CA1277936C - Microbial process for selectively plugging a subterranean formation - Google Patents

Abstract

"MICROBIAL PROCESS FOR SELECTIVELY PLUGGING A
SUBTERRANEAN FORMATION"

ABSTRACT OF THE DISCLOSURE
A microbial process is provided for selectively plugging a high permeability stratum or zone in a subterranean reservoir. Starved bacteria of reduced size are injected into the zone. A poor-nutrient media is either simultaneously or subsequently thereafter injected into the zone to substantially uniformly resuscitate the starved bacteria. Thereupon, the bacteria regain full cell size, proliferate, and commence production of biofilm-forming exocellular polysaccharides. The biofilm is functional to selectively seal off the high permeability zone of the formation and reduce aqueous flow through the zone.

Description

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2 The present invention relates to a microbial

3 process for selectiv~ly plugging a permeable subterranean

4 zone or stratum which is associated with an oil-bearing formation.

7 In the course of some secondary oil recoYery 8 operations, water is injected through an injection well to 9 sweep or drive oil toward an adjacent production well. A
serious problem that can arise in such an operation is that 11 the water preferentially moves through permeable strata in 12 the formation and bypasses oil contained in less permeable 13 strata. This narrowly focussed water movement is commonly 14 referred to as ~fingering~. As a result of fingering, the "sweep efficiency" of many water-swept operations fall far 16 short of what is sought.
Another water movement problem associated with oil 18 recovery operations is referred to as "coning". When an oil 19 well is being produced, water present in a stratum underlying the oil zone can "cone" upwardly and enter the well bore. As 21 the difference in viscosity between the oil and water is 22 usually significant, the water tends to move more easily 23 through the rock or sand matrix adjacent the well bore. As a 24 result, this flow of water excludes the oil from the well bore, which is undesirable.
26 Bec~use of these problems, there is an ongoing 27 search for an effective means for preventing the movement of 28 water through certain zones or strata associated with an oil 29 reservoir.

1 ;~77~36 1 various techniques have been applied in the past 2 for this purpose. In general, the techniques involve 3 plugging the high permeability strata with some fluidic 4 material that remains in place and diverts water movement to the less permeable zones.
6 In connection with this approach, it is desirable:
7 - that the plugging agent be 'selective', in the 8 sense of concentrating in the high 9 permeability strata;
- and that it be adapted to create an effective 11 plug that extends deeply enough along the 12 longitudinal extent of the high permeability 13 zone so that the water flow cannot quic~ly 14 bypass it and re-enter the zone.
One of the methods which has been explored for the 16 purpose of plugging a subterranean permeable zone involves 17 the use of bacteria. Live bacteria in the vegetative state, 18 when injected into a formation, can form adherent 19 microcolonies on the surfaces of the pores and channels in 20 the rock or sand matrix. These colonies produce 21 exopolysaccharides that coalesce to form a confluent biofilm.
22 This biofilm functions to impede aqueous flow through said 23 pores and channels.
24 Laboratory studies have shown that bacteria biofilm can be effective to effectively seal a simulated reservoir 26 matrix or core formed of fused glass beads. This is 27 disclosed in a paper entitled 'Bacterial Fouling in a Model 28 Core System' by J. C. Shaw et al in Applied and Environmental 29 Microbiology, March, 1985, pages 693 - 701.

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1 This paper further disclosed that when bacterial 2 cultures were passed through a cylindrical fused-glass-bead 3 core, the build-up of a thick biofilm took place at the 4 inlet end of the core, whereas bacterial colonization of surfaces was very sparse in the lower areas of the core.
6 Stated otherwise, the bacteria tended to quickly seal the 7 inlet end of the core. This has been referred to as "skin 8 pluggingl' and this result is referred to again below.
g Additionally, it has been shown that when parallel reservoir cores of differing permeability were simultaneously 11 gubjected to injection with a bacterial plugging agent, the 12 more permeable pathway was first plugged. Stated otherwise, 13 plugging with bacteria is selective of the permeable zone.
14 This was disclosed in U. S. Patent 4,558,739 issued to l 5 McInerney et al.
16 The McInerney patent went on to teach a process 17 embodiment which is of particular interest with respect to 18 the present invention. More particularly, the patent 19 disclosed:
- injecting bacterial spores into a permeable 21 stratum to be plugged;
22 - then injecting a solution (brine) which is 23 capable of sustaining spore viability while 24 being inadequate to induce spore germination;
- and then injecting nutrient solution 26 to induce spore germination and bacterial 27 proliferation.

