In-situ crosslinked and self-degrading high-strength gel temporary plugging agent and its preparation method

Abstract

The present invention discloses an in-situ cross-linked self-degradable high-strength gel temporary plugging agent and a preparation method thereof, which belongs to the field of oil and gas field development technology. The gel temporary plugging agent is composed of the following substances in parts by weight: 0.33~2.33 parts of polyvinyl alcohol, 0.67~4.67 parts of corn amylopectin, 2.8~7.0 parts of acrylamide, 1.2~3.0 parts of N-vinyl pyrrolidone, 0.05~1.05 parts of a cross-linking agent, 0.001~0.06 parts of an initiator, 0.3~2.1 parts of glycerol, 0.02~0.14 parts of calcium chloride, and the remainder being water, and the sum of the weight parts of the above substances is 100 parts. Its preparation method is simple to operate, the raw materials are cheap and easy to obtain, the preparation process is controllable, and it shows a huge prospect for industrial promotion. The gel temporary plugging agent is used for a reservoir temporary plugging and diversion process, and can self-degrade under constant temperature conditions in the formation, which is of great significance for achieving efficient development of oil fields and improving oil and gas recovery.

Description

Technical Field

[0001] The invention belongs to the technical field of oil and gas field development, and particularly relates to an in-situ cross-linking and self-degrading high-strength gel temporary plugging agent, a preparation method and application thereof. Background Art

[0002] Temporary plugging and diversion technology is widely used in shale and tight sandstone mining to enhance the density of the fracture network and increase the oil and gas production of reservoirs in undeveloped areas. Temporary plugging and diversion technology blocks the initial fractures to avoid ineffective fracturing caused by energy loss during fracturing, and forces the fracturing fluid to divert and open new fractures, thereby restoring the initial fractures and continuing production. Traditional mechanical temporary plugging methods are mainly suitable for wellbore plugging, and their scope of application is limited. Chemical temporary plugging methods mainly include solid-phase material injection and liquid rubber plug segment plugging. After the temporary plugging and diversion material is injected into the reservoir fracture, it forms an effective temporary plug segment through bridging and in-situ cross-linking, prompting the fracturing fluid to divert and open new fractures. After the temporary plug segment degrades, the initial fracture resumes production. Studies have reported that the segment plug structure constructed with fibers and degradable particles has high strength and wide temperature adaptability, but the lack of access to real reservoir fracture information limits its plugging performance. Liquid rubber plug temporary plugging materials are injected into fractures in a plugging manner and then cross-linked and degraded in situ. Currently, liquid rubber plug temporary plugging materials are mainly used for profile control and water blocking, but the non-degradability of such materials makes them difficult to apply to temporary plugging and diversion processes.

[0003] A growing number of researchers are studying self-degradable liquid plugging agents. A study titled "A Thermally Induced Supramolecular Gel Temporary Plugging and Diverting Fracturing Fluid" (CN106190087B) has developed a thermally responsive temporary plugging and diverting agent. This system is a low-viscosity, flowable liquid at room temperature. As the temperature rises to 60-90°C, its viscosity rapidly increases, losing its fluidity. When the temperature continues to rise to 90-150°C, it automatically breaks down into small molecules, returning to a low-viscosity, flowable state. Although this system achieves a thermal phase transition, the required liquid-solid-liquid phase transition temperatures are different. In actual production, it is difficult to heat the formation, so this thermal phase transition system still has certain limitations.

[0004] The present invention proposes for the first time an in-situ cross-linked and self-degradable physical and chemical interpenetrating high-strength gel temporary plugging agent, which can achieve self-degradation of the liquid plug under constant temperature conditions in the formation without the need for temperature changes. Under the action of the physical cross-linking structure, the polymer gel system has both thermal phase change properties and stronger wall adhesion properties. Summary of the Invention

[0005] The purpose of the present invention is to provide an in-situ cross-linked and self-degradable high-strength gel temporary plugging agent. The gel temporary plugging agent has the advantages of strong toughness, self-degradation, wide applicable temperature, excellent wall adhesion performance and low price, and has broad market application prospects.

