RESERVOIR TREATMENTS

Abstract

The present invention relates to the field of enhanced oil recovery and provides a method of establishing a plug in a hydrocarbon reservoir, the method comprising introducing into the reservoir a formulation comprising solid particles and a viscosifier and then reducing the viscosity of said viscosifier, thereby causing said solid particles to form a plug within said hydrocarbon reservoir. Also provided is a method of establishing a plug in a hydrocarbon reservoir, the method comprising introducing into the reservoir a formulation comprising: (a) microorganisms or cell-free enzymes; (b) solid particles; and (c) a viscosifier which is a substrate for the microorganisms or cell-free enzymes of (a).

Also provided is a formulation comprising: (a) microorganisms or cell-free enzymes; (b) solid particles made from wood or a wood derived product; and (c) a viscosifier which is a substrate for the microorganisms or cell-free enzymes of (a).

Claims

1-21. (canceled)

22. A formulation comprising: (a) microorganisms or cell-free enzymes; (b) solid particles; and (c) a viscosifier which is a substrate for the microorganisms or cell-free enzymes of (a); wherein the solid particles (i) are substantially spherical, and (ii) have a core which is not deformable and an outer layer which is deformable.

23. The formulation of claim 22, further comprising growth medium.

24. The formulation of claim 22, wherein the microorganisms or cell-free enzymes are saccharolytic or lignocellulolytic.

25. The formulation as claimed in claim 22, wherein the microorganisms are, or wherein the cell-free enzymes are from, Clostridium thermocellum or Acidothermus cellulolyticus.

26. The formulation of claim 22, wherein the viscosifier comprises, cellulose, hemicellulose or a derivative thereof or a polysaccharide gum.

27. The formulation of claim 26, wherein the viscosifier comprises a polyanionic cellulose or a microfibrillated cellulose.

28. The formulation of claim 27, wherein the viscosifier comprises carboxymethyl cellulose.

29. The formulation of claim 22, wherein the solid particles have the same density as the rest of the formulation.

30. The formulation of claim 22, wherein the solid particles are substantially uniform in size.

31. The formulation of claim 22, wherein the formulation is made up of 25-70% by volume of the solid particles.

32. The formulation of claim 22, wherein the solid particles of the formulation comprise a first population of solid particles and a second population of solid particles, wherein said first population are at least five times larger than said second population.

33. The formulation of claim 22, wherein the solid particles of the formulation comprise a population of solid particles which are 0.05 to 5 mm in diameter.

34. The formulation of claim 22, wherein the solid particles of the formulation comprise a population of solid particles which are <0.2 mm but 1 m in diameter.

35. The formulation as claimed in claim 22, wherein the solid particles of the formulation comprise a population of solid particles which are 1 to 100 m in diameter.

36. The formulation of claim 22, wherein the formulation comprises a colloid comprising a continuous phase and a dispersed phase in which the solid particles are the dispersed phase.

37. The formulation of claim 22, wherein the formulation is an aqueous formulation.

38. The formulation of claim 22, wherein the formulation has a viscosity of 5-15 cPa.

39. The formulation of claim 22, wherein the particles are made from wood or a wood derived product.

Description

[0101] The invention is further described in the following non-limiting Examples and the figures in which:

[0102] FIG. 1is a drawing showing how the formation of dominant fractures between Injector and Producer holes result in reduced Sweep Efficiency through a matrix.

[0103] FIG. 2shows the set-up of experiments performed in a modelled chalk fracture. These experiments are outlined in Example 11 and demonstrated increased pressure produced in the chalk fracture as a consequence of the formation of a plug of wooden particles. The experiments also demonstrated facilitated movement of wooden particles through the chalk fracture when said particles were suspended in a viscosifier (xanthan).

[0104] FIG. 3shows graphs describing pressure (mbar) vs rate of flow of water (ml/min) through the model chalk fracture in the absence of a plug (top graph) and with a plug formed of 1 mm diameter round wooden particles (bottom graph). A mobility reduction factor (MRF) of 861 was achieved upon formation of the wooden plug.

