A high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells and its use method

Abstract

The present invention discloses a high-efficiency and environmentally friendly low-temperature gel breaker for shallow oil and gas wells and a method of use, and relates to the technical field of chemicals for oil and gas well fracturing. When preparing the high-efficiency and environmentally friendly low-temperature gel breaker for shallow oil and gas wells, the present invention first reacts and polymerizes acrylic acid, acrylamide, sodium lignin sulfonate, polyvinyl alcohol, and a cross-linking agent, and then loads oxalic acid to prepare a slow-release capsule core; glutaraldehyde is reacted with sodium carboxymethyl cellulose, and then added to the slow-release capsule core for impregnation, and dried to obtain a catalytic slow-release capsule; persulfate, sodium thiosulfate, sodium bisulfite formaldehyde solution, a chelating agent, sodium nitrite, ammonium chloride, and a catalytic slow-release capsule are added to pure water and mixed evenly to obtain the high-efficiency and environmentally friendly low-temperature gel breaker for shallow oil and gas wells. The high-efficiency and environmentally friendly low-temperature gel breaker for shallow oil and gas wells prepared by the present invention has the advantages of self-heating, thorough gel breaking, no heavy metal ion pollution, and no impact on fracturing construction.

Description

Technical Field

[0001] The present invention relates to the technical field of chemicals for oil and gas well fracturing, and in particular to a high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells and a method of using the same. Background Art

[0002] Hydraulic fracturing is a commonly used process to increase the production of oil and gas wells. During the fracturing construction process, fracturing fluid needs to be injected into the formation. Its main purposes are: (1) low-loss, low-friction fracturing fluid is squeezed into the formation to form cracks; (2) fracturing fluid with a certain viscosity carries proppant into the cracks to form effective support and thus form a good diversion channel; after the fracturing construction, the fracturing fluid that has entered the well needs to be broken to avoid clogging the cracks and polluting the reservoir.

[0003] Currently, the commonly used fracturing fluid thickeners are mainly guar gum and polyacrylamide. Among them, the polymer fracturing fluid system based on polyacrylamide has strong C=C bond binding force. In low-temperature wells of 10~50℃ represented by coalbed methane and shallow oil and gas wells, it is difficult to break the gel and the cost is high. This has always been a technical problem in oil and gas field development.

[0004] Currently, commonly used breakers include bio-enzyme breakers and peroxide breakers. Bio-enzyme breakers are mainly used for breaking guar gum-based fracturing fluids. At the same time, the activity of bio-enzymes is greatly affected by temperature and pH value, which makes it difficult to scale up and apply them in a targeted manner due to high costs. Peroxides such as persulfate and hydrogen peroxide currently have a relatively good breaking effect above 50°C and are widely used. However, when the temperature is below 50°C, the ambient temperature is insufficient to provide the energy required for persulfate to generate oxygen free radicals, which greatly weakens the breaking ability of the persulfate breaker and fails to achieve the purpose of breaking the gel.

[0005] At temperatures between 10 and 50°C, oxidant-based breakers require the addition of a break activator. This allows the oxidant to release oxygen free radicals at low temperatures, disrupting the polymer backbone and achieving breakage. Currently, according to published patents, low-temperature break activators contain copper salts, iron salts, zinc salts, and silver ions. These activators inhibit the expansion of polyacrylamide molecular chains, resulting in difficulty in increasing the viscosity of the fracturing fluid or rapid viscosity reduction, failing to meet viscosity maintenance requirements during operation. Furthermore, these substances tend to form precipitates and complexes in the formation, seriously contaminating the reservoir. In recent years, major oilfields have restricted the use of these substances in wells. Currently, commonly used reducing agents have low activity at low temperatures, resulting in slow breakage and requiring high dosages. Therefore, a more efficient low-temperature activation system is needed that reduces the amount of persulfate and activator required, improves the breakage rate of fracturing fluids in low-temperature wells, maintains operational viscosity, and avoids heavy metal ions that contaminate the formation, ultimately achieving environmentally friendly, pollution-free, and increased production. Summary of the Invention

[0006] The purpose of the present invention is to provide a high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells and a method of use thereof, so as to solve the problems existing in the prior art.

[0007] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0008] A high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells is prepared by uniformly mixing, by weight, 0.4-0.5 parts of persulfate, 0.075-0.12 parts of sodium thiosulfate, 0.3-0.375 parts of sodium bisulfite formaldehyde solution, 0.003-0.0075 parts of a chelating agent, 8-9 parts of sodium nitrite, 6-7 parts of ammonium chloride, 0.8-1 parts of a catalytic sustained-release capsule, and 40-50 parts of pure water.

