A super heat-resistant and salt-resistant gel particle for deep flow diversion in oil reservoirs and a preparation method thereof

Abstract

The present invention discloses a super temperature-resistant and salt-resistant gel particle for deep-profile control in oil reservoirs and a preparation method thereof, belonging to the technical field of oilfield chemistry. The super temperature-resistant and salt-resistant gel particle is polymerized from a monomer, a stabilizer, a buffer, a cross-linking agent, and an initiator. The total addition amount of the stabilizer and the buffer is 10% of the molar amount of the monomer, the addition amount of the cross-linking agent is 5.5% of the molar amount of the monomer, and the addition amount of the initiator is 1% of the molar amount of the monomer. The present invention synthesizes the super temperature-resistant and salt-resistant gel particle for deep-profile control in oil reservoirs by emulsion polymerization. The main processes are: preparing an aqueous solution, preparing an oil-phase solution, preparing an initiator solution, preparing an emulsion, carrying out a polymerization reaction, and freeze-drying. The gel particle of the present invention has higher stability and mechanical strength in high-temperature and brine environments, and also performs excellently in deep migration and plugging strength, providing an effective solution for deep-profile control.

Description

Technical Field

[0001] The present invention belongs to the technical field of oilfield chemistry, and particularly relates to a super temperature-resistant and salt-resistant gel particle for deep flow regulation in oil reservoirs and a preparation method thereof. Background Art

[0002] The problem of excessive water production is the biggest problem faced by mature oilfields at present, which will lead to uneconomic oil production and even well shutdown. In conventional water injection development, water preferentially flows into high-permeability areas after entering the formation, resulting in serious fingering or tongue-in phenomena during the water flooding process. Specifically, the flow rate of the injected water exceeds that of the crude oil, causing water breakthrough channels to form in the areas where the production wells have seen water. Subsequently, the injected water migrates along these water breakthrough channels, resulting in some areas not being affected by the injected water, forming dead oil areas, which contain a large amount of residual oil or remaining oil. The high water production of oil wells has become an important obstacle restricting the improvement of the recovery rate of low-permeability oil reservoirs.

[0003] In view of the current situation of high water cut in oilfields, the commonly used solution is to block and regulate the flow, that is, inject a plugging agent into the formation to block the deep preferential channels, realize deep fluid flow diversion, increase the swept volume of the injected water, and recover the remaining oil in the secondary channels. Polymer gels are the most synthesized and widely used plugging and regulating chemical agents at present, with the characteristics of controllable particle size, good plugging effect, simple process, etc., which can effectively control the preferential channels with high permeability, improve the heterogeneity between reservoir layers, and increase the oil recovery rate. However, the traditional polymer gel particles have a relatively fast water absorption and swelling rate, will accumulate in the near-wellbore area, hinder the deep migration of the particles, and cannot effectively plug the deep reservoirs. Moreover, they are extremely easy to hydrolyze under the action of high temperature, high salt and acidic environments, and have poor long-term aging stability, so they cannot carry out long-term effective plugging.

[0004] In the above background, there is an urgent need to develop a plugging and flow-regulating material with good injectability and long-term stability in high-temperature and high-salt oil reservoir environments. Summary of the Invention

[0005] To solve the above problems, the present invention proposes a super temperature-resistant and salt-resistant gel particle for deep profile control in oil reservoirs and a preparation method thereof. The gel particle contains components such as styrene, vinyl acetic acid, ammonium bicarbonate, divinylbenzene, and ammonium persulfate, and is synthesized by emulsion polymerization, having excellent injectability and temperature-resistant and salt-resistant characteristics. During the preparation process, by optimizing the synthesis conditions, the microstructure and performance of the gel particle are optimized. The present invention introduces divinylbenzene containing a benzene ring and a double bond as a crosslinking agent, significantly enhancing the crosslinking density of the gel particle, making it have higher stability and mechanical strength in high-temperature and brine environments. Verified by experiments, the gel particle can be stable for at least 90 days under the conditions of a temperature of 150°C and a salinity of 200,000 mg / L, and its temperature-resistant and salt-resistant performance far exceeds that of traditional gel materials. In addition, the gel particle of the present invention also performs excellently in deep migration and plugging strength, providing an effective solution for deep profile control.

