NO177021B - Process for achieving improved oil recovery using microorganisms - Google Patents

Description

This invention relates to a method for improving oil recovery from petroleum deposits using microorganisms. In the method, a specific strain of bacteria is used to seal zones with high permeability, whereby the efficiency of waterflooding is increased as a result of the flow being redirected to oil-bearing zones with less permeability.

Petroleum found in underground reservoirs is brought to the surface in a number of different ways. One of the most common notions when it comes to oil extraction is the notion of the oil well "springing forth from the earth". As a result of changes in the nature of the oil reserves and economic and environmental considerations, the "oil source" is a thing of the past. Surface pumps, as can often be seen, often provide the lifting energy necessary to bring the oil to the surface from the reservoirs where the pressure is insufficient. In addition, the surface pumps can be connected to pumps which are arranged below the surface, and which contribute to lifting up the oil. However, there comes a time in the life of many oil-bearing formations when the reservoir pressure and pumping arrangements are insufficient to overcome the viscosity of the oil and the capillary forces of the formation. At this point, techniques for assisted oil recovery (EOR = Enhanced Oil Recovery) become relevant to extract additional amounts of oil

which cannot be brought to the surface in the manner described above.

Mature petroleum reservoirs are thus often subjected to waterflooding to increase oil recovery. However, when there is a heterogeneous permeability profile, the water flow sweeps poorly over the less permeable zones. By the expression "profile modification" is traditionally understood the improvement of the efficiency of waterflooding which is usually achieved by injection of soluble, crosslinkable polymers and a crosslinking agent. The injected polymer selectively penetrates the highly permeable, dewatered "thief zones", where it hardens into an insoluble, cross-linked gel, channeling the water flow to the denser, oil-bearing zones.

However, the term "EOR" covers a wide spectrum of techniques and devices used to extract the last remnants of oil from the oil reservoir. There are devices and methods for: steam injection, water injection, gas driving, emulsification, injection of sealants, etc. A "device" capable of carrying out many of these operations is a microorganism, especially a bacterium.

The idea of using bacteria to increase or improve oil recovery is not new. Many laboratory studies have been conducted, and many field trials have been conducted both in the United States and elsewhere, see generally J. Davis, Petroleum Microbiology (1967) and reports collected in J.E. Zajic et al., Microbes and Oil Recovery, Bioresource Publications, El Paso

(1985). Several technical congresses have been held exclusively devoted to microbially assisted oil recovery (MEOR = Microbial Enhanced Oil Recovery). Parts of the previous literature consist of anecdotal accounts or poorly controlled investigations, which have resulted in a skeptical attitude towards the technology, see also D. Hitzman, Petroleum Microbiology and the History of its Role in Enhanced Oil Recovery, Proe. Int'l. Conf. on Micro. Enh. Oil. Ree, p. 162, (May 16-21, 1982), and E. Donaldson et al., There are Bugs in My Oil Well, Chemtech, p. 602 (Oct. 1985).

The principle behind MEOR is based on the fact that microbes can produce most of the agents currently used in chemical EOR, i.e. water-soluble polymers, surfactants, co-surfactants and solvents such as e.g. ethanol and acetone, see M. Singer, Microbial Biosurfactants in Zajic, Microbes and Oil Recovery and US Patent Nos. 4,552,261, 2,807,570 and 2,660,550. Certain microbially produced products, e.g. xanthan biopolymer, is now used commercially for EOR. Such application is dependent on the microbial product being cost-effective compared to competing products that are not produced microbially, e.g. xanthan compared to polyacrylamide. In the present patent application, the term "MEOR" refers to processes involving the in situ application of microbial processes, while the term usually excludes EOR processes where only chemical products produced in a fermentation plant are used.

