A pH-responsive low-temperature gel temporary plugging agent and its preparation method

Abstract

The present invention discloses a pH-responsive low-temperature gel temporary plugging agent and a preparation method thereof, belonging to the technical field of oil and gas field development. The pH-responsive low-temperature gel temporary plugging agent is composed of the following substances in parts by weight: 2.8-5.7 parts of acrylamide, 1.4-2.6 parts of 2-acrylamido-2-methylpropanesulfonic acid, 0.05-1.04 parts of crosslinking agent, 0.01-0.1 part of initiator, 2-3.33 parts of branched starch, 1-1.67 parts of sodium alginate, 0.27-0.35 part of acetic acid, 2.14-2.61 parts of sodium acetate, 0.9-1.5 parts of glycerol, and the balance is water, and the sum of the parts by weight of the above components is 100 parts; the crosslinking agent is synthesized by dihydrazide and diacetone acrylamide. The gel temporary plugging agent is simple to prepare, the raw materials are cheap and easy to obtain, and the production process is controllable. Using it in the temporary plugging and diversion fracturing process of low-temperature tight reservoirs is of great significance for realizing the efficient development of oil and gas fields.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil and gas field development, and particularly relates to a pH-responsive low-temperature gel temporary plugging agent and a preparation method thereof. Background Art

[0002] Tight oil and gas resources and shale oil and gas resources are the main components of unconventional energy. However, due to their strong heterogeneity and small oil and gas seepage channels, the temporary plugging and diversion fracturing technology is widely used in the exploitation of tight oil and gas to increase the fracture network density in undeveloped areas and improve oil and gas production. The temporary plugging and diversion technology seals high-water-bearing fractures to avoid energy loss during the fracturing process, resulting in ineffective fracturing, and forces the fracturing fluid to turn and create new fractures longitudinally and horizontally, enhancing the fracture network density and improving oil and gas recovery. Currently, common fracturing diversion materials include solid-phase temporary plugging agents and liquid-phase temporary plugging agents. Solid-phase temporary plugging agents such as fibers and particles bridge and block at the fracture pore throats during the injection process with the fracturing fluid and degrade in high-temperature and high-salt environments. Degradable fibers and particles have high plugging strength and wide temperature adaptability, but the lack of obtaining real reservoir fracture information limits their plugging performance. At the same time, solid-phase temporary plugging materials cannot effectively plug distal fractures and micro-fractures, and are restricted in the process of constructing complex fracture networks in tight reservoir temporary plugging and diversion fracturing. Liquid-phase temporary plugging agents are injected into the water-producing fractures in a slug manner and crosslink and degrade in situ in the fractures. However, currently, the self-degrading temporary plugging agents generally have low strength and require a high-temperature environment to achieve effective degradation, making it difficult to apply to the temporary plugging and diversion fracturing technology for low-temperature tight oil reservoirs.

[0003] The gel network structure crosslinked by acylhydrazone is widely used in human drug release. In a low-temperature environment of 30-60 °C and a weak acid and weak base environment, acylhydrazone undergoes a reverse reaction to form acylhydrazide and ketone / aldehyde, realizing the reverse degradation of the gel network structure, and this process is usually completed within 5-6 days. The present invention synthesizes an acylhydrazone crosslinking agent through diacylhydrazide and diacetone acrylamide, and in situ gels after mixing with acrylamide and 2-acrylamido-2-methylpropanesulfonic acid, constructing a gel with low-temperature in-situ polymerization-self degradation, which fully meets the application requirements of the temporary plugging agent in the temporary plugging and diversion fracturing technology for low-temperature oil reservoirs. Summary of the Invention

[0004] The purpose of the present invention is to provide a pH-responsive low-temperature gel temporary plugging agent, which can in-situ polymerize-degrade at 30-90 °C, has the advantages of strong toughness, low-temperature self-degradation, wide applicable temperature, excellent wall adhesion performance and low price, is suitable for workover, diversion fracturing and refracturing in shallow wells, and has broad market application prospects.