[0006] The technical solution of the present invention is as follows:

[0007] A super temperature-resistant and salt-resistant gel particle for deep profile control in oil reservoirs, which is polymerized from a monomer, a stabilizer, a buffer, a crosslinking agent, and an initiator. The total addition amount of the stabilizer and the buffer is 10% of the amount of substance of the monomer, the addition amount of the crosslinking agent is 5.5% of the amount of substance of the monomer, and the addition amount of the initiator is 1% of the amount of substance of the monomer.

[0008] Further, the monomer is styrene.

[0009] Further, the stabilizer is vinyl acetic acid, and the buffer is ammonium bicarbonate.

[0010] Further, the crosslinking agent is divinylbenzene.

[0011] Further, the initiator is ammonium persulfate.

[0012] A preparation method of a super temperature-resistant and salt-resistant gel particle for deep profile control in oil reservoirs as described above, characterized by comprising the following steps:

[0013] Step 1: Prepare an aqueous solution; weigh and add the stabilizer, the buffer, and deionized water into a beaker, and place the beaker on a magnetic stirrer and stir well.

[0014] Step 2: Prepare an oil-phase solution; weigh and add the monomer and the crosslinking agent into a three-necked flask and stir well.

[0015] Step 3: Prepare an initiator solution; weigh and add the initiator and deionized water into a beaker, and place the beaker on a magnetic stirrer and stir well.

[0016] Step 4: Prepare the emulsion; thoroughly mix the aqueous solution and the oil-phase solution in a three-necked flask, and disperse them under the action of a homogenizer to obtain a uniform homogeneous emulsion;

[0017] Step 5: Conduct the polymerization reaction; place the homogeneous emulsion in a constant-temperature oil bath, stir at a temperature of 70 °C and a rotation speed of 450 rpm / min; install a condensation reflux device, and simultaneously pass N 2 to remove oxygen. After reacting for 5 minutes, add the prepared initiator, and react for 10 hours to obtain an emulsion product;

[0018] Step 6: Freeze-dry; cool the reacted emulsion product to room temperature and then put it into the freezer of a freeze-dryer, freeze at -40 °C for 24 hours, and then put it into the vacuum chamber of the freeze-dryer to evacuate and dry for 12 hours to obtain a white powdery gel particle product, and the white powdery gel particle product is the ultra-temperature-resistant and salt-resistant gel particle used for deep-profile control in oil reservoirs.

[0019] The beneficial technical effects brought by the present invention: The ultra-temperature-resistant and salt-resistant polymer gel particles of the present invention are different from conventional polymer gel particles. A divinylbenzene (DVB) cross-linking agent containing benzene rings and double bonds is introduced, which forms a high cross-linking density with styrene, has a stable three-dimensional network structure, and strong temperature resistance. Moreover, the cross-linked three-dimensional network structure is very dense. In a brine environment, dissolved salt ions and water molecules cannot easily enter the high-density cross-linking network, further improving the salt resistance. The present invention verifies that the provided temperature-resistant and salt-resistant gel particles have good temperature resistance and salt resistance by studying the changes in the particle morphology of the ultra-temperature-resistant and salt-resistant gel particles under different salinity and aging temperature conditions. The ultra-temperature-resistant and salt-resistant polymer gel particles of the present invention are different from conventional polymer gel particles. They have a small initial particle size, good injectability, reduce particle accumulation in the near-wellbore area, can enter the deep matrix pores of the formation to adjust the flow, and improve the deep migration ability of the particles. At the same time, the provided temperature-resistant and salt-resistant gel particles are verified to have good deep migration and flow control performance through core flooding experiments. Description of the Drawings

[0020] Figure 1 It is a flow chart of the preparation method of the ultra-temperature-resistant and salt-resistant gel particles for deep-profile control in oil reservoirs of the present invention.

[0021] Figure 2 It is a particle morphology diagram at different stirring speeds in Example 1 of the present invention.

[0022] Figure 3 It is a curve graph of the particle size change at different stirring speeds in Example 1 of the present invention.

[0023] Figure 4 It is a result diagram of the influence of salinity on the particle size in Example 2 of the present invention.

[0024] Figure 5 It is a comparison chart of particle aging for 0 day and 90 days under the conditions of 150 °C and 200,000 mg / L in Example 3 of the present invention; among them, (a) is the result chart of aging for 0 day; (b) is the result chart of aging for 90 days.