The specific use of microorganisms in EOR

according to the invention consists in their application for selective sealing of zones with high permeability (ie thief zones) in petroleum reservoirs. When water injection is used to extract oil, the water is injected into an injection well to drive any oil out of the formation, so that it can be extracted through a production well. The water pushes the oil out of the small spaces and pores in the rocks, but it first pushes the oil out of the larger openings and pores (ie zones of greater permeability) and leaves the smaller open areas still filled with oil. As petroleum has formed in layered, sedimentary deposits, several distinct layers of oil-bearing sand are usually present along the vertical profile of an oil well. The different layers can vary greatly with regard to permeability and porosity and with regard to other properties. As a water flow will naturally seek the zone that offers the least resistance (or that has the greatest permeability), zones with low permeability can be bypassed. After some time, the recoverable oil is "watered out" of the zones of high permeability, while the strips of low permeability still contain significant amounts of recoverable oil. "Profile modification" provides an opportunity to extract the residual oil from these zones with low permeability. The technology currently used involves the injection of water-soluble polymers, which selectively penetrate the zones of high permeability. Cationic cross-linking agents, namely Cr<+3>, Ti<+4> or Al<+3>, which are kept in solution by means of a complexing agent (namely citrate) or as a result of their oxidation state, are co-injected with the polymer or introduced after the polymer,

see US Patent No. 4,552,217. As the polymer gradually cross-links and gels into a water-insoluble three-dimensional matrix in the zones of high permeability, the water flow is channeled into zones of low permeability, whereby oil recovery is increased. There are certain problems

associated with the methods of profile modification by means of cross-linking polymers. Such polymers are relatively expensive, they can degrade when subjected to shear forces during injection at the wellhead, and they may not penetrate well enough before forming a gel. Therefore, the use of microorganisms may prove beneficial in profile modification, eliminating some of these problems.

Like other methods, the use of microbes is for

sealing zones with high permeability is not in itself new. Some early researchers are mentioned in J. Davis, "Petroleum Microbiology" (1967), and more recently published literature includes US Patent No. 4,558,739 and D. Revus, A. Study of Reservoir Selective Plugging Utilizing In Situ Growth of Bacteria to Improve Volumetric Sweep Efficiency, Masters Thesis, Univ. of Oklahoma (1982), P. Kalish et al., The Effect of Bacteria on Sandstone Permeability, 16 Jour. Pet. Tech.

805 (July 1964), and C. Brierly et al., Investigation of Microbially Induced Permeability Loss During In-Situ Leaching, Bureau of Mines (NTIS Publication) (April 1982). In these publications, microbes are used in different ways to improve oil recovery. Some researchers have used the bacteria found naturally in the formation and have simply injected nutrients down the borehole to stimulate the growth of the bacteria and seal the formation, see US Patent No. 4,475,590 and L. Allison, Effect of Microorganisms on Permeability of Soil During Prolonged Submergence, 63 Soil Science 439 (1947). Others have injected bacteria into the borehole and then injected a nutrient solution. Some of the researchers have relied on the bacteria's biomass to achieve sealing, while others have shown that exopolymers formed by the bacteria,

are effective in sealing off areas with high permeability.

. Another factor in this sealing method is the size

of the organism being injected. Small bacteria can penetrate the formation far more easily than larger bacteria. To this end, the spores of different bacteria can be used during the injection to achieve deeper penetration. Spores penetrate more easily

in a reservoir formation and remain in these permeable zones, so that when they are stimulated to grow by means of a nutrient solution, they will more effectively clog more pores.

Other problems relate to the nutrient solutions used according to known techniques. For example, down in the wellbore in a petroleum reservoir there are conditions that represent limitations for microorganisms. More specifically, native water in many formations has high concentrations of salt (NaCl), alkaline earth metal ions (Ca<+2>, Mg+2 , Ba<+2>) and heavy metal ions. Such ions can form insoluble precipitates with many of the standard constituents of nutrient media for bacteria. These deposits can clog the well hole and prevent the injection of cells or nutrient media. Moreover, some of these ions have an inhibitory or toxic effect on microbial cells, and some (eg Ca<+2>) inhibit the production of microbial biopolymer. The bacteria and nutrient medium injected through the borehole must be able to resist these ions, if they are to be able to work as intended.

The environment down the borehole is usually anaerobic and unlike the highly oxygenated environment above. In order to survive and reproduce in both environments, a bacterium must either be shielded from oxygen (which can be difficult and expensive) or be able to withstand the conditions (e.g. by being a facultative anaerobic bacterium). [Bacteria can generally be divided into three categories according to their ability to utilize and tolerate oxygen: (1) obligately aerobic bacteria, which require molecular oxygen in order to grow, (2) obligately anaerobic bacteria, for which molecular oxygen is toxic and (3) ) facultatively anaerobic bacteria, which can grow both in the presence and in the absence of atmospheric oxygen. Among the bacteria in the three categories, facultatively anaerobic bacteria appear to be the most suitable candidates for MEOR, as they are able to survive while exposed to air during storage and injection, while also having the ability to grow well under anaerobic conditions. ]

The most important component, i.e. the bacteria, sometimes has to be chosen for the very specific conditions that prevail in a reservoir. The nutrient solution will also have to be adapted to both the bacteria and the reservoir into which it will be injected. All these conditions must be taken into account in an overall assessment

in order to be able to achieve the desired result by sealing the formation's thief zones.

