Temporary plugging agent for fracturing and workover operation and preparation method thereof

Abstract

A temporary plugging agent for a fracturing and workover operation comprises 10-15 parts of N, N-methylenebisacrylamide, 10-15 parts of acrylamide, 6-30 parts of hydroxide, 5-8 parts of starch, 5-8 pa

Description

[0001] Embodiments of the present disclosure relate to the temporary plugging and production increment technology for oilfields, and in particular to a temporary plugging agent for a fracturing and workover operation and a method for preparing the temporary plugging agent. BACKGROUND CO

[0002] With the continuous deepening of development of oilfields with high water content and low CXI permeability, requirements for fracturing technology of oil layers are getting higher and higher, and -j— construction difficulty is increasing. For reservoirs with extremely low permeability, using a conventional fracturing method can only form a single symmetrical fracture, and it is difficult to CM achieve efficient development of oil and gas resources. To increase the production of a single well, it is necessary to force to expand a fracturing-modification volume and increase complexity of fractures.

[0003] A temporary plugging method is a method to pump temporary plugging agents into a destination layer under certain conditions and change inflow directions of original liquids by taking advantage of the difference in permeabilities of different layers, to achieve the purpose of generating new fractures. Most of the reservoirs with low permeability have increased production to a greater extent after the fracturing modification, but are still not very productive. At present, the temporary plugging method is an effective method of increasing the complexity of fractures, the permeabilities of the reservoirs, and the productivity of oil-gas wells.

[0004] In addition, for oil-water wells with great differences in permeabilities of oil layers and having a serious sanding problem, production operations such as workover, well killing, and sand flushing, etc., are frequently performed, and incoming fluids of the wells are seriously leaked, for example, the incoming fluid of the wells are leaked during well washing without return, sand control, and sand flushing. The leakage not only pollutes the oil-gas reservoirs, but also affects the operation processes and increases the cost of the operations. Using a leakage-proof temporary plugging technology is an effective method.

[0005] The temporary plugging agents commonly used in the fracturing modification and operation include: temporary plugging balls, polymer-based temporary plugging agents, fiber-based temporary plugging agents, etc. The temporary plugging balls are easy to dissolve, have less contamination, and are easy to operate, but the temporary plugging balls are dense and have sinking problems. The polymer-based temporary plugging agents are more troublesome to prepare and have relative high requirements for placing, and in-place responses are not obvious. The fiber-based temporary plugging CO agents are resistant to high temperature and have strong pressure-bearing ability, but the cost is high, CM the construction operability is poor, and the fiber-based temporary plugging agents have no certain "!“ elasticity and toughness, such that it is easy to cause a rise in a pumping pressure after agglomeration.

[0006] in view of the probes of difficult prepanuion, poor saU resistance, poor tenure resistance, and uneven dispersion and agglomeration of biodegradable fibers due to winding of the biodegradable fibers, our company has developed a new type of temporary plugging agent for a fracturing and workover operation. SUMMARY

[0007] To solve the above technical problems, embodiments of the present disclosure provide a temporary plugging agent for a fracturing and workover operation and a method for preparing the temporary plugging agent. The temporary plugging agent may achieve layer transfer or plugging during fracturing or workover via a temporary plugging function thereof. Different from a conventional temporary plugging agent, the temporary plugging agent according to some embodiments of the present disclosure has the elasticity and toughness of the polymeric-based temporary plugging agent and is swellable, and also has the rigidity of the biodegradable fiber and the temporary plugging ball. The temporary plugging agent is easy to prepare and operate, has a certain pressure resistance and temperature resistance, is safe, environmentally friendly, and biodegradable, and will not cause secondary damage to the formation and the environment.

[0008] In order to achieve the above objectives, in one aspect of the present disclosure, a temporary plugging agent for a fracturing and workover operation may be provided. The temporary plugging agent includes raw components in parts by mass: 10-15 parts of N, N-methylenebisacrylamide, 10-15 parts of acrylamide, 6-30 parts of hydroxide, 5-8 parts of starch, 5-8 parts of biodegradable fiber, 2-4 parts of chitin, 2-4 parts of scleroglucan, 50-68 parts of mineral oil, and 50-90 parts of distilled water.

[0009] In some embodiments of the temporary plugging agent, the mineral oil is an industrial-grade white oil with a certain dispersion function. Considering the safety, the white oil with a flash point greater than 60°C may be chosen. The hydroxide is industrial-grade sodium hydroxide with a concentration of at least 98.5%.

[0010] In some embodiments of the temporary plugging agent, the biodegradable fiber at least comprises cellulosic fiber, chitin-based fiber, or polyester fiber.

[0011] In a second aspect, a preparing method for the temporary plugging agent for a fracturing and workover operation is provided, and rough material is firstly prepared. The preparation of the rough material comprises a first stage, a second stage, and a third stage. At the first stage, solution A and solution B are prepared, respectively. At the second stage, the solution A and the solution B are mixed to prepare solution C. At the third stage, the solution C reacts (i.e., a first reaction is performed on the solution C), and the reacted solution C is processed into the rough material.

[0012] At the first stage, the solution A is prepared by mixing the N, N-methylenebisacrylamide, the acrylamide, and the distilled water, and the solution B is prepared by mixing the starch and the hydroxide. At the second stage, after the solution A and the solution B are processed, 15% ammonium persulphate aqueous solution are dropwise added, the biodegradable fiber, the chitin, and the scleroglucan are added and the mixed solution is stirred at a low speed. The hydroxide is added to adjust a PH value and the temperature of the mixed solution rapidly rises to 80°C, and a second reaction is performed on the mixed solution at a constant temperature of 80°C for 2h to obtain the solution C. At the third stage, 2-20 parts of the hydroxide is added to the solution C, such that the temperature of the solution C rises to 90°C, and the solution C reacts (i.e., the first reaction is performed on the solution C) at a constant temperature of 90°C for 6h to obtain the rough material.