1 The McInerney process was designed to emplace the spores 2 deeply into the formation. Spores were used because they are 3 small and non-adhesive in nature. The brine was used to 4 displace them deeply into the rock or sand matrix. And the nutrient was used to resuscitate the emplaced spores and 6 induce them to produce biofilm to pl ug the formation 7 channels.
8 However, the NcInerney process was subject to g certain disadvantages.
As bacterial spores, which are metabolically inert 11 spherical cells, are of lf~ m diameter, size constraints 12 restrict the penetration thereof to rocks having a 13 permeability of greater than 1 darcy. In a typical reservoir, there usually exist ~fingering~ zones, having a permeability less than 1 darcy, which require sealing off.
16 Further, in order to be successfully returned to 17 the vegetative state, a species-specific nutrient is required 18 or must be developed for each type of spore. Additionally, 19 only a relatively small number of classes of Gram-positive bacteria exhibit spore-forming capability.
~1 These factors limit the use of spores for plugging 22 purposes.
23 Digressing somewhat, by way of background, to the 24 field of marine microbiology, it has been known that, in a low nutrient environment, the cells of certain bacterial 26 strains undergo significant reductions in cell size and 27 morphological transformations during progressive cell 28 divisions. These reduced-sized cells formed under a 29 starvation regime are defined as 'ultramicrobacteria' (umbJ
or 'ultramicrocells'. The diameters of ultramicrobacteria 31 range from about 0.2~m to about O.4~m.

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1 ~he isolation of ultramicrobacteria from deep ocean 2 water was first disclosed by J. A. Novitsky and R. Y. Morita 3 in 1977 in an article entitled 'Survival of a Psychrophilic 4 Marine Vibrio under Long-Term Nutrient Starvation' in Applied Environmental Microbiology 33:635-641.
6 Subsequent experimental work has demonstrated that 7 ultramicrobacteria can be prepared in the laboratory by 8 simulating the starvation conditions found in low nutrient g environments. It has further been observed that the ultramicrobacteria, although in a dormant condition, remain 11 viable during starvation. Further, the dormant condition of 12 some starved microorganisms has been demonstrated to be 13 reversible. The supply of nutrient to the starved cells rapidly produces an increase in cell size, growth, cell division and a return to the original cell configuration.
16 Stated otherwise, once fed, the starved cells may return to 17 the vegetative adherent biofilm-forming state.
18 Applicants postulated that ultramicrobacteria had a 19 better potential, because of their small size and lack of glycocalyx coating, for penetrating deeply into a relatively 21 "tight" formation to effect plugging thereof, than had 22 bacterial spores or live vegetative bacteria.
23 However, it will be readily appreciated that at 24 this stage, although the response of marine and soil organisms to starvation and resuscitation had been explored, 26 the responses of microorganisms from other environments were 27 not understood.

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1 It was not predictable:
2 - that umb could be produced from naturally-3 occurring species of deep groundwater 4 bacteria; or - that umb were sufficiently non-adhesive to 6 penetrate the formation and be evenly distributed 7 therethrough; or 8 - that the umb could be effectively resuscitated g in situ without "skin pluggingl'; or - that the produced biofilm would be effective to 11 plug the stratum.

-13 In accordance with the present invention, a microbial process is provided for selectively plugging a subterranean permeable stratum which is usually associated 16 with an oil-bearing formation. The process comprises:
17 - injecting into the stratum ultramicrobacteria 18 having a diameter in the range of about 0.2 to 19 about 0.4J~m; and - injecting a specific nutrient controlled solution 21 into the stratum to substantially uniformly 22 resuscitate said ultramicrobacteria to the 23 vegetative state and cause them to produce 24 biofilm functional to effect plugging of said stratum.
26 Preferably such ultramicrobacteria are formed by 27 isolating a bacterial class indigenous to oil reservoir 28 waters and subjecting said isolates to a starvation regime.
29 Nost preferably, the ultramicrobacteria are selected from the 30 group consisting of the species Pseudomonas Putida and 31 Klebsiella pneumoniae.