With the present invention, there is now provided a method for achieving improved oil recovery by improving the sweep efficiency of a waterflood through an oil-producing reservoir which has zones of relatively high permeability and contains native salt water, which method is characterized by: preparing a suspension of bacterial spores and/or cells of a biologically pure strain of Bacillus licheniformis, NRRL No. B-18178, and/or of a biologically pure strain of Bacillus licheniformis. NRRL No. B-18179,

injects the suspension into an oil-producing reservoir where the spores and/or cells can locate in zones of relatively high permeability,

bringing the spores and/or cells into contact with a nutrient solution to cause them to plug the zones of relatively high permeability, which nutrient solution contains polyphosphate as a source of phosphate nutrient medium, and

introduces a flow agent into the zones of high permeability after the grooves and cells have caused sealing of the zones, so that the flow agent can push out oil that is in the reservoir.

With the present microbial method, the sweeping efficiency is improved more effectively than is usually possible with injected polymer or with the previously known methods where microorganisms are utilized. Strains of bacteria have now been isolated from saline sediments that are halogen-tolerant (capable of living in an environment of moderate salinity), spore-forming, thermotolerant (capable of living within a wide temperature range), biopolymer-forming, facultatively anaerobic bacteria, which will say they can grow and form a viscous biopolymer inside a petroleum reservoir. When spores of these bacteria, which have been pretreated according to the new techniques described here, are injected together with the specially composed nutrient solution described here, they selectively penetrate into zones of greater permeability. After spore formation and growth, their biomass and the exopolymer (the polymer secreted to the outside of the cell) formed in situ provide the desired profile modification. The efficiency is better than for injected polymers, because the non-viscous spore suspension has better penetration, and because the chemicals cost one-tenth of the polymers. As the polymer is formed in situ, no degradation occurs during storage or as a result of the impact of shear forces during injection.

With the new techniques that have now been developed for spore formation, injection, design of medium and cross-linking agent and delivery of the cross-linking agent, problems with regard to injectability, refinement and regulation that have been associated with the previously known methods are overcome, and the technique becomes applicable and practical. Fig. 1 shows an example of a continuous MEOR test unit. Fig. 2 shows the pressure response for cumulative addition of nutrient medium to growing cells in a nucleus.

MEOR requires the use of halogen-tolerant, facultatively anaerobic bacteria, as the native water in most oil fields is saline and has no oxygen tension. The cells must be thermophilic (able to grow at temperatures above 5°C) or "thermotolerant" (able to grow within

a wide temperature range including 40-55°C and to survive exposure to temperatures above 55°C), as petroleum reservoirs most often maintain temperatures within this temperature range. The cells (or spores from the cells) must be small in size and must be mobile (or motile) so that they can penetrate far into the porous rock. They must not be demanding with regard to the nature of the nutrient medium, as laboratory culture media will be far too expensive for use in an oil field, where very large volumes are injected. The cells must be able to grow and to form the desired product under in-situ conditions with regard to pH, temperature, heavy metal

ion concentration, etc. Although some oil-bearing formations are too hot, impermeable or otherwise uninhabitable for microbes, there are many within a temperature range of 20-80°C that allow the presence and growth of microbes. Microbes used for MEOR must also be non-pathogenic, and they must not form animal or plant toxins, as they may be injected close to water-bearing formation layers that serve as a source of water supply.

Although many microbes are able to utilize hydrocarbons as the only carbon and energy source, all known species capable of this are aerobic microbes, which require molecular oxygen for the initial attack on hydrocarbons. If factually anaerobic microbes are used for MEOR, carbon sources that are not of petroleum origin must be added. Unless hydrocarbon-utilizing anaerobic microbes can be created through recombinant DNA techniques, sufficient amounts of metabolizable non-hydrocarbon constituents must be present in the petroleum. Suitable carbon substrates are cheap carbohydrates, such as molasses and whey and possibly cheap synthetic products such as methanol. Nitrogen, phosphorus and other nutrients must also be added, if these are not present in the carbon substrate or in the rock. The nutrients must be supplied at the desired time when microbial activity is desired, and loss or absorption of nutrients will entail economic losses.

