyled analogs(Adams et al., 1992), several studies, conducted in both labora-tories and oilfields, have shown that FBA exhibit conservativebehavior (low adsorption on rock), high thermal stability (up to150–175 1C), and chemical and biological stability (Bjornstad,1994b, 1997, 1998; Galdiga, 1998; Gieles and Beuthan, 2004;Hernandez et al., 2002a). Similar restitution curves to those forHTO and SCN� (Bjornstad, 1994b; Dugstad et al., 1999) or Br�

(Benson and Bowman, 1994; Dahan and Ronen, 2001) wereobserved.

According to different experimental tests (Bjornstad, 1994b,1997; Seaman et al., 2007), the thermal stability of FBA is verysatisfying up to 120 1C (2-fluorinatedbenzoic acid; 2,6-difluoro-benzoic acid; 2,3,4-trifluorobenzoic acid; 2,4,5-trifluorobenzoicacid; 3,4,5-trifluorobenzoic acid). The thermal stability is judgedas acceptable up to 150 1C for 3-fluorobenzoic acid and even up to175 1C for some other FBA molecules (4-fluorobenzoic acid).

The stability of FBA can be explained by the carbon–fluor bond,which is the shortest, and thus strongest and most stable, carbon–halide bond (Stetzenbach and Farnhalm). In addition, FBA has avery low presence in the reservoir fluids and a low toxicity(Stetzenbach and Farnhalm).

All of these characteristics explain the common use of FBA inthe oil industry since the 2000s (Huseby et al., 2009; Maloneet al., 2003; Meza et al., 2007; Pritchett et al., 2010; Seccombeet al., 2010; Valestrand et al., 2010).

However, a comparison study between some FBA and HTOmolecules showed a weak delay in the restitution curves for

3-trifluoromethylbenzoic acid, 2,4-difluorobenzoic acid and2,6-difluorobenzoic acid (Seaman et al., 2007). In contrast,2-fluorobenzoic acid had a higher velocity than HTO in field testsbecause of ion exclusion.

To date, several analytical methods have been proposed(Malone et al., 2003), including LC and GC methods with LODsof approximately 10 ng/mL by HPLC/UV, 0.1–1 ng/mL by LC/MS-MS and 0.1–0.01 ng/mL by GC/MS (Galdiga and Greibrokk, 1998a,1998c; Hernandez et al., 2002b; Tyler, 2010). Recently, a simpler,faster and more robust method, with a similar LOD to GC/MS, hasbeen developed: UHPLC/MS-MS (ultra-high-performance liquidchromatography coupled with tandem mass spectrometry)(Preud’homme, 2010; Serres-Piole et al., 2011).

3.1.3. Other types of tracers

3.1.3.1. Deuterated compounds. Deuterium oxide (D2O) (Bennionet al., 1995; Fjerstad et al., 1993; Zemel, 1995) and deuteratedwater (HDO) (Anderson, 2006) have been successfully used aspassive water tracers in the oil industry and hydrology because oftheir similar properties and structure to water (Anderson, 2006).

A comparative hydrological study between HDO and Br�

(Becker and Coplen, 2001) showed that HDO had passive proper-ties. HDO, which is non-toxic, has a low natural abundance in theenvironment compared with water enriched with 18O. It iscommercially available, easy to use and sample, and has anacceptable cost. HDO can be distinguished from other waters byits low levels of deuterium or deuterium oxide. However, isotopicexchange (H) can occur and cause a delay compared to the othertracers because of the presence of minerals.

HDO can be analyzed by isotope ratio mass spectrometry(IR-MS) with an LOD of 0.1 mg/L (field tests) (Becker andCoplen, 2001). However, the analysis cost is high because of theneed for specialized equipment.

Finally, despite its high cost, this type of deuterium-labeledtracer was widely used during the early 1990 s, especially forSWTT operations (Bjornstad, 1991).

D2O is not highly recommended. It is expensive (Anderson,2006) because of the ratio of water necessary to form the product(approximately 10 tons of water to prepare 1 kg of oxide ofdeuterium) and for analysis (Rossi, 1994). D2O requires knowl-edge of the natural abundance of deuterium in the formationwater to conduct a quantitative analysis. In addition, it can bepresent in the environment (150 mg/mL), which limits its use.

D2O can be detected by GC or HPLC coupled with mass spectro-photometry, nuclear magnetic resonance (NMR) imaging (Anderson,2006), or mass spectrometry (pg/mL LOD) (Bennion et al., 1995).

