MEOR SCREENING CRITERIA FIT 27% OF U.S. OIL RESERVOIRS

April 15, 1991
Rebecca Smith Bryant National Institute for Petroleum & Energy Research Bartlesville, Okla. Criteria developed by the National Institute for Petroleum & Energy Research (Niper) indicate that 27% of reservoirs in the major oil-producing states of the U.S. have potential for microbial-enhanced oil recovery (MEOR) processes. MEOR has been recognized as a potentially cost-effective method, particularly for recovering additional oil from stripper wells.
Rebecca Smith Bryant
National Institute for Petroleum & Energy Research
Bartlesville, Okla.

Criteria developed by the National Institute for Petroleum & Energy Research (Niper) indicate that 27% of reservoirs in the major oil-producing states of the U.S. have potential for microbial-enhanced oil recovery (MEOR) processes.

MEOR has been recognized as a potentially cost-effective method, particularly for recovering additional oil from stripper wells.

Niper has conducted both laboratory research and field applications in microbial-enhanced oil recovery (MEOR) for the U.S. Department of Energy (DOE) since 1983.1-7 One of the goals of this research is to maintain a data base of field projects using MEOR technology.

The data base provides documentation of characteristics of reservoirs used for MEOR field projects and is used to revise published screening criteria for MEOR processes.

Stripper wells are particularly in need of cost-effective EOR because independent operators produce about 40% of the U.S. daily oil production, but usually cannot conduct needed EOR research.

Microbial methods for improving oil recovery are well-suited to be applied in today's economic climate. The lower price of crude oil, as well as a more general acceptance of biotechnological processes, has probably contributed to increased interest in MEOR.

The nature of the reservoir to be used for MEOR technology will strongly affect the success of the process. If the reservoir is highly channeled, injecting a micro-organism that produces only a surfactant may not recover a significant amount of oil, because the micro-organisms will flow mostly through the highly permeable zone and thus bypass much of the trapped crude oil that was bypassed during the original waterflood.

By contrast, if there is no channeling and the reservoir permeability is low, injecting a micro-organism that produces only a polymer and biomass may decrease injectivity and cause undesirable plugging.

Sometimes the mineral content of the connate water may inhibit the growth of the selected micro-organisms. If that happens, it may be necessary to selectively stimulate indigenous microbial strains that have the capability of mobilizing oil in the reservoir.

Table 1 lists screening criteria considered important for MEOR field projects.

MICROBIAL PROCESSES

The increasing number of microbial-enhanced oil recovery field projects and the variety of different microbial processes that are applicable demonstrate the difficulty and complexity of placing reservoir limitations on the technology. Some thought must be given to the type of microbial process needed, which means that first some knowledge of reservoir problems must be obtained.

Simple compatibility studies between reservoir fluids and micro-organisms can be adequate in many cases to predict whether micro-organisms can be applied successfully.

Compatibility tests are usually test tube experiments in which several microbial formulations are grown in the presence of reservoir fluids and sometimes reservoir rock. The growth and metabolite production of the microorganisms are measured to determine the optimal conditions. Determining screening criteria for MEOR processes becomes a matter of selection of particular microbial formulations for specific reservoir conditions after problems are defined. The most important reservoir criteria to be considered are listed in Table 2.

The U.S. DOE reservoir data base (public copy) was used to screen several oil-producing states for the number of reservoirs that satisfied the following criteria:

  • Injected and connate water salinities less than 100,000 ppm

  • Rock permeability greater than 75 md

  • A depth less than 6,800 ft.

This corresponds to a temperature limitation of about 75 C.

A graph of the percent of reservoirs by state that satisfy these limiting criteria and the total is shown in Fig. 1.

MICROBIAL TREATMENTS

Because of the diverse nature of MEOR technology, several different oil production problems have been addressed by microbial or nutrient injection. To differentiate among field projects using micro-organisms, they are separated according to the classification in Table 3.

The processes listed in Table 3 are used only for classification; some of them have not been used in the field, but field work has been planned based upon laboratory results.

The most practiced MEOR technique involves cyclic treatments of producing wells. These types of treatments have been conducted since 1953; however, those conducted most recently have involved some nutrients that have improved injectivity and more carefully designed microbial processes. These more recent tests were considered for this report.