Factually anaerobic, halogenophilic or halogen-tolerant, thermotolerant bacteria can be used to achieve profile modification through exopolymer formation and/or growth of cells inside highly permeable rocks, so that the rock's permeability is reduced. The exopolymer forms in the pores of the rock an insoluble mass that is resistant to biodegradation, degradation as a result of shear forces and thermal degradation. Ionic cross-linking agents or other cross-linking agents are preferably used to improve the stability of the polymer in situ. However, since the polymer is formed in situ and not injected from the surface (in which case water solubility is essential), such cross-linking agents constitute an optional feature of the invention.

The spores that are injected are sufficiently small that

they can penetrate into zones with high permeability, but not into the oil-bearing zones with low permeability. The spores are produced by growing the cells in a spore-forming medium, so that a spore concentration of 10 9 spores/ml is obtained. The spore suspension obtained is stable for a long time and is pre-treated (by ageing, lysozyme treatment or other enzyme treatment) and filtered to remove cell debris and improve injectability. The spore suspension can be diluted in a ratio of 1:1000 with nutrient medium or salt solution before injection, so that the injected spore concentration is approx. 10/ml.

The bacteria

The selected bacteria have been described in US

application EI-34A, which is incorporated herein by reference. They consist of two strains of Bacillus licheniformis (dubbed SLS-1 NRRL No. B-18179 and Salton-1, NRRL No. B-18178) and exhibit the following characteristics: motility, facultative anaero-bias, exopolymer formation, spore formation, thermotolerance and halogen tolerance. They will change the profile of a petroleum reservoir as a result of their own mass and the mass of their exopolymer, when placed in zones of high permeability. As they are motile, the bacteria can penetrate further into the permeable zone.

The biopolymer can be formed in larger quantities, if one of the following components or a mixture of such is added directly to the culture medium: phosphate in the form of tripolyphosphate, citric acid, Al+ (as aluminum citrate) or ammonium nitrate. Polyphosphate is an important component of the preferred medium, in the form of a salt such as sodium, potassium or ammonium tripolyphosphate.

BENCH SCALE CONTINUOUS CORE TESTS

Automated core testing apparatus

An automated bench-scale laboratory unit was operated continuously for core-2 testing and packed-column testing to test the feasibility of this MEOR method. A simplified flow diagram for units is shown in fig. 1. Six core experiments can be performed simultaneously and independently of each other. Oil 4 and salt solution 6, bacteria and nutrient media 8 were supplied through separate feed pump systems (oil was pumped through pump 10, and salt solution, bacteria and nutrient media were pumped through pump 12). Sterile saline solution 6, bacterial cells (or

-spores) and nutrient media 8 were pumped through a pipeline with an outside diameter of 3.175 mm using a Milton Roy pump or peristaltic pump 12. To maintain anaerobic conditions, a small stream of nitrogen was introduced into each of the containers 6 and 8. The supply rate is normally, depending on the predetermined experimental conditions, in the range 0.03-1.0 ml/min. In the field, this corresponds to a supply rate of 0.091-3.35 linear meters per day. For temperature regulation, all cores 2 were placed in

an incubator 14 which maintained a constant temperature. All attempts

was performed at 40°C. Pressure differences were recorded using transducers 16 equipped with suitable membranes. In most tests, pressure outlets 18 were arranged at specific intervals along the core 2. This was done by drilling a hole with an external diameter of 3.175 ml in and through the resin and epoxy layer (on the surface of the core 2) and into the sandstone in the core 2, so that fluid which was transferred through the core 2, flowed out of the hole. The holes in the resin were tapped and threaded and fitted with gyro lock connectors. Nylon tubing was then led from these connections to a pressure transducer. The pressure signals were processed and converted into a digital signal by means of a signal demodulator 20. Calibration of the transducers 16 was carried out using a preset nitrogen calibration pressure. A computer 22 monitored

continuously the core pressure and other readings (and printed them out every 30 minutes as well as logged them on hard copy or diskette). The printout provided a time-pressure diagram, so that the pressure differences could be followed for each core 2 throughout its length. This enabled an accurate collection of pressure measurements for each core experiment, independently of the others,

for the duration of the experiment. The flow rates were measured both from time-pump supply rates and from the collection rates for effluent. These speeds were measured continuously. The effluent samples were collected continuously using a fraction collector 24.

