WATER SHUTOFF IN HORIZONTAL WELLS USING GEL TECHNOLOGY · WATER SHUTOFF IN HORIZONTAL WELLS USING GEL TECHNOLOGY CT&F - Ciencia, Tecnología y Futuro - Vol. 1 Núm. 3 Dic. 1997 washed

WATER SHUTOFF IN HORIZONTAL WELLS USING GEL TECHNOLOGY

CT&F - Ciencia, Tecnología y Futuro - Vol. 1 Núm. 3 Dic. 1997

WATER SHUTOFF INHORIZONTAL WELLS USING

GEL TECHNOLOGY

� SPE, Texas A&M UniversityEcopetrol - Instituto Colombiano del Petróleo, A.A. 4185 Bucaramanga, Santander, Colombia

e-mail: [email protected]

N. SAAVEDRA* and D. MAMORRA�

he main objective of this research was to investigate the feasibility of setting a chemical wellbore plug ina horizontal wellbore. Two main problems associated with the wellbore plug were investigated: (a)method of placement of the plug so that slumping would not occur, and (b) selection and testing of

chemicals that could be used to make wellbore plugs with sufficiently high holding pressures. Three chemicalsused in the oil industry for gas and/or water shut-off were selected for the study, namely, a monomer, apolyacrylamide and a plastic. The experimental results indicate that (i) a chemical plug could be placed in ahorizontal wellbore with minimum slumping if the plug is introduced into a viscous pill, and (ii) the plastic is mostsuitable for use in a chemical plug because of its high holding pressure. The proposed new isolation techniquethus appears to be viable. However, further experimental studies are required, particularly to evaluate theeffectiveness of the wellbore plug in conjunction with injection of formation gels.

Se estudió la posibilidad de colocar un tapón químico en la sección horizontal de un pozo. Se analizaron dosproblemas: (a) El método para colocar el tapón químico evitando los efectos de gravedad que tienden a llevarel tapón hacia la parte inferior del hueco, y (b) Selección y evaluación de químicos que pueden ser utilizadospara formar tapones químicos que provean suficiente contra presión. Se seleccionaron tres productos químicosutilizados actualmente en la industria para aislar zonas con problemas en la producción de gas y/o agua. Estosproductos fueron un monómero, una poliacrilamida y un plástico. Los resultados experimentales mostraron queun tapón químico se puede colocar en la sección horizontal de un pozo con deformaciones mínimas por efectosgravitacionales, si el tapón en su fase líquida se inyecta dentro de un colchón viscoso. Con base en losresultados experimentales, la nueva técnica de aislamiento de zonas parece viable. Sin embargo, se requieremayor investigación para evaluar la efectividad del tapón químico en combinación con el desplazamiento degeles que penetren la formación.

Keywords: zone isolation, plugging, water shutoff, water production, gas shutoff, gel.

* To whom correspondence should be sent

T

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N. SAAVEDRA and D. MAMORA

INTRODUCTION

Two types of completion are commonly used inhorizontal wells: openhole, and cased and cemented(Aguilera et al.., 1991). Figure 1 illustrates the mostcommon completion designs for horizontal wells. Thethree types of openhole completions are true open holecompletions, slotted or pre-perforated liner/casing andsegmented uncemented liner/casing. The true openhole completion is the simplest and cheapest toexecute.

for certain conditions. However, implicit in the decisionto case and cement is a commitment to perforate andstimulate the well. Thus, costs associated with this typeof completion are so significant that it is only used ifthere are problems associated with the wellbore andformation that can not be remedied in any other way(Economides, 1993).

Thus, more than 90% of horizontal wells have open-hole completions. Many wells completed on poorlyconsolidated sands are equipped with prepackedgravelpack, wire-wrapped screens, or slotted liners,to control sand production. Consequently, in thesewells, there is complete fluid communication at thereservoir interface throughout the horizontal section(Mamora et al., 1995).

An interval(s) along the horizontal section mayencounter early gas and/or water breakthrough due tocresting, extraneous production from a high permeabilitystreak or fracture, or a horizontal interval being toonear the fluid contacts. Although only certain intervalsalong the horizontal section are affected, the total wellproduction rate may be significantly reduced due tothe following reasons. First, when water breaks throughin a horizontal well, water blockage may occur, since:(i) the flowing differential pressure in the wellbore istypically very small (Goode et al., 1987), and (ii) thelows of the undulating sections of the well act as watertraps. Water blockage thus results in reduced oilproduction rate. Second, when early gas breakthroughoccurs, oil flow in the reservoir and wellbore will beseverely impeded because gas has higher mobility thanoil. Consequently, the ability to shut off high water and/or gas producing zones in a horizontal well would resultin increased production and profitability (Odosirio etal., 1992).