[0013] The rough material obtained at the third stage is soaked in and washed with the distilled water, the washed rough material is dehydrated with ethanol, dried under reduced pressure, and pulverized by using a high-speed centrifugal grinder to obtain a composite powder.

[0014] In some embodiments of the preparing method, at the first stage, 10-15 parts of the N, N- methylenebisacrylamide and 10-15 parts of the acrylamide are mechanically ground for 20-40min, and the ground components all pass through a screen mesh having a mesh number of 210, are dried under vacuum at 50°C for 5h, and placed in a dryer for spare use. After the N, N-methylenebisacrylamide and the acrylamide are ground and dried, 40-60 parts of the distilled water are added, the N, N-methylenebisacrylamide, the acrylamide, and the distilled water are mixed evenly by using ultrasonic waves, and nitrogen gas is kept introducing for 24h, to obtain the solution A. The mesh number refers to a number of openings in one linear inch of the screen mesh.

[0015] In some embodiments of the preparing method, at the first stage, 5-8 parts of the starch are added into to a three-necked flask equipped with a stirrer, a condenser tube, and a thermometer, 10-30 parts of the distilled water are added, and 2-10 parts of the hydroxide is added, such that the temperature of the mixture rapidly rises to 90°C, and the three-necked flask is placed in a water bath with a temperature of 90°C to maintain a constant temperature of 90°C, the nitrogen gas is kept introducing, and the mixture is continuously stirred to enable the starch to be completely gelatinized, to obtain the solution B. The N, N-methylenebisacrylamide, the acrylamide, and the starch have a certain water absorbing and swelling property. In case that the N, N-methylenebisacrylamide, the acrylamide, and the starch are polymerized together, the polymeride has both a certain toughness and a certain elasticity.

[0016] In some embodiments of the preparing method, at the second stage, the solution A and the solution B prepared at the first stage are added to a four-necked flask equipped with a condensation tube, a constant-pressure dropping funnel, a thermometer, and a nitrogen inlet, and the four-necked flask is placed in a water bath with a temperature of 60°C for 30min. The nitrogen gas is continuously introduced into the four-necked flask until the oxygen is completely removed. 5-15 parts of 15% ammonium persulphate aqueous solution (“ammonium persulphate” is abbreviated as “APS”) are dropwise added for 40-60min. 5-8 parts of the biodegradable fiber, 2-4 parts of the chitin, and 2-4 parts of the scleroglucan are added, and the mixed solution is stirred at a low speed of 100-500r / min for 5h. 2-10 parts of the hydroxide is added, such that the temperature of the mixed solution rapidly rises to 80°C, the second reaction is performed on the mixed solution at a constant temperature of 80°C for 2h, the nitrogen gas is stopped introducing after the second reaction is completed, to obtain the solution C. The biodegradable fiber provides a bridging effect and is interspersed in the polymers, thereby increasing the rigidity and stiffness of the temporary plugging agent. The chitin is easily chemically modified. Stereo-functional groups of the acrylamide molecules are easily chimeric with polysaccharide molecules (glucosaminoglycans) containing p-1,4 glycosidic bonds, which breaks the intermolecular hydrogen bonds, thereby changing the arrangement of the polymer chains. In this way, it is possible to enhance the strength of the temporary plugging agent; besides, the chitin itself is a good natural degradation material, which may promote the self-degradation of the temporary plugging agent and reduce the possibility of causing damage to the reservoir and environmental pollution. The scleroglucan increases the shear resistance, salt resistance, and high-temperature resistance (i.e., stability under high temperature) of the temporary plugging agent, giving the temporary plugging agent excellent swelling and temporary plugging performance even in harsh environments.

[0017] In some embodiments of the preparing method, the hydroxide is industrial-grade sodium hydroxide with a concentration of at least 98.5%.

[0018] In some embodiments of the preparing method, the obtained composite powder is fully mixed with 50-68 parts of the mineral oil by using ultrasonic waves, and the temporary plugging agent for a fracturing and workover operation may be obtained.

[0019] The temporary plugging agent according to some embodiments of the present disclosure may have the following performances. Swell factor (water) >50 multiples, swell factor (mineralized water) >40 multiples; plugging rate >90%: breakthrough pressure >60Mpa; degradability: degradable; temperature resistance^ 180°C.

[0020] An indoor evaluation method of the temporary plugging agent for a fracturing and workover operation may include the following. 1. Swell factor: 1% temporary plugging agents are prepared with purified water and saline solution with a concentration of lOOOOmg / L, respectively. The prepared temporary plugging agents are stirred evenly and placed at 25°C, 90°C and 180°C, respectively, to CO observe swelling statuses of the temporary plugging agents. CM

[0021] 2. Degradability: the 1% temporary plugging agent is divided into three equal portions and "!“ each portion weighs 100g. The three portions are further placed and rolling aged at the room temperature of 25°C, at a temperature of 90°C, and at a temperature of 140°C respectively. Whether the three portions of the temporary plugging agent are degraded is observed at every 30min, and the duration for complete degradation is recorded.

[0022] 3. Plugging rate: 1) Natural cores are used for the test of the plugging rate. The natural cores are drilled and cut, washed off oil, dried, and measured for spare use.

[0023] 2) An initial positive standard saline permeability Kwi of a core sample is measured on a core flow tester by using a gripper.

[0024] 3) The core sample is removed from the gripper and connected to a high temperature and high pressure dynamic integrated tester of drilling fluid. The core sample is positively damaged with the temporary plugging agent, the temperature of the temporary plugging agent is 80°C, the differential pressure is 315MPa, the surrounding pressure is 5MPa, the shear rate is 150s’1, and the damage duration is 30min.

[0025] 4) The gripper is connected to the core flow tester, the positive standard saline permeability Kw2 of the core sample is measured, and the plugging rate RpiUgging is obtained by the following formation.

[0026] Rr,ussins = 1 - X 100% ‘'W2

[0027] 4. Breakthrough pressure.

[0028] The end face of the core or a fracture is plugged or sealed manually by using the temporary plugging agent. The core is determined to be broken through based on a condition that the pumping pressure is suddenly reduced (usually reduced by at least 30%), and the highest pressure is the breakthrough pressure.