Deuterated short-chained fatty acids, which are also analyzedby GC/MS, have also been considered as possible tracers (Galdigaet al.). These short-chained acids, water soluble and stable underreservoir conditions, are naturally present in the reservoirs. Thesecompounds are tagged with deuterium content above the naturallevel to allow detection (ethanoic acid, propionic acid, butanoicacid, etc.). Thus, they have the same properties as the naturalshort-chained acids. They are stable under reservoir conditions(tested at 150 1C over 8 weeks), with similar behavior to HTO.Among these compounds, pivalic acid is unstable under reservoirconditions. In addition, the costs are high, and the low number ofsuppliers indicates low available quantities.

3.1.3.2. Stable activable tracers. The stable activable tracers (stabletransition metals 51V, 59Co, 115In, etc.) are stable isotope tracers.Their properties were first used in hydrological and geothermalapplications (Behrens et al., 2001). These tracers have the same



advantages as the radioactive isotope tracers (Bingyu et al., 2002).As a result, they are used as an alternative to radioactive species(3H, 14C, 82Br, 131I, etc.). However, as with the radioactive species,special equipment is needed (Bingyu et al., 2002).

Among the stable activable tracers, non-radioactive gadoliniumwas proposed as a tracer, but the technique (pulsed neutron tool)used for its detection has disadvantages (Ramos and Stephenson,1999).

Indium (In) (Chrysikopoulos and Kruger, 1987) has beenconsidered as an excellent activable tracer for geothermal appli-cations because of its high detectability and low presence in thesubsurface formation (less 100 ng/mL). It is a conservative tracerat low temperatures, with very low sorption in natural materials,and is applicable for short-term tracer tests (20 days) at tem-peratures below 200 1C. Stable activable tracers cannot be used ascationic tracers because of the risk of hydrolysis at the basic pH ofgeothermal fluids. Thus, they are used with organic ligands(EDTA) to form chelated species. This complex form ensuresgeochemical stability, mobility, and solubility in water and pre-vents precipitation (Chrysikopoulos, 1993). However, the com-plex [InY]� is not applicable at high temperature owing to itsdegradation at 150–200 1C.

For oilfield applications, Green and Stubbs (2006) proposed anew type of non-radioactive tracer, particularly for the determi-nation of the flow of fluid from a well (SWTT test). These newtracers are composed of one metal (cesium, niobium, tantalum,tellurium, iridium, osmium, iron, platinum, palladium, rhenium,ruthenium, rhodium, hafnium, indium) or a lanthanide metal(lanthanum, cerium, terbium).

The stable activable tracers are detectable at very low con-centrations (Adams et al., 1992) using analytical techniques suchas inductively coupled plasma-mass spectrometry (ICP-MS). Thistechnique provides an LOD and LOQ of lanthanum of 35 pg/mLand 117 pg/mL, respectively (Green and Stubbs, 2006). Thesetracers can also be analyzed by neutron activation at the pg/mLlevel (Bingyu et al., 2002).

3.1.3.3. Microelements. Microelements that are very rare or absentin the reservoir water could be used as tracers (the types ofelements that fit this description are not available). Theseelements have the advantages of stable tracers, including lowcost, higher measurement precision with fg/mL detection, andstability at high temperatures, but special equipment is alsorequired (Bingyu et al., 2002).

3.1.3.4. Polymers. In enhanced oil recovery methods using apolymer (polyacrylamides), the use of the polymer itself aspotential tracer has been proposed by Manichand et al. (2010).The advantage is that the polymer is not present in the studiedreservoir. However, it has a high LOD (50 mg/mL). In addition, thetravel time of the polymer was too slow because of retention inporous media. Thus, polymers are not suitable as tracers toestablish interwell connections.

3.2. The partitioning tracers

3.2.1. For SWTT

The primary partitioning tracers used in SWTT are reactivecompounds such as esters. The injected ester in the well-borezone partitions into the immobile oil phase, which slows thetracer penetration (Buijse et al., 2010). The ester reacts, by ahydrolysis reaction, with the reservoir water to form an alcoholtracer, which does not partition with oil. After a shut-in of severaldays, the two tracers, ester and alcohol, are back produced. Theinterpretation of this test requires the knowledge of the partition

coefficient Kow and the hydrolysis rate of the ester in the reservoirconditions (determined in lab) (O’Brien et al., 1978). The differ-ence in time between the alcohol (which travels faster than theester) and ester peaks is a direct measure of the SOR, for ahomogeneous system (Buijse et al., 2010; O’Brien et al., 1978;Deans, 1978).