CORE MANUFACTURING

Berea sandstone cores, obtained from Cleveland Qarries (Amherst, Ohio), were 50.8 mm in diameter and were supplied as cylinders of specified length and permeability.

A specified core is coated with epoxy and cast in a resin spike mold. After cutting the core to a given length and planing the ends, the core was placed in a core holder, so that it was attached to the flow apparatus, after which it was vacuum saturated with salt solution for accurate determination of the pore volume. A core saturated with oil and brine was prepared by pumping several pore volumes of brine, followed by the addition of several pore volumes of crude oil to irreducible water saturation. Salt solution was then pumped through the core, until no more oil was observed in the drain. At this point, the initial permeability to saline was determined. In most of the experiments, pipelines, fittings and valves were disinfected and then completely flushed with sterile saline before each start-up. Calculations of % porosity and % pore volume oil saturation were also determined before starting each core experiment.

INJECTION OF CELLS AND NUTRIENT MEDIUM

Depending on the purpose of a given experiment, a core may or may not be oil saturated. The order of additions in an experiment is as follows:

1. Injection of saline to determine permeability. 2. Addition of 0.3-1.0 pore volumes of cell suspension or spore suspension in a concentration of 10 cells per ml. 3. Addition of 0.3-1.0 pore volumes of specifically formulated nutrient solution. This can be added together with the cells (as has been shown). 4. A cross-linking agent (Al<+3> or Cr<+3>) can also be added, either as a water rinse or in combination with cells and/or nutrient medium. The concentration of the cross-linking agent will be approx. 1000ppm. Such a cross-linking agent is preferably kept in dissolved form with polyphosphate salt.

5. Incubation ("locking in") for 5-10 days.

6. Addition of saline solution is started, or

a second portion of nutrient medium and/or cross-linking agent is added. At this point, a check is also made on the reduction in permeability, and effluent samples are analyzed to determine the concentration of biopolymer.

7. Incubation (repeated).

8. Any repetition of steps 6 and 7.

9. Final addition of salt solution to check that a permanent reduction in permeability has been achieved.

PERFORMANCE MONITORING DURING CORE TESTS

Cell growth along with biopolymer production to get

an improved and more permanent reduction in permeability is the real goal of the present MEOR process. Monitoring the rate of growth and production during an ongoing core experiment can be carried out accurately by recording the build-up of gas pressure as the cells grow, calculating and evaluating pressure gradients and making calculations of the total permeability (during pumping) during successive additions , and by carrying out analyzes of extracted sewage samples, i.e. carrying out platelet counts, determining the biopolymer concentration and measuring

the residual concentration of sucrose (the most commonly used carbon source), etc. Several demonstration experiments were completed using this basic apparatus.

Example 1 (Utilization of nutrient medium)

Fig. 2 shows the pressure response to cumulative addition

of nutrient medium on growing cells in the nucleus. Previously added cells (added to the nucleus) were added to four separate portions of nutrient medium with approx. 5 days incubation between each addition. Separate curves are shown for the pressure response at: 1) the core's intake, 2) 30% of the core's length measured from the inlet and 3) 70% of the core's length measured from the inlet. As shown, the pressure response increases as a function of time as a result of stepwise addition of nutrient medium to the cells previously added to the nucleus. The increase in static pressure during incubation is due to cell gas production (CC^), while the increase in dynamic pressure (pumping) with each injection is due to the accumulation of cell mass/biopolymer in the core along its entire length. It has been repeatedly shown that when the number of cells in the nuclear drain and the utilization of sucrose in the nutrient medium increases, a desired reduction in the permeability of the nucleus is achieved (as shown here, i.e. a reduction from 367 millidarcy or md to 58 md).

Example 2 (Transport over long distances)

To confirm the presence of biopolymer formed

in situ at long distances from the injection site was 12.2

m long "thin pipe" (crushed sandstone) of packed Berea sandstone prepared with pressure transducers and sampling openings at 3.05 m intervals. The thin tube's initial permeability for saline solution (76 ml pore volumes) was 6.7 d (6700 md), and after the supply of 2.3 pore volumes (abbreviated to PV) 6 x 10^ cells per ml at a rate of 0.11 ml/min (24.4 m/day) for 17 hours the sampling opening at the end of the tube (after 12.2 m) gave 10 <4> cells per ml with cells that were identical to the added cells, and 113 ppm biopolymer that these cells had produced. As time passed and successive additions took place, the total through-

urgency reduced to 0.4 d (400 md), and the biopolymer concentration in the effluent at the end of the thin pipe was measured at 1222 ppm.

