CA2052525C - Microbial manipulations of surfactant-containing foams to reduce subterranean formation permeability - Google Patents

Abstract

A microbial system is provided for selective plugging of permeable regions of a subterranean formation, for use in conjunction with injection of surfactant-containing foams.
Bacteria indigenous to the target formation are isolated, and selected for ability to degrade the surfactant of interest.
Small, non-adherent ultramicrobacteria, or UMB, are prepared from the selected culture by starvation. The UMB and the surfactant-containing foam are then injected into the target formation. The surfactant allows the foam to penetrate into the formation. The UMB then revive to their vegetative state, degrade the surfactant and produce exopolymer, thus plugging the formation.

Description

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1 FI~TD OF TH~ INV~TION

2 The present invention relates to a microbial system

3 for plugging permeable strata in a formation, through microbial

4 degradation of surfactants contained in injected foams. It also relates to preparation of surfactant degrading bacterial strains, 6 and ultramicrobacteria therefrom, for use in this system.

7 R~CKGROUND OF TH~ INV~NTION
8 Reduction of permeability in subterranean formations 9 is desirable in a number of fields.
In the oil industry, in the course of some secondary 11 oil recovery operations, water is injected through an injection 12 well to sweep or drive oil toward an adjacent production well.
13 A serious problem that can arise in such an operation is that the 14 water preferentially moves through permeable strata in the formation and bypasses oil contained in less permeable strata.
16 This narrowly focused water movement is commonly referred to as 17 "fingering". As a result of fingering, the sweep efficiency of 18 many water-swept operations fall far short of what is sought.
19 Another water movement problem associated with oil recovery operations is referred to as "coning". When an oil well 21 is being produced, water present in a stratum underlying the oil 22 zone can "cone" upwardly and enter the well bore. As the 23 difference in viscosity between the oil and water is usually 24 significant, the water tends to move more easily through the rock 2052S2~

1 or sand matrix adjacent the well bore. As a result, this flow 2 of water excludes the oil from the well bore.
3 Because of these problems, there is an ongoing search 4 in the oil industry for an effective means for preventing the movement of water or displacement fluids through permeable zones 6 or strata associated with an oil reservoir.
7 In addition, reducing formation permeability is also 8 desirable in other areas. These include to prevent seepage of 9 salt water or waste to water supplies, or of water from water-retaining structures.
11 Methods have been developed, particularly in the oil 12 industry, to reduce formation permeability. However, they entail 13 a number of problems.
14 One method in the prior art involved the injection of surfactant-containing foams into the target formation. Such 16 foams are normally formed using an inert gas, a surfactant, and 17 a liquid. They may be injected as a preformed foam, or by 18 sequential injections of surfactant solution and gas. The 19 surfactant causes the surface tension of the foam bubbles to drop, so that the foam can easily penetrate the permeable zones 21 in the formation. The problem with the use of such foams is that 22 the foam remains unstable and is therefore mobile or readily 23 displaceable. Thus it may be displaced by water or displacement 24 fluids used in attempting to produce the well.
Another method is disclosed in U.S. Patent 4,800,959 26 entitled "Microbial Process for Selectively Plugsing a 20~2~25 1 Subterranean Formation". This patent noted that laboratory 2 studies have shown that bacterial expolysaccharides that coalesce 3 to form a confluent biofilm can be used to effectively seal a 4 simulated reservoir matrix or core formed of fused glass beads (as disclosed in "Bacterial Fouling in a Model Core System", J.
6 C. Shaw et al, (1985) Applied and Environmental Microbiology, p.
7 693-701). However, if vegetative cells are used, "skin plugging"
8 occurs - a build-up of thick biofilm at the injection point.
9 U.S. Patent 4,558,739 issued to McInerney et al sought to eliminate this problem by injection of bacterial spores, which 11 are metabolically inert and non-adhesive in nature. However, 12 problems remained - of size constraints, as the sporas are still 13 of 1 ~ in diameter; only a few types of bacteria produce spores;
14 and specific nutrients are necessary to return the spores to the vegetative state. Those problems were attacked in U.S. Patent 16 4,800,959, by use of ultramicrobacteria, or UMB.
17 UMB are produced by certain bacterial strains in a low-18 nutrient environment. Under such a starvation regime, the cells 19 undergo significant reductions in cell size and morphological transformations during progressive cell divisions, to form the 21 reduced-size cells known as UMB. The diameter of UMB range from 22 about 0.2 ~m to about 0.4~ m. In the absence of nutrient, UMB
23 do not adhere readily to a sand matrix such as found in a 24 reservoir.
U.S. Patent 4,800,959 disclosed injecting UMB into the 26 formation, followed by a specific nutrient controlled solution 2~)525~5 1 to resuscitate the UMB to the vegetative state. The revived 2 cells then produce biofilm to plug the formation. Preferably the 3 UMB were formed by isolating the bacterial class indigenous to 4 oil reservoir waters, such as Pseudomonas putida or a Klebsiella species and subjecting them to a starvation regime.
6 One of the problems with using UMB as disclosed in this 7 patent, is that the plugging of the formation depends on the 8 continued existence of the biofilm. Another problem is that 9 there is a significant time lag before plugging takes place, as the plug does not form until the UMB resuscitate and the cells 11 produce exopolymer.