The most commonly used tracers are esters of formic andacetic acids (Keller and Linda, 1972): methyl acetate (Kow¼0.869;Kow¼1.92 (Deans, 1978; Dijk et al., 2010; Sheely and Baldwin,1982; Tang and Zhang, 2001)), ethyl acetate (Kow¼2.786 (Kellerand Linda, 1972); Kow¼4.65 (Buijse et al., 2010; Chang et al.,1988; Cockin et al., 2000; Deans, 1978; Hernandez et al., 2002b;Jerauld et al., 2010; Knaepen et al., 1990; Othman et al., 2007;Sheely, 1978; Sheely and Baldwin, 1982; Shook et al., 2009;Skretinggland et al., 2010; Tomish et al., 1973; Wellington andRichardson, 1994)), isopropyl acetate (Kow¼8.478; Kow¼8.20(Deans, 1978; Sheely and Baldwin, 1982)), ethyl formate (Bragget al., 1978; Gardien et al., 1996; Shook et al., 2009; Stoll et al.,2010; Talukdar and Instefjord, 2008), or propyl formate (Bragget al., 1978; Cayias et al., 1990; Myal and Wesson, 1981; O’Brienet al., 1978; Sheely, 1978).

Among these compounds, propyl formate was one of the firsttracers used because of its fast reaction rate (Deans and Mut,1997). The low-molecular-weight esters, particularly ethyl acet-ate, are suitable at temperatures up to 121 1C (Deans and Ghosh,1994). However, methyl formate is unsuitable because it hydro-lyzes too rapidly under reservoir conditions (Tang and Zhang,2001).

These esters are always co-injected with non-partitioningtracers, usually alcohols (NPA, IPA, n-propanol, methanol, 2-pro-panol) (Cockin et al., 2000; Garnes, 1992; Hernandez et al., 2002b;McGuire et al., 2005; Othman et al., 2007; Stoll et al., 2010; Tangand Zhang, 2001; Wellington and Richardson, 1994). They areuseful for the material balance (to identify the water thatcontained the ester, Cockin et al., 2000). They are also used as abackup tracer (n-propanol) (Hernandez et al., 2002b).

In certain tests, butanol (a partitioning tracer with a K veryclose to that of the primary tracer, methyl acetate) was also co-injected with the primary tracer (Tang and Zhang, 2001).

Tracer concentrations (alcohol, ester) are generally analyzedon site by GC methods (Deans, 1978; Dijk et al., 2010; Hernandezet al., 2002b; Othman et al., 2007; Sheely and Baldwin, 1982;Tang and Zhang, 2001), particularly headspace gas chromatogra-phy (Garnes, 1992).

3.2.2. For PITT

To date, few partitioning tracers have been developed, espe-cially for PITT. This dearth may be explained by the complexity oftheir partitioning behavior, which differs by reservoir type.

Before the use of non-radioactive chemicals, the first com-pounds were radioactive. These compounds were tritiated alco-hols with a higher molecular weight than methanol, ethanol, andpropanol; a higher Kow; and a high detectability by liquidscintillation counting and included tritiated n-butano1 (TNB)(K¼1.3) (Wood et al., 1990) and C14-tagged i-amyl alcohol (CIA)(Tang, 1992, 1995; Tang and Zhang, 2000).

Since the 1990s, these alcohols have been the most commonlyused compounds, but in a non-radioactive form (Cheung et al.,1999; Lichtenberger, 1991). The following compounds weretested in laboratory and field tests and defined as suitablepartitioning tracers to estimate the SOR: propanol (Tang, 1995),IPA (Kow¼0.04) (Lichtenberger, 1991; Oyerinde, 2004; Tang,2003a), butanol (Kow¼0.41) (Hutchins et al., 1991), tertiary butylalcohol (TBA) (Kow¼0.2) (Allison et al., 1991; Cheng et al., 2005;Du and Guan, 2005; Lichtenberger, 1991; Lliassov et al., 2002;



Oyerinde, 2004; Tang, 1995, 2002, 2003a), 2-butanol (Kow¼0.32)(Hutchins et al., 1991), 2-pentanol (Kow¼1.50) (Hutchins et al.,1991), n-pentanol (Kow¼1.4) (Yoon et al., 1999), hexanol (withKow¼5.6) (Delshad et al., 2002), n-Hexanol (Kow¼4.6) (Yoon et al.,1999), 1-heptanol (Jin et al., 1997), 2,4-dimethyl-3-pentanol(Hetterschijt et al., 2000), 2,2-dimethyl-3-pentanol (Kow¼12.9)(Yoon et al., 1999), and 2-ethyl-1-butanol (Kow depends on theexperimental conditions).