[0006] Another object of the present invention is to provide a method for preparing the in-situ cross-linking-self-degrading high-strength gel temporary plugging agent. The method has a reliable principle, is simple to operate, uses cheap and readily available raw materials, and has a controllable preparation process, showing great prospects for industrial promotion.

[0007] Another object of the present invention is to provide the application of the in-situ cross-linked and self-degradable high-strength gel temporary plugging agent in the reservoir temporary plugging and diversion process, which can achieve self-degradation of the liquid plug under constant formation temperature conditions without temperature changes, and is of great significance for achieving efficient development of oil fields and improving oil and gas recovery rates.

[0008] In order to achieve the above technical objectives, the present invention adopts the following technical solutions.

[0009] The in-situ cross-linking and self-degrading high-strength gel temporary plugging agent is composed of the following substances in parts by weight: 0.33-2.33 parts of polyvinyl alcohol, 0.67-4.67 parts of corn amylopectin, 2.8-7.0 parts of acrylamide, 1.2-3.0 parts of N-vinyl pyrrolidone, 0.05-1.05 parts of a cross-linking agent, 0.001-0.06 parts of an initiator, 0.3-2.10 parts of glycerol, 0.02-0.14 parts of calcium chloride, and the balance being water. The total weight of the above substances is 100 parts.

[0010] Furthermore, the cross-linking agent is polyethylene glycol diacrylate.

[0011] Furthermore, the initiator is tert-butyl perbenzoate.

[0012] Furthermore, the mass ratio of acrylamide to N-vinyl pyrrolidone is 7:3.

[0013] The preparation method of the in-situ cross-linking-self-degrading high-strength gel temporary plugging agent comprises the following steps:

[0014] First, a certain amount of polyvinyl alcohol (PVA) was weighed and dissolved in an appropriate amount of water. Corn amylopectin was dispersed in the PVA solution, and glycerol and CaCl2 were added. After mixing evenly at room temperature, the mixture was placed in a constant temperature environment of 90°C for 30 minutes to obtain a pre-gelatinized solution.

[0015] Secondly, a certain amount of monomers acrylamide (AM) and N-vinyl pyrrolidone (NVP) were weighed and dissolved in water to prepare a base solution of a certain concentration. A crosslinker and initiator were then added to the base solution to obtain a monomer solution.

[0016] Finally, the pre-gelatinized solution and the monomer solution are mixed evenly at room temperature to obtain a reaction solution of the gel temporary plugging agent.

[0017] The in-situ cross-linking and self-degrading high-strength gel temporary plugging agent is used in a reservoir temporary plugging and diversion process. The process is as follows: first, a pre-fluid is injected, then a prepared gel temporary plugging agent reaction solution is injected according to the requirements of the oil and gas reservoir, and finally, a displacement fluid is injected; after the gel temporary plugging agent reaction solution enters a formation environment with a temperature of 100-150°C, it is in-situ polymerized and cross-linked in the fractures for 20-200 minutes to form a high-strength gel temporary plugging agent, and then a subsequent fracturing process is immediately carried out; after the construction is completed, the gel temporary plugging agent spontaneously degrades into a low-viscosity degradation fluid in a high-temperature environment of 100-150°C after 12-90 hours, and is returned to the surface with the fracturing fluid.

[0018] The present invention forms a gel temporary plugging agent with a physicochemically interpenetrating structure by combining a physical crosslinking network constructed from polyvinyl alcohol (PVA) and starch with a chemical crosslinking network constructed from acrylamide and N-vinyl pyrrolidone. Initially, starch and PVA are mixed to form an initial physical crosslinking network (a pre-gelatinized solution). Then, under high formation temperature conditions, acrylamide and N-vinyl pyrrolidone react within the physical network formed by starch and PVA to form a chemical crosslinking network. The chemical crosslinking network then grows within the starch network and interpenetrates it, ultimately creating a uniform, high-strength crosslinking network with interpenetrating physicochemical structures.