[0025] Figure 6 It is an electron micrograph of particle aging for 0 day and 90 days under the conditions of 150 °C and 200,000 mg / L in Example 3 of the present invention; among them, (c) is the electron micrograph of aging for 0 day; (d) is the electron micrograph of aging for 90 days.

[0026] Figure 7 It is a change curve graph of displacement pressure with the particle injection volume in Example 4 of the present invention. Detailed implementation manners

[0027] The present invention will be further described in detail below with reference to the accompanying drawings and specific implementation manners:

[0028] In a first aspect, the present invention provides a super heat-resistant and salt-resistant gel particle for deep-profile control in oil reservoirs, which is polymerized from a monomer, a stabilizer, a buffer, a cross-linking agent, and an initiator. The total addition amount of the stabilizer and the buffer is 10% of the molar amount of the monomer, the addition amount of the cross-linking agent is 5.5% of the molar amount of the monomer, and the addition amount of the initiator is 1% of the molar amount of the monomer; specifically, it contains the following components: styrene, vinyl acetic acid, ammonium bicarbonate (NH 4 HCO 3 ), divinylbenzene (DVB), and ammonium persulfate (APS). Among them, the stabilizer vinyl acetic acid and the buffer ammonium bicarbonate (NH 4 HCO 3 ) together account for 10% of the molar amount of the monomer (vinyl acetic acid: ammonium bicarbonate = 2:1) (molar amount), the cross-linking agent divinylbenzene (DVB) accounts for 5.5% of the molar amount of the monomer, the initiator ammonium persulfate (APS) accounts for 1% of the molar amount of the monomer, and the balance is water.

[0029] In a second aspect, the present invention provides a preparation method of a super heat-resistant and salt-resistant gel particle for deep-profile control in oil reservoirs. Among the components, the monomer can be dispersed in water to form an emulsion by mechanical stirring and undergo a polymerization reaction to generate gel particles. In a specific implementation manner of the present invention, the monomer used is styrene. The cross-linking agent can help the system form a network three-dimensional structure and at the same time control the cross-linking density of the particles. In a specific implementation manner of the present invention, the cross-linking agent used is divinylbenzene (DVB). The buffer can maintain the stability of the pH of the emulsion system. In a specific implementation manner of the present invention, the buffer used is ammonium bicarbonate (NH 4 HCO 3)。Initiators can promote the polymerization of monomers and control the polymerization rate. In a specific embodiment of the present invention, the initiator used is ammonium persulfate (APS).

[0030] As Figure 1 shown, the steps for synthesizing polymer gel particles with good injectability, controllable particle size, acid and salt resistance, etc. by emulsion polymerization method are as follows:

[0031] Step 1: Prepare the aqueous solution. Weigh a certain amount of vinyl acetic acid, ammonium bicarbonate (NH 4 HCO 3 ), and deionized water, and add them to a beaker to dissolve vinyl acetic acid and ammonium bicarbonate (NH 4 HCO 3 ) fully in deionized water, and place the beaker on a magnetic stirrer and stir well.

[0032] Step 2: Prepare the oil phase solution. Weigh a certain amount of styrene and divinylbenzene (DVB), add them to a three-necked flask, and stir well.

[0033] Step 3: Prepare the initiator solution. Weigh ammonium persulfate (APS) and deionized water and add them to a beaker, and place the beaker on a magnetic stirrer and stir well.

[0034] Among them, the stabilizer vinyl acetic acid and the buffer ammonium bicarbonate (NH 4 HCO 3 ) together account for 10% of the molar amount of the monomers (vinyl acetic acid: ammonium bicarbonate = 2:1) (molar amount), the crosslinking agent divinylbenzene (DVB) accounts for 5.5% of the molar amount of the monomers, and the initiator ammonium persulfate (APS) accounts for 1% of the molar amount of the monomers.

[0035] Step 4: Prepare the emulsion. Mix the aqueous solution and the oil phase solution fully in a three-necked flask, and mix and disperse them under the action of a homogenizer to obtain a uniform homogeneous emulsion.

