Copyright 2003, Society of Petroleum Engineers Inc. This paper was prepared for presentation at the SPE European Formation Damage Conference to be held in The Hague, The Netherlands 13-14 May 2003. This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract The oil industry is still focusing on the optimization of hydrocarbon production with the minimum cost possible in complete HSE respect. Nowadays a continuous flow of innovative ideas generates worldwide new technological frontiers. As a result, we witness remarkable applications of new well solutions and techniques. In this scenario we would like to give an overview of the possible answers to the technological needs and actions towards Stimulation Treatments and Formation Damage Removal. The main issues to investigate are:

if the mature and worldwide applied techniques of Acidizing and Fracturing are still adequate to the new well geometry scenario

if the novel technologies are well known and cost effective

if innovative ideas on formation stimulation and damage removal are to be better investigated and implemented.

Introduction Stimulation1 is a magic word: synonym of hydrocarbon productivity increase from reservoir formation. After a stimulation job the dream is an astonish fold increase of productivity index (PI) of the well. Even though the field results could be less than the expectations the stimulation is financially unrivaled2 because it is less expensive than the other well construction activities like drilling or completions with a very high Return on Investment (ROI). In the current situation dominated by economists the stimulation is still attractive for the rapid payout of the investment in the order of days, weeks or few months.

For this beneficial characteristic of productivity increase the stimulation techniques were introduced inside the execution of some remedial jobs that now are more economically attractive. Frac & Pack technique is the most famous and successful example: a sand control remedial job that optimizes productivity. Another amazing observation is that stimulation consists, from many decades, in only two major techniques: Matrix Acidizing3 and Fracturing4 (with acid or proppant). Matrix Acidizing In fact, matrix acidizing is to be considered the oldest stimulation technique still in use to-day . The beginning of the jobs is updated more the one century ago. Herman Frasch,5 chemistry chief at Solar Refinery, is credited with the first patent on acidizing; the date was March 17, 1896. This Frasch patent Increasing the Flow of Oil Well is the base line of the modern matrix stimulation. He suggested many needs to better perform the acidizing treatments as: High injectivity: put the acid under strong pressure

pressed into the rock and made to act upon the same at a distance from the original wellbore

Wormhole: long channels can be formed Overflush: it is advantageous to displace the acid and

cause it to penetrate further into the rock by forcing a neutral or cheap liquid, such water ,into the well

Inhibitor: to introduce an alkaline liquid to neutralize any unspent acid returned

Tubing and Packer: to force the acid into the formation to be treated

Matrix acidizing history6 shows how many systems have appeared and used to solve the different problems. A unique and standard job procedure was never accepted because of some disappointing results always presented in each technique. The best and more applied treatment methods have shared the Frasch idea to put the acid under strong pressure. In fact, the matrix acid injection parameters are linked based on the Darcy relationship as: P = pwi -pe = 141.2 qi [(ln re / rw) + s] / (k h) (1) All the best and well known acidizing techniques influence on the parameters of (1) in order to maximize the value of P (that is below the fracture pressure). The most diffused techniques, that provide the maximum beneficial effect of the acid mixture on the treated interval of the formation, are the following:

SPE 82573

Beyond Acidizing and Fracturing Mauro Tambini, Eni E&P Division

2 SPE 82573

Gelled7 or viscosified or viscoelastic8 acid or viscous foam9 that increase the value of viscosity (),

ball sealers10 or the dual inflatable packers that decrease the interval length (h),

the diverted agents11 increase temporary the skin (s), MAPDIR12 technique that increases the injection rate (qi) Fracturing Fracturing was developed more recently about half century ago: the first jobs were employed to improve the productivity of marginal wells in Kansa in 1947. Until now it is fully applied in United States reaching the 85% of gas well and the 60% of oil. Unfortunately fracturing is not intensely used outside the North America due to the traditional EuropeRussia Middle East approach that limited, in the past, the application of fracturing to a very low permeable formations or desperate no-flowing wells. In the last decade the successes in North Sea and Russia fields are changing this idea. On the other hand there is no any other petroleum technology with so significant incremental asset than fracturing. M. Economides et all13 suppose a considerable worldwide production increase only by fracturing the oil wells as in U.S. with a negative final skin factor. A simple approach to show the fracturing potential may be to consider the Prats14 relation that for a infinite conductivity kfw fracture gives a equivalent well with a effective radius rw as following: rw = 0.5 xf (2) There is again a psicological barrier that obstacles the fracturing applications . If to suppose that a perforation charge should reach the distance of in the range of 20-30 m typical of a small fracturing treatment then oil industry would extensively use this technique because it is considered low risk operation. On the other hand fracturing was successfully developed and there are industrial different methods to be applied for every reservoir situations as follows: Acid Fracturing15 for carbonate formation Closed Fracture Acidinzing16 -CFA- for carbonate