1 In applications in which a rapid but shallow 2 biofilm plug is required, we use a rich complex 3 bacteriological medium designated l/2 strength brain heart 4 infusion. Where deeper and more even plugging is re~uired, we use a chemically defined salt medium containing various 6 amounts of trisodium citrate as a slowly-utilizable carbon 7 source. In this way the extent of the plugged zone can be 8 manipulated.
9 The invention is characterized ky the following advantages:
11 - the ultramicrobacteria penetrate deeply and are 12 distributed generally uniformly in the reservoir matrix - the difficulty with "skin plugging~ is resolved;
- the ultramicrobacteria return to the vegetative 16 state with Lniection of a relatively non-specific 17 nutrient; and 18 - saline-resistant bacteria which are indigenous 19 to the subterranean reservoir can be isolated, starved, injected, and evenly resuscitated in 21 situ to provide effective plugging.

22 DESCRIPTION OF ~HE DRAWINGS
23 The figure is a plot of the permeability reduction 24 (k/ko) for Pseudomonas putida versus growth time, through a sand pack core.

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2 Microorganisms which may be utilized in the 3 practice of the invention include those classes of bacteria 4 functional to adopt a reduction in cell size under starvation conditions ti.e. to produce ultramicrobacteria of diameter 6 about 0.2~m to about 0.4~ m) and, upon resuscitation to the 7 vegetative state, to secrete biofilm-producing exocellular 8 polysaccharides. Bacterial isolates from oil reservoir 9 waters are particularly suitable for the generation of suitable ultramicrobacteria. In particular, the species 11 Pseudomonas Putida and Xlebsiella pneumoniae successfully 12 produce the desired sealing biofilm.
13 The bacterial isolates are first grown to the stationary phase and harvested. The harvested isolates are then subjected to a starvation regime, until 16 ultramicrobacteria having a diameter of about 0.2 - O.4~ m 17 are detected. Typically, the time required for starvation is 18 about two to four weeks. The starvation media in which the 19 vegetative bacteria are suspended comprises a carbon-poor medium. one suitable medium is an aqueous solution 21 containing phosphate buffer salts (PBS), which contain (g 22 distilled water) NaCl, 8.5; KH2PO4, 0.61; KzHPO4, 0.96; pH, 23 7Ø
24 In ca rryin g out th e inv enti on, the ultramicrobacteria are injected into the formation through a 26 well in accordance with conventional practice.

1 Again, using conventional methods, a nutrient slug 2 is injected into the formation to resuscitate the 3 ultramicrobacteria emplaced therein. The nutrient media is 4 usually a nutritionally 'poor' medium functional to resuscitate the ultramicrobacteria substantially uniformly 6 throughout the formation, to thereby ensure growth of a deep 7 bacterial plug. A suitable nutrient comprises, for example, 8 sodium citrate solution containing (gl~l glass distilled 9 water) Na3C6HsO7.2H2O, 7.36; (NH4J2SO4, 3.30; KH2POg, 7.30;
K2HP04, 9.22; MgSOg, 0.12; FeC13, 0.0041; pH 7Ø

11 Experimental 12 The invention is further described by the following 13 examples which are provided to demonstrate and support the 14 operability of the present process.

Ultramicrobacterial Preparation 16 Klebsiella pneumoniae and Pseudomonas putida were 17 isolated from produced reservoir water and identified by 18 standard microbiological methods. The bacteria were grown, 19 in 40 litres of solution, to the stationary phase (i.e. a 20 growth of 1 x 109 cells/mL) in trisodium citrate medium (SCM) 21 by the method as disclosed by Shaw et al. 'Bacterial Fouling 22 in a Model Core System', Applied Environmental Microbiology, 23 50:693-701. The organisms were harvested by centrifugation 24 (10,000 x g, 15 min., 4 C) and washed in sterile phosphate 25 buffer salts (PBS) twice to eliminate any transfer of 26 nutrients into the starvation media. The PBS contained (gL~
27 distilled water) NaCl, 8.5; XH2PO4, 0.61; K2HPO4, 0.96; pH, 28 7. The cells were re-suspended in a sterile PBS starvation 1~7793~