These alcohols are stable at very high temperatures (between250 1C for n-butanol) (Adams et al., 2004), but some HTO or SCN�

field tests showed that TBA and IPA were partially adsorbed in thereservoir (Lichtenberger, 1991).

In addition, the literature contains contradictory findings onthe partitioning behavior of NPA and IPA; NPA and IPA were alsodefined as passive tracers and used in SWTT for material balance(Delshad et al., 2002; Jerauld et al., 2010; Skretinggland et al.,2010; Vilela et al., 1999).

Alcohols such butanol were detected by GC with a thermalconductivity detector (Wood et al., 1990), but their use is not veryinteresting because of their high LOD (mg/mL level).

Other compounds detectable by HPLC techniques were testedin 2000 with promising results: 2,6-bis(trifluoromethyl)benzoicacid, 2,4-bis(trifluoromethyl)benzoic acid, 5-fluoro-2-methyl ben-zoic acid, p-(trifluoromethyl)benzoic acid, and 3,5-di(trifluoro-methyl)benzoic acid (Chatzichristos et al.).

Phenol was used as a partitioning tracer in the development ofa two-dimensional planar flow model (Tang, 2003b).

Methyl-ethyl ketone was also used as a partitioning tracer(Lichtenberger, 1991).

Other partitioning alcohols have been used to locate and quantifynon-aqueous phase liquids (Jin et al., 1995) present in vadose andsaturated zones, including 2,2-dimethyl-3-pentanol (Annable et al.,1998) and 2-dimethyl-3-pentanol (Wilson, 1995; Yoon et al., 1999).

Khalil and Oliveira (1999) proposed the use of the sodiumderivative of fluorescein and the ester derivative of lower alcoholsfor use in the aqueous phase to characterize reservoir studies(fluorescein ester (oil soluble) and oleophilic fluorescein).

Finally, to develop a cheaper method than PITT, Sinha et al.(2004) successfully investigated the possibility of using the moresoluble organic compounds in oil naturally present (aliphatic acidand alkyl phenols) in crude oil as natural tracers to estimate oilsaturation and swept pore volumes (acid acetic, butyric acid,phenol, o-cresol, 2,4-dimethyl phenol).

Trans-stilbene, analyzed by UV absorption, was also used as atracer in the oil phase in a core flood test (Shields et al., 2008).

3.2.3. Conclusion

The use of partitioning tracers in PITT tests remains complex. Incomparison with SWTT methods, PITT methods are rarer in theliterature (Deans and Mut, 1997). Their selection is difficult becausean overly long delay compared to the passive tracer might increasethe costs of a tracing test (additional sampling and analysis).

In the 2000s, few chemicals were available and applicable aspartitioning tracers, and the data interpretation was largelyqualitative (Guan and Du, 2004).

3.3. Nanoparticles

Since the 2000s, nanotechnology has become very promisingin a large number of domains, for both fundamental research andindustrial applications (Kong and Ohadi, 2010; Tillement et al.,2006). Nanoparticles were first studied and used for medicalapplications but have become increasingly studied for differentapplications, such as formation characterization, enhanced oilrecovery methods (Kong and Ohadi, 2010; Metin et al., 2010;

Onyekonwu and Ogolo, 2010; Yu et al., 2010a, 2010b), and theSOR determination (Prodanovic et al., 2010), the latter of which isdifficult when using partitioning tracers.

Nanoparticles, with their unique optical, magnetic, paramagneticand chemical properties, can contribute to reservoir description viathe development of nano-sensors (Kong and Ohadi, 2010).Obviously, the nanoparticles must be stable in the reservoir condi-tions (temperature and pressure) and detectable directly on site.

Because nanoparticles have a nanometer (nm)-level diameter,they are waiting to have a similar behavior like molecules andmigrate easily through the pores in reservoirs without interactingwith the rock and through pore throats, even at very large particleconcentrations (Rodriguez et al., 2009). Nanoparticles may haveinteresting properties for potential application as water tracers in oilreservoirs (Prodanovic et al., 2010; Rodriguez et al., 2009). However,several experiments are necessary to understand the transport andretention of nanoparticles in an oilfield environment (with highsalinity, high temperature, etc.) (Yu et al., 2010b).