There are generally two types of well-stimulation treatments: those that are designed to improve injectivity by cleaning out the well bore, and those that can actually improve crude oil mobilization in the near-well bore region.

Improvements in oil production can result from removal of paraffinic or asphaltic deposits from the nearwell bore region or from mobilization of residual oil in the limited volume of the reservoir that is treated. Because there is a potential for improved residual oil mobilization, these treatments are distinguished from those that use micro-organisms specifically for well bore cleanup.

Well stimulation treatments generally use micro-organisms that require the addition of a nutrient to survive and thrive for periods of several months in the well, whereas micro-organisms used for well bore cleanup are those that generally do not survive for extended periods of time and are injected on a regular basis, somewhat similar to hot oil treatments. They generally do not survive outside the well bore region because they are oxygen-requiring microbes.

Typically, well stimulation treatments can be implemented with only a few minor modifications to existing surface facilities, and they are relatively inexpensive.

Use of micro-organisms in the near-well bore region can greatly improve injectivity and mitigate certain production problems. Several different companies promote microbial well bore cleanup technology; however, information from most of these production operations is proprietary.

One microbial treatment company (Micro-Bac, International Inc.) provided a listing of petroleum regions where its microbial products are in use and also several case histories of microbial well bore cleanup.8 That company has estimated that 2,500-3,000 wells have been treated using its microbial products, and this number does not include production tank or barge treatments for basic sediments and water (bs&w) or paraffin.

Oil production increases have occurred in about 50% of all wells treated, with increases in total fluid produced ranging from 100 to 10%. From the available information, it is clear that in certain instances, microbial injection in the near-well bore region can rival certain existing chemical treatments, both in efficiency and cost.9

Well stimulation treatments can be considered successful not only by improving oil production but also by decreasing the cost of maintenance and operation of a well. As an example, a microbial formulation that assists in cleaning out production lines can improve the productivity and increase the life of a well. By improving injectivity, maintenance treatments of a well, such as hot oil or solvent treatments, may not have to be implemented as often.

During the 1950s and 1960s, countries such as Czechoslovakia, Poland, Hungary, and the U.S.S.R. conducted numerous well stimulation treatments with a wide variety of micro-organisms and injection protocols.10 Underlying trends in all of these early single-well injections are that they used inexpensive sources of nutrients (usually molasses), and that they were generally successful. Increases in oil production ranging from 50 to 300% were reported.

In the 1970s and 1980s, researchers at some universities and small companies in the U.S. conducted probably as many as 300 well stimulation treatments. Unfortunately, the information resulting from all but a few of these treatments is unavailable to the public.

For a microbial-enhanced waterflood, the micro-organisms should be capable of moving through the reservoir and producing chemical products to mobilize crude oil. Micro-organisms can produce surfactants that can reduce oil/water interfacial tension (IFT) and cause emulsification. In addition, surfactants can alter the relative permeability of rock to oil by changing the wettability of the reservoir rock and thereby increasing oil recovery.

Microbes also produce gases such as CO2, N2, H2, and CH4 that could improve oil recovery by increasing reservoir pressure and by reducing the viscosity and swelling of individually trapped droplets of crude oil. Sometimes, particularly with heavy crude oils, production of CO2 may decrease the viscosity of the oil enough to lead to some improvement in oil production In carbonate formations or sandstone rocks with carbonaceous cementation, acid-producing micro-organisms can increase permeability and thereby improve oil recovery.

Care must be taken to ensure that more micro-organisms can move through the reservoir in waterfloods than in single-well stimulation treatments. However, the potential of treated waterfloods for a much greater increase in oil production is high because of the larger amount of reservoir contacted or treated.

One of the first successful MEOR field pilots was performed in 1954 and consisted of an injection well and a production well.11 More recent microbial-enhanced waterflood projects have been conducted by the National Institute for Petroleum & Energy Research (Niper), Imperial Energy Corp., Alpha Environmental, and countries such as Romania, East Germany, and the U.S.S.R.1 5-6 Many of these microbial waterfloods showed increases in oil production. The MEOR process responsible for the improved production is generally attributed to gas and surfactant production by the micro-organisms.