formation with high closure pressure. Massive Hydraulic Fracturing17 MHF for

permeability < 0.1md High Permeability Fracturing18 for permeability > 10md Tip Screen Out19 TSO to create very high

conductivity fracture Frac & Pack20 for unconsolidated formation Coiled Tubing Fracturing21 CTF for multi

layer fracturing

Real time analysis The acidizing and fracturing techniques reached the mature stage (Fig 1) in design and execution only when pressure diagnostic tools were developed to work in real time. On-site field analysis for predicting skin factor evolution22 during acidizing and fracture dimensions23 during fracturing now are available. This approach increased the number of stimulation successes decresing uncertainties of input data due

a pretreatment calibration tests: as minifrac or injection test or pretreatments24 to control excessive pressures. Emerging Stimulation Techniques The acidizing and fracturing are so mature that they have generally the possibility and the opportunity to be adapted to the different new well geometries and completions with acceptable results. An example is the successful stimulation of the horizontal wells using the matrix high rate treatments25 and traverse multi fracturing technique26 . On the other hand the perception is that both the technologies will soon be in the decline part of the technical-economical life cycle (Fig 1). For this reason the oil industry is looking for some new ideas or a completely different approachs in order to optimize the productivity of the wells. The main characteristic of the new techniques should be that they would be more closed to the new concepts of well geometry (e.g. multi lateral branches) and completions (e.g. smart wells or expandable tubular). The following emerging technologies are to be underlined:

o Explosive stimulation - Propellant o Acoustic stimulation Vibrowaves and ultrasonic o Electric stimulation

All this techniques could be used for managing the conductivity properties of the rock generally near the wellbore region. Different experiences were accumoleted as Enhanced Oil Recovery (EOR) techniques especially in Russia27 and U.S. and now all these novel technologies are completely revisited and improved with new ideas. Propellant This type of stimulation creates break-down of the formation (1-2 m depth) and improves the clean up of perforations. It is possible to use the propellant or simultaneously with the perforation guns operations or alone in already producing wells. Vaporized propellant creats in milliseconds a high pressure gas wave in the wellbore surrounds: named as the deflagration. The gas breaks through the perforation tunnel and crushes the formation damage creating small fractures near the perforation tunnel. When pressure dissipates, the gas surges back carrying fine particles into the wellbore from the formation. The application of the propellant assisted perforating system28 in the well is useful and of low risk. Moreover it is possible to perform propellant in conjunction with traditional stimulation treatments. Before carbonate matrix stimulation the less permeable or the naturally unfractured zones could be perforated with the help of propellant in order to have a more uniform treatment of the acid mixture in the entire expose interval. In hydraulic fracturing propellant pre-fractures the formation and reduces the maximum necessary pumping pressure. The propellant stimulation gives also the benefit to be applied selectively: it works only in the interval where the tool is run. The expectation29 is to minimize the skin around zero (without reaching the benefit of large negative values typical of fracturing stimulation). Only an intensive use in the field could give the true efficiency of the propellant technique.