[0019] The glycerol and CaCl2 are mainly used to provide plasticization for physical cross-linking, thereby ensuring the fluidity of the physical cross-linking.

[0020] The reaction process of the formation of physical and chemical interpenetrating network is as follows:

[0021]

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] (1) The gel temporary plugging agent of the present invention uses acrylamide monomers, N-vinyl pyrrolidone, polyvinyl alcohol and starch as reaction materials, and adds a chemical crosslinking agent. The obtained physicochemical interpenetrating crosslinked gel temporary plugging agent can effectively dissipate energy when subjected to external forces, further improving the strength and toughness of the gel temporary plugging agent;

[0024] (2) The raw materials for preparing the temporary gel plugging agent of the present invention include an unstable crosslinking agent, which contains an ester group. When the temporary gel plugging agent is heated, the ester group is hydrolyzed and the gel is automatically broken and degraded, without the need for additional gel breaking means. The present invention can control the degradation time of the temporary gel plugging agent by adjusting the amount of the unstable crosslinking agent in the system;

[0025] (3) The initiator used in the gel temporary plugging agent system of the present invention is a high-temperature initiator, which requires a higher temperature condition to have a stronger initiation activity, thereby making the system applicable to high-temperature reservoirs;

[0026] (4) The gel temporary plugging agent forms a gel structure with high-temperature thixotropy under the mixed action of polyvinyl alcohol, starch, propylene glycol and calcium chloride, so that it can reduce filtration loss while ensuring effective migration;

[0027] (5) After the physical cross-linking of polyvinyl alcohol and starch is introduced into the gel temporary plugging agent, more hydrogen bond donors can be provided for the gel temporary plugging agent, thereby enhancing the adhesion strength between the gel and the rock interface and effectively improving the plugging performance of the gel. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Microscopic morphology of gel temporary plugging agent under different cross-linking conditions.

[0029] (a) Pure chemical cross-linked gel (b) Physicochemical interpenetrating cross-linked gel

[0030] Figure 2 This is a graph showing the change in elastic modulus of chemically cross-linked gel and physicochemically interpenetrating cross-linked gel. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0032] 1. Preparation of in-situ cross-linking-self-degradable high-strength gel temporary plugging agent

[0033] Example 1

[0034] Dissolve 0.33g of PVA in 50g of water, weigh 0.67g of corn amylopectin and disperse it in the PVA solution. Add 0.3g of glycerol and 0.02g of CaCl2, mix thoroughly at 200r / min at room temperature, and then pre-gelatinize at 90°C for 30min to obtain a pre-gelatinized solution. Add 5.6g of AM and 2.4g of NVP to 20g of water, followed by 0.2g of a cross-linking agent and 0.004g of an initiator to obtain a base solution. Add the base solution to the pre-gelatinized solution, add water to 100g, and mix thoroughly at 200r / min at room temperature to prepare an in-situ cross-linking, self-degradable, high-strength gel temporary plugging agent reaction solution.

[0035] The reaction solution was placed in a simulated reservoir environment at 100°C for 90 minutes to form a gel. After gelation, the gel temporary plugging agent was further placed in a simulated reservoir environment at 100°C, and it completely degraded into a low-viscosity liquid within 5 hours.

[0036] Example 2

[0037] Dissolve 1.00g of PVA in 50g of water, weigh 2.00g of corn amylopectin and disperse it in the PVA solution. Add 0.9g of glycerol and 0.06g of CaCl2, mix thoroughly at 200r / min at room temperature, and then pre-gelatinize at 90°C for 30min to obtain a pre-gelatinized solution. Add 5.6g of AM and 2.4g of NVP to 20g of water, followed by 0.2g of a cross-linking agent and 0.004g of an initiator to obtain a base solution. Add the base solution to the pre-gelatinized solution, add water to 100g, and mix thoroughly at 200r / min at room temperature to prepare an in-situ cross-linking and self-degrading high-strength gel temporary plugging agent reaction solution.

[0038] The reaction solution was placed in a simulated reservoir environment at 100°C for 52 minutes to form a gel. Further placed in a simulated reservoir environment at 100°C, the gel temporary plugging agent completely degraded into liquid within 11 hours.