[0036] Step 5: Carry out the polymerization reaction. Place the homogeneous emulsion in a constant-temperature oil bath, stir at a temperature of 70°C and a rotation speed of 450 rpm / min, install a condensing reflux device, and simultaneously pass N 2 to remove oxygen. After reacting for 5 minutes, add the prepared initiator ammonium persulfate (APS), and react for 10 hours to obtain an emulsion product.

[0037] Step 6: Freeze-dry. Cool the reacted emulsion to room temperature and then put it into the freezer of a freeze-dryer, freeze it at -40°C for 24 hours, and then put it into the vacuum chamber of the freeze-dryer to vacuum-dry for 12 hours to obtain a white powdery gel particle product.

[0038] Thirdly, the present invention provides a new solution for improving the temperature resistance and salt tolerance of gel particles. Currently, common gel materials mostly use ester compounds as crosslinking agents. However, ester bonds are prone to breakage at high temperatures, and the bond energy of ester bonds is relatively low, making them susceptible to hydrolysis or other chemical reactions at high temperatures, resulting in molecular decomposition. Therefore, their thermal stability is poor. In view of the complex reservoir conditions, the present invention introduces divinylbenzene (DVB) containing benzene rings and double bonds as a crosslinking agent. The conjugated π electron cloud of the benzene ring endows it with high thermal stability. The aromaticity of the benzene ring structure provides additional stability, making the molecule not easily decomposed at high temperatures. Moreover, there are two double bonds in the divinylbenzene (DVB) molecule, which form a high crosslinking density with styrene, resulting in a stable three-dimensional network structure and strong temperature resistance. Additionally, the crosslinked three-dimensional network structure is very dense. This structure not only provides mechanical strength but also reduces the penetration of external chemical substances (such as salt ions). In a saline environment, dissolved salt ions and water molecules cannot easily enter the high-density crosslinked network, further improving the salt tolerance.

[0039] Fourthly, the present invention optimizes the conditions that affect the emulsion stability and the microscopic state of gel particles during the synthesis process, such as the emulsion homogenization power, the synthesis stirring speed, the synthesis reaction temperature, and the initiation conditions of the reaction system. Through the optimization of the preparation conditions of the temperature-resistant and salt-tolerant gel particles, a uniform and stable emulsion and polymer gel particles with good uniformity, good roundness, and an initial particle size meeting the deep migration conditions are finally obtained.

[0040] Fifthly, the present invention provides the superiority of the ultra-temperature-resistant and salt-tolerant gel particles for deep flow control in oil reservoirs in terms of temperature resistance, salt tolerance, deep migration performance, and deep plugging performance.

[0041] To prove the feasibility and superiority of the present invention, the following examples are given.

[0042] Example 1:

[0043] Weigh a certain amount of vinyl acetic acid, ammonium bicarbonate (NH 4 HCO 3 ), and deionized water without oxygen, and add them to a beaker to fully dissolve vinyl acetic acid and ammonium bicarbonate (NH 4 HCO 3 ) in the deionized water without oxygen, and place it on a magnetic stirrer to stir well. Weigh a certain amount of styrene and divinylbenzene (DVB), add them to a three-necked flask, and stir well. Weigh a certain amount of ammonium persulfate (APS) and deionized water without oxygen, add them to a beaker to fully dissolve ammonium persulfate (APS) in the deionized water without oxygen, and place it on a magnetic stirrer to stir well.

[0044] The molar ratio is as follows: styrene: vinyl acetic acid: ammonium bicarbonate = 30:2:1. The cross-linking agent divinylbenzene (DVB) accounts for 5% of the monomers in terms of molar amount, and the initiator ammonium persulfate (APS) accounts for 1% of the monomers in terms of molar amount.