SPE 82573 3

Active Cavitation & Ultrasonic The properties of the acoustic treatment30 are different from conventional stimulation: due to several mechanisms (as energy dissipation due to thermal slide or flowless moviment of fluid in the channel) of the interaction between the acoustic field and the saturated porous rock it is possible to cause changes in permeability or to remove the damage plugging materials. Everyday life the use of a simple ultrasonic wave source proves itself in the scale removal cleaning of potable water filters or dentist apparatus. The most used ultrasonic frequency is 20-40 kHz that in the wellbore gives an effective treatment radius of maximum value of 2 rw. The tools of this techinque use an active cavitation jetting31 (4-6 kHz) or a fluidic oscillator effect32 . They are very simply and applicable with coiled tubing. The expectation, as shown by the first applications and yard tests, is the removal ofg organic or inorganic deposits or drilling solids during a slowly motion of the tool in the treatment zone. The efficiency of the tools should be better evaluated using an active fluid like solvent or acid mixture. In combination with hydrochloric acid the tools should be very powerful in boosting the productivity of long intervals in carbonate reservoirs. Electric stimulation The stimulation effects ofthe electric current in the formation, are electrothermal and electrodynamic type. The electrothermal effect33 is evident in the near wellbore zone during heating with infrared or high frequency or microwaves. The electrodynamic34 effects create a cleaning of the bottom whole formation zone from clay particles restoring or improving the permeability. At the moment the applications are limited on viscous oils for the electrothermal or in some field of Former Soviet Union for the electrodynic. The improvement of this new technology is correlated with better understanding of the interaction of the electric current with the rock in order to use the optimal treatment parameters for every well and formation scenario. Interwell Stimulation The above new technologies vibrowave, electric and explosive treatments could have the possibility to extend their effects far from the wellbore. The first case studies on the influence of vibration on the oil field production were carried out after natural earthquakes. The most evident significant change in oil production was noted in different fields in Daghestan and Caucasus after the 1972 earthquake of 6.5 magnitudes. The critical point35 of this interwell stimulation is the ability to generate deeply in the formation an activeeffect36. For the elastic waves the theoretic calculations (Fig.2) show that the penetration depth of the treatment increases with the low frequency waves (>20 40 Hz). For a deep hydrodynamic treatment it is recommended to use a generator with infrasonic frequencies (0.5 5 Hz). Different types of vibration tools at surface or hydraulic hammers down hole are developed 32 to obtain maximum acoustic coupling with the reservoir. For the electric treatment the wide range of the amplitude, shape and frequencies of the waves sent into the formations impose deeper studies and field tests to understand the

mechanims of interaction electrical current of with rock and choose the right methodology to pump the electricity into the reservoir. In some wells, the oil flow increase was accompagnied with simultaneous water cut decrease. The perspective of the use the above mentioned technologies in the rejuvenate of mature fields creates exciting expectations and R&D investigations. Conclusions The stimulation is considered as the most effective

production optimization technique: is less expensive than other well construction activities and provides a very high Return of Investment (ROI).

During many decades, stimulation was consistsed of only

two major techniques: matrix acidizing and fracturing. They are so mature are so mature that they give the possibility and the opportunity to be adapted to the different new well geometries and innovative completions

The following emerging technologies are to

be underlined: o Explosive stimulation - Propellant o Acoustic stimulation Vibrowaves & Ultrasonic o Electric stimulation All this technologies could be used for managing the conductivity properties of the rock generally near the wellbore. On the other hand more investigations must be carried out to understand the impact the novel technologies on the reservoir.

Acknowledgement The author is grateful to Eni E&P Division for granting permission to publish this article. Thanks are due to colleagues Gilberto Toffolo and Vladimir Reutov for useful discussions and their positive motivation to investigate the emerging technologies. Nomenclature Pw i = bottom hole injection pressure, psi pe = constant outer reservoir pressure, psi qi = injection rate, bbl/d = treated fluid viscosity, cp k = rock permeability , md kf = fracture permeability , md h = reservoir thickness, ft re = reservoir radius, ft rw = wellbore radius, ft rw = effective wellbore radius, ft s = total skin factor, dimensionless w = fracture width, inch xf = fracture half length, ft

4 SPE 82573

References 1. Economides, M. and Nolte, K.: "Reservoir Stimulation,

Prentice Hall, Englewood Cliffs, NJ (2000). 2. Danashy, A: "Economics of Damage Removal and

Production Enhancement, paper SPE 30234 (1995). 3. Williams, B.B., Gidley, J.L. and Schechter, R.S.:

Acidizing Fundamentals, SPE monograph series (1979).

4. Gidley, J.I., Holditch, S.A., Nierode,D.E. and Veatch, R.W.Jr: Recent Advances in Hydraulic Fracturing SPE monograph series (1989)

5. Frasch, H: Increasing the Flow of Oil Well, U.S. Patent No.556,669 (1896).

6. Kalfayan, L: Production Enhancement with Acid Stimulation PennWell Corporation, OK (2000)

7. Menzies, N.A. et al. : Modeling of Gel Diverter Placement in horizontal Wells paper SPE 56742 (1999)

8. Alleman, D. et al.: The Development and Successful Field Use of Viscoelastic Surfactant based Diverting Agents for Acid Stimulation paper SPE 80222 (2003)

9. Thompson, K. and Gdansky, R.D.: Laboratory Study Provides Guidelines for Diverting Acid with Foam SPE Production & Facilities (Nov.1993)