[0039] Example 3

[0040] Dissolve 1.67g of PVA in 50g of water, weigh 3.33g of corn amylopectin and disperse it in the PVA solution. Add 1.5g of glycerol and 0.1g of CaCl2, mix thoroughly at 200r / min at room temperature, and then pre-gelatinize at 90°C for 30min to obtain a pre-gelatinized solution. Add 5.6g of AM and 2.4g of NVP to 20g of water, followed by 0.25g of a cross-linking agent and 0.002g of an initiator to obtain a base solution. Add the base solution to the pre-gelatinized solution, add water to 100g, and mix thoroughly at 200r / min at room temperature to prepare an in-situ cross-linking, self-degradable, high-strength gel temporary plugging agent reaction solution.

[0041] The reaction solution was placed in a simulated reservoir environment at 120°C for 23 minutes to form a gel. The gel temporary plugging agent was then completely degraded into liquid within 6 hours after being placed in a simulated reservoir environment at 120°C.

[0042] Example 4

[0043] Dissolve 2.33g of PVA in 50g of water, weigh 4.67g of corn amylopectin and disperse it in the PVA solution. Add 2.1g of glycerol and 0.14g of CaCl2, mix thoroughly at 200r / min at room temperature, and then pre-gelatinize at 90°C for 30min to obtain a pre-gelatinized solution. Add 5.6g of AM and 2.4g of NVP to 20g of water, followed by 0.95g of a cross-linking agent and 0.001g of an initiator to obtain a base solution. Add the base solution to the pre-gelatinized solution, add water to 100g, and mix thoroughly at 200r / min at room temperature to prepare an in-situ cross-linking, self-degradable, high-strength gel temporary plugging agent reaction solution.

[0044] The reaction solution was placed in a simulated reservoir environment at 150°C for 20 minutes to form a gel. The gel temporary plugging agent was then completely degraded into liquid within 10 hours after being placed in a simulated reservoir environment at 150°C.

[0045] Example 5

[0046] Dissolve 1.67g of PVA in 50g of water, weigh 3.33g of corn amylopectin and disperse it in the PVA solution. Add 1.5g of glycerol and 0.1g of CaCl2, mix thoroughly at 200r / min at room temperature, and then pre-gelatinize at 90°C for 30min to obtain a pre-gelatinized solution. Add 2.8g of AM and 1.2g of NVP to 20g of water, followed by 0.65g of a cross-linking agent and 0.06g of an initiator to obtain a base solution. Add the base solution to the pre-gelatinized solution, add water to 100g, and mix thoroughly at 200r / min at room temperature to prepare an in-situ cross-linking, self-degradable, high-strength gel temporary plugging agent reaction solution.

[0047] The reaction solution was placed in a simulated reservoir environment at 100°C for 25 minutes to form a gel. Further placed in a simulated reservoir environment at 100°C, the gel temporary plugging agent completely degraded into liquid within 37 hours.

[0048] 2. Performance test of in-situ cross-linking-self-degradable high-strength gel temporary plugging agent

[0049] Comparative Example 1

[0050] 5.6 g of AM and 2.4 g of NVP were added to 50 g of water, and then 0.25 g of a cross-linking agent and 0.002 g of an initiator were added to obtain a base solution, and clean water was added to 100 g to obtain a chemically cross-linked gel reaction solution.

[0051] The chemical cross-linking gel reaction solution of Comparative Example 1 and the physical and chemical interpenetrating cross-linking gel reaction solution of Example 3 were placed in a simulated reservoir environment at 120°C to form gels, and gel temporary plugging agents under different cross-linking conditions were prepared. The microscopic morphology of the gels is shown in FIG. Figure 1 (a) Figure 1 (b).

[0052] Figure 2 It shows that the maximum elastic modulus of pure chemical cross-linked gel is 415.25Pa, while the elastic modulus of physicochemical interpenetrating cross-linked gel can reach up to 2637.8Pa, which is 6.3 times that of pure chemical cross-linked gel.