12 SuMMARy OF TH~! INV~NTION
13 The present invention provides a system to plug a 14 permeable stratum in a formation without the problems noted above associated with the prior art.
16 According to the present invention:
17 - UMB which are competent to degrade a surfactant 18 upon resuscitation to the vegetative state under 19 stratum conditions are injected into the stratum;
and 21 - a foam containing the surfactant is injected into 22 the stratum;
23 - such that the UMB resuscitate to the vegetative 24 state, and degrade the surfactant to effectively 2 5 plug the stratum.

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1 The surfactant allows the foam to deeply penetrate the 2 permeable zones by reducing the surface tension of the bubbles.
3 The foam provides an initial plug. After it is in place, the UMB
4 resuscitate or revive to their vegetative state. This revival, or induction period, may take some time as the bacteria adapt to 6 the nutrients and environment. The length of the induction 7 period may be manipulated by adjusting formation conditions, such 8 as salinity, or by injection of additional nutrients. The cells 9 resuscitate by degrading the surfactant, which raises the surface tension of the foam bubbles. This renders the foam less 11 displaceable and provides a long-lasting plug to reduce the 12 permeability of the formation.
13 One aspect of the invention relates to preparation of 14 the surfactant-competent bacteria, and UMB therefrom. First of all, bacteria which can degrade a particular surfactant are 16 selected. One way this may be done is by cultivation in a 17 recirculating perfusion column in which the surfactant in 18 question is the only organic nutrient. Once enrichment has taken 19 place, those micro-organisms that have developed the capacity to degrade the surfactant are isolated, for instance by the spread 21 plate technique. Surfactant-degrading isolates are then assessed 22 for their ability to form UMB, through starvation of their cells.
23 "Starvation" is achieved by placement of the bacteria in a 24 carbon-free environment, such as a phosphate buffer salts solution (PBS), for at least two weeks. Facultative 20~2525 1 cultures, which may grow both in the presence and absence of 2 oxygen, are preferred.
3 Another aspect of the invention relates to the process 4 of injecting the UMB and surfactant-containing foam into the formation. The surfactant-stabilized foam may be prepared and 6 injected as is known in the art, with thè addition that UMB are 7 also injected. The surfactant containing foam may be injected 8 as a preformed foam or by sequential injections of surfactant 9 solution and gas. The UMB may be injected before, after or with the foam. Preferably the UMB are mixed with foam and the mixture 11 injected. Alternatively the UMB could be mixed with surfactant 12 solution and alternate injections of UMB/surfactant solution and 13 gas made. However, when UMB are premixed with surfactant 14 solution or surfactant containing foam, then injection must take place before any significant resuscitation of the UMB can take 16 place.
17 UMB-producing surfactant-degrading bacterial cultures 18 which can tolerate different formation conditions can be 19 developed. Particular conditions that are important are formation temperature and salinity. The appropriate 21 concentration of surfactant may also be varied, depending on 22 these conditions.
23 A surfactant concentration which is sensitive to 24 provide as large a change in surface tension with as little degradation as possible is desired. Surfactants lower the 26 surface tension of water. As more surfactant is added, the ~5~52~