[0029] The mechanism of action of the present disclosure may be as follow. The biodegradable fiber may be grafted in the polymer, such that the polymer may retain the swelling, toughness, and elasticity of the polymer itself, and also introduce the rigidity and stiffness of the biodegradable fiber. Thus, the temporary plugging agent may have certain ductility, hardness, and pressure resistance. To increase an adaptability of the temporary plugging agent to harsh environments, the scleroglucan is grafted, such that the temporary plugging agent has a good thermal stability, salt resistance, and resistance to trivalent ions, to ensure that the temporary plugging agent is able to rapidly swell in the harsh environments. Considering that the product is a temporary plugging agent, the so-called temporary plugging agent, as the name suggests, is an agent configured to temporarily plug and should be degraded within a short duration or period. In order to increase the degradability of the temporary plugging agent, the chitin is grafted in the formulation. The chitin is easily chemically modified and the stereo-functional groups of the acrylamide molecule are easily chimeric with polysaccharide molecules (glucosamine) containing P-1,4 glycosidic bonds, which breaks the intermolecular hydrogen bonds, thereby changing the arrangement of the polymer chains. In this way, it is possible to enhance the strength of the temporary plugging agent; besides, the chitin itself is a good natural degradation material, which may promote the self-degradation of the temporary plugging agent and reduce the possibility of c causing damage to the reservoir and environmental pollution.

[0030] The temporary plugging agent may utilize the soluble biodegradable fiber and scleroglucan to build and bridge to form a plugging agent layer on the end face of the matrix and the fracture and the inner side of the fracture, utilize acrylamide and starch to absorb water and swell to further fill a gap between particles, thereby improving the plugging rate. When the temporary plugging agent enters the fracture, it may form a bridge plug in the fracture, and effectively prevent the subsequent temporary plugging agents from continuing to enter the fracture, such that the subsequent temporary plugging agents may be accumulated at a position forming the bridge plug. With the continuous injection of subsequent temporary plugging agents, more and more temporary plugging agents will be accumulated and bridged at the position forming the bridge plug, thereby forming a bridging zone with certain strength in the fracture. Under the effect of certain temperature and time, the temporary plugging agent may be self-degraded. CO

[0031] Embodiments of the present disclosure may have the following technical effects. The CM temporary plugging agent has very good swelling property, elasticity, strength, pressure resistance, "!“ salt resistance, and temperature resistance. The temporary plugging agent is able to be self-degraded, will not cause damage to the reservoir, and is able to promote rapid formation of oil-gas channels. The mineral oil is added to and dispersed in the temporary plugging agent, such that the surface of the temporary plugging agent is hydrophobic, thereby reducing problems such as agglomeration and increased pumping pressure, caused by hydrating or absorbing water and swelling on the surface of the plugging agent, during the pumping process. The temporary plugging agent has good flowability and is easy to handle and pump in the field. The temporary plugging agent is non-ionic and has good compatibility with existing injection fluids. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] In one aspect of the present disclosure, a temporary plugging agent for a fracturing and workover operation may be provided. The temporary plugging agent includes raw components in parts by mass: 10-15 parts of N, N-methylenebisacrylamide, 10-15 parts of acrylamide, 6-30 parts of hydroxide, 5-8 parts of starch, 5-8 parts of biodegradable fiber, 2-4 parts of chitin, 2-4 parts of scleroglucan, 50-68 parts of mineral oil, and 50-90 parts of distilled water.

[0033] In some embodiments of the temporary plugging agent, the mineral oil is an industrial-grade white oil with a certain dispersion function. Considering the safety, the white oil with a flash point greater than 60°C may be chosen. The hydroxide is industrial-grade sodium hydroxide with a concentration of at least 98.5%.

[0034] In some embodiments of the temporary plugging agent, the biodegradable fiber at least comprises cellulosic fiber, chitin-based fiber, or polyester fiber.

[0035] In a second aspect, a preparing method for the temporary plugging agent for a fracturing and workover operation, rough material is firstly prepared. The preparation of the rough material comprises a first stage, a second stage, and a third stage. At the first stage, solution A and solution B are prepared, respectively. At the second stage, the solution A and the solution B are mixed to prepare solution C. At the third stage, the solution C reacts (i.e., a first reaction is performed on the solution C), and the reacted solution C is processed into the rough material.

[0036] At the first stage, the solution A is prepared by mixing the N, N-methylenebisacrylamide, the acrylamide, and the distilled water, and the solution B is prepared by mixing the starch and the hydroxide. At the second stage, after the solution A and the solution B are processed, 15% ammonium persulphate aqueous solution are dropwise added, the biodegradable fiber, the chitin, and the scleroglucan are added and the mixed solution is stirred at a low speed. The hydroxide is added to adjust a PH value and the temperature of the mixed solution rapidly rises to 80°C, and a second reaction is performed on the mixed solution at a constant temperature of 80°C for 2h to obtain the solution C. At the third stage, 2-20 parts of the hydroxide is added to the solution C, such that the temperature of the solution C rises to 90°C, and the solution C reacts (i.e., the first reaction is performed on the solution C) at a constant temperature of 90°C for 6h to obtain the rough material.

[0037] The rough material obtained at the third stage is soaked in and washed with the distilled water, the washed rough material is dehydrated with ethanol, dried under reduced pressure, and pulverized by using a high-speed centrifugal grinder to obtain a composite powder.

[0038] In some embodiments of the preparing method, at the first stage, 10-15 parts of the N, N-m ethylenebisacrylamide and 10-15 parts of the acrylamide are mechanically ground for 20-40min, and the ground components all pass through a screen mesh having a mesh number of 210, are dried under vacuum at 50°C for 5h, and placed in a dryer for spare use. After the N, N-methylenebisacrylamide and the acrylamide are ground and dried, 40-60 parts of the distilled water are added, the N, N-methylenebisacrylamide, the acrylamide, and the distilled water are mixed evenly by using ultrasonic waves, and nitrogen gas is kept introducing for 24h, to obtain the solution A. The mesh number refers to a number of openings in one linear inch of the screen mesh.