1 media. The starved cell suspension was stirred at 22 C at 2 200 rev. min~l for 2 - 4 weeks, until the cell sizes had 3 reached a diameter of about 0.2 - 0.4~ m as determined by 4 direct light and electron microscopy.
After 24 days of starvation cell sizes were 6 determined by transmission electron microscopy (TEN).
7 Confirmation of the viability of ultramicrobacteria was made 8 by resuscitating the starved cells by inoculation (2% v/v) g into both SCM and half strength Brain Heart Infusion medium (1/2 ~HI) and the now-ascertainable cell sizes were monitored 11 thereafter. The ultramicrobacteria solutions tested 12 contained about +1.0 x 108 cells/ml and dilutions of this 13 stable umb suspension can be made readily.
14 Example I
Glass bead (6 - 7 darcy) and sandstone (200 and 400 16 md) cores were used to determine plugging properties of a 17 vegetative culture of Pseudomonas putida, isolated from a 18 produced water source. The sintered glass bead cores were 19 prepared as described by Shaw et al in Applied and 2Q Environmental Microbiology,March, 1985, pages 693 - 701, and 21 the rock cores as described by Cusack et al. in the Journal 22 of Petroleum Science and Engineering, volume 1, 1987, pages 23 39 - 50. Effluent volumes and flow rates were measured during 24 vegetative culture injections to determine percent plugging 25 by dividing the initial flow rate (Ki) by the final flow 26 rates (K). Experiments were performed under a constant 27 nitrogen pressure of 3.5 psi (24 KPA). The cultures were 28 grown to stationary phase as described by Shaw et al in 6 1 29 batch cultures of trisodium citrate medium prior to injection.

~m93~5 1 To examine biofilm development at the different 2 core depths, the cores were fractured at various lengths and 3 prepared for scanning electron microscopy, viable cell 4 concentrations and carbohydrate analysis, as described by Shaw et al, supra.
6 The results of three experimental core runs using 7 the Pseudomonas species are summarized in Table I. Core 1 8 was a 6 - 7 darcy glass bead core, Core 2 was a 200 md 9 sandstone core, and core 3 was a 400 md sandstone core. All three cores were 5 cm in length. The sandstone cores were 11 1.5 cm in dia~eter and the glass bead core was 1.0 cm in 12 diameter, Initial concentrations of bacterial cultures were:
4 core 1 - 1.0 x 108 cells/ml; core 2 - 4.4 x 108 cells/ml; and core 3 - 4.4 x 108 cells/ml.
16 With respect to all three cores, it was evident 17 that the greatest concentration of cells and carbohydrate 18 production occurred at the top end of each core. The cell 19 population and slime production rapidly decreased about 1.0 cm from the inlet surface. Visual observation of the scanning 21 electron micrographs indicated a mass of bacteria encased in 22 slime on the top section and scant throughout the length.
23 It was confirmed that injection of vegetative cells 24 resulted in a skin plug at the inlet faces of the cores.

4 Viable Cells Carbohydrate cell-ml ~g/piece**
6 Core l* top72.7 x 107 3469 7 1 cm72.7 x 107 2542 8 2 cm3.4 x 106 1410 9 3 cm1.1 x 106 1530 4 cm1.8 x 106 1650 11 Core 2 top> 3.0 x 108 14000 12 1 cm1.04 x 107 8640 13 2 cm1.04 x 107 8640 14 3 cm9.7 x 106 7920 4 cm1.08 x 107 8320 16 Core 3 top73.0 x 108 15520 17 1 cm1.24 x 107 10320 18 2 cm1.04 x 107 8320 19 3 cm1.14 x 107 8200 4 cm1.22 x 107 8320 21 * Core 1, 6 - 7 D glass bead core 22 Core 2 - 200 md sandstone core 23 Core 3 - 400 md sandstone core 24** Core 1 piece = .39 cm3, Cores 1 & 2 piece = 2.01 cm3 ~27793S

1 ExamPle I I
2 This example is provided to illustrate the 3 penetration profiles of the ultramicrobacteria into cores and 4 to show the effects of nutrient resuscitation thereof in situ.
6 Each core was prepared by packing a stainless steel 7 cell (10 cm in length by 1.27 cm in diameter) with -200 mesh 8 Ottawa sand. The sand pack was purged with CO2 gas and then 9 saturated with deionized water. The permeability of the sand pack was 3.3 darcys.
11 Ultramicrobacteria of the Pseudomonas putida 12 species were starved for 40 days in the manner descri~ed 13 above.
14 10 pore volumes of the ultramicrobacteria were injected into the sand pack at a rate of 2 ml min. for 16 approximately twenty-five minutes to ensure saturation of the 17 core therewith.
18 Following umb injection 2 - 3 pore volumes of 19 trisodium citrate nutrient solution, as previously described, was injected and locked in.
21 Changes in core permeabilities were determined by 22 changes in pressure across the core during nutrient injection 23 (10 minutesJ at a constant flow rate of 2 ml/min. An 24 increase in pressure indicated a decrease in permeability.
The experimental results of permeability reduction 26 during resuscitation under nutrient stimulation are given in 27 Table II herebelow and are illustrated in the figure 28 appended hereto.