[0045] The aqueous solution and the oil-phase solution were fully mixed in a three-necked flask and mixed and dispersed under the action of a homogenizer to obtain a uniform emulsion. The homogenized emulsion was placed in a constant-temperature oil bath. At a temperature of 70 °C, the rotation speeds were set to 300 rpm / min, 350 rpm / min, 400 rpm / min, 450 rpm / min, 500 rpm / min, and 550 rpm / min for stirring. A condensation reflux device was installed, and at the same time, N 2 was deoxygenated. After reacting for 5 min, the prepared initiator ammonium persulfate (APS) was added, and after reacting for 10 h, an emulsion product was obtained. The obtained emulsion was diluted 20 times in deionized water with oxygen removed, and the particle morphology was observed and the particle size was measured using a transmission electron microscope. The particle morphology at each stirring speed is shown in Figure 2 and the change in particle size at each stirring speed is shown in Figure 3 . From Figure 2 , it can be seen that when the stirring speed is 450 rpm / min, the particle roundness is the best and the reaction is sufficient. Figure 3 It can be seen that the particle size first increases and then decreases with the increase of the stirring speed. This is because a faster stirring speed will provide a higher shear force, causing the droplets of the dispersed phase to break into smaller particles, resulting in smaller gel particle sizes formed. At the same time, a higher stirring speed helps to disperse the monomers and the initiator more evenly, promoting the uniform progress of the reaction and reducing the formation of large particles and agglomeration phenomena during the reaction, thus generating more uniform and smaller gel particles. However, when the stirring speed is too high, although the droplets continue to break, the excessive shear force may cause the droplets to coalesce, that is, small droplets re-aggregate into larger droplets, resulting in an increase in the gel particle size formed. When the stirring speed is 500 rpm / min, the particle size is the smallest, which is 200 nm.

[0046] Example 2:

[0047] Take the emulsion synthesized by reaction at 70 °C and 450 rpm in Example 1. After cooling to room temperature, put it into the freezer of a freeze dryer, freeze at -40 °C for 24 h, and then put it into the vacuum chamber of the freeze dryer for vacuum drying for 12 h to obtain a white powdery gel particle product. Prepare formation simulation waters with salinities of 50000 mg / L, 100000 mg / L, 150000 mg / L, and 200000 mg / L respectively. Prepare suspensions of the provided temperature-resistant and salt-resistant gel particles with a concentration of 2 wt% in the above formation simulation waters and deionized water. After fully dispersing for 6 hours, observe the particle morphology and measure the particle size with a transmission electron microscope, study the change of gel particle size with salinity, and verify the salt resistance of the provided temperature-resistant and salt-resistant gel particles. The results are as Figure 4 shown. From Figure 4 the result analysis, it can be seen that with the increase of salinity, the particle size of the gel particles does not change significantly, and the influence of salinity on the particle size is small. This shows that the cross-linking density of the gel particles is high, the three-dimensional network structure is stable, reducing the penetration of salt ions and water molecules, weakening the influence of mineral ions in the formation on the particle size of the gel particles, and thus realizing the salt resistance of the gel particles.

[0048] Example 3:

[0049] Take the emulsion synthesized by reaction at 70 °C and 450 rpm in Example 1. After cooling to room temperature, put it into the freezer of a freeze dryer, freeze at -40 °C for 24 h, and then put it into the vacuum chamber of the freeze dryer for vacuum drying for 12 h to obtain a white powdery gel particle product. After fully dispersing the obtained gel particles for 6 hours under the condition of a salinity of 200000 mg / L, age them at 150 °C. By observing the change of particle morphology with aging time, study the temperature resistance of the provided temperature-resistant and salt-resistant gel particles. The results are as Figure 5 、 Figure 6 shown. From Figure 5 、 Figure 6 the analysis, it can be seen that with the increase of aging time, the morphology of the provided temperature-resistant and salt-resistant gel particles changes little, there is a normal caking phenomenon, and they can be dispersed after homogenization, indicating strong stability under high temperature conditions and good temperature resistance.

[0050] Example 4:

[0051] Through core flooding experiments, the change of displacement pressure with injection volume during the injection process of the provided temperature-resistant and salt-resistant gel particles was analyzed. The specific steps are as follows:

[0052] (1) Place the outcropping core into a wide-mouth bottle, add deionized water to submerge the core, and ultrasonicate it in an ultrasonic cleaner for 90 min. After washing, place it in a constant-temperature vacuum drying oven at 90 °C for 24 h. Weigh the dried core to obtain its dry weight, and use a vernier caliper to measure the length and diameter of the core. Measure the permeability of the core with an automatic permeability measuring instrument to be 200 mD.