[0039] In some embodiments of the preparing method, at the first stage, 5-8 parts of the starch are CO added into to a three-necked flask equipped with a stirrer, a condenser tube, and a thermometer, 10-C\J 30 parts of the distilled water are added, and 2-10 parts of the hydroxide is added, such that the "!“ temperature of the mixture rapidly rises to 90°C, and the three-necked flask is placed in a water bath with a temperature of 90°C to maintain a constant temperature of 90°C, the nitrogen gas is kept introducing, and the mixture is continuously stirred to enable the starch to be completely gelatinized, to obtain the solution B. The N, N-methylenebisacrylamide, the acrylamide, and the starch have a certain water absorbing and swelling property. In case that the N, N-methylenebisacrylamide, the acrylamide, and the starch are polymerized together, the polymeride has both a certain toughness and a certain elasticity.

[0040] In some embodiments of the preparing method, at the second stage, the solution A and the solution B prepared at the first stage are added to a four-necked flask equipped with a condensation tube, a constant-pressure dropping funnel, a thermometer, and a nitrogen inlet, and the four-necked flask is placed in a water bath with a temperature of 60°C for 30min. The nitrogen gas is continuously introduced into the four-necked flask until the oxygen is completely removed. 5-15 parts of 15% ammonium persulphate aqueous solution (“ammonium persulphate” is abbreviated as “APS”) are dropwise added for 40-60min. 5-8 parts of the biodegradable fiber, 2-4 parts of the chitin, and 2-4 parts of the scleroglucan are added, and the mixed solution is stirred at a low speed of 100-500r / min for 5h. 2-10 parts of the hydroxide is added, such that the temperature of the mixed solution rapidly rises to 80°C, the second reaction is performed on the mixed solution at a constant temperature of 80°C for 2h, the nitrogen gas is stopped introducing after the second reaction is completed, to obtain the solution C. The biodegradable fiber provides a bridging effect and is interspersed in the polymers, thereby increasing the rigidity and stiffness of the temporary plugging agent. The chitin is easily chemically modified. Stereo-functional groups of the acrylamide molecules are easily chimeric with polysaccharide molecules (glucosaminoglycans) containing P-1,4 glycosidic bonds, which breaks the intermolecular hydrogen bonds, thereby changing the arrangement of the polymer chains. In this way, it is possible to enhance the strength of the temporary plugging agent; besides, the chitin itself is a good natural degradation material, which may promote the self-degradation of the temporary plugging agent and reduce the possibility of causing damage to the reservoir and environmental pollution. The scleroglucan increases the shear resistance, salt resistance, and high-temperature resistance (i.e., stability under high temperature) of the temporary plugging agent, giving the temporary plugging agent excellent swelling and temporary plugging performance even in harsh environments.

[0041] In some embodiments of the preparing method, the hydroxide is industrial-grade sodium hydroxide with a concentration of at least 98.5%.

[0042] In some embodiments of the preparing method, the obtained composite powder is fully mixed with 50-68 parts of the mineral oil by using ultrasonic waves, and the temporary plugging agent for a fracturing and workover operation may be obtained.

[0043] Example 1

[0044] A temporary plugging agent for a fracturing and workover operation includes the following components in parts by mass: 12 parts of N, N-methylenebisacrylamide, 10 parts of acrylamide, 5 parts of starch, 8 parts of biodegradable fiber, 3 parts of chitin, 2 parts of scleroglucan, 60 parts of mineral oil, 25 parts of sodium hydroxide, and 70 parts of distilled water.

[0045] The preparation method of this example is achieved by the following operations. 12 parts of the N, N-methylenebisacrylamide and 10 parts of the acrylamide are mechanically ground for 30min. The ground components are sieved with and all pass through a screen mesh having a mesh number of 210, dried under vacuum at 50°C for 5h, and then placed in a dryer for spare use. After 12 parts of the N, N-methylenebisacrylamide and 10 parts of the acrylamide are ground and dried, 50 parts of the distilled water are added. The N, N-methylenebisacrylamide, the acrylamide, and the distilled water are mixed evenly by using ultrasonic waves, and nitrogen gas is kept introducing for 24h. Thus, solution A is prepared. The mesh number refers to a number of openings in one linear inch of the screen mesh.

[0046] 5 parts of the starch are added into to a three-necked flask equipped with a stirrer, a condenser CO tube, and a thermometer. 20 parts of the distilled water are added into the three-necked flask, and then CM 8 parts of sodium hydroxide is further added into the three-necked flask, such that the temperature of "!“ the mixture rapidly rises to or is rapidly increased to 90°C. Then the three-necked flask is placed in a water bath with a temperature of 90°C to maintain a constant temperature of 90°C, the nitrogen gas is kept introducing throughout, and the mixture is continuously stirred, such that the starch is completely gelatinized, and the solution B is prepared.

[0047] In the above operations, the N, N-methylenebisacrylamide, the acrylamide, and the starch have a certain water absorbing and swelling property. In case that the N, N-methylenebisacrylamide, the acrylamide, and the starch are polymerized together, the polymeride has both a certain toughness and a certain elasticity.

