Copyright 2001, Society of Petroleum Engineers Inc.

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AbstractThe Libwa 4 well offshore the Democratic Republic of Congowas stimulated in November 1999 using an enzyme-basedprocess that generated acid in-situ. The results to date havebeen excellent. Well tests as high as 759 BOPD have beenrecorded after the treatment compared to less than 100 BOPDbefore the treatment.

IntroductionThe Libwa field is located off the coast of the DemocraticRepublic of Congo (formerly Zaire) West Africa. The fieldwas discovered in June 1981 and declared commercial in1983. Development was initiated soon after, but was deferreddue to the subsequent discovery at the Lukami field. TheLibwa field was put on production in January 1990.

The Libwa field was formed in response to growth-faulting.The structure is a fault block anticline plunging northwest-southeast. The faults bounding Libwa field in the Aptian toAlbian age section formed in response to movement of theunderlying salt. They extend upward into the lower Kinkasi,and are believed to extend downward into the underlyingLoeme salt layer. Structural dip within the Libwa Upper Pindareservoir averages about 14°. The depositional environment ofthe Libwa field is that of a shallow carbonate shelf, adjacent toa clastic-dominated coastline with the terrigenous materialperiodically supplied by the river systems of deltas, asobserved in other nearby Upper Pinda fields.

The Libwa field is characterized as a low permeability (2millidarcy) limestone with about 240 feet of oil columnoverlain by about 310 feet of gas cap. The oil leg is laterallycontinuous through the field and the gas zone is very thin at thedowndip portions of the field that thickens towards the crest ofthe structure. Core samples indicated that there is virtually novertical permeability and minimal natural fracturing. All of theLibwa wells currently on production required acid stimulationin order to establish economic production rates.

The Libwa Upper Pinda reservoir is a saturated oil systemwith a large gas cap. Original oil in place is estimated to be350 MMSTB and the original gas in place in the gas cap isestimated to be 114 BCF. PVT analysis indicated a solutiongas-oil ratio of approximately 470 scf/STB at a bubble pointpressure of 2540 psia. Cumulative oil recovery from the fieldthrough year 2000 is 5.2 MMSTB or only 1.5% of the originaloil in place. Libwa 4 is a horizontal well that was drilled in1990 into the low permeability (2 md) limestone of the Libwafield [Figure 1].

Figure 1. Libwa Field Schematic

SPE 68911

Stimulation of a Producing Horizontal Well Using Enzymes that Generate Acid In-Situ -Case HistoryRalph E. Harris SPE, Cleansorb Ltd., Ian D. McKay SPE, Cleansorb Ltd., Justin M. Mbala, Chevron Oil Congo. Ltd. andRobert P. Schaaf SPE, Chevron Oil Congo Co. Ltd.

Gas cap

Oil bounds

2 R.E. HARRIS, I.D. MCKAY, J. M. MBALA, R.P. SCHAAF SPE 68911

Due to initial low production, Libwa 4 was acidized with15% hydrochloric acid via jet washing with a work stringimmediately after the completion. A post stimulationproduction of 2361 BOPD declined to 800 BOPD during thefirst month with an increase in gas production. This may havebeen the result of the downward coning of gas leading to areduction in oil production. There has been evidence that thewell was suffering from the presence of residual drillingdamage. Fluid production was occurring from only 150 feet ofthe 2306 feet open hole wellbore when a production log wasrun in October 1990.

Since 1990 oil production from the Libwa 4 well hasdeclined at an annual rate of 15% to the pre-treatment rate ofunder 100 BOPD. The oil production decline has beenaccompanied by an increase in the gas-oil ratio to over 6000SCF/bbl. The well has always produced a negligible amount ofwater. The well was considered to be a good candidate for astimulation treatment.

Well Stimulation OptionsDifferent methods of stimulation were considered to increasethe production of the well. These included (a) placing 15%hydrochloric acid (HCl) throughout the well bore using coiltubing (b) pumping 15% HCl from the surface, and (c) usingenzymes that generated acid in-situ. When the job was firstbeing planned, early 1999, oil prices were at very low levels.

The equipment and materials required to do a 15% HClacid placement via coil tubing is expensive. The tripod welljackets used in the offshore Democratic Republic of Congo arerelatively small. Additionally the waters are rough having astrong current and constant swells. In order to use a coil tubingunit, a jack-up work barge is required to support the unit andplace the coil tubing well head on the well. Being in WestAfrica, to get the equipment and materials to do this coiltubing job would be expensive. Coupled with the then low oilprices, the economics of this type of job would be marginal atbest.