1 surface tension drops lower. This continues until the critical 2 micellular concentration (CMC) of the surfactant is reached. If 3 surfactant in excess of the CMC is added, the surface tension is 4 unaffected. Thus, a surfactant concentration which is less than the CMC should be selected, so that small changes in surfactant 6 concentration due to bacterial degradation will effect surface 7 tension.
8 The bacteria may preferably be originally taken from 9 populations indigenous to the targeted formation. This may reduce environmental concerns. More preferably, they may be 11 taken from a formation where the surfactant of interest has been 12 used in the past.
13 The bacteria used preferably will be a variety which 14 produce exopolymer and form a biofilm. Produced biofilm will add lS to the plugging effect of the foam. Some specific species which 16 may be used include Pseudomonas Klebsiella, Enterobacter, 17 Bacillus and Serratia species. Mixed cultures are preferred as 18 they may grow more rapidly, and if one species dies, others may 19 remain.
The bacteria used preferably form UMB which will revive 21 to vegetative state due to the presence of the surfactant in the 22 injected foam. However bacteria that form UMB which require 23 nutrients in addition to surfactant to revive may also be used.
24 In thiS case, the additional nutrients will be injected into the formation either along with the foam or separately from it, to 26 revive the UMB. If additional nutrients are used, preferably 20~2525 1 they will be chemically defined nutrients which will revive the 2 UMB slowly. Rich nutrients lead to rapid resuscitation and may 3 cause skin plugging. A nutrient which may be used to avoid skin 4 plugging is trisodium citrate.
Any surfactant which is foam forming and for which a 6 bacterial strain may be selected to degrade it may be used.
7 However, particular surfactants of interest include various 8 sulphonates and phosphates. Petroleum sulphonate, produced by 9 the reaction of sulphuric acid with petroleum by-products, is one example, and is available from Witco Canada Inc., of Calgary, 11 Alberta.
12 By "degradation" of surfactant, it is meant to include 13 any action by the bacteria or their products on enough of the 14 surfactant that the surface tension of the surfactant-containing foam will rise sufficiently to effectively plug the formation.
16 This would include simple disruption of the polar moieties of the 17 surfactant, or any other action with that effect.

18 RRI~F D~SC~TPTION OF TH~ DRAWINGS
19 Figure 1 is a graph showing concentration versus surface tension of petroleum sulphonate and determination of the 21 CMC;
22 Figure 2 is a graph showing effluent surface tension 23 and percent permeability with time of a Berea sandstone core 24 injected with a mixed culture and a surfactant solution;

20~2~27 1 Figure 3 is a graph showing percent permeability with 2 time of a Berea sandstone core injected with UMB and surfactant;
3 and 4 Figure 4 is a graph showing percent permeability with time of a Berea sandstone core injected with UMB.

6 D~SCRIPTION OF TH~ PR~F~R~D ~MRODIM~NT
7 Briefly, bacterial cultures are isolated from a 8 suitable source, such as oil well reservoir water or reservoir 9 rock. A range of isolates may be obtained and assessed.
Potential surfactant degraders of the surfactant of interest are 11 then grown at 23C +/- 2C in a chemostat. The chemostat is 12 modified to consist of a single reservoir containing the 13 surfactant as the sole organic nutrient. Cultures are assessed 14 for ability to grow in aerobic and anaerobic environments.
Facultative cultures are selected and inoculated into a 16 surfactant solution to assess growth, biodegradation and 17 production of expolysaccharides. Cultures which are viable, 18 produce exopolymer and degrade surfactant are assessed for 19 ability to form UMB of size less than about 0.4 ~ ~ under starvation conditions. The UMB are then assessed for ability to 21 resuscitate to vegetative growth upon addition of surfactant or 22 surfactant and chemically defined nutrients. Finally, those 23 cultures which may be successfully resuscitated are assessed to 24 determine the range of temperature and salinity under which they 20~2S25 1 may grow. The result is a library of cultures which are useful 2 for the present invention under a range of conditions.
3 UMB from a culture produced as above are then mixed 4 with a foam containing the surfactant which the culture is competent to degrade. The surfactant concentration should be 6 less than the critical micellular concentration for the 7 surfactant. The mixture is then injected into the target 8 formation soon afterwards, allowing insufficient time before 9 injection for any significant UMB resuscitation. The mixture should be injected less than about four hours after mixing. The 11 surfactant lowers the surface tension of foam so that it may 12 deeply penetrate the formation and form an initial plug. The UMB
13 resuscitate in the presence of the surfactant in the formation 14 and degrade enough of the surfactant to form a long-lasting plug.
The long-lasting plug is enhanced by the production of exopolymer 16 and biofilm by the culture.