[0048] The prepared solution A and solution B are added to a four-necked flask equipped with a condensation tube, a constant-pressure dropping funnel, a thermometer, and a nitrogen inlet. The fournecked flask is placed in a water bath with a temperature of 60°C for 30min. The nitrogen gas is continuously introduced into the four-necked flask until the oxygen is completely removed. 5 parts of 15% ammonium persulphate aqueous solution, i.e., the ammonium persulphate aqueous solution with the concentration of 15%, (“ammonium persulphate” is abbreviated as “APS”) are dropwise added for 42min. 8 parts of the biodegradable fiber, 3 parts of the chitin, and 2 parts of the scleroglucan are added, and the mixed solution is stirred at a low speed of 100-500r / min for 5h. 8 parts of the sodium hydroxide is added, such that the temperature of the mixture or mixed solution rapidly rises to 80°C. The reaction is performed on the mixed solution at a constant temperature of 80°C for 2h. After the reaction is completed, the nitrogen gas is stopped introducing. Thus, solution C is prepared. The biodegradable fiber includes but is not limited to, i.e., at least includes, cellulosic fiber, chitin-based fiber, polyester fiber, etc. The biodegradable fiber provides a bridging effect and is interspersed in the polymers, thereby increasing the rigidity and stiffness of the temporary plugging agent. The chitin is easily chemically modified. Stereo-functional groups of the acrylamide molecules are easily chimeric with polysaccharide molecules (glucosaminoglycans) containing P-1,4 glycosidic CO bonds, which breaks the intermolecular hydrogen bonds, thereby changing the arrangement of the CM polymer chains. In this way, it is possible to enhance the strength of the temporary plugging agent; "!“ besides, the chitin itself is a good natural degradation material, which may promote the self-degradation of the temporary plugging agent and reduce the possibility of causing damage to the C\J reservoir and environmental pollution. The scleroglucan increases the shear resistance, salt resistance, and high-temperature resistance (i.e., stability under high temperature) of the temporary plugging agent, giving the temporary plugging agent excellent swelling and temporary plugging performance even in harsh environments.

[0049] 9 parts of the sodium hydroxide is added to the solution C, such that the temperature of the solution C rises to 90°C rapidly. A reaction is performed on the solution C at a constant temperature of 90°C for 6h.

[0050] The reaction product is soaked in and washed with the distilled water, to wash off the residues from the reaction product. The washed reaction product is dehydrated with ethanol, dried under reduced pressure, and pulverized by using a large high-speed centrifugal grinder to obtain a composite powder.

[0051] The obtained composite powder is fully mixed with 60 parts of the mineral oil by using ultrasonic waves. The mineral oil is an industrial-grade white oil with certain dispersion function. After evenly mixing, a new type of temporary plugging agent for a fracturing and workover operation may be obtained.

[0052] An indoor evaluation method of the temporary plugging agent for a fracturing and workover operation may include the following. 1. Swell factor: 1% temporary plugging agents are prepared with purified water and saline with a concentration of lOOOOmg / L, respectively. The prepared temporary plugging agents are stirred evenly and placed at 25°C, 90°C and 180°C, respectively, to observe swelling statuses of the temporary plugging agents.

[0053] 2. Degradability: the 1% temporary plugging agent is divided into three equal portions and each portion weighs 100g. The three portions are further placed and rolling aged at the room temperature of 25°C, at a temperature of 90°C, and at a temperature of 140°C respectively. Whether the three portions of the temporary plugging agent are degraded is observed at every 30min, and the duration for complete degradation is recorded.

[0054] 3. Plugging rate: 1) Natural cores are used for the test of the plugging rate. The natural cores are drilled and cut, washed off oil, dried, and measured for spare use.

[0055] 2) An initial positive standard saline permeability Kwi of a core sample is measured on a core flow tester by using a gripper.

[0056] 3) The core sample is removed from the gripper and connected to a high temperature and high pressure dynamic integrated tester of drilling fluid. The core sample is positively damaged with the temporary plugging agent, the temperature of the temporary plugging agent is 80°C, the differential pressure is 315MPa, the surrounding pressure is 5MPa, the shear rate is 150s'1, and the damage duration is 30min.

[0057] 4) The gripper is connected to the core flow tester, the positive standard saline permeability K W2 of the core sample is measured, and the plugging rate RpiUgging is obtained by the following formation.

[0058] Rpluggtng = 1 - x 100%

[0059] 4. Breakthrough pressure.

[0060] The end face of the core or a fracture is plugged or sealed manually by using the temporary plugging agent. The core is determined to be broken through based on a condition that the pumping pressure is suddenly reduced (usually reduced by at least 30%), and the highest pressure is the breakthrough pressure.

[0061] According to the indoor evaluation method of the temporary plugging agent, the performances of the temporary plugging agent for a fracturing and workover operation in this example are as follows.

[0062] Performance Temporary plugging agent of Example 1 Conventional temporary plugging agent Swell factor (purified water), multiple 25°C 85 42 90°C 102 44 180°C 92 0 Swell factor (saline), multiple 25°C 60 22 90°C 73 25 Degradability 25°C 26h self-degraded 3-5days 90°C 12h self-degraded 36h 140°C 5h self-degraded lOh Plugging rate, % 94 80-91 Breakthrough pressure, Mpa 72 40-60

[0063] Example 2

[0064] A temporary plugging agent for a fracturing and workover operation includes the following components in parts by mass: 12 parts of N, N-methyl enebisacryl ami de, 15 parts of acrylamide, 7 parts of starch, 5 parts of biodegradable fiber, 4 parts of chitin, 4 parts of scleroglucan, 53 parts of mineral oil, 6 parts of sodium hydroxide, and 50 parts of distilled water.

[0065] The preparation method of this example is achieved by the following operations. 12 parts of the N, N-methylenebisacrylamide and 15 parts of the acrylamide are mechanically ground for 30min. The ground components are sieved with and all pass through a screen mesh having a mesh number of 210, dried under vacuum at 50°C for 5h, and then placed in a dryer for spare use. After 12 parts of the N, N-methylenebisacrylamide and 15 parts of the acrylamide are ground and dried, 40 parts of the distilled water are added. The N, N-methylenebisacrylamide, the acrylamide, and the distilled water are mixed evenly by using ultrasonic waves, and nitrogen gas is kept introducing for 24h. Thus, solution A is prepared. The mesh number refers to a number of openings in one linear inch of the screen mesh.

[0066] 7 parts of the starch are added into to a three-necked flask equipped with a stirrer, a condenser CO tube, and a thermometer. 10 parts of the distilled water are added into the three-necked flask, and then CM 2 parts of the sodium hydroxide is further added into the three-necked flask, such that the temperature T"— of the mixture rapidly rises to or is rapidly increased to 90°C. Then the three-necked flask is placed in a water bath with a temperature of 90°C to maintain a constant temperature of 90°C, the nitrogen gas is kept introducing throughout, and the mixture is continuously stirred, such that the starch is completely gelatinized, and the solution B is prepared.