Pumping 15% HCl acid from the surface was anotheroption. The equipment required and costs would be much lessthan a coil tubing job. A vessel that had pumps and tankswould be needed for the job. For this particular well, theproblem with this method would be limited coverage of theexposed formation. The horizontal section of the well is 2306feet long. The HCl acid is highly reactive and will reactimmediately with the first carbonate rock that it encounters.Thus, only a small section of the well bore would actually gettreated. A more disturbing possibility, though, is that the HClmight penetrate into a fracture network and open a channel tothe gas cap. Increasing gas production would be detrimental.The well already had a high gas-oil ratio and any increase ingas production could inhibit oil production. Since gas is not

currently marketed in the Congo, there would only be negativeeconomic consequences if this channeling were to occur.

The chosen alternative was to use a process in whichenzymes produce acid in-situ.

Enzyme-Generated Acid ProcessThe process used to treat well Libwa 4 was first described inSPE paper 301231. The process uses an enzyme (a biologicalcatalyst) to produce organic acid at a controlled rate in-situ.SPE 30123 referred to the process as “Biologically GeneratedAcid”. However, in order to adequately differentiate theprocess from previous less efficient acid production methodsbased on bacteria we would like to propose that the term“Enzyme Generated Acid” be adopted in preference.

The basis of the process is that a suitable enzyme (one withactivity as an ester hydrolyzing enzyme) is mixed with anaqueous solution of suitable esters (the “acid precursor”). Thefluid is then introduced into the wellbore. The enzymehydrolyses the ester to produce acetic acid (Figure 2).Hydrolysis occurs over a period, normally between 24 and 72hours. The treatment fluid is neutral to slightly acidic whenplaced so is much lower reactivity than conventional acids.

Figure 2. Basis of the Enzyme Generated Acid Process

ESTERENZYME

ACID +ALCOHOL

Hydrolysis of ester

With the enzyme-based acid treatment, the majority of theacid (typically >95%) is produced after placement of the fluiddownhole giving better zonal coverage of horizontal wellbores.Problems such as worm holing associated with the use ofreactive acids such as HCl are avoided. As the acid isproduced from the precursor, it reacts with acid-reactivematerial present in the wellbore or formation. This means thatthere is never a high concentration of unreacted acid present inthe fluid, but over the period of treatment sufficient acid isdelivered to have a substantive acidizing effect.

SPE 30123 presented laboratory data that demonstratedthat Enzyme Generated Acid could be used to remove drillingdamage caused by water based drilling mud. The authorsspeculated that the process could also be used to removedrilling damage caused by oil based drilling mud if a suitablesurfactant was incorporated into the treatment fluidformulation. It has subsequently been established that this isindeed the case.

Treatment ObjectivesThe objectives of the enzyme-based acid treatment were toremove residual damage (oil based mud filter cake containingcarbonate arising from drilling), reduce draw down along thewhole of the wellbore, minimize gas coning, and increase oil

STIMULATION OF A PRODUCING HORIZONTAL WELL USINGSPE 68911 ENZYMES THAT GENERATE ACID IN-SITU - CASE HISTORY 3

production. An important consideration in any stimulationtreatment at the Libwa field is to minimize any increase in gasproduction due to the current high gas-oil ratio.

For this reason a near well bore stimulation using EnzymeGenerated Acid was chosen to avoid significant penetration ofacid into fracture networks and thereby prevent gas productionbeing inadvertently increased.

Treatment ExecutionThe Enzyme Generated Acid formulation selected for thetreatment was one which had been shown to be effective forthe removal of oil based drilling fluid damage and whichgenerates about 5% w/v acetic acid in-situ. The acid precursorwas supplied in 55-gallon drums and the enzymes in 25-literjerry cans. The treatment was performed by the “Falcon Tide”stimulation vessel. The boat was ideally set-up for thetreatment of offshore wells. It was equipped with mixing tanksand pumps that allowed for the efficient mixing of thecomponents of the treatment. The sequence of operations issummarized in Figure 3.

Figure 3. Sequence of Operations in Enzyme GeneratedAcid Treatment

Mix acid precursor into seawater

Mix enzyme into acidprecursor solution

Pump complete treatmentfluid downhole

Shut in well for 48 hours to allowenzyme to produce acid in-situ

Return well to production

All tanks and lines were thoroughly cleaned before thetreatment. The acid precursor was mixed into seawater. Theacid precursor needs a reasonable degree of agitation todissolve it in water. Dissolution was achieved using paddleblenders and re-circulation of the mixed fluid through hoses.

When the acid precursor was completely mixed withseawater the enzyme (a stabilized aqueous solution) was alsomixed into the fluid. It took only about 2 hours to mix all ofthe chemicals. A volume of fluid of 208 barrels (33 m3) wasprepared. This was 30% higher than that required tocompletely fill the openhole wellbore.

