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Microbially Enhanced Oil Recovery Field Pilot, Payne County, Oklahoma
197
Microbially Enhanced Oil Recovery Field Pilot, Payne County, Oklahoma J.D. Coates", J.L. Chisholmb, R.M. Knappc, M.J. McInerney', V .K . Bhupathiraju '
D.E. Menziec, and
'Department of Botany and Microbiology, University of Oklahoma, 770 Van Vleet Oval, Room 135, Norman, OK 73019-0245 bChemical & Natural Gas Engineering, Texas Ad1 University, Campus Box 1 9 3 , Kingsville, TX 78363 'College of Engineering, School of Petroleum & Geological Engineering, University of Oklahoma, Energy Center, Suite T 301, Norman, OK 73019-0628 Abstract A multi-well MEOR field study was performed in the Vassar Vertz Sand Unit in Oklahoma. The purpose of the field trial was to determine whether microorganisms could be used to preferentially plug high permeability zones to improve waterflood sweep efficiency. Laboratory studies determined that a nutrient system based on molasses and ammonium nitrate would induce the growth of nitrateutilizing bacteria indigenous to the Vertz unit reservoir while limiting sulfate reduction. Tracer studies were done to determine the predominant flow pattern from the pilot injection well within the reservoir. The molasses and ammonium nitrate nutrients were injected over a four-monthperiod. Samples were routinely analyzed for sulfate-reducing bacteria, molasses-nitrate utilizing bacteria, general fermentative bacteria, microbial metabolites, carbohydrates, sulfide, sulfate, nitrite, and nitrate. Over the period of nutrient injection, 56.2 tonnes of molasses and 18.8 tonnes of ammonium nitrate were injected into the pilot area. The results show that large-scale injection of readily metabolizable carbohydrates didnot detrimentally affect the ongoing operation in the fieldbut did result in an alteration of the existing flow patterns and reduction in transmissibility within the pilot area. 1.
INTRODUCTION
The United States remains dependent on crude oil and crude oil products to fuel its economy and although it is one of the world's largest producers of oil, its imports constitute over 40% of current U.S. consumption. Nearly two-thirds of all of the known U.S. oil ( 3 3 0 billion barrels) will be abandoned after conventional primary and secondary recovery techniques have been exhausted (Department of Energy, 1985). Enhanced oil recovery (EOR) methods increase the total recovery of oil beyond that achievable with primary and secondary methods by increasing the proportion of the reservoir affected (sweep efficiency), reducing the amount of residual oil in the swept zones, and reducing the viscosity of heavy oils. Unfortunately, conventional EOR technologies have only been marginally effective and the strong dependence of their economics on the price of oil has limited widespread application. Microbially enhanced oil recovery (MEOR) is a newer technology that has evolved in recent years, with a great potential for cost-effective recovery of residual oil. Crawford (1,2] hypothesized that injected bacteria could preferentially plug high-permeability zones, and that this plugging could correct variations in permeability in oil
198
reservoirs. Recent results from field pilot studies on interwell MEOR floods have been encouraging [3-61. The Department of Botany and Microbiology and the School of Petroleum and Geological Engineering at University of Oklahoma have been involved in a field pilot study at the Southeast Vassar Vertz Sand Unit (SEWSU), Payne County, OK. The objective of this study was to determine whether stimulation of indigenous bacteria in the reservoir by injecting molasses and ammonium nitrate could selectively plug high-permeability regions of a reservoir and improve volumetric sweep efficiency.
2.
DESCRIPTION OF STUDY AREA AND THE FIELD PILOT TEST
The Southeast Vassar Vertz Sand Unit (SEWSU) is located 10 miles southwest of Stillwater, in Payne County, Oklahoma. A plat map of the reservoir is shown in Figure 1. The field is currently operated using a line drive waterflood with wells 3 - 2 , 3 - 3 ,7 - 3 , 7-5, and 9 - 2 active as the brine injection wells. The flood front is believed to move in a northeasterly direction. The field pilot was performed in the southwest quarter of section 13, behind the line of active brine injection wells. Wells 5 - 1 , 5-2,and 7-1 were converted to production wells, and well 7-2 served as the injector for the pilot trial. Daily rates of fluid production in the pilot area were set at 150 barrels per day (bbl/d) each for wells 5-1 and 5-2, and at 300 bbl/d for well 7-1. About 300 bbl/d of brine was injected into 7-2. The production of 300 bbl/d more from wells 5-1, 5-2, and 7-1 than injected into 7-2 was done to ensure that the fluids injected into well 7-2 would mostly flow to the pilot production wells. Previous studies determined that the floodwater fromthe reservoir was highly saline containing 11 to 19% NaCl and 1 to 2 % calcium and magnesium ions. Diverse populations of anaerobic bacteria capable of growing with simple nutrient additions at these salinities are present throughout the reservoir. An active sulfate-reducing population was present in the waterflooded areas of the reservoir and in the water tanks at the field tank battery. Both sulfatereducing and nitrate-reducing bacteria were isolated and characterized. Of these, five nitrate-reducing bacteria were chosen for more extensive study. These were shown to be new obligate halophiles with a definite requirement for salt of concentrations greater than 6%. Of the various carbohydrate and inorganic nutrient mixtures tested, a molasses-ammonium nitrate mixture best stimulated their growth [7]. Several core experiments were performed in which permeability was altered and residual oil was recovered as a result of in-situ microbial growth at the temperature and pressure of the field using oil, brine, core material, and bacteria from the S E W S U [ 8 ] .
3.