Nano-tracers are composed of a core and a coating (McDanielet al., 2009). The nature of the coating depends on the application:hydrophilic if the objective is to follow the water phase orhydrophobic if the objective is to reach the non-aqueous phase(Prodanovic et al., 2010).

The coating, consisting of an inert polymer, surfactant ordispersant (Prodanovic et al., 2010; Zhang et al., 2009) (forinstance, polysiloxane or polyethylene glycol), is necessary toprotect the core and to allow the nanoparticles to remain stableand dispersed in water, without aggregation or adsorption, and tochemically protect the core during the test without a loss ofluminescence intensity (Prodanovic et al., 2010; Rodriguez et al.,2009; Zhang et al., 2009). The adsorption of polymers to particlesenhanced their mobility in both calcite and porous sand media, inwhich nanoparticles showed similar migration to an inert tracer(Zhang et al., 2009).

Depending on their intended application, the encapsulatingagent can be oils, polymers, or a mixture and can be natural orsynthetic (Malone et al., 2003).

The core can be either organic (fluorophores encapsulated andprotected by a shell from tens to hundreds of nanometers thick)or inorganic (oxides or metals inserted in balls of a few hundrednanometers in diameter). An inorganic core provides a higherstability (Tillement et al., 2006).

During the experiment, the coating, in contact with the fluids,can break and release an encapsulating agent (e.g., a fluorescentmolecule) used as the tracer. The tracer is diffused in response tothe progressive degradation of the polymer (Malone et al., 2003).

Some core tests using a limestone rock have shown thatnanoparticles could be transported through sedimentary rocks,even those of low permeability (Rodriguez et al., 2009).

To date, several materials have been studied (with HTO as areference tracer).

Paramagnetic nanoparticles, induced by an imposed magneticfield, are potentially useful (Ryoo et al., 2010). These nanoparticlesare characterized by a long-term dispersion stability in brines withminimal retention in reservoir rock and with preferential adsorptionat the oil–water interface. When exposed to a magnetic field,paramagnetic nanoparticles generate sufficient interfacial move-ments for external detection. Because of their small size, paramag-netic nanoparticles are mobile even in magnetic fields.

The subsurface applications of super-paramagnetic nanoparti-cles, ferrofluids, began in the late 1990s through a porousformation by exposure to a strong magnetic field (Prodanovicet al., 2010).

Optical nanoparticles characterized by the combination of a coreof gadolinium oxide (Gd2O3) doped with luminescent and magneticrare-earth ions (Tb3þ ions) or organic dyes have been studied



(Tillement et al., 2006). These nanoparticles are ultra-sensitive,multicolored and represent highly photostable luminescent tracers.The use of lanthanides, such as Tb3þ , instead of organic dyes isinteresting because of their attractive optical properties and highphotostability, suggesting that they may be appropriate for long-term studies and multi-tracing (Tillement et al., 2006).

Recently, additional materials have been studied. Zinc phospho-nate nanoparticles (Zhang et al., 2010), Si–Zn–DTPMP (diethylenetriamine pentakis (methylene phosphonic acid)) were tested in thelaboratory in a column containing calcite and sandstone porousmedia. These nanoparticles showed reasonable migration in thesecore materials. Nanometer-sized metal-phosphonate particles, cal-cium chloride and zinc chloride, mixed with a basic phosphonatesolution were tested in the laboratory (Zhang et al., 2009).

Carbon nanoparticles (NPs) tested in the laboratory (dolomitecore material) showed retardation in comparison with HTO (Yuet al., 2010b). High-ionic-strength and multivalent ions (Ca2þ ,Mg2þ) could have affected carbon NP breakthrough.

Thus far, few works have addressed the transport of nanopar-ticles in reservoir rock formations (Yu et al., 2010b).

3.4. Discussion

The selection of tracers with suitable characteristics can bedifficult because it depends on the study objectives and thereservoir characteristics.

Generally, in the case of mature oilfields, artificial tracers arepreferable to natural tracers. Among these artificial tracers, radio-active tracers are not recommended for HSE reasons. Non-radioactive chemical species are clearly preferable.

However, the lack of variety of dyes and chemicals remains aproblem. Among chemicals, alcohols, inorganic anionic ions, andrelated compounds, scientists have focused on fluorinated com-pounds for several years because they are generally moredetectable (Adams et al., 2004). Among these compounds, bro-mide and benzoates have been the most widely used and areconsidered reference tracers for geothermal applications (Hirtzet al., 2001). Dyes and alcohols were rejected because of theobserved adsorption phenomena in the rock reservoir, chemicalor microbial degradation and interference or natural contamina-tion, all of which hinder the analysis of these types of tracers.