However, zone isolation in a horizontal well withopen hole completion or uncemented pipe is not a trivialjob due to three main reasons. First, since there is fluidcommunication at the reservoir interface, spotting andsqueezing a zone isolation material (e.g. a gel) acrossthe zone of interest could result in the material beingsqueezed into other intervals that are to be left untou-ched. Second, zone isolation materials are generallydenser than the workover brine, so that the materialmay slump on the low side of the hole. To alleviatethese problems, inflatable straddle packers have beenused to isolate the zone to be treated. These packersmay not seal against the wellbore wall if the hole is

Slotted or preperforated liners can be run to protectthe borehole from collapse. Slotted liners are generallyused in unconsolidated formations, while preperforatedliners are used for consolidated formations. The useof uncemented liners has the disadvantage that, forremedial treatments, there is no way to control the in-jection of fluids into the formation.The use of segmen-ted uncemented liners is the most sophisticated of thepseudo openhole completion methods. They allow openhole evaluation of the wellbore and subsequent selec-tive stimulation, if necessary. Using these liners,segments of the wellbore may be isolated if port collartools are run in conjunction with external casing packers(ECP�s). Cased and cemented completions are used

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washed out or very irregular. Third, for horizontal wellscompleted with slotted liners, there is practically nomechanical way of sealing off the liner-boreholeannulus (Andersen et al., 1995).

To alleviate the aforementioned problems, a newzone isolation method for horizontal wells using achemical wellbore plug has been proposed (Mamorraet al., 1995). The technique involves three stages(Figure 2). First, a chemical plug is set upstream ofthe problem zone. Second, gel is spotted and squeezedinto the problem zone. Third, the excess gel and plug

are washed or drilled out before producing the well.The three stages are carried out using coiled tubing.This paper describes the research which investigatedthe viability of setting a chemical wellbore plug forzone isolation in horizontal wells.

EXPERIMENTAL APPARATUS

Two main types of experiments were performed inthis study: (i) evaluation of placement techniques, and(ii) measurement of holding pressure of the chemical

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wellbore plug. An X-Ray (CT) scanner was used tovisualize the shear profile of the chemical plug to un-derstand better the shear mechanisms involved in theprocess.

Placement Technique - Experimental Apparatusand Procedure

The apparatus for investigating placementtechniques consisted of three main components whichare:1. A 0,102 m (4 in) long by 0.0381 m (1.5 in) I. D.

straight plexiglass tube, which represented thehorizontal wellbore. One end of the tube wascapped. The other end had a right-angle-extensionwhich was open to ensure the tube was completelyfull when placed in the horizontal position.

2. A 0,00635 m (1/4 in) plastic tubing - representingthe coiled tubing - attached to a positive displace-ment pump.

3. An electrically heated constant-temperature jacketwhich provided the required reservoir temperature338,6 K (65,6 ºC, 150 ºF) , into which the plexiglasstubing (wellbore) was inserted.A schematic diagram of the apparatus is shown in

Figure 3. A typical experiment was conducted asfollows. Completion brine was prepared by dissolvingsodium chloride in water for the required salt concen-

tration. If the experiment also required the use ofcarboxylmethylcelulose (CMC) as a placement fluid,then a CMC mixture in water of 11,41 kg·m-3 (4 lb/bbl) was prepared. The completion fluid was pouredinto the plexiglass tube which was filled to excess.The plexiglass tube was placed completely in a horizon-tal constant-temperature jacket, whose temperaturehad been already stabilized at 338,6 K (65,6 ºC, 150 ºF)for several hours. A thermocouple, placed in the plexi-glass tube, measured the gel and brine temperature.