MATERIALS AND METHODS
3.1. Nutrient injection Nutrient injection into well 7-2 was initiated at the end of August 1991. In total, 56.2 tonnes of molasses and 18.8 tonnes of ammonium nitrate were injected in four phases over four months. On August 29, 2.6 tonnes of ammonium nitrate were injected into the 7-2 well as previous studies had shown inhibition of sulfide production in both oil field samples and sewage sludge samples by high
199
samples by high nitrate concentrations [9, 101. Beginning on September 2 7 , 10.9 tonnes of molasses and 3 . 2 tonnes of ammonium nitrate were injected into well 7-2. Injection was stopped on October 4 for a 30-day incubation period to allow metabolism of the nutrients injected in the pilot area. A third nutrient treatment was initiated on November 9, during which 2 2 . 7 tonnes of molasses and 6 . 3 tonnes of ammonium nitrate were injected into the pilot area. This injection was completed on November 25. A fourth and final nutrient treatment of 22.7 tonnes of molasses and 6 . 7 tonnes of ammonium nitrate was injected from December 3 to 2 0 . Injection into well 7-2 and production from wells 5-1,5-2,and 7-1 was stopped for a second incubation period of 14 days on January 2 8 , 1992.
3.2. Biochemical analysis Brine samples were routinely collected from wells 5-1, 5-2,7-1,1A-1,and 1A9. Nitrate and nitrite were analysed using chemical test kits (Spectroquant). Hydrogen sulfide was determined colorimetrically [ll]. Sulfate concentration was
@ Active production w e l l
.
A Actlve Injection w e l l Inactive production w e l l j Inactive Injectfon w e l l 4 Dry hole
'
-
Southeast Vassar-Vertz Sand U n i t
I320 I t
Figure 1. Plat Map of the Southeast Vassar Vertz Sand Unit (SEWSU).
200
determined by high-performance liquid chromatography using an ion exchange column and a conductivity detector. Volatile fatty acids were determined by gas chromatography. Alkalinity was determined as outlined by Clesceri et al. [12] and total carbohydrates were determined by the phenol-sulfuric acid method [ 1 3 ] . Bacteriological analyses Enumeration studies were done by the three-tube most probable number (MPN) technique. Aerobic heterotrophic bacteria, anaerobic heterotrophic bacteria, anaerobic bacteria using molasses-nitrate, and sulfate-reducing bacteria were enumerated using media containing 15% NaCl as previously outlined [ 1 4 ] . 3.3.
4.
RESULTS AND DISCUSSION
4.1. Pre-treatment data Before treating the pilot area, baseline concentrations of salinity, alkalinity, sulfide, sulfate, nitrite, nitrate, carbohydrate, and volatile fatty acids were determined on a regular basis. From February 1990 to August 1991, the sulfide concentration in the reservoir increased from an average of 11 mg/l to over 52 mg/l. This increase in souring was the result of an increase in the volume of make-up water used, which has a sulfate concentration of 2 , 5 0 0 mg/l. Two fluorescein tracer studies were also completed before nutrient injection to determine the predominant flow path of the injected brine in the pilot area. The results indicated a significant flow channel existed from 7 - 2 to the 1A section of the reservoir, although it was originally believed that the preferential flow path was from well 7-2 to 7-1. On both occasions, the tracer was observed in wells 1 A - 1 , 1A-5, and 1A-9, a distance of 1870 feet from 7-2, within 18 days o f initial injection, thus, traveling over 100 feet per day. The calculated rate of travel of the tracer through the reservoir suggested the existence of a fracture system. 4.2. Microbial activation by nutrient treatment Following the first injection on August 29 of 2.6 tonnes of ammonium nitrate, the nitrate concentration produced in the brine o f the pilot production wells increased from undetectable levels to about 5 to 10 mg/l, 50 days later. Coproduced brine sulfide concentrations in the pilot area decreased from an average of 52 mg/l to 28 mg/l during the same period (Table 1). However, because of the high sulfide concentrations in the original brine and the high sulfate concentration in the make-up water, the amount of ammonium nitrate added was not sufficient to inhibit transient increases in the sulfide concentration of the coproduced brines due to molasses metabolism. About 56 days after the second nutrient injection, the sulfide concentrations increased and peaked at 203 mg/l for 5-1 and 5-2, respectively, and 334 mg/l for 7-1 before returning to 65 mg/l. A similar peak in sulfide concentration occurred in 5-1, 5-2, and 7 - 1 brines 57 days after the final nutrient injection. This time, however, the sulfide concentration of 7-1 brine attained a maximum of 195 mg/l while values for 5-1 and 5-2 were 251 mg/l and 184 mg/l, respectively (Table 1). Sulfide levels also increased in the brine from wells in the 1A tract although the levels were It should be emphasized that although large substantially lower (Table 1). increases in sulfide production were noted, these increases were transient and did not affect the long-term sulfide concentrations in the reservoir which had returned to a baseline value of 4 4 mg/l by March, 1992. The numbers of sulfate-reducingbacteria in the coproduced brines from 5-1 and 5-2 also were observed to increase with nutrient injection and sulfide
201 Table 1 Sulfide results of coproduced brines Date