Finally, few tracers that meet the selection criteria are cur-rently available for use as water tracers (Huseby et al., 2010). FBA,with a negligible background noise in the reservoir waters, hasseen the broadest use as a passive water tracer. These compoundscan be detected at very low concentrations using a rapid andsensitive technique, by direct injection, and without samplepreparation (UHPLC/MS-MS). Eighteen passive FBA and one par-titioning FBA are commercially available as water tracers(McCarthy et al., 2000).

In comparison with SWTT methods, PITT methods have beenless frequently reported (Deans and Mut, 1997). The use of thepartitioning tracer is quite difficult. Its selection must be appro-priate because a high degree of retardation in comparison withpassive tracers can considerably increase the costs of the tracingtest (as a result of the additional sampling and analysis required).In the 2000s, few chemicals were available for use as partitioningtracers, and the analysis of the tracer response curves was stillqualitative (Guan and Du, 2004).

4. Molecules analytically studied at the laboratory scale

Currently, the lack of chemical tracers prevents the repetitionof tracer tests in oilfields already being monitored or the imple-mentation a tracing test in a reservoir with multiple injecting

wells. Thus, the development of new types of water tracers isincreasingly necessary.

The selection of new tracers for reservoir applications requiresa careful assessment of the positive and negative features ofpotential candidates. Among the main selection criteria (seeSection 2.3.1) (Wu et al., 2008) – stability, passive behavior,detectability, and toxicity – detectability is key and must be thefirst parameter considered in the tracer development phase; verylow LOD and LOQ are required to inject very low quantities in theoil reservoirs and hence to limit the environmental impact(‘‘environmentally friendly’’ tracers). To prevent a high analyticalcost, the aim is to increase the number of tracers withoutchanging the analytical technique between the current FBAtracers and the new tracers. Achieving this cost parity is animportant consideration for tracing campaigns.

In this investigation, the strategy defined was as follows: (1)consider FBA as ‘‘reference water tracers’’; (2) use the sameanalytical tool, which is sensitive, selective, simple (without samplepreparation), rapid and robust, for the simultaneous detection ofdifferent types of tracers used in tracing campaigns, including FBAand other families of tracers. The UHPLC/MS-MS technique recentlydeveloped for the detection of FBA in reservoir waters (Serres-Pioleet al., 2011) addresses all of these analytical criteria: it is rapid(5 min analysis), robust, ultra-sensitive and selective. In addition,this method is reliable with regard to the representativeness of theresults because chemical modifications are not necessary. Directinjection prevents problems such as losses of some compounds orcross contamination during sample preparation.

The objective here was to propose tracers that are alsodetectable by UHPLC/MS-MS, by direct injection, with similarLOD to FBA.

A first analytical study was conducted with molecules alreadylargely used in geothermal applications: NSA.

Next, other molecules, the halogenated boronic acids (seeFig. 1), which had never been used as tracers in any discipline(hydrology, geothermal energy, etc.), were tested for potential useas tracers. These molecules were interesting because they havesimilar structures to FBA. The only differences are the acid group(useful for good solubilization) and the halogenated substituentsin the ring. As a result, these molecules are expected to have thesame chemical and physical properties as FBA (thermal stability,passive behavior in the reservoir, etc.).

Although these molecules, which are commercially available, arerelatively unknown (with few articles available in literature), theyare of general interest because the presence of an aromatic ring inthe molecules allows them to express a number of isomers withsimilar characteristics that can be chromatographically separated.

Several analytical techniques in addition to UHPLC/MS-MSwere tested in this work to compare their respective sensitivity,including UHPLC/ICP-MS (presenting excellent performance interms of sensitivity) and UHPLC/FLD.

For the selection of the MRM (multiple reaction monitoring)transitions for the MS-MS detection of the tested compounds, ahybrid mass spectrometer ESIqQqTOF (ESI, electrospray source; qQq,triple quadripole; TOF, time of flight) (QSTAR XL, Sciex Canada) wasfirst used to identify daughter ions of compounds by high-resolutionMS with the direct infusion of individual compound solutions in

Fig. 1. Example of halogenated boronic acids.



both modes, negative and positive (20 ml min�1 (25 mL of thestandard solutionþ1000 mL acetonitrile (CH3CN)þ3975 mL ultra-pure water (H2O), 2 mg/mL)). This procedure leads to very accuratemass measurements for the identification of compounds. ESI-QQQwas applied based on the previously found daughter ions, and oncethe setting parameters were optimized, the choice of the MRMtransitions was confirmed.