17 Example 1 - Determination of CMC of Petroleum Sulphonate.
18 The critical micellular concentration (CMC) of 19 Petroleum Sulphonate was determined, by adding varying concentrations of Petroleum Sulphonate to water and measuring the 21 surface tension. Surface tension was measured using a Fisher 22 Autotensionmat with a denoy ring. Results are shown in Figure 1.
23 The surface tension levelled off at Petroleum Sulphonate 24 concentration of 1%, indicating that the CMC of Petroleum 20~2525 1 sulphonate is 1%. Therefore, concentrations of petroleum 2 sulphonate below 1% were used in the following examples.

3 Example 2 - Isolation and Development of Cultures 4 Reservoir water and reservoir rock were obtained from a variety of oil formations in Alberta, Canada. The reservoir 6 rocks were sonicated in water for 45 seconds to remove any cells 7 from the rock surface. Samples of the reservoir water and 8 reservoir rock supernatant were plated onto half strength brain 9 heart infusion (BH1) agar plates, 1/10 BHI agar plates and 0.01%
petroleum sulphonate surfactant agar plates, all plates 11 containing 15% ~ifco Agar as the solidifying agent. The plates 12 were incubated at room temperature and 60C, both aerobically 13 anaerobically. Colonies of differing morphology were picked off 14 plates and transferred to new 1/2 BHI, 1/10 BHI and surfactant plates. The colonies picked from aerobically grown plates were 16 grown anaerobically and the colonies picked from anaerobically 17 grown plates were grown aerobically, to ensure cultures were 18 facultative. The facultative anaerobes were identified by 19 standard microbiological methods at Universite de Montreal, Service de diagnostic. Mixtures of the cultures were grown 21 together in a Chemostat, consisting of a single reservoir 22 containing 0.01% petroleum sulphonate in solution as the sole 23 organic nutrient. The chemostat was sampled periodically to 24 obtain isolates capable of growth on surfactant alone. Single and mixed cultures were added to flasks containing 0.01%

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1 Petroleum Sulphonate, and grown to 109 cells/ml. A 2% (v/v) 2 inoculum was added to flasks containing 0.01% Petroleum 3 Sulphonate. Growth was assessed by measuring optical density at 4 490 nm, and viable cell numbers by the spread plate technique.
Degradation was assessed by increases in surface tension using 6 a Denoy ring. Exopolymer production was assessed by carbohydrate 7 assay, using the phenol sulphuric acid method of Dubois, as 8 described in M. Dubois et. al. "Colorimetric Method for 9 Determination of Sugars and Related Substances," (1956) Anal.
Chem. 28, 350 - 356. Mixed cultures exhibiting rapid growth, 11 surfactant degradation and exopolymer production were chosen for 12 the following examples.
13 Table 1 shows growth of various mixed cultures isolated 14 as above in 0.1% petroleum sulphonate with time.