FLUID DIVERSION

Another application for micro-organisms in a waterflood is fluid diversion. Because many types of micro-organisms produce polymers, it has been suggested that some micro-organisms could be used in situ to plug high-permeability zones in reservoirs preferentially and thus improve sweep efficiency.12 Injected microorganisms remain in the water phase and may act to increase relative permeability to oil and decrease relative permeability to water.

In 1958, researchers in The Netherlands conducted a selective plugging experiment using Betacoccus dextranicus and reported significant increases in oil production as well as improved water/oil ratio.10

Micro-organisms that produce polymers, biomass, and slimes have been shown to reduce core permeability under reservoir conditions in the laboratory. 13 14

Few data have been published regarding MEOR processes where the amount of injected micro-organisms that produce polymer is actually equivalent to that of a conventional polymerflood. Moses" has conducted laboratory research in this area, but no field test results have been published. Researchers in China recently reported on laboratory tests involving novel micro-organisms that produce polymer which they intend to field test sometime later in 1990.16

Researchers at the University of Calgary have patented a methodology for using ultramicrobacteria to plug the area around the production well and thus alleviate water coning problems. 17 No field trials have been reported to the public.

RESERVOIR CONDITIONS

MEOR field projects have been conducted under a wide range of reservoir conditions. Well stimulation microbial treatments have been done in wells with connate brine salinities greater than 11%. Microbial-enhanced waterflooding projects have been conducted in waters with a total dissolved solids (TDS) concentration value as high as 32%. Although in the high-salinity case, it is probably not entirely sodium chloride that is responsible for the high value. In the past, most reservoir screening criteria used a TDS limit of 10%, or 100,000 ppm.

Reservoir rock permeability ranges of 1-1,000 md have been reported for MEOR field projects.

A crucial factor for single-well treatments should be good injectivity. Good injectivity may be limited to some extent by permeability in the near-well bore region.

In microbial-enhanced waterflooding, reservoir rock permeability becomes a more important consideration; however, successful field tests have been demonstrated in rock that was previously considered too tight for microbial treatment (< 100 md).5-6

A single-well injectivity test can provide valuable information to those producers considering microbial-enhanced waterflooding. If injectivity is unaffected after microbial injection, then permeability may not be a limiting factor for that particular reservoir. In these screening criteria, therefore, no limitations will be placed on permeability, although it is recommended that a single-well injectivity test be conducted prior to a multiwell microbial waterflood.

If a particular formation is known to have low permeability, microbial EOR may not be a viable process.

No MEOR field projects have been reported where pressures and temperatures were too high for microbial growth. The usual biological limitation for temperature is about 158 F. (70 C.), and the pressure limitation is about 20,000 psi. Testing of microbial compatibility with reservoir fluids under reservoir conditions is recommended prior to any microbial field test, even well stimulation treatments.

Temperature constraints for microbial growth occur with individual microbial species and therefore will not be considered under revised screening criteria. The presence of indigenous micro-organisms, as cited in previous screening criteria, is still a major concern. Micro-organisms used for crude oil mobilization must survive and thrive in the reservoir.

Compatibility testing using indigenous micro-organisms of a particular reservoir is also highly recommended. In reservoirs with more harsh environmental characteristics for microbial survival, the possibility of stimulating indigenous micro-organisms is feasible.

OIL PROPERTIES

Although most MEOR field projects have been conducted with light crude oils having API gravities around 30-40, successes have been reported with heavy crudes having gravities around 20 API. Obviously, the higher the viscosity of a crude oil, the more difficult it will be to mobilize; yet, the principal mechanisms of micro-organisms for improved displacement efficiency, emulsification, reduction in interfacial tension, reduction in oil viscosity, increase of reservoir permeability, and wettability alteration should still apply.

Many options are available to oil producers interested in microbial-enhanced oil recovery. Because of the nature of microbial growth and the ability of micro-organisms to utilize relatively inexpensive chemicals as nutrients, the economics should be attractive under almost any circumstance.