Day
5-1
5-2
7-1
1A-1
Feb. 15, 1990 Aug. 29, 1991 Sep. 27, 1991 Oct. 14, 1991 Oct. 22, 1991 Nov. 18, 1991 Nov. 22, 1991 Dec. 06, 1991 Dec. 10, 1991
0 29 46 54 81 85 100 104
1A-9
6.60
14.23
59.79
16.96
30.56
3s.59
64.82
48.93
50.37
33.93
22.49
23.25
26.19
21.10
23.20
ND
37.96
52.82
46.10
73.16
50.04
5.94
68.17
63.12
86.02
58,43
33.42
153.95 203.03
131.88 203.03 60.75 ND
429.44 334.49
124.45 99.84
92.17 22.53
65.88 64.12
47.28 29.65
28.94 27.39
52.23
Jan. 03, 1992
128
62.35
Jan. 17, 1992
142 170
144.69 251.16
157.30
170.95
132.73
93.08
184.34
195.13
87.62
112.97
60.19
64.48
25.09
37.70
49.90
22.89
35.80
Feb. 14, 1992 Mar. 20, 1992
201
60.19
May
262
67.51
20, 1992
ND
concentration although the changes in bacterial counts were relatively small (Table 2). The number of sulfate-reducingbacteria in the 7-1brine samples also appeared to peak although the increase occurred 25 days after that of both 5-1 and 5-2. The numbers of sulfate-reducingbacteria in the coproduced brine from the 1A tract remained relatively stable throughout the entire trial (Table 2). Table 2 Numbers of sulfate-reducing bacteria in the coproduced brines (cells/ml) Date
5-1
5-2
Aug. 29, 1991
12.40x10'
9.33~10'
Nov. 05, 1991
9.33~10'
4.27~10'
7-1
1A-1
1A-9
4.27~10'
9.30~10'
4.27~10'
4.77~10'
4.27~10'
9.33~10'
Dec. 06, 1991
4.27~10'
2 . 4 0 ~ 1 0 ~ 2.30~10'
2.40x10'
9.33~10'
Jan. 17, 1992
2 .4Ox1O2
9.30~10'
4.62~10'
2 .4Ox1O2
9.33~10'
Feb . 11, 1992
9.3Ox1O2
4.27~10'
4.27~10'
2.05x10'
9.33~10'
Mar. 03, 1992
2.31~10'
2.31~10'
9.20~10'
4.27~10'
9.20~10'
Apr. 22, 1992
9.33~10'
ND
9.33~10'
2 .40x102
9.33~10'
202 Table 3 Molasses-nitrate utilizing bacteria in the coproduced brines (cells/ml) Date
5-1
5-2
7-1
Aug. 29, 1991
1.10x103
4.62~10’
Nov. 05, 1991
2.31~10’
2.31~10’
Dec. 0 6 , 1991
2.31~10’
2.31~10’
Jan. 17, 1992
2.31~10’
2 . 3 1 ~ 1 0 ~ 2.31~10’
ND
ND
Feb. 11, 1992
2.31~10’
2.31~10’
2.31~10’
ND
ND
Apr. 22, 1992
9.33~10‘
9.33~10’
2.4Ox1O3
ND
1A-1
1A-9
4.27~10’
2 .4Ox1O2
2.34x105
2.31~10’
2.40~10~
2.31~10’
2.31~10’
ND
2.31x10’
4.27~10’
The numbers of molasses-nitrate utilizing bacteria in 5-1 and 5-2 brines decreased 10-fold over the first 70 days of the trial and then remained constant at 23 cells per ml. These bacteria in the 1A-9 brine decreased by lo4 during the same period while those in the 1A-lbrine remained relatively constant throughout the trial (Table 3 ) . The low counts of the molasses-nitrate utilizing bacteria after the injection of molasses and ammonium nitrate during this trial may be the result of sulfide toxicity due to the high concentrations attained. During the latter part of 1991, analyses of the alkalinities of coproduced brine were started. Alkalinity increases of almost 50 mg/l CaCO, for 5-1 and 100 mg/l for 5-2 and 7-1, respectively, were observed from April 1991 baseline concentrations. Regular analyses thereafter showed an increasing trend until concentrations peaked on January 17 (day 142) for 5-2, and 7-1, and on February 14 for 5-1 at 375, 275, and 275 mg/l CaCO, respectively. Similar profiles were noted for the coproduced 1A tract brines. Increases of about 100 mg/l CaCO, were observed in the 1A-1 and 1A-9 brines in January 1992 over a baseline value of 1 5 0 to 175 mg/l CaCO, determined between July 1989 and February 1990. By February 1992, these values had decreased and were close to baseline values. These increases in alkalinity are believed to result from microbial production of GO, due to metabolism of molasses in the reservoir. As none of the expected metabolic products, such as nitrite or volatile fatty acids, were detected in the brine waters from the production wells during the pilot study, it would appear that the rate-limiting step in the complete mineralization of the molasses was the initial metabolism of the carbohydrate. Preferential permeability reduction A permeability reduction factor (PRF), defined as the ratio of the permeability between wells in the reservoir after the microbial process (or any other process) to the initial interwell permeability, was determined as outlined previously [15] using pressure interference tests. The results of these tests, conducted before the trial and during the second incubation after the final nutrient injection in December 1991, indicate that there had been reduction in permeability in the pilot area. The initial permeability between wells 7 - 2 and 5-2 was three-fold higher than that between wells 7 - 2 and 7 - 1 (Table 4). The largest reduction in permeability was observed between wells 7-2 and 5-1 and between 7-2 and 5-2 which were also the regions with the highest initial permeabilities (Table 4). The results also indicate that permeability distribution throughout the pilot area became more uniform after the molasses and ammonium nitrate treatments. 4.3.
203
Table 4 Results of interference pressure test Well/Test Date 5-1
5-2
7-1
Permeability bd)
1991
154
1992
56
1991
181
1992
49
1991
60
1992
43
PRF
0.37
0.27
0.72
The results of a third tracer study started on February 18, 1992 also indicated that a substantial alteration had occurred in the flow channel from well 7-2 to well 1A-9 following nutrient treatment. The two prior tracer studies had breakthrough times for the tracer of 16-18 days in the 1 A tract. By the time of writing, no breakthrough had been observed in any of the sampled production wells.