[0005] Another object of the present invention is to provide a preparation method of the pH-responsive low-temperature gel temporary plugging agent. The method has a reliable principle, is easy to operate, the raw materials are cheap and easy to obtain, the preparation process is controllable, and it shows great prospects for industrial promotion.

[0006] Another object of the present invention is also to provide the application of the pH-responsive low-temperature gel temporary plugging agent in the reservoir temporary plugging and diversion process. It can realize the self-degradation of the liquid plug under the constant temperature condition of the formation without temperature change, can meet the technical requirements of the temporary plugging and diversion fracturing technology for low-temperature tight oil reservoirs, and is of great significance for realizing the efficient development of oilfields and improving oil and gas recovery.

[0007] To achieve the above technical objectives, the present invention adopts the following technical solutions.

[0008] A pH-responsive low-temperature gel temporary plugging agent is composed of the following substances in parts by weight:

[0009] 2.8 - 5.7 parts of acrylamide, 1.4 - 2.6 parts of 2-acrylamido-2-methylpropanesulfonic acid, 0.05 - 1.04 parts of crosslinking agent, 0.01 - 0.1 parts of initiator, 2 - 3.33 parts of branched starch, 1 - 1.67 parts of sodium alginate, 0.27 - 0.35 parts of acetic acid, 2.14 - 2.61 parts of sodium acetate, 0.9 - 1.5 parts of glycerol, and the balance is water. The sum of the parts by weight of the above components is 100 parts.

[0010] Furthermore, the crosslinking agent is synthesized from dihydrazide and diacetone acrylamide.

[0011] The crosslinking agent is prepared by the following steps:

[0012] Weigh the corresponding masses of dihydrazide and diacetone acrylamide according to the molar ratio of dihydrazide:dihydroxyacetone acrylamide of 1:2, dissolve them in a water-ethanol mixture, the mass of the water-ethanol mixture is 10 times the total mass of dihydrazide and diacetone acrylamide, the mass ratio of water to ethanol is 1:1, then add appropriate amounts of acetic acid and inhibitor, and react in a N2 environment at 60 °C for 8 h, and then obtain the crosslinking agent through cooling, crystallization, filtration and recrystallization.

[0013] The synthesis mechanism of the crosslinking agent is as follows:

[0014]

[0015] Furthermore, the dihydrazide is oxalic acid dihydrazide or succinic acid dihydrazide.

[0016] Furthermore, the inhibitor is inhibitor 701.

[0017] Furthermore, the initiator is ammonium persulfate.

[0018] Furthermore, the amylopectin is corn amylopectin.

[0019] The preparation method of the pH-responsive low-temperature gel temporary plugging agent comprises the following steps:

[0020] First, weigh a certain amount of sodium alginate and dissolve it in an appropriate amount of water. Disperse the amylopectin in the sodium alginate solution, and at the same time add glycerol. After mixing evenly at room temperature, place it in a constant temperature environment of 90 °C for 30 min to obtain a pre-gelatinized solution;

[0021] Second, weigh a certain amount of monomer acrylamide and 2-acrylamido-2-methylpropanesulfonic acid and dissolve them in water to prepare a base solution. Add a crosslinking agent, an initiator, acetic acid and sodium acetate to the base solution to obtain a monomer solution;

[0022] Finally, mix the pre-gelatinized solution and the monomer solution evenly at room temperature to obtain a reaction solution of the low-temperature gel temporary plugging agent.

[0023] The application of the pH-responsive low-temperature gel temporary plugging agent in the reservoir temporary plugging and diversion process is as follows: First, inject a preflush fluid, then inject the prepared gel temporary plugging agent reaction solution according to the requirements of the oil and gas reservoir, and finally inject a displacement fluid; when the gel temporary plugging agent reaction solution enters a formation environment with a reservoir temperature of 30-90 °C, in-situ polymerization and crosslinking occur in the fracture for 20-200 min. After forming a high-strength gel temporary plugging agent, the 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 after 6-168 h in an environment of 30-90 °C and returns to the ground with the fracturing fluid.

