SPE Introduction Secondary recovery operations utilizing water flooding represent the majority of oil fields in the United States.

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1 SPE Improved Water Flood Operation in the Permian Basin Through Microbial Culture Treatment T. Ratliff, Bio-Tech, Inc.; B.C. Hoskins, SPE, and D.R. Schneider, SPE, Micro-Bac International, Inc. Copyright 1996, Society of Petroleum Engineers, Inc. This paper was prepared for presentation at the Permian Basin Oil & Gas Recovery Conference held in Midland, Texas March 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. Permission to copy is restricted to an abstract of not more than 3 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 , Richardson, TX , U.S.A., fax Abstract Microbial cultures can be used to reduce or eliminate scale formation in water flood injection systems. Several water floods have been treated for over a year with microbial culture products and the effects of the treatment on operating performance has been studied. The injection systems initially showed significant scale problems from calcium sulfate and calcium carbonate. Tracking water flood performance parameters such as injection pressures and volumes over the treatment period showed that, as compared with prior treatment historical data, a variety of positive results of treatment were evident. For example, the flood injection volumes were stabilized or increased, injection pressures were stabilized and oil and gas production was increased. Laboratory studies of injection water showed that filtration times could be significantly reduced by microbial treatment, indicating that near well-bore occlusion could be reduced by such treatments. The microbial treatments were considered more effective than prior alternative treatment technologies. This new technology offers a cost-effective superior alternative to conventional technologies. Introduction Secondary recovery operations utilizing water flooding represent the majority of oil fields in the United States. The practice is particularly common in mature producing regions such as the Permian Basin. The handling of the large volumes of water used in such projects leads to specific operational problems related to water chemistry. Demand for water leads to the use of various aqueous sources which may contain differing amounts of cations and anions. These incompatible waters lead to the formation of mineral salt deposits such as calcium carbonate, calcium sulfate, barium sulfate or iron sulfide. These deposits may form at any point in the injection system from surface facilities to downhole. Occlusion of water injection wells by either preformed or in situ formed scale is a principal cause of reduced water flood efficiency. Skin damage and blockage of the formation by scale leads to decreased injection volumes and increased injection pressures. The former instance may reduce injection volumes and the latter increases operating costs. Conventional treatments for injection system scale include acid and chemical treatments. Both technologies suffer from significant drawbacks. Acid treatments can promote corrosion, utilize a hazardous chemical and require significant downtime. Acid alone is not effective on calcium or barium sulfate scale. Chemical treatments are expensive and may not be cost-effective or successful. They may also not be successful in treating downhole permeability problems. An alternative technology is the use of microbial culture products to improve water flood operations. Microbial culture products have been used successfully for a number of years in the oil industry to control paraffin, corrosion and to enhance production 1,2. Such products are composed of non-pathogenic, naturally occurring microbes that are specifically selected and adapted for a given industrial purpose. Microorganisms are small (1-2 micrometers in length), free-living organisms that display remarkable biochemical diversity. Specific microbial culture products are designed for the control of mineral deposition. Such products control scale by containing and producing chelating agents such as organic acids and cell 583