MEOR Oil Production Southeast Vassar Vertz Sand Unit 90-
g
cf' 80v
g
.........................................................................
70- .........................................................................
. 0a
$ 60-........................................................................ 2 50-........................................................................ ~
.......................................................................
E2 430-..................................................................... 9
.................................................................... ....................................
.W.e11.5.:2 .dasad..................................
02/06 02/26 03/17 04/06 04/26 05/16 06/05 06125 07/15 08/04 Date 1992 Figure 2.
Cumulative tertiary o i l production.
204
4.4.
Tertiary oil production Production of 22.5 barrels of tertiary oil was observed during the first half of 1992 before the shut down of the pilot wells 5-2 and 7-1 in April 1992. An additional 60 barrels of oil had been produced by well 5-1 prior to its shut down on June 3 0 , 1992 (Figure 2). No oil was produced from the pilot wells during two months of waterflooding of the pilot area prior to the injection of nutrients, suggesting that the oil produced was a result of microbial stimulation in the reservoir pilot area. Significant tertiary oil recovery was not expected because of the near theoretical maximum recovery of oil (60% of original oil in place) from the swept portion of the field [16]. 5.
CONCLUSIONS
The original goal of the project was achieved, that is to preferentially plug zones of high permeability in the reservoir resulting in a more homogeneous distribution of permeability throughout the pilot area. Interference and tracer studies indicate that the permeability in the pilot area was preferentially reduced and the existing flow channel to the 1A tract was partially plugged as a result of the injection of 56.2 tonnes of molasses and 18.8 tonnes of ammonium nitrate into well 7-2. The alkalinity changes, resulting from CO, production in the reservoir, imply that the observed modifications of permeability were probably due to microbial metabolism of the molasses and ammonium nitrate by indigenous bacteria. The initial injection of ammonium nitrate resulted in a significant decrease (about 50%) in the sulfide levels in the coproduced brines from the 1 A tract as well as from the pilot area. However, insufficient nitrate was injected with the molasses to inhibit transient increases in sulfide production due to carbohydrate metabolism. Finally, it should be emphasized that the overall biological effects of the stimulation of in-situ microbial populations in an oil reservoir were transitory and that 4 months after the last injection of nutrients both sulfide and alkalinity values had returned to pre-test levels with no permanent alteration in the coproduced brines being noted. 6.
ACKNOWLEDGEMENTS
The authors wish to thank Sullivan and Company, Tulsa, Oklahoma for the use of the field in this experiment, Halliburton Services Research Center for the use of the injection pump, and Alan B. Erwin, Dale E. Dawson, and H.G. "Pete" O'Kelly of Sullivan and Company for their advice during this project. Financial support for this work was provided by U.S. DOE contract N o . DE-FG22-89BC14246. 7.
REFERENCES
1. P.B. Crawford, Prod. Mon. 25 (1961) 10. 2. P.B. Crawford, Prod. Mon. 26 (1962) 12. 3 . R.S. Bryant and T.E. Burchfield, Microbial Enhanced Oil Recovery - Recent Advances, E.C. Donaldson (ed.), Elsevier, Amsterdam, 1991. 4 . M.V. Ivanov and Belyaev, S.S., In: Microbial Enhanced Oil Recovery Recent Advances, E.C. Donaldson (ed.), Elsevier, Amsterdam, 1991.
205 5.
6.
7. 8. 9. 10
11. 12. 13.
14. 15. 16.
I. Lazar, S . Dobrota, M. Stefanescu, L. Sandulescu, P. Constantinescu, C. Morosandu, N. Botea, and 0. Iliescu, In: Microbial Enhanced Oil Recovery Recent Advances, E.C. Donaldson (ed.), Elsevier, Amsterdam, 1991. Recent Advances, E.C. M. Wagner, In: Microbial Enhanced Oil Recovery Donaldson (ed.), Elsevier, Amsterdam, 1991. V.K. Bhupathiraju, P.K. Sharma, M.J. McInerney, R.M. Knapp, K. Fowler, W. Recent Advances, E.C. Jenkins, In: Microbial Enhanced Oil Recovery Donaldson (ed.), Elsevier, Amsterdam, 1991. R.M. Knapp, M.J. McInerney, D.E. Menzie, R.A., Raiders, Final Report, DOE/BC/14084-6, U.S. Department of Energy (1989). T.R. Jack and E. DiBlasio, In: Microbes and Oil Recovery, J.E. Zajic and E.C. Donaldson (eds.), Petroleum Bioresources, El Paso, 1985. G.E. Jenneman, M.J. McInerney, and R.M. Knapp, Appl. Environ. Microbiol. 51 (1986) 1205. R.S. Tanner, J. Micro. Methods. 10 (1989) 83. L.S. Clesceri, A.E. Greenberg, and R. Rhodes Trussell, Standard Methods for the Examination of Water and Wastewater, APHA-AWWA-WPCF, 17th Edition (1989). P. Gerhardt, R.G.E. Murray, R.N. Costilow, E.W. Nester, W.A. Wood, N.R. Krieg, and G. Briggs Phillips, Manual of Methods for General Microbiology, American Society for Microbiology (1981). R.M. Knapp, M.J. McInerney, D.E. Menzie, and J.L. Chisholm, Annual Report for the Period Ending December 31, 1989, U . S . Dept. of Energy, Contract No. DE-FG22-89BC14246 (1990). R.M. Knapp, M. J. McInerney, J.D. Coates, J.L. Chisholm, D.E. Menzie, and V.K. Bhupathiraju, SPE 24818, (1992). S.E. Buckley and M.C. Leverett, Transactions of AIME, 146 (1942) 107.