In the meantime, liquid gel (to form the chemicalwellbore plug) was prepared. The gel concentrationsvaried depending on the chemicals used. When thetemperature of the completion fluid had stabilized at338,6 K (65,6 ºC, 150 ºF), the liquid gel was placed ina reservoir connected to a positive displacement pump.A 0,00635 m (1/4 in) plastic tubing was attached tothe outlet of the pump, the open end of the tubing beingplaced roughly at the center of the plexiglass tube.The liquid gel was injected at 400 cc/min. Total gelvolume placed into the plexiglass tube varied between50 to 300 cc. The plastic tubing was removed and thegel was left to cross-link and age, to form the chemicalwellbore plug. The plexiglass was later removed fromthe jacket and the profile of the wellbore plug wasobserved and its dimensions measured.

Holding pressure - Experimental apparatus andprocedure

Apparatus for determining of the holding pressureof the chemical wellbore plug consisted of the followingmain components.1. PVC pipe of various sizes and lengths. The inside

wall of these PVC pipes were lined with sand tosimulate the surface roughness of a wellbore in asandstone formation. PVC cement was evenlyapplied on the inside wall followed by sand.

2. A positive displacement pump for injecting waterinto the PVC pipe wellbore. A pressure transducerwas used to measure the injection fluid pressure.The pressure transducer was frequently calibratedagainst a dead-weight tester.

3. A data logger and personal computer system torecord pressure transducer readings at 5-secondintervals.

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A schematic diagram of the apparatus is shown inFigure 4. A typical experiment was conducted asfollows. The PVC tube (wellbore) was filled with geland then placed in the oven whose temperature wasfixed at 65.6 ºC (150 ºF). After the gel cured to forma chemical wellbore plug, one end of the PVC pipewas exposed to atmospheric conditions and the otherend was connected to the positive displacement pump.Water was injected into the PVC pipe. Pressure trans-ducer readings were recorded every 5 seconds. Thetest was ended when the injected fluid broke thewellbore plug. The injection pressure began to fall afterreaching a peak value corresponding to the holdingpressure of the wellbore plug. On completing the holdingpressure experiment, the PVC pipe was CT scannedto obtain a profile of the wellbore plug and the channelthrough which the injection fluid traversed the wellboreplug.

The X-Ray CT scanner used is a high resolution,4th generation Universal HD200 with 760 diodedetectors. Samples with diameters up to 0,50 m maybe scanned. After the holding pressure test, the chemi-cal plugs were scanned and the fracture profile wasvisualized. First a deltaview image was taken and thena set of slices covering all the plug was selected. Then,the CT scanner automatically generated the cross sec-tional slices. The images were briefly studied on thescreen of the scanner. Then, the images were trans-

ferred to a SUN workstation in which they wereanalyzed using the VoxelCalc software.

Chemicals UsedThree chemicals were used in the experiments: a

monomer, a polyacrylamide and a plastic. Monomersare the units that form a polymer, and thus presentlower molecular weight and lower viscosity than thepolymer. Monomer systems are used to improve oil-displacement efficiency in water injection wells(Halliburton, 1993). Specifically, they improve the arealconformance of the zone. Monomer treatments areplaced in the formation as a low-viscosity fluid. Howe-ver, as the fluid polymerizes, its viscosity increasessignificantly. As more monomers enter the higherpermeability layers of the formation, more polymersare formed in those areas. In a waterflood, for example,when the high-viscosity polymer is formed in theformation, floodwater is diverted into parts of theformation that have received less water. Thus, waterflood of these formations tends to be more uniformand efficient. The main advantage of a monomer overa polymer is its lower initial viscosity which stems fromits lower molecular weight.

Polyacrylamides consist of a carbon-carbon chainto which are attached amine groups, possibly tens ofthousands of them, resulting in molecular weights inthe millions. In its pure form, the polymer is electricallyneutral, and would appear to preclude any crosslinkingby ionic bonding. However, when mixed with a smallamount of alkaline solution (e.g. sodium hydroxide), orwhen heated to a high temperature, some of the aminegroups react to form carboxylate groups. Each of thesecarboxylate groups carries a negative charge. Thefraction of amine groups converted to carboxylategroups is called the degree of hydrolysis (DH), withvalues typically varying from 0% to 60%. The polymerresulting is called partially hydrolyzed polyacrylamide(PHPA). Because of its negatively charged carboxy-late groups, the polymer thus becomes susceptible toionic crosslinking (Borling et al., 1994).