~ART,~ 1 16 Growth of surfactant in batch culture by various mixed 17 isolates. OD = optical density at 490 nm. The control 18 value of OD was 0.06.
19 0.1% Petroleum Sulphonate 1% inoculum 8 hr. 24 hr. 168 hr.
21 in solution OD OD OD
22 Pseudomonas sp. & Klebsiella sp. .22 .33 1.32 23 Pseudomonas sp. & Bacillus sp. .25 .38 1.42 24 Bacillus sp. & Klebsiella sp. .22 .33 1.42 Pseudonomas sp., Bacillus sp. &
26 Klebsiella sp. .25 .38 1.42 27 Enterobacter sp. & Serratia 28 liquefaciens .26 .48 1.56 20~2~2~

1 Example 3 - Surfactant Degradation and Bacterial Growth Within 2 a Sand Pack 3 The mixed culture of Bacillus sp., Klebsiella sp. and 4 Pseudomonas sp. isolated and identified in Example 2 was grown in a chemostat containing petroleum sulphonate in solution as the 6 sole organic nutrient. The temperature was regulated by flowing 7 water through the outer chamber.
8 In these examples, a Fisher Autotensiomat with a denoy 9 ring was used to measure the surface tension of the various solutions. Surfactant degradation was indicated by an increase 11 in surface tension of the solution. Viable cell counts were 12 determined by the surface spread technique.
13 A sand pack was prepared by packing a core holder which 14 measured 358 mm in length and 73 mm in diameter with 1.86 Kg of Kitscoty sand. The sand pack was saturated with brine followed 16 by 0.5 pore volumes of a solution of 0.2% (v/v) petroleum 17 sulphonate and the mixed bacterial culture (1% v/v) at a 18 concentration of 1.25 x 106 CFU/ml. The experiment was performed 19 at 23 +/- 2~C. Effluent samples were taken for surface tension measurements and viable cell counts.
21 The results, as shown in Tables 1 and 2, indicate that 22 bacterial growth occurred in the sand pack, as the viable cell 23 count rose from 1.25 x 106 CFU/ml to 1.5 x 108 CFU/ml. Also, the 24 surface tension rose from 29 dynes/cm to 67 dynes/cm, indicating that the surfactant was degraded.

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2Viable Cells (CFU/ml) 3 0 hr. 24 hr. 96 hr. 168 hr.
Surfactant <l.OxlOI ~l.OxlOl <l.OxlOI <l.OxlO
7 Surfactant+
8 bacteria 1.25x106 - - -Effluent - 6.0x106 2.6xlO/ 1.5xlO~

13 Sandpack Core 14Surface Tension (dynes/cm) 0 hr. 24 hr. 96 hr. 168 hr.

17 Surfactant 30 30 30 30 19 Surfactant +
bacteria 29 22 Effluent - 62 67 67 24 Example 4 - Surfactant Degradation and Plugging Within a 25Sandstone Core 26The mixed culture of Enterobacter sp. and Serratia 27 liquefaciens isolated and identified in Example 2 was developed 28 for competency to degrade petroleum sulphonate in the same manner 29 as described in Example 3.
30A solution of 0.001% petroleum sulphonate and the 31competent mixed culture (3.4 x 108 cells/ml) was injected into 32 a 400 milli-Darcy Berea sandstone core under a constant pressure 20~2S25 1 of 1 psi. After 60 minutes, the core was injected with 0.001%
2 petroleum sulphonate only. Effluent samples were collected for 3 surface tension measurements and plugging rate. A control was 4 performed by injecting surfactant only into the core.
The results are shown in Figure 2. The increase in 6 surface tension above the control value of 48 dynes/cm indicates 7 surfactant degradation. The decrease in permeability to less 8 than 20% of the original permeability within 90 hours indicates 9 plugging of the core has taken place.
11 Example 5: Resuscitation of Surfactant-Degrading 12 Ultramicrobacteria on Petroleum Sulphonate in a 13 Sandstone Core and Subsequent Plugging 14 The mixed bacterial culture of Enterobacter sp and Serratia liquefaciens from Example 4 was grown in petroleum 16 sulphonate (0.001%) to stationary phase. The organisms were 17 harvested by centrifugation (10,000 x g, 15 min., 4C) and washed 18 in sterile phosphate buffered saline (PBS) five times to 19 eliminate any transfer of nutrients into the starvation media.
The PBS contained (g/L distilled water), NaCl, 8.5; KH2PO4, 0.61;
21 K2HPO4, 0.96; pH 7. The cells were re-suspended in a sterile PBS
22 starvation media in acid-washed glassware. The starved cell 23 suspension was stirred at 22C at 200 rev. minl, for 2 weeks, 24 until the cell sizes had reached a diameter of about 0.2 to 0.4 m as determined by direct light and electron microscopy. The UMB
26 were filtered to remove any dead vegetative cells.