-
-
Microbially Enhanced Oil Recovery Field Pilot, Payne County, Oklahoma J.D. Coates", J.L. Chisholmb, R.M. Knappc, M.J. McInerney', V .K . Bhupathiraju '
D.E. Menziec, and
'Department of Botany and Microbiology, University of Oklahoma, 770 Van Vleet Oval, Room 135, Norman, OK 73019-0245 bChemical & Natural Gas Engineering, Texas Ad1 University, Campus Box 1 9 3 , Kingsville, TX 78363 'College of Engineering, School of Petroleum & Geological Engineering, University of Oklahoma, Energy Center, Suite T 301, Norman, OK 73019-0628 Abstract A multi-well MEOR field study was performed in the Vassar Vertz Sand Unit in Oklahoma. The purpose of the field trial was to determine whether microorganisms could be used to preferentially plug high permeability zones to improve waterflood sweep efficiency. Laboratory studies determined that a nutrient system based on molasses and ammonium nitrate would induce the growth of nitrateutilizing bacteria indigenous to the Vertz unit reservoir while limiting sulfate reduction. Tracer studies were done to determine the predominant flow pattern from the pilot injection well within the reservoir. The molasses and ammonium nitrate nutrients were injected over a four-monthperiod. Samples were routinely analyzed for sulfate-reducing bacteria, molasses-nitrate utilizing bacteria, general fermentative bacteria, microbial metabolites, carbohydrates, sulfide, sulfate, nitrite, and nitrate. Over the period of nutrient injection, 56.2 tonnes of molasses and 18.8 tonnes of ammonium nitrate were injected into the pilot area. The results show that large-scale injection of readily metabolizable carbohydrates didnot detrimentally affect the ongoing operation in the fieldbut did result in an alteration of the existing flow patterns and reduction in transmissibility within the pilot area. 1.
INTRODUCTION
The United States remains dependent on crude oil and crude oil products to fuel its economy and although it is one of the world's largest producers of oil, its imports constitute over 40% of current U.S. consumption. Nearly two-thirds of all of the known U.S. oil ( 3 3 0 billion barrels) will be abandoned after conventional primary and secondary recovery techniques have been exhausted (Department of Energy, 1985). Enhanced oil recovery (EOR) methods increase the total recovery of oil beyond that achievable with primary and secondary methods by increasing the proportion of the reservoir affected (sweep efficiency), reducing the amount of residual oil in the swept zones, and reducing the viscosity of heavy oils. Unfortunately, conventional EOR technologies have only been marginally effective and the strong dependence of their economics on the price of oil has limited widespread application. Microbially enhanced oil recovery (MEOR) is a newer technology that has evolved in recent years, with a great potential for cost-effective recovery of residual oil. Crawford (1,2] hypothesized that injected bacteria could preferentially plug high-permeability zones, and that this plugging could correct variations in permeability in oil
198
reservoirs. Recent results from field pilot studies on interwell MEOR floods have been encouraging [3-61. The Department of Botany and Microbiology and the School of Petroleum and Geological Engineering at University of Oklahoma have been involved in a field pilot study at the Southeast Vassar Vertz Sand Unit (SEWSU), Payne County, OK. The objective of this study was to determine whether stimulation of indigenous bacteria in the reservoir by injecting molasses and ammonium nitrate could selectively plug high-permeability regions of a reservoir and improve volumetric sweep efficiency.
2.
DESCRIPTION OF STUDY AREA AND THE FIELD PILOT TEST
The Southeast Vassar Vertz Sand Unit (SEWSU) is located 10 miles southwest of Stillwater, in Payne County, Oklahoma. A plat map of the reservoir is shown in Figure 1. The field is currently operated using a line drive waterflood with wells 3 - 2 , 3 - 3 ,7 - 3 , 7-5, and 9 - 2 active as the brine injection wells. The flood front is believed to move in a northeasterly direction. The field pilot was performed in the southwest quarter of section 13, behind the line of active brine injection wells. Wells 5 - 1 , 5-2,and 7-1 were converted to production wells, and well 7-2 served as the injector for the pilot trial. Daily rates of fluid production in the pilot area were set at 150 barrels per day (bbl/d) each for wells 5-1 and 5-2, and at 300 bbl/d for well 7-1. About 300 bbl/d of brine was injected into 7-2. The production of 300 bbl/d more from wells 5-1, 5-2, and 7-1 than injected into 7-2 was done to ensure that the fluids injected into well 7-2 would mostly flow to the pilot production wells. Previous studies determined that the floodwater fromthe reservoir was highly saline containing 11 to 19% NaCl and 1 to 2 % calcium and magnesium ions. Diverse populations of anaerobic bacteria capable of growing with simple nutrient additions at these salinities are present throughout the reservoir. An active sulfate-reducing population was present in the waterflooded areas of the reservoir and in the water tanks at the field tank battery. Both sulfatereducing and nitrate-reducing bacteria were isolated and characterized. Of these, five nitrate-reducing bacteria were chosen for more extensive study. These were shown to be new obligate halophiles with a definite requirement for salt of concentrations greater than 6%. Of the various carbohydrate and inorganic nutrient mixtures tested, a molasses-ammonium nitrate mixture best stimulated their growth [7]. Several core experiments were performed in which permeability was altered and residual oil was recovered as a result of in-situ microbial growth at the temperature and pressure of the field using oil, brine, core material, and bacteria from the S E W S U [ 8 ] .
3.