Efficient cross-linkers for PHPA are trivalent metalions such as aluminum (Al+3) and chromium (Cr+3).These cross-linkers may be in the form of simple inorga-nic ions in solution or within soluble chemical complexesin which the trivalent ion is associated with small

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inorganic or organic groups called ligands. Some ofthe first polymer-gel systems developed in the early1970�s used crosslinking aluminum in the form of alu-minum sulfate (Dovan et al., 1987). In any case, thetrivalent metal ion easily links carboxylate groups ondifferent polymer molecules, or possibly on the samemolecule. Relatively few cross links are needed tocreate a gel from the polymer-cross-linker mixture.

Thermosetting plastics are used to seal off formationpores. When properly applied, these plastics havesufficient physical strength to seal fractures, vugs,channels and perforations. These thermosetting plasticsare also used for sand consolidation. They are used, inless concentrated form, for zone isolation applications.The plastic is placed into a well in such a manner thatit completely fills the formation pores. Phenolic, epoxyand furfuryl alcohol plastics are commonly used forthis purpose in the field (Sparling et al., 1984).

RESULTS

Two types of experiments were carried out. Thefirst type of experiment consisted of finding a place-ment technique to prevent slumping of the chemicalplug. The second type of experiment involved measure-ment of the holding pressure of the plug. The resultsof these experiments are described as follows.

Placement TechniqueInitial tests showed slumping of the gel in a horizontal

section containing water. Then, the problem ofcrosslinker dispersion and slumping of the gel wereaddressed. The first thoughts about slumping of thegel were that it was caused by gravity segregationand that reducing the difference in density betweenthe completion fluid and the gel would solve the problem.This possible solution was investigated as follows.Sixteen 20 cm3 test tubes were filled with 15 cm3 ofcompletion KCl brine of increasing density, from 50,000mg/kg of KCl up to a maximum of 250,000 mg·kg-1KCl. 5 cm3 of gel was then placed in each of the testtubes. The sixteen test tubes were placed in an ovenat 338,6 K (65,6 ºC, 150 ºF) for eight hours.

For polyacrylamide gel at a weight concentrationof 5%, the gel location varied depending on the brineconcentration, from being at the bottom of the test tubefor those containing brine concentration of less than85.000 mg·kg-1 to the gel being at the top of the testtube for those with brine concentration greater than150.000 mg·kg-1 of KCl (Table 1). For the test tubesfilled with KCl brine between 5.000 to 100.000mg·kg-1 a weak gel was formed due to the dispersionof the crosslinker resulting in the gel filling almost allthe tube.

Similar results were obtained using polyacry-lamideat a weight concentration of 10%. But in this case, the

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gel formed at the bottom of the test tube when thebrine concentration was less than 150.000 mg·kg-1,while the gel was formed at the top of the test tubewhen the brine concentration was greater than 175.000mg·kg-1. Also, for brine concentration of 150.000mg·kg-1 and 175.000 mg·kg-1 the gel was dispersedthroughout the test tube.

The plastic gel was more uniform in size, and nodispersion was observed. For brine concentration ofless than 160.000 mg·kg-1 the gel was formed at thebottom of the test tube. For brine concentrations greaterthan 160.000 mg·kg-1 the gel formed at the top of thetest tube. For brine concentration of 160.000 mg·kg-1the gel was formed near the top but not at the verytop, showing floatation (Table 2).

The tests indicate therefore that a plastic gel wouldnot slump in a wellbore if it is placed in a completionbrine with concentration of 160.000 mg·kg-1. How-ever, the required brine density is too high for practicalapplication since this would cause severe fluid lossesinto the formation, damaging the producing zones.Consequently, it was decided to investigate the feasi-bility of placement techniques that take advantage ofthe yield point of the completion fluid in combinationwith its density.

Calvert et al., 1995, solved the problem of place-ment cement plugs in horizontal wells by increasingthe density and the yield point of the completion fluid.This technique was applied in the gel placement expe-riments and a new type of tests were developed. Toincrease the yield point, CMC was added to the brine.

Experiments were carried out to determine variationof yield point with CMC concentration. The resultsare shown in Figure 5. The effect on yield point bythe addition of NaCl to increase the density was alsostudied (Figure 5). For the range of NaCl concentrationstudied, the effect on yield point by the addition of NaClcan be seen to be practically negligible.

The completion fluid was prepared as follows. First,700 g of water was placed into a blender and the desiredamount of NaCl was added while blending at low rpmto avoid introducing air into the fluid. When the saltwas completely dissolved, the desired amount of CMCwas added blending for five minutes at low rpm.