ACKNOWLEDGMENTS

The author thanks Edith Allison, Bartlesville Project Office, U.S. Department of Energy, and Dr. Thomas E. Burchfield, Dr. Min Tham, and Bill Linville of Niper for their suggestions and review of this article. This work was supported by the U.S. Department of Energy under Cooperative Agreement DE FC22 83FE60149.

REFERENCES

  1. Bryant, R.S., "Screening Criteria for Microbial EOR Processes," U.S. Dept. of Energy Report No. NIPER-478, August 1990. NTIS order No. DE91002208.

  2. Bryant, R.S., "MEOR Data Base and Evaluation of Reservoir Characteristics for MEOR Projects," U.S. Dept. of Energy Report No. NIPER-272, August 1989, NTIS order No. DE89000764.

  3. Bryant, R.S., and Burchfield, T.E., "Review of Microbial Enhanced Oil Recovery Technology," SPE Reservoir Engineering, Vol. 4, May 1989, pp. 151-154.

  4. Bryant, R.S., and Douglas, J., "Evaluation of Microbial Systems in Porous Media for Enhanced Oil Recovery," SPE Reservoir Engineering, Vol. 3, May 1988, pp. 489-495.

  5. Bryant, R.S., Burchfield, T.E., Dennis, M.D., and Hitzman, D.O., "Microbial-Enhanced Waterflooding: Mink Unit Project," SPE Reservoir Engineering, Vol. 5, February 1990.

  6. Bryant, R.S., Burchfield, T.E., Dennis, M.D., Hitzman, D.O., and Porter, R.E., "Microbial-Enhanced Waterflooding: A Pilot Study," International Conference on Microbial Enhancement of Oil Recovery, Norman, Okla., May 27-31, 1990.

  7. Chase, K.L., Bryant, R.S., Bertus, K.M., and Stepp, A.K., "Investigations of Microbial Mechanisms for Oil Mobilization in Porous Media," 1990 International Conference on Microbial Enhancement of Oil Recovery, Norman, Okla., May 2731, 1990.

  8. Schneider, Dennis R., Micro-Bac International Inc., personal communication, Aug. 20, 1990.

  9. Pelgar, J., "MEOR Research on Wellbore Stimulation," International Conference on Microbial Enhancement of Oil Recovery, Norman, Okla., May 27-31, 1990.

  10. Hitzman, D.O., "Review of Microbial Enhanced Oil Recovery Field Tests," Symposium on Applications of Microorganisms to Petroleum Technology, U.S. Dept of Energy Report No. NIPER-351, September 1988. NTIS order No. DE88001232.

  11. Yarbrough, H.F., and Coty, V.F., "Microbially Enhanced Oil Recovery from the Upper Cretaceous Nacatoch Formation, Union County, Arkansas," International Conf. on Microbial Enhanced Oil Recovery, Afton, Okla., May 16212 1982, Dept. of Energy Report No. Conf-8205140, 1982, pp. 149-153.

  12. Jack, T.R., Thompson, B.G., and DiBlasio, E., "The Potential for Use of Microbes in the Production of Heavy Oil," International Conference on Microbial Enhanced Oil Recovery, Afton, Okla., May 16-21, 1982, Dept. of Energy Report No. Conf-8205140, 1982, pp. 88-93.

  13. Knapp, R.M., McInerney, M.J., Menzie, D.E., and Raiders, R.A., "Microbial Field Pilot Study, U.S. Dept. of Energy Report No. DOE/BC/14084-6, January 1989.

  14. Jack, T.R., "Field and Laboratory Results for a Bacterial Selective Plugging Systems," International Conference on Microbial Enhancement of Oil Recovery, Norman, Okla., May 27-31, 1990.

  15. Moses, V., personal communication, April 1989.

  16. Wang, X.Y., "Studies of the Thermally Generated Gel-agent Produced by Ps. aeruginosa for EOR," International Conference on Microbial Enhancement of Oil Recovery, Norman, Okla., May 27-31, 1990.

  17. Costerton, J.W., Cusack, F., and MacLeod, F.A., "Microbial Process for Selectively Plugging a Subterranean Formation," U.S. Patent No. 4,800,959, Jan. 31, 1989.

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