This experiment made it possible to measure the polymer production penetration and adsorption over a long distance corresponding to a "thief zone" in the formation. The results show that the cells and subsequently produced biopolymer needed complete re-

nom to the end of the tube, demonstrating the effectiveness of the MEOR technique. That total "resealing" of the thin pipe was achieved is further confirmed by the following facts:

1. Throughout the course of the experiment, the supply pump's delivery speed had to be reduced. The progressively slower speed was necessary to keep the pressure below the equipment's pressure limitations. The pressure build-up and the lower supply rate show the efficiency of the build-up of biopolymer throughout the entire length of the thin tube. 2. The reduction in permeability over the total length of the thin pipe was 94%.

3. Significant increases in speed took place

with which the pressure differences changed during the course of the experiment.

4. In the experiment with "thin tubes" were samples of the product

taken out at specific times and locations along the packed core during the experiment. The 12.2 m long Berea packed column was equipped with both transducers and sampling ports at the inlet and every subsequent 3.0 m.

These results show that the reduction in total permeability continues to increase due to the increase in the amount of in situ formed biopolymer throughout the length of the 12.2 m thin tube. By continuously monitoring the pressure response and the concentration of biopolymer at the sampling openings, you always had a good overview of the progress of the experiment (with regard to the reduction of permeability).

This experiment also showed that the Salton bacteria cells will be transported through a high permeability Berea core in a reasonably high concentration to achieve an efficient process.

CELL AND SPORE PENETRATION EXPERIMENTS

Example 3 (Cell injection phase)

A cell penetration experiment using a moderately permeable Berea sandstone core (146 md; length 9 cm) and a highly permeable core (1361 md; length 9 cm) was carried out to measure the flow properties of the cell suspension. Three pore volumes with 4x10 cells per ml was added to each core. Good retention of the cells on the sandstone was achieved in each knee. Thus, practically 100% of the injected cells were retained in the respective nuclei.

A second cell migration experiment was performed using a Berea core of length 9 cm and diameter 5 cm to obtain more information about the penetration ability of the Salton cell. In this case too, the majority of the cells were retained in the nucleus. The following material balance was calculated:

Total liquid recovery = 99.4%

Total number of vegetative cells counted in supply

selenium = 1.06 x 10 g cells per ml

Total amount of recovered cells counted

5

= 3.06 x 10 cells per ml.

This showed that more than 99% of the cells in the feed were retained on the nucleus. From these two experiments it became clear that spores and not cells should be the particle that penetrates and travels through the sandstone. The cells appear to be too large and/or to stick to the core material, and they are unable to move any significant distance through the network of pores in the rock.

Example 4 (Pretreatment of spores)

When cells of Bacillus bacteria are induced to form spores, examination of healthy spores under a microscope will reveal small, optically refracting spores plus a significant amount of adherent cell debris. When such "fresh" spores were diluted to approx. 10 tracks per ml and injected into a Berea core with a permeability of 1000 md, poor injectability was achieved with almost immediate clogging of the inlet face. A visual examination revealed a film of sticky proteinaceous material on the inlet surface of the nucleus.

The concentrated spore suspension was then treated by the addition of one mg/ml lysozyme at 40°C for 2 hours. The spores were then filtered through No. 1 Whatman paper. Examination under a microscope showed that the previously adherent material was no longer present. Subsequent injection of these treated spores into a core provided good injectability without build-up of any inlet surface clogging layer.

It was later found that aging a fresh spore suspension for more than 2 months has a beneficial effect similar to that which can be achieved with lysozyme within a few hours.

Example 5 (spore injection)

Tracer transport properties and adsorption and desorption properties were investigated in a core with high permeability and a core with low permeability. Pre-treated spores were in a concentration of 10 7 spores per ml injected into each core for 23.33 hours. The low permeability core received 18.1 pore volumes (PV) of spores and then the same amount of saline. The high permeability core received 8.5 PV spores and then the same amount of saline.