[0105] FIG. 4is a graph showing the reduction of xanthan viscosity over the course of three days when xanthan is incubated at 30 C. in anoxic conditions with an anaerobic xanthan degrading bacteria. A concomitant increase in bacterial cell growth is observed as the xanthan is degraded.

[0106] FIG. 5shows that the turbidity of Exilva (microfibrillated cellulose) is decreased (correlating with decreased viscosity) when incubated with Clostridium thermocellum (CT). Bottles from leftright show Exilva+CT, Exilva (settled) and Exilva dispersed.

[0107] FIG. 6is a schematic representation of apparatus for a two component plug experiment (described in Example 14). The top image is a schematic drawing of the particle and solvent (viscosifier) inlet. This inlet set up allows the viscosifier to wooden particle paste ratio to be adjusted easily during the course of the experiment. The bottom image is a schematic diagram of the experimental set up for investigating the formation of a two component plug consisting of larger particles interspersed with smaller particles. The smaller particles may be introduced at the same time as the larger particles or as a slug of secondary particles.

[0108] FIG. 7is a graph showing pressure (mbar) over time (s) at a flow rate of 20 ml/min for a 2 component plug made up of 1 mm diameter particles and 0.2 mm diameter particles. The performed test indicated that the two component plug tested could withstand a pressure of 11400 mbar and above.

EXAMPLES

Example 1Degradation of Carbon/Methyl Cellulose by Clostridium thermocellum Bacteria

[0109] Clostridium thermocellum (CT) JW20; ATCC 31549

Growth Medium

[0110] CT were cultured according to the methodology described by Freier et al. in Applied and Environmental Microbiology [1988] vol 54, No. 1, p 204-211 but with carbon/methyl cellulose (CMC) present as the carbon source. The CMC product used was CELPOLRX, a highly viscous polyanionic cellulose (CAS number 9004-32-4) available from Kelco Oil Field Group.

[0111] Specifically, the culture medium contained (per liter of deionized water)

1.5 g KH.sub.2PO.sub.4
4.2 g Na.sub.2HPO.sub.4. 12 H.sub.2O

0.5 g NH.SUB.4.Cl

[0112] 0.5 g (NH.sub.4).sub.2SO.sub.4
0.09 g MgCl.sub.2. 6H.sub.2O

0.03 g CaCl.SUB.2

0.5 g NaHCO.SUB.3

[0113] 2 g of yeast extract
0.5 ml of vitamin solution. The vitamin solution contained (per liter of distilled water) 40 mg of biotin, 100 mg of p-aminobenzoic acid, 40 mg of folic acid, 100 mg of pantothenic acid calcium salt, 100 mg of nicotinic acid, 2 mg of vitamin B12, 100 mg of thiamine hydrochloride, 200 mg of pyridoxine hydrochloride, 100 mg of thioctic acid, and 10 mg of riboflavin.

[0114] 5 ml of mineral solution. The mineral solution contained (per liter of distilled water) 1.5 g of nitriloacetic acid, 3 g of MgSO.sub.4-7H.sub.20, 0.5 g of MnSO.sub.4. H20, 1 g of NaCl, 0.1 g of FeSO.sub.4 7H20, 0.1 g of Co(NO.sub.3).sub.2.6H.sub.20, 0.1 g of CaCl.sub.2 (anhydrous), 0.1 g of ZnSO.sub.4. 7H.sub.20, 50 mg of NiCl.sub.2, 10 mg of CuSO.sub.4*5H.sub.20, 10 mg of AlK.sub.2(SO.sub.4).sub.3 (anhydrous), 10 mg of boric acid, 10 mg of Na.sub.2MoO.sub.4*2H.sub.20, 10 mg of Na.sub.2WO.sub.4-2H.sub.20, and 1 mg of Na.sub.2SeO.sub.3 (anhydrous)

1% carboxymethyl cellulose (CMC).

[0115] Growth Experiments

[0116] A culture of CT bacteria was inoculated and allowed to grow in a flask containing the above growth medium as described in Freier et al. supra for 5 days (referred to herein as Freier medium).