2 wall components. They also contain and produce scale dispersing and antinucleation agents such as biosurfactants. Other, different microbial culture products have been used extensively for several years in the control of paraffin, corrosion and production enhancement. This paper describes three cases of treatment of water injection systems with microbial culture products for the control of scale. Methodology of Treatment and Monitoring Prior to treatment, supply water samples should be tested to determine the nature of the scaling problem, its potential severity, other relevant water chemistry phenomena such as elevated chlorides, solids content, and the appropriate type of microbial culture product to use in treatment. Control of conventional chemical treatment is also necessary to prevent possible adverse effects on the viability of the microorganisms. Regular monitoring of water flood treatment is necessary to ascertain changes in the water system and optimize product application as well as document successful treatment. Changes in water supply may occur which will affect the type and amount of scale deposited. Typical microbial treatment technique involves batch treatment into water supply tanks on a monthly or biweekly basis. Samples were obtained from operators on a regular basis for analysis. Three water floods from three different operators were subjected to a detailed analysis. Results Water Flood #1. The first water flood discussed has approximately 5, barrels of water per month delivered to six injection wells. The system had a history of calcium sulfate and iron sulfide deposition. Downhole deposition was accompanied with periodic plugging of the injection pump with scale. Prior to microbial culture product treatment the system was treated with scale inhibitors at the water tanks by continuous injection. At the initiation of microbial culture treatment, the water analysis showed a strong tendency for formation of calcium sulfate. Comparison of the pre- and post-treatment filtration of water samples from a similar flood showed a marked improvement in the rate at which the water would pass through a three micrometer filter (Figure 1). This strongly indicated that the treatment was reducing the capacity of the water to produce skin damage. When the pattern of water volume injected and the pressure of injection over time was analyzed, a significant reduction in injection pressures was noted with a corresponding stabilization of injection rates (Figure 2). The cost of conventional treatment utilizing chemicals was $2,884/month. This treatment was not considered effective. The microbial treatment cost was $l,44/month. Water Flood #2. The second water flood examined has a larger injection volume of over 1, barrels per month into 26 injectors. This flood had a history of calcium carbonate, iron sulfide, with occasional barium sulfate problems stemming from the source of supply water. Conventional treatment was continuous injection with chemical scale inhibitors at the central battery. Microbial culture treatments began in September, Analysis of injection parameters revealed a modest increase in injection pressure with a significant increase in the volume of water injected over nine months of treatment (Figure 3). Scaling deposition was reported as significantly reduced. The cost of conventional treatment was $3,671/month. This treatment was not considered effective. The operator of the flood requested a microbial treatment of comparative cost in order to evaluate the technology strictly on the basis of performance alone. The microbial cost was $3,7/month and was considered effective. Water Flood #3. The third water flood under study had a history of calcium carbonate and iron sulfide scale. The system had a significant history of flow line leaks as well. Prior treatment was a continuous injection of a chemical scale inhibitor with periodic acid treatments. Prior to beginning microbial culture product treatment the water analysis suggested a scaling tendency for calcium carbonate. After treatment commenced the water injection rate stabilized and pressures declined (Figure 4). Chemical and acid treatments were no longer required. Anecdotal evidence suggested that flow line leaks were reduced as well. Interestingly, a significant increase in the oil to water ratio was also seen on the production side (Figure 5). The cost of treatment using conventional chemicals was $1,64 per month and was not considered effective by the operator. The cost of the microbial treatment was $l,5/month, a savings of $1,68 per year. This did not include the savings due to reduced flow line leaks or any increases in oil production. Discussion. The results presented demonstrate the feasibility of using microbial culture products to improve water flood injection operation. The products were capable of addressing the same problems handled by conventional chemical products. The products performed successfully over long periods of time indicating that the effects produced were not transient. They were considered cost-effective. Some of the concerns that have been raised about the use of microbial culture products include the possibility of microbial biomass reducing reservoir permeability through plugging. No evidence of this was seen in any of these treatments and there are several reasons why this is unlikely to occur. The absolute amount of microbes added to these systems is very small as the typical batch dose of product is around 2 parts per million. As the product used contained 584