The pressure drop data for the high permeability core showed that the spores penetrated faster in the low permeability core. After completing the material balance for the high permeability core, the following conclusions were drawn: 1. Breakthrough of injected spores into the effluent from the core occurred at 0.5 PV, which is a measure of the "inaccessible pore volume" for spore transport.

2. During the experiment, the spore concentration increased i

2 6

the drain from 4 x 10 to 1.8 x 10 spores per ml.

3. 95% of the injected spores were retained by the core with high permeability, even after extensive flushing with saline. An insignificant concentration of spores was detected in the effluent from the core with low permeability during the injection and during subsequent flushing with saline solution.

Significant reduction of the total permeability was observed for both cores, but the pressure drop data indicate some accumulation of spores in the front part of each

core.

Example 6 (Spore injection)

When using the method out in the field, large volumes of fluids will pass through the surface of the wellbore, where possible surface clogging mechanisms will cause problems.

A Salton spore suspension (concentration = 1.3 x 10 spores per ml) was injected into a Berea core of low cross-section and low permeability (141 md) at a rate of 1 PV/h in two different ways, namely with continuous recycling and with a single pass.

The results presented in Table 2 show the permeation calculations for the single pass and recirculation experiments. It was further observed that no spores from the supply were detected in the effluent that was recycled • It can be safely assumed that during the 365 hours of recycling, 337 PV spores, i.e. practically 100% of the spores, remained retained on the core . An in-depth examination of the pressure drop data along the core also showed that the predominant fraction of the supply was in the front part of the core. This is almost certainly the reason for the 6% reduction in permeability after 337 PV had been recycled.

The same core was then used in an experiment where a single review was carried out, with 184 PV of a supply of 1.35 x 10 spores per minute being continuously injected. ml. In this case too, no spores were detected in the drain. After 184 PV had been added in this single-pass design, and 337 PV in the recycling design (521 PV added in total), a reduction in permeability of 22% was measured. After application of more than 500 PV of the spore suspension, there was no sign of any clogging of the inlet face, with only a pressure change at the core inlet face of +2.07 kPa being measured.

Example 7 (Retention of traces)

Spores were injected by the single pass embodiment into a Berea core with relatively high initial permeability to saline solution (about 2100 md). The most important core parameters are listed below:

Core length = 10.85 cm

Diameter = 5.04 cm

Pore volume = 4 9.7 ml

Only two pressure transducers, namely at the inlet and at the outlet, were attached to the core. 145 PV spores were injected (mean conc. = 1.9 x 10 spores per ml) into the core over a period of 339 hours, at a mean delivery rate of 0.36 ml/min. The results show that penetration of spores (2.5 x 10 <2> spores per ml) took place after 20.7 ml (0.42 PV), and that the concentration of spores continued to increase during pumping to a concentration in the effluent of 5.6 x 10^ spores per ml at the time when the experiment was terminated. An accurate material balance was completed for this experiment which was as follows: Total Spores Applied (145 PV) = 1.43 x 10<10 >Total Spores Recovered (Effluent) = 5.73 x 10o Spores Recovered = 4%

Spores retained in core = 96%

This result shows that most of the spores are retained by the nucleus after the application of 145 PV. The permeability has continued to decrease throughout the duration of the experiment from approx. 2100 months at the start of the experiment to 60 months at the end of the experiment. No clogging of the inlet surface of the core was observed. The injection rate decreased only 13%, and the injection pressure decreased by less than 6.9 kPa.

It has been shown experimentally that cells and spores under the conditions of the present invention can easily penetrate Berea core material. These conditions are: 1) Use of selected bacteria, such as e.g. the SLS and Salton-1 strains described here form small, compact spores and motile cells. 2) Pretreatment of the spores with a proteolytic enzyme, such as lysozyme (or with autogenous proteases formed during prolonged aging of spores) to remove adherent cell debris and sticky proteins. 3) Use of polyphosphates in the nutrient solution, which chelates and prevents precipitation with ions present in the native salt solution, such as e.g. calcium and magnesium ions.