[0024] The present invention forms a gel temporary plugging agent with a physical and chemical interpenetrating structure through a physical crosslinking network constructed by sodium alginate and starch and a chemical crosslinking network constructed by acrylamide and 2-acrylamido-2-methylpropanesulfonic acid. At the beginning, the starch and sodium alginate are mixed to form an initial physical crosslinking network (pre-gelatinized solution). Then, under the formation environment conditions, acrylamide and 2-acrylamido-2-methylpropanesulfonic acid react in the physical network formed by starch and sodium alginate to form a chemical crosslinking network. The chemical crosslinking network grows and penetrates each other in the starch network, and finally a high-strength crosslinking network with a uniform physical and chemical structure interpenetration can be obtained.

[0025] The glycerol is mainly used to provide plasticization for physical crosslinking, so as to ensure the fluidity of physical crosslinking.

[0026] The acetic acid and sodium acetate mainly change the pH environment of the gel. When the addition amount of acetic acid is 0.27-0.35% and the addition amount of sodium acetate is 2.14-2.61%, the pH environment of the prepared gel is 4.5-6.5. In an environment of 30-90 °C, the acylhydrazone bond will break, realizing the pH response.

[0027] The formation process of physical and chemical interpenetrating network is as follows:

[0028]

[0029] The self-degradation mechanism of the pH-responsive low-temperature gel temporary plugging agent is as follows:

[0030]

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] (1) The gel temporary plugging agent of the present invention uses acrylamide, 2-acrylamido-2-methylpropane sulfonic acid, sodium alginate and starch as reaction materials, and adds a chemical cross-linking agent. The obtained physicochemical interpenetrating cross-linked gel temporary plugging agent can effectively dissipate energy when subjected to external forces, thereby further improving the strength and toughness of the gel temporary plugging agent;

[0033] (2) The raw materials for preparing the temporary gel plugging agent of the present invention include an unstable cross-linking agent, which is prepared under weakly acidic conditions using dihydrazide and diacetone acrylamide as reaction materials. The unstable cross-linking agent contains an acylhydrazone bond. The temporary gel plugging agent automatically breaks and degrades under certain pH conditions and thermal environments, and no additional breaking means are required. The present invention can control the degradation time of the temporary gel plugging agent by adjusting the amount of the unstable cross-linking agent in the system and the amount of acetic acid and sodium acetate;

[0034] (3) The gel temporary plugging agent forms a thixotropic gel structure under the mixed action of sodium alginate, starch and glycerol, so that it can reduce filtration loss while ensuring effective migration;

[0035] (4) After the physical cross-linking of sodium alginate 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

[0036] Figure 1 Microscopic morphology of the gel temporary plugging agent under different cross-linking conditions.

[0037] (a) Pure chemical cross-linked gel; (b) Physical and chemical interpenetrating gel DETAILED DESCRIPTION

[0038] In order to make the purpose, technical solution 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 used to limit the present invention.

[0039] Preparation of pH-responsive low-temperature gel temporary plugging agent

[0040] To avoid redundancy, in the following embodiments of the present invention, the crosslinking agent is obtained through the following process:

[0041] Weigh the corresponding masses of dihydrazide and diacetone acrylamide according to the molar ratio of dihydrazide: diacetone acrylamide of 1:2, dissolve them in a water-ethanol mixture, the mass of the water-ethanol mixture is 10 times the total mass of the former two, the mass ratio of water to ethanol is 1:1, then add appropriate amounts of acetic acid and 701 inhibitor, and react in a N2 environment at 60 °C for 8 h. Subsequently, the crosslinking agent is obtained through cooling, crystallization, filtration, and recrystallization.

[0042] Example 1

[0043] Dissolve 1 g of sodium alginate in 50 g of water, weigh 2 g of corn branched starch and disperse it in the sodium alginate solution. At the same time, add 0.9 g of glycerol, mix well at 200 r / min at room temperature, and then place it in a constant temperature environment at 90 °C for pregelatinization for 30 min to obtain a pregelatinized solution. Prepare a base solution by dissolving 4.5 g of acrylamide and 2.1 g of 2-acrylamido-2-methylpropanesulfonic acid in water. Add 0.1 g of crosslinking agent and 0.04 g of initiator to the base solution. After complete dissolution, add 0.31 g of acetic acid and 2.33 g of sodium acetate to obtain a monomer solution. Mix the pregelatinized solution with the monomer solution, add clear water to make up to 100 g, mix well at 200 r / min at room temperature to prepare a reaction solution of a pH-responsive low-temperature gel temporary plugging agent, pH value.