MATERIALS AND METHODS
3.1. Nutrient injection Nutrient injection into well 7-2 was initiated at the end of August 1991. In total, 56.2 tonnes of molasses and 18.8 tonnes of ammonium nitrate were injected in four phases over four months. On August 29, 2.6 tonnes of ammonium nitrate were injected into the 7-2 well as previous studies had shown inhibition of sulfide production in both oil field samples and sewage sludge samples by high
199
samples by high nitrate concentrations [9, 101. Beginning on September 2 7 , 10.9 tonnes of molasses and 3 . 2 tonnes of ammonium nitrate were injected into well 7-2. Injection was stopped on October 4 for a 30-day incubation period to allow metabolism of the nutrients injected in the pilot area. A third nutrient treatment was initiated on November 9, during which 2 2 . 7 tonnes of molasses and 6 . 3 tonnes of ammonium nitrate were injected into the pilot area. This injection was completed on November 25. A fourth and final nutrient treatment of 22.7 tonnes of molasses and 6 . 7 tonnes of ammonium nitrate was injected from December 3 to 2 0 . Injection into well 7-2 and production from wells 5-1,5-2,and 7-1 was stopped for a second incubation period of 14 days on January 2 8 , 1992.
3.2. Biochemical analysis Brine samples were routinely collected from wells 5-1, 5-2,7-1,1A-1,and 1A9. Nitrate and nitrite were analysed using chemical test kits (Spectroquant). Hydrogen sulfide was determined colorimetrically [ll]. Sulfate concentration was
@ Active production w e l l
.
A Actlve Injection w e l l Inactive production w e l l j Inactive Injectfon w e l l 4 Dry hole
'
-
Southeast Vassar-Vertz Sand U n i t
I320 I t
Figure 1. Plat Map of the Southeast Vassar Vertz Sand Unit (SEWSU).
200
determined by high-performance liquid chromatography using an ion exchange column and a conductivity detector. Volatile fatty acids were determined by gas chromatography. Alkalinity was determined as outlined by Clesceri et al. [12] and total carbohydrates were determined by the phenol-sulfuric acid method [ 1 3 ] . Bacteriological analyses Enumeration studies were done by the three-tube most probable number (MPN) technique. Aerobic heterotrophic bacteria, anaerobic heterotrophic bacteria, anaerobic bacteria using molasses-nitrate, and sulfate-reducing bacteria were enumerated using media containing 15% NaCl as previously outlined [ 1 4 ] . 3.3.
4.
RESULTS AND DISCUSSION
4.1. Pre-treatment data Before treating the pilot area, baseline concentrations of salinity, alkalinity, sulfide, sulfate, nitrite, nitrate, carbohydrate, and volatile fatty acids were determined on a regular basis. From February 1990 to August 1991, the sulfide concentration in the reservoir increased from an average of 11 mg/l to over 52 mg/l. This increase in souring was the result of an increase in the volume of make-up water used, which has a sulfate concentration of 2 , 5 0 0 mg/l. Two fluorescein tracer studies were also completed before nutrient injection to determine the predominant flow path of the injected brine in the pilot area. The results indicated a significant flow channel existed from 7 - 2 to the 1A section of the reservoir, although it was originally believed that the preferential flow path was from well 7-2 to 7-1. On both occasions, the tracer was observed in wells 1 A - 1 , 1A-5, and 1A-9, a distance of 1870 feet from 7-2, within 18 days o f initial injection, thus, traveling over 100 feet per day. The calculated rate of travel of the tracer through the reservoir suggested the existence of a fracture system. 4.2. Microbial activation by nutrient treatment Following the first injection on August 29 of 2.6 tonnes of ammonium nitrate, the nitrate concentration produced in the brine o f the pilot production wells increased from undetectable levels to about 5 to 10 mg/l, 50 days later. Coproduced brine sulfide concentrations in the pilot area decreased from an average of 52 mg/l to 28 mg/l during the same period (Table 1). However, because of the high sulfide concentrations in the original brine and the high sulfate concentration in the make-up water, the amount of ammonium nitrate added was not sufficient to inhibit transient increases in the sulfide concentration of the coproduced brines due to molasses metabolism. About 56 days after the second nutrient injection, the sulfide concentrations increased and peaked at 203 mg/l for 5-1 and 5-2, respectively, and 334 mg/l for 7-1 before returning to 65 mg/l. A similar peak in sulfide concentration occurred in 5-1, 5-2, and 7 - 1 brines 57 days after the final nutrient injection. This time, however, the sulfide concentration of 7-1 brine attained a maximum of 195 mg/l while values for 5-1 and 5-2 were 251 mg/l and 184 mg/l, respectively (Table 1). Sulfide levels also increased in the brine from wells in the 1A tract although the levels were It should be emphasized that although large substantially lower (Table 1). increases in sulfide production were noted, these increases were transient and did not affect the long-term sulfide concentrations in the reservoir which had returned to a baseline value of 4 4 mg/l by March, 1992. The numbers of sulfate-reducingbacteria in the coproduced brines from 5-1 and 5-2 also were observed to increase with nutrient injection and sulfide
201 Table 1 Sulfide results of coproduced brines Date