[0117] The Freier approach to CT culturing was modified with bottles containing 3 different fractions of oil. One set of bottles contained 90% of oil, one set of bottles contained 50% of oil and the last set of bottles contained 10% of oil. The remaining liquid contained the Freier medium with CMC at 1%. Bottles containing 100% Freier medium with CMC (1%) were provided as control. The bottles were shaken every 3 hours during the day. The bottles were opened after 1 week.

[0118] Ethanol was produced in concentrations corresponding to the volume and concentration of growth medium containing CMC, indicating degradation of CMC.

[0119] The oil did not have a inhibiting effect on the culture. The culture does not grow and metabolize within the oily fraction. We concluded that the growth medium was effectively removed in the high concentration of oil due to the fact that oil and water are not soluble. We also concluded that the culture could not utilize hydrocarbons as a carbon source.

Viscosity Test

[0120] The media collected after the above degradation was added to a glass cylinder and the time taken for a lead ball to sink through the liquid was measured. The experiment was repeated with pure water in place of the growth medium and with a sample of the growth medium which had not been inoculated with CT.

[0121] Media which had been exposed to CT as described above allowed the lead ball to move through it (acceleration, time and maximum velocity) in a similar fashion to the pure water. The media which had not been contacted with CT, on the other hand, offered significant resistance to the passage of the ball.

[0122] Using this test it was no longer possible to detect CMC in the sample which had been in contact with CT.

[0123] These experiments indicated that most CMC is degraded by CT and that the resultant viscosity is similar to that of pure water.

Example 2Generation of Wooden Particles

[0124] Sandpaper (different grades of sandpaper result in different sized particles) was applied to pieces of wood (Corylus avellana) to generate particles of about 200, 500 or 1-2 mm in diameter.

Example 3Plugging Experiments Using Wooden and Sand Particles

[0125] Wood particles of 0.5 mm and 1-2 mm in diameter produced as described in Example 2 were inserted with water into a transparent hose of 15 mm diameter, 150 cm in length with a downstream valve. After filling the hose, a slowly increasing pressure differential was applied by injecting water from one end of the hose to provide a backpressure of 8 bars. The plug was pushed to become more compact and resisted the backpressure until the backpressure reached a maximum. The plug length could be controlled by the volume of particles added and plug lengths of 10 and 15 cm were generated. Before the maximum backpressure was reached the plug started moving in the flow direction, the particles sticking together and the plug gliding along the hose. Shaking the hose tended to release the plug, enabling it to slide further. The larger (1-2 mm) particles allowed a greater flow of water through the plug.

[0126] The experiments were repeated with sand particles. These were much less effective at plugging and the particles did not stick together as the plug accelerated through the hose. When a 25 mm plug was generated then there was a plugging effect but it was very easy to release by shaking.

[0127] The results of these tests are summarised in Table 1 below.

TABLE-US-00001 TABLE 1 Max Particle Hose Plug Diff Size diam. size Pres. Quality Shape (mm) (mm) (mm) (bar) Observations Wood Rounded 0.5 15 10/15 8 10: Slip pres. <8 bar particle 15: Slip pres. >8 bar Maintained plug integrity Wood Rounded 1-2 15 10/15 8 10: Slip pres. <8 bar particle 15: Slip pres. >8 bar Maintained plug integrity Sand Rounded <1 15 8 No plug integrity up to 25 bar

Example 4Acid Treatment of Particles

[0128] Particles generated as described in Example 2 were exposed to concentrated hydrochloric acid for 15 hours and then an alkaline wash was used to increase the pH and establish a stable pH of about 7. Water alone could be used to remove the acid.

[0129] The particles were added to Freier medium as described in Example 1 but with no carbon source (CMC absent) and contacted with CT. The bacteria attacked and partially degraded the particle.

[0130] The particles were studied under a microscope. The treated particles were much more deformed and hairy or fluffy in appearance than those which were not acid treated.

[0131] The cell walls of wood have lignin and cellulose. The above acid treatment attacks the lignin layer making the cellulose parts accessible to degradation by cellulolytic bacteria such as CT.