3 approximately 2 X 1 8 cells/milliliter, this makes the typical dose 4 X 1 5 cells/milliliter. The indigenous population of microorganisms in supply water is frequently this high. It is therefore very unlikely that microbial culture products could produce plugging. Most importantly, extensive field operations utilizing these products in a wide variety of formations have found no instances of plugging due to the products and, as can be seen in the case histories described, the reverse is seen. Also, the type of microorganisms used in these products are not slime forming or capsule producing bacteria. The products are not produced with sulfate reducing bacteria and have been used to control problems with sulfate reducing bacteria by competing with them for nutrients. 5. Forbes, A.D.: Hydrocarbons in Biotechnology. Heyden and Sons, Ltd. London.(198) Some comment should be made as to the change seen in the water/oil ratio in water flood #3. Considerable research has been done on microbial enhanced oil recovery (MEOR) 3,4,5. The principles behind this technology incorporate the concepts of bacteria producing biosurfactants which change reservoir wettability as well as gases and other small molecules which reduce oil viscosity. Although the treatment was not designed specifically for this purpose, the observation that oil to water ratios increased may indicate a microbial induced change in reservoir wettability or in oil viscosity although this aspect was not examined in this field study. In conclusion, it can be said that microbial culture products offer an effective alternative to conventional chemical treatments in the control of calcium carbonate, calcium sulfate and iron sulfide scale. Such products are shown to be effective over an extended period of time, did not produce any adverse side-effects and were cost-effective to conventional technologies. Acknowledgments We thank Steve Davis for valuable assistance in gathering field data and Dr. Lee Entzeroth for valuable comments. References 1. Nelson, L., and Schneider, D.R.,: Microbial Enhancement of Oil Recovery-Recent Advances. Developments in Petroleum Science 39. Elsevier, Amsterdam Pelger, J.W.: "Wellbore Stimulation Using Microorganisms to Control and Remediate Existing Paraffin Accumulations," paper SPE presented at the 1992 SPE International Symposium on Formation Damage Control, Lafayette, Louisiana, Feb Stepp, A.K., Bryant, R.S., Bertus, K.M. and Chang, M.M.: Microbial Enhancement of Oil Recovery-Recent Advances. Developments in Petroleum Science 39. Elsevier, Amsterdam, (1992) Lazar, I.: Microbial Enhanced Oil Recovery. Pennwell Books, Tulsa, Oklahoma. (1983)

4 Time Elapsed (sec) 8 6 2/16/95 2/23/95 3/1/ Volume Filtered (ml) Figure One. One hundred milliliters of water was filtered through a three micrometer filter and rate of filtration was timed. Microbial treatment began on 2/16/95 on this unit Daily Injection Pressures (psi) Daily Injected Volume (BWIPD) 5 System under microbial treatment Dec-92 Jan-93 Feb-93 Mar-93 Apr-93 May-93 Jun-93 Jul-93 Aug-93 Sep-93 Oct-93 Nov-93 Dec-93 Jan-94 Feb-94 Mar-94 Apr-94 May-94 Jun-94 Jul-94 Time (mon) Figure Two. Water Flood #1. Daily injection pressures are indicated along with daily injected volumes. 586

5 Daily Injected Volume (BWIPD) Daily Injection Pressures (psi) 3 2 System under microbial treatment 1 Jul-92 Aug-92 Sep-92 Oct-92 Nov-92 Dec-92 Jan-93 Feb-93 Mar-93 Apr-93 May-93 Jun-93 Jul-93 Aug-93 Sep-93 Oct-93 Nov-93 Dec-93 Jan-94 Feb-94 Mar-94 Apr-94 May-94 Jun-94 Time (mon) Figure Three. Water Flood #2. Daily injection pressures are indicated along with daily injection volumes. 6.E+5 5.E+5 Initial Microbial Treatment Pressure x Time (psi x days) 4.E+5 3.E+5 2.E+5 1.E+5.E+.E+ 2.E+5 4.E+5 6.E+5 8.E+5 1.E+6 1.2E+6 Cumulative Water Injected (BBLS) Figure Four. Water Flood #3. Hall plot of effect of microbial treatment on pressure required to inject a given volume of water per day. Declining slope indicates lower energy requirement per injection volume. 587

6 Production Rate (BOPM) Water-Oil Ratio BOPM WOR 1 System under microbial treatment 4. 5 Jan-92 Jul-92 Jan-93 Jul-93 Jan-94 Jul-94 Jan-95 Jul Time (mon) Figure Five. Water Flood #3. Oil production rate versus time along with water to oil ration (WOR) versus time. Significant drop in WOR over time of treatment is evident. A corresponding increase in oil production is evident as well. BOPM= Barrels of Oil per Month, WOR= Water oil ratio 588