Day
5-1
5-2
7-1
1A-1
Feb. 15, 1990 Aug. 29, 1991 Sep. 27, 1991 Oct. 14, 1991 Oct. 22, 1991 Nov. 18, 1991 Nov. 22, 1991 Dec. 06, 1991 Dec. 10, 1991
0 29 46 54 81 85 100 104
1A-9
6.60
14.23
59.79
16.96
30.56
3s.59
64.82
48.93
50.37
33.93
22.49
23.25
26.19
21.10
23.20
ND
37.96
52.82
46.10
73.16
50.04
5.94
68.17
63.12
86.02
58,43
33.42
153.95 203.03
131.88 203.03 60.75 ND
429.44 334.49
124.45 99.84
92.17 22.53
65.88 64.12
47.28 29.65
28.94 27.39
52.23
Jan. 03, 1992
128
62.35
Jan. 17, 1992
142 170
144.69 251.16
157.30
170.95
132.73
93.08
184.34
195.13
87.62
112.97
60.19
64.48
25.09
37.70
49.90
22.89
35.80
Feb. 14, 1992 Mar. 20, 1992
201
60.19
May
262
67.51
20, 1992
ND
concentration although the changes in bacterial counts were relatively small (Table 2). The number of sulfate-reducingbacteria in the 7-1brine samples also appeared to peak although the increase occurred 25 days after that of both 5-1 and 5-2. The numbers of sulfate-reducingbacteria in the coproduced brine from the 1A tract remained relatively stable throughout the entire trial (Table 2). Table 2 Numbers of sulfate-reducing bacteria in the coproduced brines (cells/ml) Date
5-1
5-2
Aug. 29, 1991
12.40x10'
9.33~10'
Nov. 05, 1991
9.33~10'
4.27~10'
7-1
1A-1
1A-9
4.27~10'
9.30~10'
4.27~10'
4.77~10'
4.27~10'
9.33~10'
Dec. 06, 1991
4.27~10'
2 . 4 0 ~ 1 0 ~ 2.30~10'
2.40x10'
9.33~10'
Jan. 17, 1992
2 .4Ox1O2
9.30~10'
4.62~10'
2 .4Ox1O2
9.33~10'
Feb . 11, 1992
9.3Ox1O2
4.27~10'
4.27~10'
2.05x10'
9.33~10'
Mar. 03, 1992
2.31~10'
2.31~10'
9.20~10'
4.27~10'
9.20~10'
Apr. 22, 1992
9.33~10'
ND
9.33~10'
2 .40x102
9.33~10'
202 Table 3 Molasses-nitrate utilizing bacteria in the coproduced brines (cells/ml) Date
5-1
5-2
7-1
Aug. 29, 1991
1.10x103
4.62~10’
Nov. 05, 1991
2.31~10’
2.31~10’
Dec. 0 6 , 1991
2.31~10’
2.31~10’
Jan. 17, 1992
2.31~10’
2 . 3 1 ~ 1 0 ~ 2.31~10’
ND
ND
Feb. 11, 1992
2.31~10’
2.31~10’
2.31~10’
ND
ND
Apr. 22, 1992
9.33~10‘
9.33~10’
2.4Ox1O3
ND
1A-1
1A-9
4.27~10’
2 .4Ox1O2
2.34x105
2.31~10’
2.40~10~
2.31~10’
2.31~10’
ND
2.31x10’
4.27~10’
The numbers of molasses-nitrate utilizing bacteria in 5-1 and 5-2 brines decreased 10-fold over the first 70 days of the trial and then remained constant at 23 cells per ml. These bacteria in the 1A-9 brine decreased by lo4 during the same period while those in the 1A-lbrine remained relatively constant throughout the trial (Table 3 ) . The low counts of the molasses-nitrate utilizing bacteria after the injection of molasses and ammonium nitrate during this trial may be the result of sulfide toxicity due to the high concentrations attained. During the latter part of 1991, analyses of the alkalinities of coproduced brine were started. Alkalinity increases of almost 50 mg/l CaCO, for 5-1 and 100 mg/l for 5-2 and 7-1, respectively, were observed from April 1991 baseline concentrations. Regular analyses thereafter showed an increasing trend until concentrations peaked on January 17 (day 142) for 5-2, and 7-1, and on February 14 for 5-1 at 375, 275, and 275 mg/l CaCO, respectively. Similar profiles were noted for the coproduced 1A tract brines. Increases of about 100 mg/l CaCO, were observed in the 1A-1 and 1A-9 brines in January 1992 over a baseline value of 1 5 0 to 175 mg/l CaCO, determined between July 1989 and February 1990. By February 1992, these values had decreased and were close to baseline values. These increases in alkalinity are believed to result from microbial production of GO, due to metabolism of molasses in the reservoir. As none of the expected metabolic products, such as nitrite or volatile fatty acids, were detected in the brine waters from the production wells during the pilot study, it would appear that the rate-limiting step in the complete mineralization of the molasses was the initial metabolism of the carbohydrate. Preferential permeability reduction A permeability reduction factor (PRF), defined as the ratio of the permeability between wells in the reservoir after the microbial process (or any other process) to the initial interwell permeability, was determined as outlined previously [15] using pressure interference tests. The results of these tests, conducted before the trial and during the second incubation after the final nutrient injection in December 1991, indicate that there had been reduction in permeability in the pilot area. The initial permeability between wells 7 - 2 and 5-2 was three-fold higher than that between wells 7 - 2 and 7 - 1 (Table 4). The largest reduction in permeability was observed between wells 7-2 and 5-1 and between 7-2 and 5-2 which were also the regions with the highest initial permeabilities (Table 4). The results also indicate that permeability distribution throughout the pilot area became more uniform after the molasses and ammonium nitrate treatments. 4.3.
203
Table 4 Results of interference pressure test Well/Test Date 5-1
5-2
7-1
Permeability bd)
1991
154
1992
56
1991
181
1992
49
1991
60
1992
43
PRF
0.37
0.27
0.72
The results of a third tracer study started on February 18, 1992 also indicated that a substantial alteration had occurred in the flow channel from well 7-2 to well 1A-9 following nutrient treatment. The two prior tracer studies had breakthrough times for the tracer of 16-18 days in the 1 A tract. By the time of writing, no breakthrough had been observed in any of the sampled production wells.