20~2525 1 A solution of 0.001% petroleum sulphonate and the 2 filtered mixed UMB (2.3 x 106 cells/ml) were injected into a 400 3 milliDarcy Berea sandstone core under a constant pressure of 1.0 4 p.s.i. After 5 pore volumes, the core was injected with 0.001%
petroleum sulphonate only and was locked in. Effluent flow rates 6 were monitored as a measure of core permeability. Decreases in 7 permeability indicated resuscitation of the UMB to vegetative 8 cells and subsequent plugging of the pore spaces. A decrease in 9 permeability to approximately 30% of the original permeability was noted within 200 hours (Figure 3).
11 As a control, the filtered mixed UMB (2.3 x 106 12 cells/ml) were injected into a 400 milliDarcy Berea sandstone 13 core under a constant pressure of 1.0 p.s.i., without addition 14 of petroleum sulphonate. The results as shown in Figure 4 indicate there was no significant decrease in permeability over 16 96 hours.

17 Example 6: Halotolerance and Thermotolerance of the 18 Surfactant-Degrading Mixed Culture 19 The surfactant-degrading strains must be able to survive over a wide variety of salinities and temperatures to be 21 of most use in environmental applications. Halotolerance of the 22 mixed culture in Examples 4 and 5 was tested by adding a 2%
23 inoculum of the mixed vegetative cell culture (8.0 x 108 24 cells/ml) into flasks of 1/2 BHI medium (half strength Brain Heart Infusion medium), with a NaCl concentration of 0, 2.5, 5, 2Q~2525 17.5, 10, 12.5 or 15% w/v. Viable cell count data were obtained 2 by plating cells onto 1/2 BHI plates and incubating at 23 +/- 2C
3 for 24 hours. The two species were equally represented on the 4 plates over the range of salinities tested (Table 3).
5Thermotolerance was tested by adding a 2% inoculum of 6 the mixed vegetative cell culture (8.0 x 108 cells/ml) into 7 flasks of 1/2 BHI medium, and incubating for 24 hours at 8temperatures of 4, 21, 37 and 60C. Viable cell counts data were 9 obtained for the above, by the spread plate technique. The two species were again equally represented on the plates over the 11 range of temperatures tested tTable 4).
12The results show that the mixed culture grows well 13 over a wide range of salt concentration and temperatures.
14 Halotolerance at least up to 15% NaCl was noted, and the culture was thermotolerant up to 37C, with some growth at 60C.

19 % NaCl Viable Cell Counts (w/v) (CFU/ml) 22 0 3.2 X 106 24 2.5 5.0 X 10 26 5.0 2.5 X 10 28 7.5 1.7 X 10 10.0 2.7 X 10 332 12.5 2.6 X 10 34 15.0 1.2 X 10 4 Temperature Viable Cell Counts (C) (CFU/ml) 7 4 1.2 X 105 9 21 2.5 X 107 11 37 5.0 X 107 13 60 1.0 X 103 4 APPT. ICA~IONS
While the inventors believe that the present invention 16 may be used to reduce the permeability of any desired formation, 17 they foresee particular applications.
18 One proposed application is in water flooding in oil 19 production. As mentioned above, in the course of some secondary oil recovery operations, water is injected through an injection 21 well to sweep or drive oil towards an adjacent production well.
22 The present invention may be used to prevent fingering, which 23 occurs when water channels preferentially through the most 24 permeable zones.