Tests were carried out to check gel slumping inwhich three parameters were varied: completion brinedensity, yield point, and injection rate. Table 3 lists theparameter values required to prevent gel slumping. Animportant result is that the density of the brine at whichthe gel formed without slumping was considerably lo-wer than that in which the yield point of the brine wasnot increased. A reduction from 100.000 mg·kg-1 to50.000 mg·kg-1 was observed in the case of polyacry-lamide, and from 160.000 mg·kg-1 to 100.000 mg·kg-1(9,0 lb/gal) in the case of plastic. Because the den-sityof the plastic is greater than that of the polyacrylamideor the monomer, a completion brine of higher densityand yield point is required to avoid slumping of theplastic.

It is important to note that the injection rate affectsthe development of the plug, as indicated in Table 3.

Brine Concentration(mg·kg )-1 Gel Position

100.000 Botton

Botton

Middle-top

150.000

160.000

170.000 top

175.000 top

180.000 top

190.000 top

200.000 top

Table 2. Floatation Test for Plastic.

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Likewise, the minimum injection rate for the plasticshould be greater than that of polyacrylamide or themonomer. It is necessary to inject above the minimumrate in order to form a cylindrical pill in the wellbore.The completion brine would maintain the shape of thepill until it is cross-linked to form a plug.

Holding PressureThe three chemicals used, a polyacrylamide, a mo-

nomer and a plastic, were cured at 338,6 K (65,6 ºC,150 ºF) except for the plastic, which was cured at roomtemperature for safety reasons. For a particularwellbore I. D. the length of the plug was increased insteps, from as short as 0,0254 m (1 in) to as long as0,9398 m (37 in).

Polyacrylamide. Polyacrylamide gel was preparedand then placed in a PVC tube lined with sand. ThePVC tube was placed in an upright position in an ovenset at 338,6 K (65,6 ºC, 150 ºF). After the gel wasformed, water was injected at one end of the PVCtube, while the other end was sawn off and exposedto atmospheric pressure. Change in injection pressurewith time was recorded at 5 second-interval on a perso-nal computer. Injection of water continued until waterbroke through the outlet end of the PVC tube.

Figure 6 shows a graph of injection pressure versustime for a typical polyacrylamide plug. Since the injec-tion rate is known, the time scale can be transformedinto injection volume by multiplying the time scale bythe rate used in these experiments, 2,0 cc/min. As itcan be seen from these two graphs, the injection pressu-re increases up to a maximum and then drops withcontinued injection. The maximum pressure corres-ponds to the holding pressure of the plug. At the holdingpressure, the gel failed and water broke through thegel.

An interesting observation was the effect of agingtime on the holding pressure. Figure 7 shows that theoptimum aging time for the polyacrylamide is aboutfive hours. After one to two hours, the gel was formedbut it only achieved its optimum consistency after aboutfive hours. Therefore eight hours of aging were usedin all subsequent tests. The effect of plug length onholding pressure was also studied using initially polya-crylamide concentration of 50 g·kg-1 (5% by weight).Figure 8 shows the results for a 0,0127 m (0,5 in) I. D.pipe. It can be seen that the holding pressure increaseswith length up to an optimum length beyond which theholding pressure remains constant with increasing pluglength. The optimum plug length is about six to eighttimes the diameter of the plug. The same result wasobtained with a 1,0 in ID pipe. The optimum lengthwas found to be about eight times the diameter of theplug.

Experiments were conducted with different polymerand cross-linker concentrations. In one experiment, thepolymer concentration was increased from 50 g·kg -1to 100 g·kg-1 (5% to 10% by wt), while the weightratio of polymer to cross-linker was kept constant at11:1. In another experiment the polymer concentrationwas increased to 100 g·kg-1 (10% by weight), with apolymer to cross-linker weight ratio of 5,5:1. Figure 9shows that increasing polymer concen-tration increasesthe holding pressure and decreases the optimum length.However, the holding pressure does not increase tohigh-enough values required in field operations.

The holding pressure does not increase for plug

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the body of the gel. The results indicate that the holdingpressure increases with length until the gel shears andthe fluid goes though a fracture. The existence of afracture throughout the plug after break through wascorroborated by a CT scan cross-sectional image takenat the center of the plug. The CT scan image (Figure10) clearly shows a fracture going through the gel.