Example 5Industrial Scale Production of Particles

[0132] Slurrification machinery (National Oil Well Slurrification Unit) used in oil drilling to process coarse cuttings can be used to process wood to generate particles suitable for use in the present invention. The wood is run through a mill with water under high pressure and after about 10 minutes the suspension is forced through a mesh of the desired size. These mesh filters effectively size the particles with the larger particles which cannot pass through the filter being recycled for further milling. The resultant particles can be as small as 200 in diameter. The particles are adequately uniform and solid but saturated.

Example 6In Situ Set Up

Features of Exemplary Carbonate Hydrocarbon Reservoir:

[0133] Volume of fracture 100 barrels (1 m.sup.3=6.29 barrels)
Length from injection well to production well: 2000 feet
Volume of injection liquid: 20000 barrels per day
Flow rate: 30 min
Differential pressure max: 6000 PSI
Expected width in fractured structure: 1-5 mm

[0134] The flow rate is the time taken for liquid to pass through the reservoir from injection well to production well. This example reservoir which may be treated according to the invention exhibits an extreme flow rate indicative of an extensive system of well developed fractures.

[0135] The differential pressure maximum is the maximum pressure differential that it is desired generate across the plug.

[0136] If required the fractures can be pre-treated by hydrochloric acid to increase the resistance of the fracture walls, i.e. to increase the potential for friction.

[0137] The suspension of particles, bacteria etc. is injected into the injection well system. This is hydraulically forced further into the fracture system by back pressuring with injection water. The suspension will displace all alkaline water.

[0138] The bacteria attack any pure cellulose and the carboxymethyl cellulose within the suspension. The cellulose inside the particles is only degraded if the particles are pre-treated for the purpose.

Example 7Further Tests to Study Plugging Effects in Sandpacks and Hoses

Ceramic Particles:

[0139] Long transparent hoses of 13 mm diameter were used as equivalents to a magnified connected pore system. Particles of different sizes where flooded through the hoses as slugs to observe if the particles were able to form plugs. The particles were of same size and same shape and were of solid ceramics. The volume of particles introduced was equivalent to an 8 cm long plug. The test system was capable of supporting a backpressure to the plug of 15 Bars.

[0140] Particles of the following size and shape were tested:

[0141] 1. <0.5 mm

[0142] 2. =1-2 mm

[0143] 3. >5 mm

Observations:

[0144] No plugs formed.

Wooden Particles:

[0145] The same test was performed with watersoaked particles of the same shape size and shape made of woodSpruce. No formation of plug was observed of particles sized <0.5 mm and >5 mm. However plugs were formed by use of particles sized =1-2 mm. The plug was flushed out of the hose by a backpressure exceeding 8 bars.

[0146] All the tests including the non-plugging tests were repeated 10 times and showed the same results.

Test to Understand the Mechanisms by which the Wooden Particles Plugged:

[0147] Similar test setups as described above were performed using the wooden particles. The structure of the particles were studied through the transparent hose showing that the particles deformed slightly, they were originally round shaped and soaked by water.

[0148] The backpressure led to the wooden particles forming an oval shape with a slightly soft surface.

[0149] Comparing the surface of ceramic particles to the surface of the wood particles clearly revealed the potential for higher surface friction on the wood particles. Studying the surface of the wood particles with a microscope revealed small fiber threads more or less as hair on the surface of the particle, while the ceramic particle was smooth.

[0150] Thus effective plugging is dependent on the particle diameter (relative to the diameter of the hose/fracture), deformability and the surface friction of the particles.

Sandpack Test

[0151] A sandpack, 10 cm in length, 5 cm in diameter containing grains of 1 mm diameter was set up and soaked wooden particles of 0.05 mm diameter were added. The particle size was selected in correlation to the relative size of the pore mouth. Pores are formed as a series of interconnecting voids between the particles, the size of the pores and thus the pore mouth being dependent on the size/diameter of the grains. The system was first flooded with water and after flooding, particles were added to the water. The particles blocked the pores immediately, i.e. did not enter the pack and form a plug.