~2m36 2 Growth Pressure Permeability k/ko 3 Time~hr) Drop(psi)(kPa) (ko) 4 1 0.00 1.0 6.894760 3.30 1.000000 2 45.00 6.947.573844 0.47 0.142424 6 3 50.25 9.766.879172 0.34 0.103030 7 4 141.50 9.162.742316 0.36 0.109~91 8 5 147.00 7.753.089652 0.42 0.127273 9 6 164.5013.794.458212 0.24 0.072727 Species - Pseudomonas putida - starved for 40 days - start 11 culture had a concentration of 2.7 x 107 cells/ml.

12 During the experimental run, effluent samples were 3 analyzed for viable cells passing through the core.
4 The following data were obtained:
Viable cell counts:

16 During bacteria injection: Day 1 17 10 minutes 3.8 x 106 cells/ml 18 20 minutes 5.1 x 106 cells/ml 19 During nutrient injection: Day 2

5 minutes 2.5 x 106 cells/ml 21 Samples taken durinq pluqqinq phase:
22 Date Cell Counts 23 A June 25 a.m. Day 2 3.4 x 106 cells/ml 24 B June 25 p.m. Day 2 2.21 x 108 cells/ml ~ June 26 a.m. Day 3 4.14 x 108 cells/ml 26 D June 26 p.m. Day 3 1.14 x 108 cells/ml 27 E June 30 a.m. Day 7 6.7 x 107 cells/ml 28 F June 30 p.m. Day 7 9.7 x 107 cells/ml 29 G July 1 a.m. Day 8 3.2 x 107 cells/ml lm~

l When the run was completed (permeability reduction 2 had reached 93%), the core was cut into 11 sections. sach 3 such section was sub-divided in 3 sub-sections. The 3 sub-4 sections were separately used for one of viable cell count carbohydrate analysis and scanning electron microscopy, to

6 determine cell penetration through the length of the core,

7 the stimulation of cell and polymer production in the core

8 and the relative distribution of cells. The results are

9 recorded as per gram of sand in Table III.

TABLE III
11 Core Sections l2 Section Viable CellsCarbohYdrate 13 1. 2.30 x 108 cells/gram 1789.47 mg/g (glucose*) 14 2. 5.00 x 108 cells/gram 4166.67 mg/g 3. 3.24 x 108 cells/gram 3200.00 mg/g l6 4. 8.18 x 108 cells/gram 1176.47 mg/g l7 5. 7.27 x 108 cells/gram 1488.37 mg/g l8 6. 3.75 x 108 cells/gram 1304.35 mg/g l9 7. 3.75 x 108 cells/gram 1478.26 mg/g 8. 3.53 x 108 cells/gram 1090.91 mg/g 21 9 2.46 x 108 cells/gram 1463.41 mg/g 22 10. 1.32 x 108 cells/gram 447.76 mg/g 23 11. 1.60 x 108 cells/gram 536.01 mg/g 24 * Carbohydrate assay: numbers expressed as (mg/g) of glucose.

12~7936 1 The results indicate:
2 (1) that the starved cells penetrated 3 the full length of the core;
4 (2J that the nutrient stimulated the growth and polymer production of the starved cells;
6 (3) that substantially even growth and concomitant 7 plugging occurred through the entire length of 8 the core; and 9 (4) that each of the viable cell and carbohydrate counts were very high, indicating that most of ll the available pore space was occupied by l2 biofilm material.
l3 It follows that injecticn of metabolically-inert, l4 substantially spherical, generally non-adhesive ultramicrobacteria will penetrate reservoir sand relatively l6 deeply and with substantially even distribution, when l7 compared to the results obtained when vegetative bacteria are l8 injected. Also, it has been demonstrated that the l9 ultramicrobacteria can be returned to the vegetative state in situ with nutrient and then function to effectively plug the 2l sand matrix.

22 Example III
23 This example is given to provide a comparison of 24 the effects of utilizing a 'poor' nutrient solution and a 'rich' nutrient solution, to resuscitate in situ 26 ultramicrobacteria.