In the examples, cells and viscous biopolymer could be detected in the core throughout its length. As a core with high permeability was used in the experiment, it can be said with certainty that the field's "thief zones" can be changed in a satisfactory manner using this method of microbial profile modification. In no experiment where the present preferred procedures were used, did any significant plugging of the inlet surface occur. Cells and spores easily penetrated the respective nuclei with little or great permeability. If continuous flow measurements are made in cores with low permeability using cells or spores, little or no penetration is observed through pressure drop data, but no clogging of the inlet surface shows itself after the supply of many pore volumes of nutrient medium or recycled solution. In contrast to this, in experiments with continuous injection of spores into cores with high permeability, a spore concentration gradient is developed (through observation of pressure drop data), and a certain fraction (e.g. 5%) of added spores is collected in the effluent after an appropriate time.

The proposed MEOR process will give the newly formed cells time to grow, multiply and produce biopolymer. This "incubation time" is - if all components have been chosen carefully - less than one week. A significant formation of incubation gas has been observed in the core over its entire length, cell growth with subsequent biopolymer formation and a permanent reduction in permeability by continuous flushing with saline solution.

The degree of profile modification accomplished by the present method can easily be made as low as 65% reduction in permeability and as high as 95% reduction in permeability. The degree of reduction depends on several factors, namely the permeability at the start, the amount of added bacteria, the incubation time and, of course, the correct nutrient medium. The ease with which the permeability can be reduced seems to increase with the use of cores with permeability above 600 md, which is desirable, because the "thief zones" that are desired to be plugged are zones of high permeability. A certain level (concentration) of cells may also be required before any significant amount of biopolymer is formed and permeability is reduced.

The first amount of injected cells can act as a "conditioner" for the sandstone, enabling further amounts of injected spores/cells to do their job, i.e. spore formation, reproduction and production of bipolymer. Incubation times of 5-10 days enable completion of the process steps. The composition of the nutrient medium and the added quantity are "decisive" process conditions. All nutritional preparations must be optimized to maximize the production of biopolymer. Also when more than one supply of nutrient medium is made, more than one supply of cells/spores may be required to achieve the maximum reduction in permeability. Alternatively, profile modification can be achieved by injecting the nutrient solution into an injection well and the spores or cells or mixtures thereof into a nearby production well or vice versa. If all steps have been carried out so that a significant profile modification has been achieved, the reduction in permeability will be more permanent and more permanent against erosion as a result of constant water flooding.

Claims (12)

1. Method for achieving improved oil recovery by improving the sweep efficiency of a waterflood through an oil-producing reservoir that has zones of relatively high permeability and contains native salt water, characterized in that: preparing a suspension of bacterial spores and/or cells of a biologically pure strain of Bacillus licheniformis, NRRL No. B-18178, and/or of a biologically pure strain of Bacillus licheniformis, NRRL No. B-18179, inject the suspension into an oil-producing reservoir where the spores and/or cells can settle in zones of relatively high permeability, contacting the spores and/or cells with a nutrient solution to cause them to plug the relatively high permeability zones, which nutrient solution contains polyphosphate as a source of phosphate nutrient medium, and introducing a flux into the high permeability zones after the spores and the cells have caused clogging of the zones so that the phlegm can push out oil that is in the reservoir. 2. Method according to claim 1, characterized in that water is fed down through a borehole in the oil-producing reservoir, before the bacterial spores and/or cells are injected. 3. Method according to claim 1, characterized in that water is introduced into the oil-producing reservoir after the nutrient solution. 4. Method according to claims 1-3, characterized in that the nutrient solution is injected into a production well and the bacterial spores and/or cells are injected into a nearby injection well, or that the nutrient solution is injected into an injection well and the bacterial spores and/or cells are injected into a nearby pro- douche on well. 5. Method according to claims 1-4, characterized in that spores which have been pre-treated with proteolytic enzymes are used. 6. Method according to claims 1-4, characterized in that spores which have been aged in suspension for at least one month are used. 7. Method according to claims 1-6, characterized in that a nutrient solution is injected into the relatively highly permeable zones. 8. Method according to claim 7, characterized in that a nutrient solution comprising polyphosphate and a polyvalent cross-linking agent is injected into the relatively highly permeable zones. 9. Method according to claim 8, characterized in that a polyvalent cross-linking agent is used which is an ion selected from aluminium, titanium and chromium ions. 10. Method according to claim 9, characterized in that water is introduced into the relatively highly permeable zones before the bacterial spores and/or cells are introduced. 11. Method according to claim 9, characterized in that water is introduced into the relatively highly permeable zones after the nutrient medium has been introduced. 12. Method according to claim 1, characterized in that the polyphosphate is also used as a chelating agent.