[0067] The prepared solution A and the solution B are added to a four-necked flask equipped with a condensation tube, a constant-pressure dropping funnel, a thermometer, and a nitrogen inlet. The fournecked flask is placed in a water bath with a temperature of 60°C for 30min. The nitrogen gas is continuously introduced into the four-necked flask until the oxygen is completely removed. 10 parts of 15% ammonium persulphate aqueous solution are dropwise added for 50min. 5 parts of the biodegradable fiber, 4 parts of the chitin, and 4 parts of the scleroglucan are added and the mixed solution is stirred at a low speed of 100-500r / min for 5h. 2 parts of the sodium hydroxide is added, such that the temperature of the mixture or mixed solution rapidly rises to 80°C. A reaction is performed on the mixed solution at a constant temperature of 80°C for 2h. After the reaction is completed, the nitrogen gas is stopped introducing. Thus, solution C is prepared.

[0068] 2 parts of the sodium hydroxide is added to the solution C, such that the temperature of the solution C rapidly rises to 90°C. A reaction is performed on the solution C at a constant temperature of90°Cfor 6h.

[0069] The reaction product is soaked in and washed with the distilled water, to wash off the residues from the reaction product. The washed reaction product is dehydrated with ethanol and dried under reduced pressure, and pulverized by using a large high-speed centrifugal grinder to obtain a composite powder.

[0070] The obtained composite powder is fully mixed with 53 parts of the mineral oil by using ultrasonic waves, and a new type of temporary plugging agent for a fracturing and workover operation may be obtained.

[0071] According to the indoor evaluation method of the temporary plugging agent, the performances of the temporary plugging agent for a fracturing and workover operation in this example are as follows.

[0072] Performance Temporary plugging agent of Example 2 Conventional temporary plugging agent Swell factor (purified water), multiple 25°C 98 42 90°C 125 44 180°C 112 0 Swell factor (saline), multiple 25°C 85 22 90°C 97 25 Degradability 25°C 24h self-degraded 3-5days 90°C 9h self-degraded 36h 140°C 2.5h self-degraded lOh Plugging rate, % 95 80-91 Breakthrough pressure, Mpa 68 40-60

[0073] Example 3

[0074] A temporary plugging agent for a fracturing and workover operation includes the following components in parts by mass: 15 parts of N, N-methylenebisacrylamide, 15 parts of acrylamide, 8 parts of starch, 8 parts of biodegradable fiber, 4 parts of chitin, 2 parts of scleroglucan, 47 parts of mineral oil, 30 parts of sodium hydroxide, and 90 parts of distilled water.

[0075] The preparation method of this example is achieved by the following operations. 15 parts of the N, N-methylenebisacrylamide and 15 parts of the acrylamide are mechanically ground for 30min. The ground components are sieved with and all pass through a screen mesh having a mesh number of 210, dried under vacuum at 50°C for 5h, and then placed in a dryer for spare use. After 15 parts of the N, N-methylenebisacrylamide and 15 parts of the acrylamide are ground and dried, 60 parts of the distilled water are added. The N, N-methylenebisacrylamide, the acrylamide, and the distilled water are mixed evenly by using ultrasonic waves, and nitrogen gas is kept introducing for 24h. Thus, solution A is prepared. The mesh number refers to a number of openings in one linear inch of the screen mesh.

[0076] 8 parts of the starch are added into to a three-necked flask equipped with a stirrer, a condenser tube, and a thermometer. 30 parts of the distilled water are added into the three-necked flask, and then 10 parts of the sodium hydroxide is further added into the three-necked flask, such that the temperature of the mixture rapidly rises to or is rapidly increased to 90°C. Then the three-necked flask is placed in a water bath with a temperature of 90°C to maintain a constant temperature of 90°C, the nitrogen gas is kept introducing throughout, and the mixture is continuously stirred, such that the starch is completely gelatinized, and the solution B is prepared.

[0077] The prepared solution A and the solution B are added to a four-necked flask equipped with a condensation tube, a constant-pressure dropping funnel, a thermometer, and a nitrogen inlet, and the four-necked flask is placed in a water bath with a temperature of 60°C for 30min. The nitrogen gas is continuously introduced into the four-necked flask until the oxygen is completely removed. 15 parts of 15% ammonium persulphate aqueous solution are dropwise added for 57min. 8 parts of the biodegradable fiber, 4 parts of the chitin, and 2 parts of the scleroglucan are added and the mixed solution is stirred at a low speed of 100-500r / min for 5h. 10 parts of the sodium hydroxide is added, such that the temperature of the mixture or solution rapidly rises to 80°C. A reaction is performed on the mixed solution at a constant temperature of 80°C for 2h. After the reaction is completed, the nitrogen gas is stopped introducing. Thus, solution C is prepared.

[0078] 10 parts of the sodium hydroxide is added to the solution C, such that the temperature of the solution C rapidly rises to 90°C. A reaction is performed on the solution C at a constant temperature of 90°C for 6h.

[0079] The reaction product is soaked in and washed with the distilled water, to wash off the residues from the reaction product. The washed reaction product is dehydrated with ethanol and dried under reduced pressure, and pulverized by using a large high-speed centrifugal grinder to obtain a composite powder.

[0080] The obtained composite powder is fully mixed with 47 parts of the mineral oil by using ultrasonic waves, and a new type of temporary plugging agent for a fracturing and workover operation may be obtained.

[0081] According to the indoor evaluation method of the temporary plugging agent, the performances of the temporary plugging agent for a fracturing and workover operation in this example are as follows.