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1 Ten cylindrical sandstone cores (5.0 cm in length 2 and 1.0 cm in diameterJ of known permeability (200 - 400 mD) 3 were prepared. Each core was gas sterilized for 4 to 5 h 4 with ethylene oxide and soaked in filtered PBS in a vacuum chamber for 40 mins. The cores were then wrapped in Teflon 6 tape, covered in silicon grease and inserted into rubber core 7 holders.
8 Ultramicrobacteria prepared as described 9 hereinabove were injected through the cores under nitrogen pressure of 25 kiloPascals (kPaJ using the apparatus 11 described by Shaw et al. supra.
12 The cores were subjected to separate treatments as 3 summarized in Table IV given herebelow.

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E _ C~ cr o z - 18a -lZ77936 1 Cores l to lO were injected with starved K.
2 pneumoniae. Numbers l and 6 were control experiments and 3 were treated with starved cells only until the maximum 4 reductions in core permeability were achieved. Cores 2 to 5 and l7 were plugged with starved cells until the permeability 6 was approximately 20%. Each core was then injected with 7 either SCM or half strength Brain Heart Infusion medium (l/2 8 BHI). The nutrients were injected either as a continuous g flow until the cores were plugged or as a single injected amount and then locked into the core.
11 The permeabilities of cores 8, 9 and lO were 12 reduced to approximately 40%~ 60% and 80% respectively with 13 starved cells. Each core was then injected with a continuous 14 flow of SCM until the final core permeability was less than 2~.
16 After each core treatment was completed the cores 17 were removed from the holders and cut along the length by 18 shallow scoring with a diamond blade saw followed by a rasor 19 blade. Each section was further divided into subsections.
Core pieces were prepared for scanning electron microscopy 21 (SEM) by critical point drying and gold-palladium coating as 22 is known in the art. Viable cell counts of core pieces of 23 known weight were undertaken by sonicating the crushed core 24 pieces in 5.0 ml PBS for 3 x 30 seconds bursts then spread plating the solution onto l/2 BHI agar. The plates were 26 incubated before the colony forming units counted.
27 The results are given in Table V herebelow.

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2Viable Counts of R. pneumoniae at Various Core Depths 3 core Depth Viable Cell counts x 108 ml1*
4 (cm)Core 5 Core 7Core 8Core 9Core 10 1.0 8.4 15.3 22.7 28.6 42.7 2.0 2.1 8.9 22.0 23.8 23.1 7 3.07.6 x 106 7.4 21.7 24.7 21.7 8 4.03.1 x 106 4.0 21.6 27.5 17.9 9 5.03.6 x 105 6.0 16.4 22.4 19.4 * 1.0 g of sandstone core pieces were crushed and sonicated 11 in 5.0 ml PBS.

12The viable count data (Table V) clearly show that 13 flooding of the core with umb and subsequent nutrient 14 stimulation with a rich complex medium (core 5) plugs the 15core to 2% permeability but produces a thin plug - 8.4 x 108 16cells/ml at the inlet and 3.6 x 105 cell/ml at 5 cm depth.
17 In contrast, flooding of the cores with umb, to varying 18 levels of permeability reduction (24 - 81% of original), and 19 subsequent nutrient stimulation using a less rich trisodium 20 citrate "package", produced a very effective plugging that 21 extended through the 5 cm length of the core (cores 7 - 10).
22These data illustrate that poor nutrients provide 23 deep plugging in this bacterial modification of stratum 24 permeability.

Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for plugging a permeable subterranean stratum which comprises:
injecting ultramicrobacteria into the stratum, said ultramicrobacteria having a diameter in the range of about 0.2µm to about 0.4µm; and injecting a nutrient-containing solution into the stratum, said solution being adapted to substantially uniformly resuscitate the ultramicrobacteria to the vegetative state and cause them to produce sufficient biofilm to effectively plug said stratum.

2. A process for plugging a permeable subterranean stratum which comprises:
isolating from oil reservoir waters a class of bacteria indigenous thereto, starving the bacteria until the cells have reached a diameter less than about 0.4µm;
injecting the produced ultramicrobacteria into the stratum; and injecting a nutrient-containing solution into the stratum, said solution being adapted to substantially uniformly resuscitate the ultramicrobacteria to the vegetative state and cause them to produce sufficient biofilm to effectively plug said stratum.

3. The process as set forth in claims 1 or 2 wherein:
the bacteria are selected from the group consisting of Pseudomonas Putida and Klebsiella pneumoniae; and trisodium citrate is the nutrient.