4.1. Experimental

4.1.1. Reagents and chemicals

NSA and standard halogenated boronic acids (purity 497%)were purchased from Sigma Aldrich. The solvents (HPLC grade)and products were also purchased from Sigma Aldrich, includingacetonitrile (CH3CN, Fluka, LC/MS, 99.9%), methanol (CH3OH, LC/MS, Chromasolv, 99.9%), formic acid (HCOOH, Fluka, 98%, MSgrade), sodium hydroxide (NaOH, Rectapur, Prolabo, 98% min),ammonium bicarbonate (NH4HCO3, ReagentPlus, 99%), andammonium acetate (CH3COONH4, 99%). Ultra-pure water(18.2 MO cm), obtained from a Millipore system (systems Elix3 and Advantage, Millipore, Saint-Quentin, France), was usedthroughout.

4.1.2. Standards and samples

Standard solutions (300 mg/mL) were prepared by dissolvingdifferent amounts of compounds (between 15 and 30 mg,depending on the compound), accurately weighed, in 40 mL ofH2O and 10 mL of 3975 mg/mL NH4HCO3 solution. These solutionswere stored at 4 1C in the dark for up to 4 months (the stability oftheir concentrations after 4 months of storage was confirmedwithin 10%).

Working solutions were prepared by the appropriate dilutionof the stock solutions with water. Water samples spiked with amixed standard solution at different concentrations (between0.5 mg/mL and 100 pg/mL) were used for this study. The sampleswere filtered through a GHP Acrodisc 13-mm syringe filter(0.2-mm GHP membrane, Pall Life Sciences, Interchim, France).

4.2. Example applications

4.2.1. NSA, alternative analytical techniques

The comparative study of the different analytical techniques(Table 1) for NdSA showed that UHPLC/MS-MS is also applicablefor NdSA, with similar LOD to the FBA, by direct injection (seeFig. 2). In addition, as a result of the optimization of the elutiongradient and the selection of specific MRM transitions, thistechnique enabled the simultaneous detection of FBA and NdSAin water (data not shown).

The most abundant MRM between the precursor ion and theproduct ion corresponds to the loss of the SO3

� (m/z¼80) frag-ment corresponding to the sulfonic group. The optimum transi-tions monitored are summarized in Table 1. In UHPLC/MS-MS, theselectivity, which is a key factor in preventing false-positiveresults, is excellent (the retention time of each analyte in sampleswas within 72 s of the RT of the analyte in the aqueous standardmixture). The positive confirmation of the target analyte insamples is achieved by the combination of a specific transitionand the retention time relative to the compound.

The analysis by UHPLC/ICP-MS, in which 32S and 34S weremonitored, was not appropriate for the detection of NdSA inwater samples: the low ionization degree of the sulfur element(14%) leads to a high LOD.

4.2.2. Halogenated boronic acids

The sensitivity of four compounds (2.5 difluorophenyl boronicacid; 3.5 difluorophenyl boronic acid; 5-bromo-2-fluorophenylboronic acid; 5-chloro-2-fluoro phenyl boronic acid) was evalu-ated by testing different analytical techniques. This studyrevealed that these compounds are difficult to detect by directinjection, with LODs similar to FBA and NS (ng/mL level).

Among the tested techniques, only UHPLC/MS (single mode),with the electrospray in negative mode, allows the detection ofhalogenated boronic acids (see Fig. 3); this technique leads, bydirect injection, to an LOD of between 5 and 300 ng/mL.A preconcentration at the SPE phase would improve the sensitiv-ity by a factor of approximately 20.

The use of neither UHPLC/FLD nor UHPLC/ICP-MS (Bore 10Band 11B monitored, ionization degree of 58%) is appropriate, even

Table 1Detection of NSA in water.

Detection parameters

Compounds HPLC/FLDa (excitation and

emission wavelengths)

UHPLC/FLDb (excitation and

emission wavelengths)

UHPLC/MS-MSn

MRM transitions

UHPLC/ICP-MSe

Monirored element

1-NSA 217/333

2-NSA 220/336

1,5-NdSA 218/334 223/330 286,904206,90 32S and 34S

1,6-NdSA 224/337 223/330 286,904206,90 32S and 34S

2,6-NdSA 225/342 223/330 287,004206,90 32S and 34S

2,7-NdSA 226/339

1,3,5-NtSA 224/340

1,3,6-NtSA 228/342 223/330

1,3,6,8-pyrenetetra-NSA 346/386

LOD 200 pg/mL 200–600 pg/mL 200–900 pg/mLc /70 pg/mLd 10 ng/mL

Techniques compared in the same sample preparation conditions, ie., in ultra-pure water.