[0082] Performance Temporary plugging agent of Example 3 Conventional temporary plugging agent Swell factor 25°C 105 42 (purified water), 90°C 140 44 multiple 180°C 112 0 Swell factor 25°C 75 22 (saline), multiple 90°C 90 25 Degradability 25°C 21h self-degraded 3-5days 90°C 6h self-degraded 36h 140°C 2.2h self-degraded lOh Plugging rate, % 95 80-91 Breakthrough pressure, Mpa 70 40-60

[0083] In the above embodiments, the sodium hydroxide may be replaced by potassium hydroxide or calcium hydroxide. The mechanism of action of the present disclosure may be as follow. The biodegradable fiber may be grafted in the polymer, such that the polymer may retain the swelling, toughness, and elasticity of the polymer itself, and also introduce the rigidity and stiffness of the biodegradable fiber. Thus, the temporary plugging agent may have certain ductility, hardness, and pressure resistance. To increase an adaptability of the temporary plugging agent to harsh environments, the scleroglucan is grafted, such that the temporary plugging agent has a good thermal stability, salt resistance, and resistance to trivalent ions, to ensure that the temporary plugging agent is able to rapidly swell in the harsh environments. Considering that the product is a temporary plugging agent, the so-called temporary plugging agent, as the name suggests, is an agent configured to temporarily plug and should be degraded within a short duration or period. In order to increase the degradability of the temporary plugging agent, the chitin is grafted in the formulation. The chitin is easily chemically modified and the stereo-functional groups of the acrylamide molecule are easily chimeric with polysaccharide molecules (glucosamine) containing P-1,4 glycosidic bonds, which breaks the intermolecular hydrogen bonds, thereby changing the arrangement of the polymer chains. In this way, it is possible to enhance the strength of the temporary plugging agent; besides, the chitin itself is a good natural degradation material, which may promote the self-degradation of the temporary plugging agent and reduce the possibility of c causing damage to the reservoir and environmental pollution.

[0084] The temporary plugging agent may utilize the soluble biodegradable fiber and scleroglucan to build and bridge to form a plugging agent layer on the end face of the matrix and the fracture and the inner side of the fracture, utilize acrylamide and starch to absorb water and swell to further fill a gap between particles, thereby improving the plugging rate. When the temporary plugging agent enters the fracture, it may form a bridge plug in the fracture, and effectively prevent the subsequent temporary plugging agents from continuing to enter the fracture, such that the subsequent temporary plugging agents may be accumulated at a position forming the bridge plug. With the continuous injection of subsequent temporary plugging agents, more and more temporary plugging agents will be accumulated and bridged at the position forming the bridge plug, thereby forming a bridging zone with certain strength in the fracture. Under the effect of certain temperature and time, the temporary plugging agent may be self-degraded.

[0085] Embodiments of the present disclosure may have the following technical effects. The temporary plugging agent has very good swelling property, elasticity, strength, pressure resistance, salt resistance, and temperature resistance. The temporary plugging agent is able to be self-degraded, will not cause damage to the reservoir, and is able to promote rapid formation of oil-gas channels. The mineral oil is added to and dispersed in the temporary plugging agent, such that the surface of the temporary plugging agent is hydrophobic, thereby reducing problems such as agglomeration and increased pumping pressure, caused by hydrating or absorbing water and swelling on the surface of the plugging agent, during the pumping process. The temporary plugging agent has good flowability and is easy to handle and pump in the field. The temporary plugging agent is non-ionic and has good compatibility with existing injection fluids.

[0086] Of course, the present disclosure may have various other embodiments, without departing from the spirit and the essence of the present disclosure. Those skilled in the art may make various corresponding changes and variations according to the present disclosure, but these corresponding changes and variations should fall within the scope of protection of the claims appended to the present disclosure.

Claims

1. A temporary plugging agent for a fracturing and workover operation, characterized in that,the temporary plugging agent comprises raw components in parts by mass: 10-15 parts of N, N-methylenebisacrylamide, 10-15 parts of acrylamide, 6-30 parts of hydroxide, 5-8 parts of starch, 5-8 parts of biodegradable fiber, 2-4 parts of chitin, 2-4 parts of scleroglucan, 50-68 parts of mineral oil, and 50-90 parts of distilled water; wherein the biodegradable fiber at least comprises cellulosic fiber, chitin-based fiber, or polyester fiber;wherein the raw components are processed and prepared into rough material, and the preparation of the rough material comprises a first stage, a second stage, and a third stage; at the first stage, solution A and solution B are prepared, respectively; at the second stage, the solution A and thesolution B are mixed to prepare solution C; and at the third stage, a first reaction is performed on theCO solution C and the reacted solution C is processed into the rough material;CMat the first stage, the solution A is prepared by mixing the N, N-methylenebisacrylamide, the"!“ acrylamide, and the distilled water, and the solution B is prepared by mixing the starch and the hydroxide; at the second stage, after the solution A and the solution B are processed, 15% ammonium persulphate aqueous solution are dropwise added, the biodegradable fiber, the chitin, and the scleroglucan are added, and the mixed solution is stirred at a low speed, the hydroxide is added to adjust a PH value and the temperature of the mixed solution rapidly rises to 80°C, and a second reaction is performed on the mixed solution at a constant temperature for 2h to obtain the solution C; at the third stage, 2-20 parts of the hydroxide is added to the solution C, such that the temperature of the solution C rises to 90°C, and the first reaction is performed on the solution C at a constant temperature for 6h to obtain the rough material; andthe rough material obtained at the third stage is soaked in and washed with the distilled water, the washed rough material is dehydrated with ethanol, dried under reduced pressure, and pulverized by using a high-speed centrifugal grinder to obtain a composite powder.

2. The temporary plugging agent for a fracturing and workover operation as claimed in claim 1,wherein the hydroxide is industrial-grade sodium hydroxide with a concentration of at least 98.5%,and the mineral oil is an industrial-grade white oil with a flash point greater than 60°C.