The stimulation vessel was equipped with high pressureCoflexive hose that attached the boat's pumps to the well head.The plan for pumping was to keep pressures well below

formation fracture pressure. The initial rate was 3 barrels perminute. After pumping at this rate for 30 minutes, the surfacetubing pressures were so low, 50 psi, that the pumping rate wasincreased to 5 barrels per minute. The surface tubing pressureonly increased to 150 psi at this higher rate. The pressures didnot get any higher and started to decrease towards the end ofthe job. The final surface tubing pressure during pumping was120 psi. After pumping the treatment fluid downhole thetubing was displaced with potassium chloride then diesel. Allpressures were well under the calculated maximum allowablesurface pressure of 2500 psi.

The entire treatment including the mixing of the chemicalstook only 8 hours. The actual pumping time for the 8750gallons of treatment fluid, 2722 gallons of 2% potassiumchloride, and 2722 gallons of diesel was less than 1.5 hours.

The well was shut in for 48 hours after the treatment toallow for the acid to be generated and react with the formation.After the shut-in period, the well was put back on production.Initially the well did not produce any fluid. The well is aflowing well with no artificial lift mechanism. In order to getthe well to produce required "rocking" the well. Gas fromadjacent wells on the jacket was used to pressure up the tubing.The gas and the release of pressure to the flow line helped liftthe oil and the residual fluid from the stimulation. After 2 to 3hours of "rocking" the well, the fluid started to flow. The wellhas continued to flow without problem since then.

Post Treatment ResponseLibwa 4 experienced a large and sustained increase in oilproduction after the stimulation. Well tests as high as 759BOPD have been recorded after the treatment compared to lessthan 100 BOPD before the treatment. The increase in oilproduction has also been accompanied by a correspondingincrease in gas production, however the gas-oil ratio camedown by approximately 40%. The production tests takenbefore and after the stimulation are listed in Table 1. Thevalues in italics are post treatment values.

DiscussionThe presence of near wellbore damage can significantly reducethe production rate of a well. Damage can include filter cakeand filtrate damage which arise during drilling, or productionrelated damage such as scaling,

After completion of Libwa 4 the well was immediatelyacidized with 15% hydrochloric acid via jet washing with awork string due to its initial low productivity. However, afterthis treatment a production log indicated that fluid was onlybeing produced from 150 feet of the 2306 feet of the open holewellbore. This strongly suggested the continued presence ofnear wellbore damage even after application of the HClthrough spotting with a work string, which is generally

4 R.E. HARRIS, I.D. MCKAY, J. M. MBALA, R.P. SCHAAF SPE 68911

Table 1. Production Test Results

Date BS&W BFPD BOPD Water Tot. Gas Gas Inj. Form.Gas TGLR FGOR TP psi Tbg. Chk.4-Nov-99 0.40% 20 20 0 790 0 790 39500 39659 440 48/645-Nov-99 0.40% 66 66 0 706 0 706 10697 10740 40 48/642-Dec-99 7.00% 816 759 57 2385 0 2385 2923 3143 310 48/64

17-Dec-99

0.30% 505 503 2 1849 0 1849 3661 3672 270 48/64

9-Feb-00 0.40% 440 438 2 1362 0 1362 3095 3108 240 48/644-Jul-00 1.00% 312 309 2 1729 0 1729 5542 5598 265 48/64

Results in italics are post treatment values.

considered to be a better method of placing HCl thanbullheading. The damage present in Libwa 4 was probably aconsequence of incomplete clean up of drilling fluid damage.

An important reason for choosing to produce throughhorizontal wells is to reduce the draw down of the well relativeto vertical wells and so minimize the coning of gas or water,which can substantially reduce oil production. A reduced drawdown can only be achieved if drilling fluid damage iseffectively removed from the whole of the length of thewellbore. If damage is substantively removed from only part ofthe horizontal wellbore, it is more likely that coning will occur.The increase in GOR during the lifetime of Libwa 4, beforetreatment with Enzyme Generated Acid, suggests that gasconing was taking place and that this was worsening with time.

The effective removal of near wellbore damage along mostor preferably all of the length of the open hole is therefore aprerequisite to obtaining the maximum benefit from thedrilling of horizontal wells and is generally viewed as highlydesirable.

Acid treatments are commonly used in attempts to treatnear wellbore damage including drilling fluid damagefollowing drilling. Acid treatments are also used to stimulateproduction after the well has been on production for severalmonths or years, to remove residual drilling fluid damage orscale.

Hydrochloric acid (HCl) is one of the acids mostcommonly used to treat formation damage. In wells withrelatively short producing intervals HCl used in conjunctionwith diversion techniques and coiled tubing is generallyeffective. However, the difficulty of applying HCl in longhorizontal producing intervals to uniformly remove drillingdamage has been identified as a very serious problem. Pooruniformity of treatment (poor zonal coverage) may result indisappointing well productivity.