10. Gabiel, G.A. and Erbstoesser S.R,,the Design of Buoyant Ball Sealer Treatment paper SPE 13085 (1984)

11. Harrison, N.W.: Diverting Agents History and Application JPT (May 1972) 593-98

12. Paccaloni, G. and Tambini, M. Advance in matrix Stimulation Technology, JPT (March 1993) 256

13. Economides, M. et al. : Unified Fracture Design Orsa Press, TX (2002)

14. Prats, M.: Effect of vertical Fractures on Reservoir Behavior Incompressible Fluid Case JPT (Sept.1961)

15. Olsen, T.N. and Karr, G.K.: Treatment Optimization of Acid Fracturing in Carbonate Formations paper SPE 15165 (1986)

16. Fredrickson, S.E.: Stimulation Carbonate Formation Using a Closure Fracture Acidizing Technique paper SPE 14654 (1986)

17. Agawam, R.G. et al.: Evaluation and performance Prediction of Low Permeability Gas Wells Stimulated by Massive Hydraulic Fracturing JPT (March 1979) 362

18. Hunt, J.L. et al.: Performance of Hydraulic Fractures in High Permeability Formations paper SPE 28530 (1994)

19. Martin, J.P. et al.: Tip Screen out Fracturing Applied to Revenspurn South Gas Field Development paper SPE 19766 (1989)

20. Ayoub, J. A. et al.: Evaluation of Frac & Pack Completion and Future Outlook paper SPE 65063 (2000)

21. Zemlak, W. Et al.: Selective Hydraulic Fracturing of Multiple Perforated Interval with a Coiled Tubing Conduit paper SPE/ICOTA 54474 (!999)

22. Paccaloni, G,: new Method Proves Value of Stimulation Planning Oil Gas Journal (Nov. 19,1979)

23. Nolte, K. G, and Smith, M. B.: Interpretation of Fracturing Pressure JPT (Sept.1981) 1767

24. Cleary, M. et al,: Field Implementation of Proppant Slugs to Avoid Premature Screen out of Hydraulic

Fracture with Adequate Proppant Concentration paper SPE 25892 (1993)

25. Tambini, M.: An Effective Matrix Stimulation Technique for Horizontal Wells paper SPE 24993 (1992)

26. Raghavan, R. et al.: An Analysis of Horizontal Wells Intercepted by Multiple Fractures paper SPE 27652 (1994)

27. Nikolaevskiy, V.N.: Geomechanics and Fluid dynamics Kluwer Academic Publishers (1996)

28. Cuthill, D.A.: Propellant Asssisted Perforating An Effective Method for Reducing Formation Damage when Perforating paper SPE 68920 (2001)

29. Gilliat, J.et al.: A Review of Field Performance of New Propellant Perforating Technologies paper SPE 56469 (1999)

30. Robert, P.M.: Ultrasonic Removal of Organic Deposit and Polimer Induced Formation Damage paper SPE 62045 (2000)

31. Bakker, T.W. and Ivannikov, V.I.: Cavitator for Effective Well Cleaning paper SPE 75352 (2002)

32. Jackson, S. et al.: Advances in Seismic Stimulation Tecnologies PTTC Volume7,2nd Q (2001)

33. Sierra, R. et al,: Promising Progress in Field Application of Reservoir Eletrical Heating Methods paper SPE 68709 (2001)

34. Selyakov, V.I. et al.: Application of technology of electroinfluence for intensification of an oil recovery in Russia and abroad Russia, Oil Industry (November 2002) 92

35. WestermarK, R.V. et al.: Enhanced Oil Recovery with Downhole Vibration Stimulation paper SPE 67303 (2001)

36. Beresnev,I.A. et al.: Elastic Wave Stimulation of Oil Production: A Review of Methods and Results Geophysics (June 1994)

SPE 82573 5

Fig . 1 Technical Economic Life Cycle of Stimulation

Fig. 2 Vibrowave Active Penetration in-side Sandstone

VVVAAALLLUUUEEE

R&D EMERGING GROWTH MATURITY DECLINE

1

BULLISH RISING STAR CASH COW HARVEST TTTIIIMMMEEE

1 ACID & FRAC

2 EXPLOSIVE ACUSTIC ELECTRIC

2

0.01 m 0.1 m 1 m 10 m 100 m 1000 m

0,00010,0010,010,1

110

1001000

10000100000

Distance from the wellbore (m)

Frequency (s-1)