Monomer. The monomer can easily penetrate theformation pores because of its low yield point, whichin turn renders it unsuitable for creating a strongchemical plug. As a wellbore plug, the monomer pro-ved to be unsatisfactory. From all experiments only50% of the monomer plugs were formed, and whenformed the plugs showed a lower holding pressure thanthe polyacrylamide. This is probably due to the gel beingbrittle and having minute internal fissures that permitfluid communication through the plug.

Different concentrations of the product were tested.An experiment was carried out in a 1 inch I.D. pipe,using monomer concentrations from one to four timesthe manufacturer�s recommendation and a constantlength of 0,1651 m (6,5 in). In weight percent, theseconcentrations were 150, 260, 350 and 410 g·kg-1.Only the plug with a concentration of 150 g·kg-1 showedsome holding pressure, which was also very low (lessthan 27,58 kPa, 4 psig). Plugs with concentrations of260, 350 and 410 formed gels which were so fracturedthat none of them supplied any resistance to the injectedwater.

lengths greater than the optimum length because theinjected water shears through the gel body. This wasevident when a pipe was cut at the outlet end, revealingthe path followed by the pressurizing fluid to be through

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Other experiments were conducted using 0,0254 mI.D. pipes and a monomer concentration of 150 g·kg-1and 260 g·kg-1 and different plug lengths. For a 150g·kg-1 monomer concentration, the holding pressurefor a plug measuring 0,40 m (16 in) was about 48,26kPa (7 psig). Figure 11 indicates the general behaviorof the monomer plug to be very similar to that of polya-crylamide but with a lower holding pressure.

Figure 12 shows the results for a product concen-tration of 260 g·kg-1. In this case the holding pressurewas 140 kPa (20,3 psi) for a 0,40 m (16 in) plug. Dueto the low holding pressure, further tests with the mono-mer were not carried out.

Plastic. For safety reasons, this product was allo-wed to cure at room temperature. This may producepessimistic results, since the plug developed at reservoirtemperature is shown to be stronger that that cured atroom temperature.

Figure 13 shows the effect of plug length on holdingpressure. The holding pressure, significantly greaterthan that of the polyacrylamide or the monomer, wasabout 275,8 kPa (40 psig) for a plug length of about0,20 m (8 in) when cured at atmospheric pressure.The plug had the consistency of a very strong solidmaterial. Since Sydansk tests carried out at roomtemperature showed shrinking of the gel due to evapo-ration of volatile components in the gel, it was suspectedthat this was occurring in the holding pressure experi-ments, reducing the adhesion of the plug to the pipewall and thus the holding pressure.

To alleviate the plug-shrinking problem, the gel wascontinuously subjected to an hydraulically appliedpressure of 344,7 kPa (50 psig) throughout the curingperiod (about two weeks). Curing the plug in thismanner resulted in an increase in holding pressure ofabout twice that without application of pressure. Mo-re important, the holding pressure increased linearlywith plug length (Figure 13), reaching a value about896 kPa (130 psig) for a plug length of about 0,381 m(15 in).

A 896 kPa (50 psig) head of pressure was used inall the experiments, although at reservoir conditionsthe pressure head would be far greater due to thehydrostatic column in the well. The plug length wasincreased in a geometric manner. A 0,9398 m (37 in)plug length was set and tested. Figure 14 shows theholding pressure test, while Figure 15 shows that the

holding pressure increases linearly with plug length andmay be described by the following equation:

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Holding pressure (psig) = 9,3 x Plug length (in). . .(1)

The effect of plug diameter on holding pressurewas studied. Experiments were conducted using twodifferent lengths for pipes of 0,0381 m, 0,0508 m and10,16 m (1,5; 2 and 4 in) I.D. The results are shown inTable 4 and plotted in Figure 14, including results forall plug diameters. Figure 15 indicates that the holdingpressure increases linearly with plug length, followingthe relationship:

Holding pressure (psig) = 9 x plug length (in). . . (2)

This equation confirms the linear relationshipdescribed in Eq. (1) and allows an estimate of the pluglength required to obtain a desired holding pressure.