The poor uniformity of treatment during conventional acidtreatments is largely a consequence of the rapid reaction rate ofHCl with carbonate rock or scale, which prevents its uniformplacement along a wellbore or deep into the formation. A

number of approaches to “retarding” acid to slow the rate ofreaction of acid with acid soluble material including carbonaterock, scale or carbonate components of filter cakes have beendeveloped by the oil industry1. In practice these methods canbe problematic when applied in the field.

The Enzyme Generated Acid process produces acid in-situand is an example of a highly retarded acid system in whichmost of the acid is produced after placement of the fluiddownhole. Use of this enzyme based process offered thelikelihood of obtaining excellent zonal coverage along all ormost of the 2306 ft of openhole wellbore even when placedwithout the use of coiled tubing.

Treatment of Libwa 4 with Enzyme Generated Acid wassuccessful and resulted in a significant and sustained increasein production and reduced the GOR by 40%. The treatmentobjectives of increasing oil production without increasing theGOR were therefore achieved. The volume of EnzymeGenerated Acid used to treat Libwa 4 was only sufficient totreat the near wellbore region. This indicates that the damagewas indeed near wellbore and that the Enzyme Generated Acideffectively removed the damage.

The decrease in GOR following the treatment with EnzymeGenerated Acid suggests that gas coning was reduced by thetreatment. This result would be expected if the treatmentremoved near wellbore damage from all or at least part of theopen hole interval leading to reduced draw down.

Samples of the produced water taken as the well returnedto production were analyzed for soluble calcium content.Elevated concentrations of calcium, above that present in theformation water and seawater were observed. This is consistentwith dissolution of calcium carbonate downhole by the acidproduced in-situ by the process. Insufficient samples weretaken to be able to estimate the total amount of calciumcarbonate dissolved during treatment of Libwa 4 but typicallya peak of elevated calcium is observed in a volume ofproduced water 2 to 3 times the volume of treatment fluidused.

In addition to achieving a successful stimulation of Libwa4 additional benefits arise from the use of Enzyme Generated

STIMULATION OF A PRODUCING HORIZONTAL WELL USINGSPE 68911 ENZYMES THAT GENERATE ACID IN-SITU - CASE HISTORY 5

Acid. These benefits include operational, health, safety andenvironmental benefits. Handling and mixing of the chemicalsis straightforward and requires only simple equipment. Thechemicals are low hazard. Handling of the individualcomponents does not pose any excessive safety problem. Themixed treatment fluid is initially non-reactive so does not posethe safety hazards that strong acids do. Due to the slow rate ofacid production, there is no need for high rate, high pressurepumping such as is often used in placing conventional acids.All materials have low environmental impact. The non-reactivenature of the unmixed chemicals along with the slow rate ofproduction of acid allows for adequate dispersion before anydamage may occur e.g. in the event of an accidental spillage orleak. These additional benefits are important when treatingwells, especially those in remote or challenging locations.

In the Libwa 4 treatment, the Enzyme Generated Acidprocess was used to stimulate a mature producing horizontalwell. The process may also be used to remove near wellboredamage from newly drilled horizontal and other wells. Whentreating such wells the drill string and mud pumps mayconveniently be used to place the fluid, rather than bullheadingor using coiled tubing. There is generally no requirement topull pumps or other downhole equipment due to the lowcorrosivity of the treatment fluid. In addition, there is no needto include corrosion inhibitors in the formulation.

Enzyme Generated Acid may also be used to stimulateproduction by increasing the permeability of a carbonateformation to a depth of several feet around the wellbore. In along well such as Libwa 4 the volumes required would besubstantial. Increasing the matrix permeability alone will notstimulate a well to the same extent as can be achieved by theeffective removal of near wellbore damage. In the case ofLibwa 4 a relatively low volume of treatment fluid was used,

as the principle objectives were to remove near wellboredamage without stimulating gas production. Use of a largervolume of treatment fluid has the potential to deliver furtherproduction increases due to deep matrix stimulation.

Conclusions

• Libwa 4 probably suffered from residual drilling damageand gas coning even after treatment with HCl by jetwashing with a work string.

• Treatment with Enzyme Generated Acid allowed the

efficient delivery of acid to the openhole section and ledto a significant and sustained increase in production and adecrease in the GOR

• Enzyme Generated Acid treatments have minimal

equipment requirements and are low hazard andenvironmentally acceptable.

Acknowledgement

The assistance of William G. Crouch (previously of ChevronOil Congo Ltd. and now with Chevron Nigeria Ltd.) in theplanning and execution of the treatment is gratefullyacknowledged.

References

1. Almond, S. et al.: “Utilization of Biologically Generated Acid forDrilling Fluid Damage Removal and Uniform Acid PlacementAcross Long Formation Intervals”, paper SPE 30123 pp 465-478in Proceedings of the European Formation Damage ControlConference, 15-16 May 1995, The Hague, The Netherlands.