3. The temporary plugging agent for a fracturing and workover operation as claimed in claim 1, wherein at the first stage, 10-15 parts of the N, N-methylenebisacrylamide and 10-15 parts of the acrylamide are mechanically ground for 20-40min, and the ground components all pass through a screen mesh having a mesh number of 210, are dried under vacuum at 50°C for 5h, and placed in a dryer for spare use; after the N, N-methylenebisacrylamide and the acrylamide are ground and dried, 40-60 parts of the distilled water are added, the N, N-methylenebisacrylamide, the acrylamide, and the distilled water are mixed evenly by using ultrasonic waves, and nitrogen gas is kept introducing for 24h, to obtain the solution A; wherein the mesh number refers to a number of openings in one linear inch of the screen mesh.CO 4. The temporary plugging agent for a fracturing and workover operation as claimed in claim 1,CMwherein at the first stage, 5-8 parts of the starch are added into to a three-necked flask equipped with "!“ a stirrer, a condenser tube, and a thermometer, 10-30 parts of the distilled water are added, and 2-10 parts of the hydroxide is added, such that the temperature of the mixture rapidly rises to 90°C, and the three-necked flask is placed in a water bath with a temperature of 90°C to maintain a constant temperature, the nitrogen gas is kept introducing, and the mixture is continuously stirred to enable the starch to be completely gelatinized, to obtain the solution B.

5. The temporary plugging agent for a fracturing and workover operation as claimed in claim 1, wherein at the second stage, the solution A and the solution B prepared at the first stage are added to a four-necked flask equipped with a condensation tube, a constant-pressure dropping funnel, a thermometer, and a nitrogen inlet, the four-necked flask is placed in a water bath with a temperature of 60°C for 30min, the nitrogen gas is continuously introduced into the four-necked flask until the oxygen is completely removed, 5-15 parts of 15% ammonium persulphate aqueous solution are dropwise added for 40-60min, 5-8 parts of the biodegradable fiber, 2-4 parts of the chitin, and 2-4 parts of the scleroglucan are added, and the mixed solution is stirred at a low speed of 100-500r / minfor 5h, 2-10 parts of the hydroxide is added, such that the temperature of the mixed solution rapidly rises to 80°C, the second reaction is performed on the mixed solution at a constant temperature for 2h, the nitrogen gas is stopped introducing after the second reaction is completed, to obtain the solution C.

6. A preparing method for preparing the temporary plugging agent for a fracturing and workover operation as claimed in claim 1, characterized by the method comprising:preparing rough material, wherein the preparation of the rough material comprises the first stage, the second stage, and the third stage; at the first stage, the solution A and the solution B are prepared, respectively; at the second stage, the solution A and the solution B are mixed to prepare the solution C; and at the third stage, the first reaction is performed on the solution C and the reacted solution C is processed into the rough material; wherein at the first stage, the solution A is prepared by mixing CO the N, N-methylenebisacrylamide, the acrylamide, and the distilled water, and the solution B is CMprepared by mixing the starch and the hydroxide; at the second stage, after the solution A and the T"— solution B are processed, 15% ammonium persulphate aqueous solution are dropwise added, thebiodegradable fiber, the chitin, and the scleroglucan are added and the mixed solution is stirred at a low speed, the hydroxide is added to adjust a PH value and the temperature of the mixed solution rapidly rises to 80°C, and the second reaction is performed on the mixed solution at a constant temperature for 2h to obtain the solution C; at the third stage, 2-20 parts of the hydroxide is added to the solution C, such that the temperature of the solution C rises to 90°C, and the first reaction is performed on the solution C at a constant temperature for 6h to obtain the rough material; and soaking the rough material obtained at the third stage in the distilled water and washing the rough material with the distilled water, dehydrating the washed rough material with ethanol, drying the dehydrated rough material under reduced pressure, and pulverizing the dried rough material by using a high-speed centrifugal grinder to obtain a composite powder.

7. The method as claimed in claim 6, wherein at the first stage, 10-15 parts of the N, N-methylenebisacrylamide and 10-15 parts of the acrylamide are mechanically ground for 20-40min,and the ground components all pass through a screen mesh having a mesh number of 210, are dried under vacuum at 50°C for 5h, and placed in a dryer for spare use; after the N, N-methylenebisacrylamide and the acrylamide are ground and dried, 40-60 parts of the distilled waterare added, the N, N-methylenebisacrylamide, the acrylamide, and the distilled water are mixed evenly by using ultrasonic waves, and nitrogen gas is kept introducing for 24h, to obtain the solution A; wherein the mesh number refers to a number of openings in one linear inch of the screen mesh.

8. The method as claimed in claim 6, wherein at the first stage, 5-8 parts of the starch are added into to a three-necked flask equipped with a stirrer, a condenser tube, and a thermometer, 10-30 parts of the distilled water are added, and 2-10 parts of the hydroxide is added, such that the temperature of the mixture rapidly rises to 90°C, and the three-necked flask is placed in a water bath with atemperature of 90°C to maintain a constant temperature, the nitrogen gas is kept introducing, and the CO mixture is continuously stirred to enable the starch to be completely gelatinized, to obtain the solution CMB."!“ 9. The method as claimed in claim 6, wherein at the second stage, the solution A and the solutionB prepared at the first stage are added to a four-necked flask equipped with a condensation tube, a constant-pressure dropping funnel, a thermometer, and a nitrogen inlet, the four-necked flask is placed in a water bath with a temperature of 60°C for 30min, the nitrogen gas is continuously introducedinto the four-necked flask until the oxygen is completely removed, 5-15 parts of 15% ammonium persulphate aqueous solution are dropwise added for 40-60min, 5-8 parts of the biodegradable fiber, 2-4 parts of the chitin, and 2-4 parts of the scleroglucan are added, and the mixed solution is stirred at a low speed of 100-500r / min for 5h, 2-10 parts of the hydroxide is added, such that the temperature of the mixed solution rapidly rises to 80°C, the second reaction is performed on the mixed solution at a constant temperature for 2h, the nitrogen gas is stopped introducing after the second reaction is completed, to obtain the solution C.

10. The method as claimed in claim 6, wherein the hydroxide is industrial-grade sodium hydroxide with a concentration of at least 98.5%, and the mineral oil is an industrial-grade white oilwith a flash point greater than 60°C.COCM