MEOR Oil Production Southeast Vassar Vertz Sand Unit 90-
g
cf' 80v
g
.........................................................................
70- .........................................................................
. 0a
$ 60-........................................................................ 2 50-........................................................................ ~
.......................................................................
E2 430-..................................................................... 9
.................................................................... ....................................
.W.e11.5.:2 .dasad..................................
02/06 02/26 03/17 04/06 04/26 05/16 06/05 06125 07/15 08/04 Date 1992 Figure 2.
Cumulative tertiary o i l production.
204
4.4.
Tertiary oil production Production of 22.5 barrels of tertiary oil was observed during the first half of 1992 before the shut down of the pilot wells 5-2 and 7-1 in April 1992. An additional 60 barrels of oil had been produced by well 5-1 prior to its shut down on June 3 0 , 1992 (Figure 2). No oil was produced from the pilot wells during two months of waterflooding of the pilot area prior to the injection of nutrients, suggesting that the oil produced was a result of microbial stimulation in the reservoir pilot area. Significant tertiary oil recovery was not expected because of the near theoretical maximum recovery of oil (60% of original oil in place) from the swept portion of the field [16]. 5.
CONCLUSIONS
The original goal of the project was achieved, that is to preferentially plug zones of high permeability in the reservoir resulting in a more homogeneous distribution of permeability throughout the pilot area. Interference and tracer studies indicate that the permeability in the pilot area was preferentially reduced and the existing flow channel to the 1A tract was partially plugged as a result of the injection of 56.2 tonnes of molasses and 18.8 tonnes of ammonium nitrate into well 7-2. The alkalinity changes, resulting from CO, production in the reservoir, imply that the observed modifications of permeability were probably due to microbial metabolism of the molasses and ammonium nitrate by indigenous bacteria. The initial injection of ammonium nitrate resulted in a significant decrease (about 50%) in the sulfide levels in the coproduced brines from the 1 A tract as well as from the pilot area. However, insufficient nitrate was injected with the molasses to inhibit transient increases in sulfide production due to carbohydrate metabolism. Finally, it should be emphasized that the overall biological effects of the stimulation of in-situ microbial populations in an oil reservoir were transitory and that 4 months after the last injection of nutrients both sulfide and alkalinity values had returned to pre-test levels with no permanent alteration in the coproduced brines being noted. 6.
ACKNOWLEDGEMENTS
The authors wish to thank Sullivan and Company, Tulsa, Oklahoma for the use of the field in this experiment, Halliburton Services Research Center for the use of the injection pump, and Alan B. Erwin, Dale E. Dawson, and H.G. "Pete" O'Kelly of Sullivan and Company for their advice during this project. Financial support for this work was provided by U.S. DOE contract N o . DE-FG22-89BC14246. 7.
REFERENCES
1. P.B. Crawford, Prod. Mon. 25 (1961) 10. 2. P.B. Crawford, Prod. Mon. 26 (1962) 12. 3 . R.S. Bryant and T.E. Burchfield, Microbial Enhanced Oil Recovery - Recent Advances, E.C. Donaldson (ed.), Elsevier, Amsterdam, 1991. 4 . M.V. Ivanov and Belyaev, S.S., In: Microbial Enhanced Oil Recovery Recent Advances, E.C. Donaldson (ed.), Elsevier, Amsterdam, 1991.
205 5.
6.
7. 8. 9. 10
11. 12. 13.
14. 15. 16.
I. Lazar, S . Dobrota, M. Stefanescu, L. Sandulescu, P. Constantinescu, C. Morosandu, N. Botea, and 0. Iliescu, In: Microbial Enhanced Oil Recovery Recent Advances, E.C. Donaldson (ed.), Elsevier, Amsterdam, 1991. Recent Advances, E.C. M. Wagner, In: Microbial Enhanced Oil Recovery Donaldson (ed.), Elsevier, Amsterdam, 1991. V.K. Bhupathiraju, P.K. Sharma, M.J. McInerney, R.M. Knapp, K. Fowler, W. Recent Advances, E.C. Jenkins, In: Microbial Enhanced Oil Recovery Donaldson (ed.), Elsevier, Amsterdam, 1991. R.M. Knapp, M.J. McInerney, D.E. Menzie, R.A., Raiders, Final Report, DOE/BC/14084-6, U.S. Department of Energy (1989). T.R. Jack and E. DiBlasio, In: Microbes and Oil Recovery, J.E. Zajic and E.C. Donaldson (eds.), Petroleum Bioresources, El Paso, 1985. G.E. Jenneman, M.J. McInerney, and R.M. Knapp, Appl. Environ. Microbiol. 51 (1986) 1205. R.S. Tanner, J. Micro. Methods. 10 (1989) 83. L.S. Clesceri, A.E. Greenberg, and R. Rhodes Trussell, Standard Methods for the Examination of Water and Wastewater, APHA-AWWA-WPCF, 17th Edition (1989). P. Gerhardt, R.G.E. Murray, R.N. Costilow, E.W. Nester, W.A. Wood, N.R. Krieg, and G. Briggs Phillips, Manual of Methods for General Microbiology, American Society for Microbiology (1981). R.M. Knapp, M.J. McInerney, D.E. Menzie, and J.L. Chisholm, Annual Report for the Period Ending December 31, 1989, U . S . Dept. of Energy, Contract No. DE-FG22-89BC14246 (1990). R.M. Knapp, M. J. McInerney, J.D. Coates, J.L. Chisholm, D.E. Menzie, and V.K. Bhupathiraju, SPE 24818, (1992). S.E. Buckley and M.C. Leverett, Transactions of AIME, 146 (1942) 107.
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