[0053] (2) Place the dried core into a suction flask, add mineralized water to submerge the core, and evacuate and saturate it for 24 h. Weigh the core saturated with mineralized water to obtain its wet weight, and calculate the water-measured pore volume and porosity of the core using the following formula;

[0054]

[0055] In the formula, V p is the water-measured pore volume of the core, cm 3 ; w s is the wet weight of the core, g; w d is the dry weight of the core, g; ρ w is the density of the simulated formation water, g·mL -1 ; φ is the porosity of the core, %; V f is the volume of the core, cm 3 ; l is the length of the core, cm; D is the diameter of the core, cm.

[0056] (3) Prepare a suspension of the provided temperature-resistant and salt-resistant gel particles with a concentration of 2 wt% in 200000 mg / L formation simulation water, and stir it thoroughly for 6 hours. At 90 °C, inject 1 PV (1 PV is the pore volume of the core) of simulated formation water, 1 PV of the salt-resistant swelling-resistant gel particle suspension, and 3 PV of simulated formation water continuously at an injection flow rate of 0.3 mL·min -1 , and record the change of injection pressure with injection volume.

[0057] (4) Plot the curve of the change of injection pressure with injection volume as shown in Figure 7 . Calculate the core plugging rate of the gel particles according to the following formula:

[0058]

[0059] In the formula, η is the core plugging rate, %; P w1 is the first water flooding equilibrium pressure, kPa; P w2 is the second water flooding equilibrium pressure, kPa.

[0060] From Figure 7It can be seen that during the initial water injection, the pressure gradually increases with the increase of the injection volume. After reaching the steady state, the pressure difference is small. During the injection of particles, the injection pressure increases rapidly. This is because the gel particles gradually accumulate in the core pores, forming a plugging effect and increasing the flow resistance. After the second water flooding, the equilibrium pressure is significantly higher than that of the first water flooding. The calculated plugging rate of the provided temperature-resistant and salt-resistant gel particles is 89%, indicating that the particles can effectively plug the deep reservoir.

[0061] The above examples show that the super temperature-resistant and salt-resistant polymer gel particles of the present invention have good injectability and controllable particle size, can maintain good stability under high-temperature and high-salt conditions, and can effectively achieve the plugging and flow regulation of deep reservoirs.

[0062] Certainly, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by those skilled in the art within the scope of the essence of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for preparing ultra-temperature-resistant and salt-resistant gel particles for deep flow regulation in oil reservoirs, characterized in that: A super-temperature-resistant and salt-resistant gel particle for deep flow regulation in oil reservoirs is prepared. The super-temperature-resistant and salt-resistant gel particle is polymerized by a monomer, a stabilizer, a buffer, a cross-linking agent, and an initiator. The total amount of the stabilizer and the buffer added is 10% of the amount of the monomer, the amount of the cross-linking agent added is 5.5% of the amount of the monomer, and the amount of the initiator added is 1% of the amount of the monomer; The monomer is styrene; The stabilizer is vinyl acetic acid, and the buffer is ammonium bicarbonate; The cross-linking agent is divinylbenzene; The initiator is ammonium persulfate; The preparation method comprises the following steps: Step 1, prepare an aqueous solution; weigh a stabilizer, a buffer, and deionized water and add them to a beaker, and place the beaker on a magnetic stirrer and stir thoroughly; Step 2, prepare an oil phase solution; weigh the monomer and the cross-linking agent, add them into a three-necked flask, and stir thoroughly; Step 3, prepare an initiator solution; weigh the initiator and deionized water and add them into a beaker, and place the beaker on a magnetic stirrer to stir thoroughly; Step 4, preparing an emulsion; fully mixing the aqueous phase solution and the oil phase solution in a three-necked flask, and mixing and dispersing them under the action of a homogenizer to obtain a uniform homogeneous emulsion; Step 5, performing a polymerization reaction; placing the homogeneous emulsion in a constant temperature oil bath, stirring at a temperature of 70°C and a speed of 450r / min; installing a condensation reflux device, and passing N2 for deoxygenation at the same time, adding the prepared initiator after reacting for 5 minutes, and reacting for 10 hours to obtain an emulsion product; Step 6, freeze drying; cool the emulsion product after the reaction to room temperature and then put it into the freezer of a freeze dryer, freeze it at -40°C for 24 hours, and then put it into the vacuum chamber of the freeze dryer for vacuum drying for 12 hours to obtain a white powdery gel particle product. The white powdery gel particle product is the ultra-temperature-resistant and salt-resistant gel particle used for deep flow regulation in oil reservoirs.

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