[0053] Comparative Example 2

[0054] The cross-linking agent polyethylene glycol diacrylate in Example 4 was replaced by N,N-methylenebisacrylamide of equal mass. The other components, amounts and reaction conditions were the same as those in Example 4. The obtained gel temporary plugging agent was not completely degraded into a liquid after aging at 150° C. for 3 days.

[0055] The temporary plugging agent formulas of Examples 1 to 5 were used as research objects, respectively denoted as 1#-5#, to investigate their plugging capabilities. Standard cores were prepared using shale outcrop cores by wire cutting. The plugging performance test method is as follows: first, the shale fracture core was placed in a clamp, the confining pressure was set to 6.0 MPa, the back pressure was 1.0 MPa, and the temperature was set to 120°C. First, 7wt% KCl brine was injected at a rate of 0.5 mL / min, and then the gel temporary plugging agent reaction solution was injected at a rate of 0.5 mL / min. The injection pressure was maintained at 1 MPa, the confining pressure was constant at 6.0 MPa, and the back pressure was 1.0 MPa. After reacting at an ambient temperature of 120°C for 1 hour, the core was removed after depressurization, placed in reverse, and injected with 7wt% KCl brine at a rate of 1 mL / min, with a confining pressure constant at 6.0 MPa and a back pressure of 1.0 MPa. The displacement pressure change trend was recorded, and the sudden drop in pressure was the breakthrough pressure.

[0056]

[0057] The results show that the plugging strength of the gel temporary plugging agent prepared by the present invention is significantly higher than that of the pure chemically cross-linked gel temporary plugging agent. This is attributed to the fact that the shale surface contains a large number of silanol groups, hydroxyl groups and amide groups that form metal coordination bonds and hydrogen bonds. The doping of physical cross-linking provides more hydrogen bond donors for the gel, further enhancing the adhesion strength between the gel and the rock interface and improving the plugging performance of the gel.

[0058] During actual use, the system components can be appropriately adjusted according to the well conditions, well type, formation water quality, etc.

[0059] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with this technical field within the technical scope disclosed by the present invention should be covered by the scope of protection of the present invention.

Claims

1. A method for preparing an in-situ cross-linked and self-degradable high-strength gel temporary plugging agent, comprising the following steps: First, a certain amount of polyvinyl alcohol was weighed and dissolved in an appropriate amount of water. The corn amylopectin was dispersed in the polyvinyl alcohol solution. Glycerol and CaCl2 were added at the same time. After mixing evenly at room temperature, the mixture was placed in a constant temperature environment of 90°C for 30 minutes to obtain a pre-gelatinized solution. Secondly, a certain amount of monomers acrylamide and N-vinyl pyrrolidone are weighed and dissolved in water to prepare a base liquid, and a crosslinking agent and an initiator are added to the base liquid to obtain a monomer solution; Then, the pregelatinized solution and the monomer solution are mixed evenly at room temperature to obtain a reaction solution of the gel temporary plugging agent; Finally, the gel temporary plugging agent reaction solution enters the formation environment with a temperature of 100-150°C, and undergoes in-situ polymerization and cross-linking in the fractures for 20-200 minutes to form a high-strength gel temporary plugging agent. The high-strength gel temporary plugging agent is composed of the following substances in parts by weight: 0.33-2.33 parts of polyvinyl alcohol, 0.67-4.67 parts of corn amylopectin, 2.8-7.0 parts of acrylamide, 1.2-3.0 parts of N-vinyl pyrrolidone, 0.05-1.05 parts of a crosslinking agent, 0.001-0.06 parts of an initiator, 0.3-2.1 parts of glycerol, 0.02-0.14 parts of calcium chloride, and the balance being water. The total weight of the above substances is 100 parts. The crosslinking agent is polyethylene glycol diacrylate.

2. The preparation method according to claim 1, wherein The initiator is tert-butyl perbenzoate.

3. The preparation method according to claim 1, wherein The mass ratio of the acrylamide to N-vinyl pyrrolidone is 7:3.

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