In UHPLC/FLD, to develop a single method the simpliest as possible applicable for reservoir waters and allowing the simultaneous detection of the NSA, a compromise was

defined: the wavelengths for excitation and emission were selected to be the same for all the studied NSAs. Only one channel of wavelength was selected, because it allows

to have a better resolution of the peaks, and hence better signal on noise: The channel 223/330 nm leads to signals the less interfered.n UHPLC/MS-MS (negative electrospray ionization), analysis by direct injection. (See conditions in Serres-Piole et al., 2011).a Data from Rose et al. (2001b,a, 2003).b UHPLC/FLD, 10 mL injected.c Xevo TQ MS instrument, 15 mL injected.d Xevo TQ MS instrument, 50 mL injected.e ICP/MS, not matrix dependent.



at high concentrations (mg/mL level). High background noise(from the vials of glass containing the bore) was constant in thecase of ICP-MS detection. The use of inert vials (Teflon) with aninert injector and chamber could solve this problem, but thisapproach is impractical for routine use.

Halogenated boronic acids are not detectable by UHPLC/MS-MS (in multiple-reaction monitoring mode), even at high con-centrations (mg/mL level). The determination of the MRM transi-tions by the two instruments showed that the molecular ion wasnot detectable, regardless of the compound. Thus, these com-pounds are characterized by a poor ionization in comparison tothe FBA. Few publications are available about the halogenatedboronic acids. Their poor ionization may also indicate a lowreactivity, which would be an advantage in terms of chemicalstability but a drawback in terms of analysis by MS.

Different parameters were studied to try to improve theionization of the compounds but without success, includingorganic solvents constituting the mobile phase (CH3CN andCH3OH), ionization sources (electrospray ionization in negativeand positive mode), the pH value (with ammonium bicarbonate(3975 mg/mL) (pH 7–8), ammonium acetate (pH 4.8), formic acid(AF) (pH 2), and sodium hydroxide (pH 9–10) (sample preparedas follows: 1 mL CH3CNþ4 mL water with NaOHþ25 mLsample)).

A derivatization step comprising the addition of diols beforeanalysis would improve the ionization and hence the sensitivity,with lower LODs. However, this type of procedure did notcorrespond with the initial strategy.

Finally, these molecules were not retained as potential watertracers.

Fig. 2. Example of detection of di-NSA by UHPLC/MS-MS (15 mL injected).

Fig. 3. Example of detection of halogenated boronic acids by UHPLC/MS (10 mL injected).



5. Conclusion

Tracers are a very efficient diagnostic tool for the oil industryto obtain information about the characteristics of a reservoir(interwell connections, heterogeneities, etc.) (Huseby et al., 2009).By providing more detailed information on the movement ofreservoir water, tracers can improve oil recovery efforts. Tracershave proven their effectiveness for more than 40 years (Bjornstadet al., 1990), but their selection is not always appropriate. Toaddress this issue, a review providing guidelines for the selectionof water tracers in a tracing campaign was proposed as thesubject this paper.

An alternative technique to the fluorescence techniques, theUHPLC/MS-MS, was successfully proposed for the detection ofnaphthalene sulfonic acids in waters. This original tool had beenrecently developed for the direct determination of FBA in reser-voir waters (Preud’homme, 2010; Serres-Piole et al., 2011). Withthis method, a sample preparation or chemical modification is notnecessary; it allows the detection of FBA and NSA in only 5 min bydirect injection. With a minimum sample manipulation (0.2 mmfiltration), this method exhibits an excellent robustness and isreliable with regard to the representativeness of the results.

New types of chemical tracers were also investigated here, butthey were not retained for further study (e.g., on their stability,passive behavior, etc.) because of their high LOD in all analyticaltechniques tested.

Acknowledgments

The authors gratefully acknowledge the support for thisproject from the TOTAL Exploration Production Department. Theauthors are very grateful to the LCABIE laboratory, FEDER andAquitaine Region funds for mass spectrometry facilities (UPLCAcquity, ICPMS, FLD, TQD MS and XevoTQ MS instruments).

Appendix A. Supporting information

Supplementary data associated with this article can be foundin the online version at http://dx.doi.org/10.1016/j.petrol.2012.08.009. These data include Google maps of the most important areasdescribed in this article.

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Documents

Water tracers in oilfield applications: Guidelines