[0152] A further test was performed using a sandpack of larger diameter and pebbles of 10-17 mm in size. These pebbles formed an enlarged pore system generating a channel through the sandpack. Sand was packed around the pebbles to provide a single channel through the sandpack.

[0153] Flooding was initiated with a viscosity 10 cP and the particles flooded through the system. A series of floodings with different viscosities were performed (reduced by 2 cP per flooding). The system started to plug when flooded with viscosity below 4 cP. The agent used to control viscosity was Carboxyl Methyl Cellulose (CMC) dissolved in the water.

Example 8Testing Viscosity Reduction

[0154] Clostridium thermocellum JW20 represents an example bacteria which has enzymatic capabilities to degrade Carboxyl Methyl Cellulose CMC and Poly Anionic Cellulose (PAC). A product of CMC was used to viscosify the carrier fluid. 5% was added to bring up viscosity of the water to 10 cP. Before adding CMC to the water a solution of nutrients equivalent to 1% Vol was added to the water. The nutrients is a defined composition based on the FreierMedium in which the cellobiose is replaced with CMC 1:1 Vol %:Vol %.

[0155] Viscosity measurements shows that the viscosity of the fluid is altered from 10 cP to 1.5 cP.

Example 9Blocking Connected Pore Systems

[0156] A test was set up equivalent to the Sandpack test of Example 7 and flooded with the carrier fluid including modified Freier media with CMC, CT and wooden particles at 35% Vol of the liquid. The composition was injected into the sandpack and shut in for 2 weeks. The system was flooded with pure water, the injection pressure had to be elevated to 12 bars to resume flooding through the system. When opening the Sandpack it was observed that the particles were blocking the connected pore systems.

[0157] The test demonstrates that it is possible to introduce particles into the sandpack via the injection water, transport and permanently locate them. This operation is possible where a viscous fluid can transport the particles and where the viscosity can be reduced by microbial activity thereby aggregating particles in the pores. An operation of this kind reduces permeability dramatically in the sandpack.

Example 9Investigating Plug Length

[0158] A test set up as first described in Example 7 was prepared using wooden particles of 1-2 mm and the same results were observed. Then the plug length was increased to 15 cm and a backpressure of 15 Bars was exceeded before the plug was forced out of the hose (the test was repeated 6 times with the same result).

Example 10Testing a Different Shaped Hose

[0159] The test described in Example 7 was repeated using a 100 cm hose with a 5 cm diameter that has been reshaped to be an oval (width 1.3 cm and height 7.15 cm). Plugging was shown under the same circumstances as seen in Example 7.

Example 11Chalk Fracture Experiment

[0160] An artificial fracture was created in natural Austin Chalk (see FIG. 2). Wooden particles of 1 mm diameter in water only (no viscosifier) were flowed through the fracture and formed a plug immediately at the inlet over the first 10 cm of the fracture.

[0161] Reduction of particle mobility and pressure build up upon plug formation in the chalk core was measured (see FIG. 3). The differential pressure in the fracture before the plug was generated was 0.00023 mbar and after plug formation was 0.19803 mbar equalling a mobility reduction factor of 861 (see FIG. 3).

[0162] In order to demonstrate the ability of a viscosifier to facilitate transport of wooden particles to fracture sites distant from the site of injection, an experiment was performed in the same fractured chalk comparing 1 mm diameter particles suspended in water and the viscosifier xanthan. The viscosity of the water containing the 1 mm wooden particles was increased to 2000 centipoise by the addition of xanthan. When the resultant suspension was introduced into the chalk fracture the particles were able to move through the fracture relatively unimpeded. This demonstrates the ability of a viscosifier to facilitate movement of plugging particles to sites remote from the injection well in a chalk fracture.

Example 12Buoyancy of Wooden Particles

[0163] In order to adjust the buoyancy of wooden particles for optimum suspension at different viscosities, the particles may be soaked in water or brine.

[0164] Wood particles are filled into a pressure cylinder containing brine. Pressure is increased at a rate of 2 bar/hour up to a desired pressure, typically between 2 and 20 bar. The particles are kept at the given pressure for minimum 2 days, maximum 1 week. Pressure is then reduced to atmospheric pressure over a time period of 1 hour. The composition of brine, the absolute pressure and the pressure exposure period is varied to adjust wood particle density.