[0044] Place the reaction solution in a simulated reservoir environment at 70 °C for 1.0 h to form a gel. After gel formation, continue to place it in a simulated reservoir environment at 70 °C. The gel temporary plugging agent can be completely degraded into a low-viscosity liquid within 65 h, and the plugging strength of the formed temporary plugging gel is 8.86 MPa / m.

[0045] Example 2

[0046] Dissolve 1.67 g of sodium alginate in 50 g of water, weigh 3.33 g of corn branched starch and disperse it in the sodium alginate solution. At the same time, add 1.5 g of glycerol, mix well at 200 r / min at room temperature, and then place it in a constant temperature environment at 90 °C for pregelatinization for 30 min to obtain a pregelatinized solution. Prepare a base solution by dissolving 4.1 g of acrylamide and 2.6 g of 2-acrylamido-2-methylpropanesulfonic acid in water. Add 0.2 g of crosslinking agent and 0.06 g of initiator to the base solution. After complete dissolution, add 0.35 g of acetic acid and 2.61 g of sodium acetate to obtain a monomer solution. Mix the pregelatinized solution with the monomer solution, add clear water to make up to 100 g, mix well at 200 r / min at room temperature to prepare a reaction solution of a pH-responsive low-temperature gel temporary plugging agent.

[0047] The reaction solution was placed in a simulated reservoir environment at 50 °C and reacted for 1.5 h to form a gel. After gel formation, it was continuously placed in a simulated reservoir environment at 50 °C. The gel temporary plugging agent could be completely degraded into a low-viscosity liquid within 88 h, and the plugging strength of the formed temporary plugging gel was 19.84 MPa / m.

[0048] Example 3

[0049] 1.67 g of sodium alginate was dissolved in 50 g of water. 3.33 g of corn amylopectin was weighed and dispersed in the sodium alginate solution. At the same time, 1.5 g of glycerol was added. After mixing evenly at 200 r / min at room temperature, it was placed in a constant temperature environment at 90 °C for pregelatinization for 30 min to obtain a pregelatinized solution. 5.5 g of acrylamide and 1.4 g of 2-acrylamido-2-methylpropanesulfonic acid were dissolved in water to prepare a base solution. 0.3 g of crosslinking agent and 0.1 g of initiator were added to the base solution. After complete dissolution, 0.35 g of acetic acid and 2.61 g of sodium acetate were added to obtain a monomer solution. The pregelatinized solution was mixed with the monomer solution, and water was added to make up to 100 g. After mixing evenly at 200 r / min at room temperature, a reaction solution of pH-responsive low-temperature gel temporary plugging agent was prepared.

[0050] The reaction solution was placed in a simulated reservoir environment at 30 °C and reacted for 3 h to form a gel. After gel formation, it was continuously placed in a simulated reservoir environment at 30 °C. The gel temporary plugging agent could be completely degraded into a low-viscosity liquid within 168 h, and the plugging strength of the formed temporary plugging gel was 18.87 MPa / m.

[0051] Example 4

[0052] 1.67 g of sodium alginate was dissolved in 50 g of water. 3.33 g of corn amylopectin was weighed and dispersed in the sodium alginate solution. At the same time, 1.5 g of glycerol was added. After mixing evenly at 200 r / min at room temperature, it was placed in a constant temperature environment at 90 °C for pregelatinization for 30 min to obtain a pregelatinized solution. 2.8 g of acrylamide and 1.4 g of 2-acrylamido-2-methylpropanesulfonic acid were dissolved in water to prepare a base solution. 1.04 g of crosslinking agent and 0.06 g of initiator were added to the base solution. After complete dissolution, 0.31 g of acetic acid and 2.52 g of sodium acetate were added to obtain a monomer solution. The pregelatinized solution was mixed with the monomer solution, and water was added to make up to 100 g. After mixing evenly at 200 r / min at room temperature, a reaction solution of pH-responsive low-temperature gel temporary plugging agent was prepared.