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1 One way this could be done is:
2 - when water breaks through to the producing well, 3 foam containing surfactant and UMB would be 4 injected at the injection well;
- preferably this would be followed by an injection 6 of water, to push the foam a desired distance 7 along the flood path;
8 - it is left an appropriate period of time to allow 9 the UMB to revive, and the cells to digest the surfactant;
11 - then water flooding would be recommenced.
12 Another proposed application, also in the oil industry, 13 is in the prevention of water coning. As mentioned above, water 14 present in a stratum underlying an oil zone can cone upwardly into an oil well bore, thus excluding the oil from the well bore.
16 The present invention can be used to reduce the permeability of 17 the formation to avoid water coning. One way this could be done 18 is:
19 - once the well begins to produce water, foam containing surfactant and UMB would be injected;
21 - preferably this would be followed by injection of 22 some water, so that the foam will not plug any of 23 the well perforations;
24 - it would be left an appropriate period of time to allow the UMB to revive, and the cells to digest 26 the surfactant;

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1 - then production from the well would be 2 recommenced.
3 Further proposed applications of the present invention 4 include plugging formations in order to:
- prevent seepage of salt water to wells producing 6 potable water;
7 - prevent seepage of leacheate from garbage dumps, 8 waste tips or other disposal areas to water 9 sources; and - prevent seepage of water from water-retaining 11 structures, and subsequent weakening of these 12 structures.

Claims (10)

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, the ultramicrobacteria having diameter less than about 0.4 µm and being competent to degrade a surfactant upon resuscitation to the vegetative state under stratum conditions; and injecting a foam containing the surfactant into the stratum, the surfactant being operative to lower the surface tension of the foam bubbles so that the foam will penetrate the stratum;
such that the ultramicrobacteria will resuscitate to the vegetative state and degrade the surfactant to effectively plug the stratum.

2. A process for plugging a permeable subterranean stratum which comprises:
isolating bacteria from stratum waters;
developing competency to degrade a surfactant by growing the bacteria with the surfactant present as the sole organic nutrient;
starving the resulting competent bacteria until the cells reach a diameter less than about 0.4 µm;

injecting the produced ultramicrobacteria into the stratum; and injecting a foam containing the surfactant into the stratum, the surfactant being operative to lower the surface tension of the foam bubbles so that the foam will penetrate the stratum;
such that the ultramicrobacteria will resuscitate to the vegetative state and degrade the surfactant to effectively plug the stratum.

3. The process as set forth in claim 1 or 2 in which the process is for microbially enhanced oil recovery and the stratum is a permeable zone in a oil-producing formation.

4. The process as set forth in claim 1 or 2 in which a nutrient solution adapted to substantially uniformly resuscitate the ultramicrobacteria to the vegetative state is also injected.

5. The process as set forth in claim 1 in which ultramicrobacteria are selected which are produced from bacteria which have the characteristic that they produce expolysaccharide biofilm in the vegetative state.

6. The process as set forth in claim 2 in which bacteria are selected from those isolated from the stratum waters which produce expolysaccharide biofilm in the vegetative state.

7. The process as set forth in claim 1 or 2 in which the surfactant is petroleum sulphonate.

8. A process for microbially enhanced oil recovery for plugging permeable subterranean strata in an oil-producing formation, which comprises:
isolating bacteria which produce expolysaccharide biofilm from formation waters;
developing competency to degrade the surfactant petroleum sulphonate by growing the bacteria with said surfactant present as the sole organic nutrient;
starving the resulting competent bacteria until the cells reach a diameter less than about 0.4 µm ;
mixing the produced ultramicrobacteria with a foam containing said surfactant; and injecting the mixture into the formation before resuscitation of appreciable numbers of the UMB can take place;
such that the ultramicrobacteria will resuscitate to the vegetative state and degrade enough of said surfactant to effectively plug the permeable strata together with the biofilm produced by the vegetative cells.

9. The process as set forth in claim 1 in which the ultramicrobacteria are prepared from a culture comprising one or more species of Enterobacter, Serratia, Bacillus, Klebsiella, or Pseudomonas.

10. The process as set forth in claim 2 or 7 in which the bacteria are a culture comprising one or more species of Enterobacter, Serratia, Bacillus, Klabsiella, or Pseudomonas.