An Overview of Field Specific Designs of Microbial EOR

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  An Overview of Field Specific Designs of Microbial EOR . - Eric P. Robertson Gregory A. Bala Sandra L. Fox Janice D. Jackson Charles P. Thomas Idaho National Engineering Laboratory Abstract The selection and design of a microbial enhanced oil recovery WOR) process for application in a specific field involves geological, reservoir, and biological characterization. Microbially mediated oil recovery mechanisms (biogenic gas, biopolymers, and biosurfactants) are defined by the types of microorganisms used. The engineering and biological character of a given reservoir must be understood to correctly select a microbial system to enhance oil recovery. The objective of this paper is to discuss the methods used to evaluate three fields with distinct characteristics and production problems for the applicability of MEOR technology. Reservoir characteristics and laboratory results indicated that MEOR would not be applicable in two of the three fields considered. The development of a microbial oil recovery process for the third field appeared promising. Development of a bacterial consortium capable of producing the desired metabolites was initiated and field isolates were characterized. ,. *-* This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed. or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process. or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or deflect those of the United States Government or any agency thereof. Presented at the Fifth International Conference on Microbial Enhanced Oil Recovery and Related Biotechnology for Solving Environmental Problems, September 11-14,1995, Plano, Texas. ~ I --- _ ~ ST€R 1 DISTRIBUTION OF THIS DOCUMENT IS UNLIi4lTED~  . I Introduction klicroorg'anisms, most commonly bacteria, haire been em$Gj%a in the recovery of crude oil for decades.' Bacteria assist in oil recovery by the in situ production of metabolites (Le., by-products as a result of growth) and biomass. Some bacterial products that may be useful in oil recovery include gases, surfactants, polymers, biomass, acids, solvents, and alcohols?' Biogases, if produced in sufficient quantities, can reduce oil viscosity, displace immobile oil, and swell oil in place. ,Biosurfactants reduce interfacial tension; thus, improving pore-scale displacement efficiency. They can also alter wettability, which may affect oil recovery? Purified and concentrated biosurfactants have been reported to reduce interfacial tension between oil and water to 10 dyne/cm.6 Bacterial polymers and biomass are used to improve the sweep efficiency of waterfloods by plugging high permeability strata or water invaded zones. Recent field work has shown promise in the area of microbial plugging for improving sweep efficiency.7p8 Less is hown about the effectiveness of acids, solvents, and alcohols produced by bacteria on oil recovery. Acids may improve permeability by altering the reservoir rockg and can also create C02 in situ by the dissolution of carbonate. Solvents may help remediate damaged wellbores resulting fiom paraftin deposition, dissolve crude oil, and act as co-surfactants. Alcohols may also assist oil recovery by acting as co-surfactants and solvents. Every producing oil field possesses its own set of production problems. In order for microbial EOR to be successful the treatment must be specifically designed to overcome problems associated with a given field. The objective of this paper is to discuss the methods used to evaluate three different fields for their suitablilty to microbial enhanced oil recovery. Schuricht Field The Schuricht is a small shgle-well field in the Powder River Basin, Crook County, Wyoming. The well (21-24) was completed in 1983 in the Minnelusa A sand between 6500 to 6508 ft subsurface. Fluid expansion is the probable drive mechanism. The well is currently producing 100% oil at about 80 bbl/month and is near the economic limit. There is no associated gas or water production. Reservoir and crude oil characteristics are summarized in Table 1. Figure 1 shows the production history of the well. There is no H2S associated with the oil; however, there is 3.24 wt elemental sulfur in Schuricht crude. This field is similar o many Minnelusa fields in the Powder River Basin. If microbial EOR could be successfully demonstrated in this field then the technology could hold promise for other fields in the arealo Moreover, this field has had no other EOR operations or off-pattern wells that could interfere with interpretation of results. Demonstrating the effectiveness of a multi-well process in a single well hufY'n' puff test is difficult. However, a technical success in this field would hold promise for economical successes in multi-well 2  pattern floods elsewhere. Determmation of MEOR method to be applied - A single-well pump-idpump-out test of the microbial system is the only available option in the Schuricht field because there are no plans to dk second well in this field in the foreseeable future. For this field, two options for microbial EOR were evaluated: in situ biosurfactant production and in situ gas generation. A simplified microbial soak process was evaluated using Buckley-Leverett fluid displacement principles to determine the results that could be expected under ideal circumstances. The Buckley-Leverett fluid displacement method uses relative permeability curves and viscosities of the flowing fluids as he basis of saturation front advancement. Relative permeability data needed for the Buckley-Leverett simulations were not available for the Schuricht field, but initial and irreducible oil saturations were known. Relative permeability curves for the Schuricht field were taken fiom similar neighboring Minnelusa fields with similar end-point saturations. Simplifyins assumptions were made for both microbial options before running the simulations. The formation was assumed to be homogeneous and no bacterial plugging occurs. The bacterial injection slugs were dilute and the relative permeability curves for water were used during injection of the slug. No attempt was made to model bacterial transport, growth, or metabolite production. The following assumptions were made for the biosurfactant case. 1) Oil and water viscosity remain constant. 