[0053] The reaction solution was placed in a simulated reservoir environment at 50 °C and reacted for 2 h to form a gel. After gel formation, it was continuously placed in a simulated reservoir environment at 50 °C. The gel temporary plugging agent could be completely degraded into a low-viscosity liquid within 124 h, and the plugging strength of the formed temporary plugging gel was 14.26 MPa / m.

[0054] Example 5

[0055] Dissolve 1.67 g of sodium alginate in 50 g of water. Weigh 3.33 g of corn amylopectin and disperse it in the sodium alginate solution. At the same time, add 1.5 g of glycerol. Mix well at 200 r / min at room temperature and then place it in a constant temperature environment of 90 °C for pregelatinization for 30 min to obtain a pregelatinized solution. Prepare a base solution by dissolving 4.3 g of acrylamide and 1.8 g of 2-acrylamido-2-methylpropanesulfonic acid in water. Add 0.3 g of crosslinking agent and 0.01 g of initiator to the base solution. After complete dissolution, add 0.31 g of acetic acid and 2.52 g of sodium acetate to obtain a monomer solution. Mix the pregelatinized solution and the monomer solution, add clear water to make up to 100 g, and mix well at 200 r / min at room temperature to prepare a reaction solution of pH-responsive low-temperature gel temporary plugging agent.

[0056] Place the reaction solution in a simulated reservoir environment at 90 °C for 0.5 h to form a gel. After gel formation, continue to place it in a simulated reservoir environment at 90 °C. The gel temporary plugging agent can be completely degraded into a low-viscosity liquid within 6 h, and the plugging strength of the formed temporary plugging gel is 15.54 MPa / m.

[0057] II. Performance Test of pH-Responsive Low-Temperature Gel Temporary Plugging Agent

[0058] The microscopic morphology diagrams of the gel temporary plugging agent under different crosslinking conditions are shown in Figure 1 .

[0059] Figure 1 (a) is a chemical crosslinking network constructed by acrylamide free radical initiation, Figure 1 (b) is a temporary plugging gel prepared by modifying the chemical crosslinking network with a physical crosslinking network constructed by sodium alginate / amylopectin. It can be seen from the figure that the physical crosslinking network constructed by amylopectin / sodium alginate fills the void structure of the chemical crosslinking gel network, improving the mechanical properties of the temporary plugging gel.

[0060] Comparative Example 1

[0061] Replace the crosslinking agent in Example 3 with polyethylene glycol diacrylate, and keep other components, dosages and reaction conditions the same as those in Example 3. The gelation time is 1.5 h, the plugging strength of the formed gel temporary plugging agent is 18.92 MPa / m, and no degradation phenomenon is found after the gel temporary plugging agent is placed at 50 °C for 10 d.

[0062] Comparative Example 2

[0063] Replace the corn amylopectin in Example 3 with the same mass of corn amylose, and keep other components, dosages and reaction conditions the same as those in Example 3. The curing time is 1.5 h, and the plugging strength of the formed gel temporary plugging agent is 4.65 MPa / m.

[0064] Comparative Example 3

[0065] Replace the preparation process of the pre-gelatinized solution in Example 5 with dissolving 0.33 g of sodium alginate in 50 g of water, weighing 0.67 g of corn amylopectin and dispersing it in the sodium alginate solution, adding 0.3 g of glycerol at the same time, mixing at 200 r / min at room temperature and then placing it in a constant temperature environment of 90 °C for pre-gelatinization for 30 min to obtain the pre-gelatinized solution. Other components, dosages and reaction conditions are the same as those in Example 5. The gelation time is 0.5 h, the plugging strength of the formed gel temporary plugging agent is 3.42 MPa / m, and after gelation, it is continuously placed in a simulated reservoir environment at 90 °C, and the gel temporary plugging agent can be completely degraded into a low-viscosity liquid within 6 h.