[0165] In one particular example, 50 g of wood particles were filled into a 200 ml stainless steel pressure cylinder. Pressure was increased by injecting brine at constant pressure step wise until reaching 20 bar. The pressure was increased at a rate of 2 bar/hour. Pressure was maintained by injection pump at 20 bar for 1 week. Pressure was then released with a gradient of 20 bar/hour to atmospheric pressure.

Example 13

[0166] A) Degradation of Xanthan by Anaerobic Xanthan-Degrading Bacteria

[0167] A microbial system for degradation of the viscosity of a slug has been established. The slug consists of a xanthan based biopolymer, anaerobic xanthan-degrading bacteria and surplus of mineral nutrients, trace elements, vitamins and nitrate. The microbes operated optimally at mesophilic conditions (20-30 C.) and sea water salinity. In a test system with 500 ppm xanthan biopolymer, a complete degradation of viscosity was observed within 2 days (FIG. 4). The concomitant increase in cell number verifies that the biopolymer was utilized for anaerobic growth of the bacteria. The degradation time of the slug may be optimized for different biopolymer concentrations by adjusting the initial cell number and essential nutrients in the slug.

[0168] B) Degradation of Exilva (Cellulose) by Clostridium thermocellum

[0169] Clostridium thermocellum is able to degrade the microfibrillated cellulose polymer product named Exilva. Exilva is visible in the growth medium as a turbid phase at the start of incubation. As degradation occurs, the turbidity decreases and finally leaves the growth medium clear at end of the growth phase (FIG. 5).

Example 14Two Component Plug

[0170] In certain circumstances plugging of fractures may be optimised by use of a two component plug comprising particles of a larger size in combination with smaller particles which are capable of filling the void space between the larger particles, thus decreasing permeability of the plug. Previous results showed that a plug consisting of the 1 mm round wood particles gave a MRF value of 700-1100. However, the permeability of such plug may be further reduced by the injection of a second slug of smaller particles (smaller wood particles 0.2 mm in diameter (seived)) which are introduced to fill the void space between the larger 1 mm round wood particles.

[0171] In order to demonstrate this principle the following experiment was performed.

[0172] A transparent tube of 0.6 cm in diameter and 50 cm length was used as a laboratory analogue to a fracture (see FIG. 6). Two differential pressure transducers were placed at either end of the tube.

[0173] Initially the tube was filled by the larger primary particles, 1 mm round particles, transported into the tube in a viscous slug (see FIG. 6). To enter the tube, particles had to pass through a 0.45 cm diameter front restriction. A viscous slug was necessary to avoid plugging of the front restriction; xanthan (700 cP@10.1/s) was used for this purpose. The particles did not pass through the end restriction. The filling of primary particles was performed by gravity drainage. The plug length formed was about 24.5 cm at the end of the tube. A MRF of 700 was recorded using the primary particles only after flooding of the system with 500 ppm xanthan diluted in brine with a viscosity of 28 cP@10, 1/s.

[0174] The secondary particles, 0.2 mm diameter wood particles (7.2 wt %) were injected by a viscous slug consisting of 500 ppm xanthan. To obtain a homogeneous slug of secondary particles the injection was performed by co-injection of solvent (viscosifier) and wood chip paste and an inline mixer was used to combine the particles with the viscosifier (see FIG. 6, top image). Co-injection of the separate components of the slug is practical for adjusting the particle to viscosifier ratio during the experiments.

[0175] The introduction of secondary particles plugged in the first part (12.1 cm) of the plug measured by the DP1. The calculated MRF value, compared to the primary particle plug, was 728 for the first part of the plug (0-12.1 cm), demonstrating considerably decreased permeability compared to the primary plug alone

[0176] FIG. 7 is a graph showing pressure (mbar) over time (s) at a flow rate of 20 ml/min for the 2 component plug made up of 1 mm particles and 0.2 mm particles. The performed test indicated that the two component plug can withstand a pressure of 11400 mbar and above.