2) Relative permeability end points are altered der bacterial incubation corresponding to a 40% reduction in residual oil saturation resulting from biosdactant production. The following assumptions were made for the microbial gas production case. 1) No change in the relative permeability curve end points. 2) Enough C02 is produced to saturate all the contacted oil. Saturating the Schuricht crude oil with C02 would swell the contacted oil and lower its viscosity ficom 15 CP o 6 CP. These assumptions represent best-case scenarios and may not be representative estimates of actual field results. Simulated incremental oil, recovery for the microbial gas mechanism over the biosurfactant mechanism is shown in Figure 2.  In this figure, normalized incremental oil recovery of the C02 process over the surfactant process is plotted against the normalized total fluid production. Normalized incremental oil production is calculated by the equation: Normalized incremental - co2 Process - sdactmt Process oil production contacted oil in place Normalized total fluid production is calculated by the following equation: Normalized total total produced fluid volume fluid Production - total injected fluid volume . Figure 2  is a good way of comparing one process to another and, as can be seen, the option of saturating the contacted oil with CO appears to recover more oil than generating a biosurfactant in situ for this particular single-well field. The reason for the poorer performance of the biosurfactant 3  option is because the mobilized oil is in the high water saturation portion of the relative permeability curves. Oil mobility is low in this region and, consequently, the additional mobilized oil moves Thr6uglil.he formation to the producing well.Very 5lowly.73nthe other hand, the microbially generated carbon dioxide lowers the Viscosity of all the contacted oil, which changes its mobility or relative permeability allowing the oil to flow faster to the producing well. Swelling the oil by saturating it with carbon dioxide also aids in its recovery. This simple analysis indicates that for the Schuricht field it would be more advantageous to employ bacteria capable of producing enough C02 o saturate the contacted oil instead of bacteria that reduce interfacid tension between oil and water. Based upon this result, it was decided to fmd and evaluate bacteria that could produce a copious quantity of C02 or other similar gas to be applied in a possible microbial EOR demonstration in the Schuricht field. Preparatory Microbiological Work for Field Test A successful field application of microbial EOR is highly dependent on the selection of the bacterial strain or consortium to be used in the field. Enrichment procedures to isolate microorganisms indigenous to the Powder River Basin of Wyoming began by collecting and screening Schuricht field fluids and solids. Specific characteristics sought for the enriched microbial cultures were: 1) growth at the reservoir temperature of 138°F (60°C), 2) growth with and without oxygen present ie., facultative anaerobe, microaerophilic, denitrifying), 3) salt tolerant to 3% KCl to match the reservoir rock and fluid compatibility, 4) compatible with Schuricht crude oil, and 5) productibn of large amounts of C02 under reservoir conditions, Soil samples retrieved from the Powder River Basin were enriched for bacteria capable of gas production. A 1 O g sample of oil-laden cow manure was added to liquid media [trypticase soy broth (TSB), potato dextrose yeast (PDY) medium (ATCC 337), and potato medium (ATCC 1 126)]12 and incubated aerobically at 60°C. The medium was amended by adding 3% KCl and 1% NaN03. After 24 hours of growth, the organisms were subcultured to identical media and incubated anaerobically at 60°C. Growth (turbidity) and gas production (bulging septa) were observed within 24 hours. Potato medium continuea.&,show a positive response for gas production and was used for further study. Organisms were &eked on agar plates and colonies were picked and transferred to isolate the organisms. Four Schuricht enrichment cultures were obtained and identified as 1,3a, 3b and 4. All four isolates are similar; however, the 1 isolate produced more gas and was singled out for further studies. This organism was renamed Powder River Basin #1 (PRB 1) and was stored in maintenance broth with 2% glycol for long term ~t0rage.l~ Medium development. A potato base medium was used initially to study the bacteria because of the availability of a cheap carbon source (potato starch). The potato based growth medium was analyzed to determine bacterial utilization of the components. The carbon and energy sources provided in potato medium were varied along with other constituents to determine the necessary components required for bacterial growth. Potato was added to provide minerals and a starch substrate (carbon source) for PRB l; whereas, sodium nitrate serves as he terminal electron acceptor of the electron transport chain, and yeast extract was added as a source of water soluble vitamins, amino acids, peptides, and an additional source of carbon. Results indicated that PRB #1 requires yeast extract 4
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