[0066] Plugging performance test:

[0067] Taking the plugging agent formulas of Examples 1 to 5 as the research objects, denoted as 1# to 5# respectively, investigate the temporary plugging ability of the studied pH-responsive low-temperature temporary plugging gel for fractures. The experimental steps are as follows: Using 1-5 mm steel cores as the research objects, injecting the pH-responsive low-temperature temporary plugging gel into the steel cores at a displacement rate of 1 mL / min, curing to form a plugging at 30-70 °C, injecting water for displacement at a rate of 1 mL / min, and after the pressure is stable, obtaining the breakthrough pressure gradient after temporary plugging, degrading at a constant temperature of 30-90 °C, and investigating the degradation performance of the temporary plugging gel. The experimental results show that the pH-responsive low-temperature temporary plugging gel has excellent temporary plugging performance in a low-temperature environment.

[0068] Table 1 Temporary plugging performance of temporary plugging agents with different systems

[0069]

[0070] The results show that: The pH-responsive low-temperature temporary plugging gel has excellent plugging performance for cores with different fracture widths, and at the same time, the degradation time can be adjusted by adjusting the ratio of each component in the low-temperature temporary plugging gel to adapt to different reservoir temperatures.

[0071] The gel temporary plugging agent described in the present invention is a liquid homogeneous system, with a low initial viscosity, no particles in the temporary plugging agent system, excellent penetration performance, and can enter tiny cracks for plugging.

[0072] In the actual use process, the system components can be appropriately adjusted according to well conditions, well types, formation water quality conditions, etc.

[0073] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A pH-responsive low-temperature gel temporary plugging agent, which is composed of the following substances in parts by weight: 2.8 - 5.7 parts of acrylamide, 1.4 - 2.6 parts of 2-acrylamido-2-methylpropanesulfonic acid, 0.05 - 1.04 parts of cross-linking agent, 0.01 - 0.1 part of initiator, 2 - 3.33 parts of branched starch, 1 - 1.67 parts of sodium alginate, 0.27 - 0.35 part of acetic acid, 2.14 - 2.61 parts of sodium acetate, 0.9 - 1.5 parts of glycerol, and the balance is water. The sum of the parts by weight of the above components is 100 parts; the cross-linking agent is synthesized from dihydrazide and diacetone acrylamide, and the dihydrazide is oxalic acid dihydrazide or succinic acid dihydrazide.

2. The pH-responsive low-temperature gel temporary plugging agent according to claim 1, wherein The cross-linking agent is prepared by the following steps: Weigh the corresponding masses of dihydrazide and diacetone acrylamide according to the molar ratio of dihydrazide:diacetone acrylamide of 1:2, dissolve them in a water-ethanol mixture, the mass of the water-ethanol mixture is 10 times the total mass of dihydrazide and diacetone acrylamide, the mass ratio of water to ethanol is 1:1, then add appropriate amounts of acetic acid and inhibitor, and react in an N2 environment at 60 °C for 8 h, and then obtain the cross-linking agent through cooling, crystallization, filtration and recrystallization.

3. The pH-responsive low-temperature gel temporary plugging agent according to claim 2, wherein, The inhibitor is inhibitor 701.

4. The pH-responsive low-temperature gel temporary plugging agent according to claim 1, wherein The initiator is ammonium persulfate.

5. The pH-responsive low-temperature gel temporary plugging agent according to claim 1, characterized in that The branched starch is corn branched starch.

6. The preparation method of the pH-responsive low-temperature gel temporary plugging agent according to claim 1, 2, 4 or 5, comprising the following steps: First, weigh a certain amount of sodium alginate and dissolve it in water, disperse the branched starch in the sodium alginate solution, and at the same time add glycerol. After mixing evenly at room temperature, place it in a constant temperature environment at 90 °C for 30 min to obtain a pre-gelatinized solution; Second, weigh a certain amount of monomer acrylamide and 2-acrylamido-2-methylpropanesulfonic acid and dissolve them in water to prepare a base solution, and add a cross-linking agent, an initiator, acetic acid and sodium acetate to the base solution to obtain a monomer solution; Finally, mix the pre-gelatinized solution and the monomer solution evenly at room temperature to obtain a reaction solution of the low-temperature gel temporary plugging agent.

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