Several CT scan cross-sectional images of a plugwere taken. Figure 16 is typical of these images. Itcan be seen that the injected fluid leaks through a gapcreated between the plug and the inner wall of thepipe, and not through a fracture in the plug as in thecase of the polyacrylamide and the monomer. Thisconfirms the different behavior of the plastic and provi-des insight into the mechanisms of holding pressure.In other words, the plug created with the plastic isstronger and does not shear with pressure. Instead, atthe holding pressure, part of the outer surface of theplug is dislodged from the inner wall of the pipe. Thisfailure in plug-pipe adhesion occurs at a weak area.Consequently, the adhesive force along the weak areais a function of plug length and hence, the holdingpressure is only a function of plug length and not plugdiameter. This probably explains why the plug doesnot slip but stays in place after the holding pressure isreached.

Holding pressure tests for the plastic were conduc-ted under very unfavorable conditions compared tothose found in the reservoir. First, the plastic gels werecured at room temperature instead of reservoir tempe-rature. Plug strength is greater at reservoir tempera-ture. Second, at the high holding pressures, the PVCpipe could have bulged, contributing to a lower-than-actual holding pressure. Third, the pressure headapplied to avoid vaporization of light components isvery low compared to the hydrostatic pressure in awell. Fourth, there would be some penetration of the

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plastic gel into a formation, so that the holding pressurewould be greater than that obtained in the study. Allof these factors tend to reduce the holding pressure.However, a holding pressure for plastic of 2,413 kPa(350 psig) for a plug length of only 0,9398 m (37 in)appears to be very promising for field applications,where the formation fluids provide a back pressure onthe plug, so that the holding pressure is actually a diffe-rential pressure.

SUMMARY AND CONCLUSIONS

· Slumping of the gel in the horizontal section can beavoided by proper adjustment of the properties ofthe completion fluid· The use of CMC to increase the viscosity of the

completion brine is particularly appropriate, sinceas a polymer it reacts with the crosslinker in thegel, avoiding further crosslinker migration. Conse-quently the gel can form a stronger wellbore plug.· A sufficiently high injection rate is required during

placement of the plug to quickly form a gel thatstraddles the wellbore, and thereby minimize geldispersion and slumping.

· The roughness of a wellbore in a sandstone forma-tion was reproduced fairly closely in the experimentsby using PVC pipes internally lined with sand.· For all chemicals tested, temperature was found to

reduce the gelation time significantly.· Polyacrylamide and monomer wellbore plugs

showed an increase in holding pressure with increa-sing length up to a maximum point, beyond whichan increase in plug length did not increase theholding pressure. This maximum length (and the-refore the optimum plug length) was found to beabout six to eight times the internal diameter of thewellbore plug.· At the holding pressure, when the injected liquid

began to leak out of the plugs, there was no slippageof the polyacrylamide and monomer plugs in thewellbore. Instead, the injected liquid leaked througha fracture created in the body of the plug.· Because of their low holding pressures, the polya-

crylamide and the monomer do not make good che-mical wellbore plugs. They are, however, excellentcandidates for use as formation gels, since theypenetrate deep into the formation, blocking the pathsfor flow of gas and/or water· Plastic formed chemical wellbore plugs of signifi-

cantly higher holding pressures than polyacrylamideand monomer did.· When the holding pressure was reached, the injected

liquid leaked through a gap between the surface ofthe plastic and the surface of the wellbore. Nofracture was formed in the body of the plastic plug.The holding pressure thus increases with increasingplug length.· For plastic chemical wellbore plugs, the holding

pressure increased linearly with plug length, inde-pendently of plug diameter. The holding pressure(in psig) was found to be equal to about nine timesthe length (in inches) of the plug. The maximumholding pressure obtained in the laboratory was2.365 kPa (343 psig) for a plug length of 0,9398 m(37 in).· In summary, a placement technique using high-

viscosity brine was developed and tested to be

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viable. This technique, together with the use ofplastic as a wellbore plug, augurs well for the propo-sed zone isolation technique. Further research ishowever required, particularly in the placement andinjection of the formation gel.

ACKNOWLEDGMENTS

The authors thank the following companies formaking possible this study: Ecopetrol for sponsoringNestor Saavedra�s graduate study at Texas A&MUniversity; and Shell, Halliburton, Texaco, and Chevronfor direct sponsorship of the study under project CEA88.

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WATER SHUTOFF IN HORIZONTAL WELLS USING GEL TECHNOLOGY · WATER SHUTOFF IN HORIZONTAL WELLS USING GEL TECHNOLOGY CT&F - Ciencia, Tecnología y Futuro - Vol. 1 Núm. 3 Dic. 1997 washed