[0009] The chelating agent is glucose;

[0010] The catalytic sustained-release capsule is prepared by reacting glutaraldehyde with sodium carboxymethyl cellulose, adding the reacted glutaraldehyde to the sustained-release capsule core, impregnating the core with the reacted glutaraldehyde, and drying the resulting capsule.

[0011] The sustained-release capsule core is prepared by polymerizing acrylic acid, acrylamide, sodium lignin sulfonate, polyvinyl alcohol, and a cross-linking agent, and then loading oxalic acid;

[0012] The cross-linking agent is N,N'-methylenebisacrylamide.

[0013] As an optimization, the persulfate is one of ammonium persulfate, potassium persulfate, and sodium persulfate.

[0014] As an optimization, the sustained-release capsule core is prepared according to the following process: 1-1.2 parts of polyvinyl alcohol and 40-50 parts of pure water are mixed uniformly by mass, stirred at 85-90°C and 300-400 r / min for 50-60 minutes until dissolved, cooled to 50-60°C, 1-1.2 parts of sodium lignin sulfonate and 0.2-0.4 parts of ammonium persulfate are added, and stirring is continued for 30-40 minutes, and 4-6 parts of oxalic acid, 80-90 parts of acrylic acid mixed solution were stirred for 30-40 minutes, 0.04-0.06 parts of N,N'-methylenebisacrylamide were added, the temperature was raised to 70-75°C, and the mixture was stirred at 300-400 r / min for 4-5 hours. The mixture was vacuum dried at 60-70°C for 18-20 hours, soaked and washed with ethanol for 3-4 times, and vacuum dried at 60-70°C for 5-6 hours. The mixture was crushed and passed through a 40-50 mesh sieve to obtain a sustained-release capsule core.

[0015] As an optimization, the acrylic acid mixed solution is prepared by uniformly mixing 20 to 30 parts of acrylic acid and 50 to 60 parts of 3 mol / L sodium hydroxide solution by mass, then adding 2 to 3 parts of acrylamide, and stirring at 200 to 300 r / min at room temperature until completely dissolved.

[0016] As an optimization, the catalytic sustained-release capsules are prepared according to the following process: by mass, 2 to 3 parts of 1 wt% glutaraldehyde aqueous solution are added to 50 to 60 parts of 3 wt% sodium carboxymethyl cellulose aqueous solution in a water bath at 50 to 60°C and 50 to 100 r / min at a rate of 2 to 3 drops per second. After the addition is completed, stirring is continued for 5 to 6 hours, and the mixture is naturally cooled to room temperature. 2 to 3 parts of sustained-release capsule cores are added thereto under stirring conditions of 50 to 100 r / min and immersed for 20 to 30 minutes. The mixture is taken out and vacuum-dried at 60 to 70°C for 16 to 18 hours to obtain catalytic sustained-release capsules.

[0017] As an optimization, the mass fraction of the sodium bisulphite formaldehyde solution is 35%.

[0018] As an optimization, the model of the polyvinyl alcohol is PVA1788.

[0019] As an optimization, the model of the sodium lignin sulfonate is sodium lignin sulfonate-M03, which was purchased from Shanghai Tingruo Chemical Co., Ltd.

[0020] As an optimization, the model of the sodium carboxymethyl cellulose is FVH9, which was purchased from Shanghai Changguang Enterprise Development Co., Ltd.

[0021] A method for using a high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells in petroleum fracturing fluid. The high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells is added to the fracturing fluid at a mass fraction of 1% to 10% and the operating temperature is 20 to 50°C.

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

[0023] The invention prepares a high-efficiency and environment-friendly low-temperature gel breaker for shallow oil and gas wells. The method comprises the following steps: firstly, acrylic acid, acrylamide, sodium lignin sulfonate, polyvinyl alcohol and a cross-linking agent are reacted and polymerized, and then oxalic acid is loaded to prepare a slow-release capsule core; glutaraldehyde is reacted with sodium carboxymethyl cellulose, and then added to the slow-release capsule core for impregnation therein, and then dried to prepare a catalytic slow-release capsule; and persulfate, sodium thiosulfate, sodium bisulfite formaldehyde solution, a chelating agent, sodium nitrite, ammonium chloride and the catalytic slow-release capsule are added to pure water and uniformly mixed to prepare the high-efficiency and environment-friendly low-temperature gel breaker for shallow oil and gas wells.

[0024] First, acrylic acid, acrylamide, sodium lignin sulfonate, polyvinyl alcohol, and a cross-linking agent are reacted and polymerized, and then oxalic acid is loaded to prepare a sustained-release capsule core. Acrylic acid and acrylamide undergo free radical reaction through double bonds. Sodium lignin sulfonate forms a large free radical under the action of the initiator ammonium persulfate, and undergoes a grafting reaction with the polymer chain segments formed by acrylic acid and acrylamide. N,N'-methylenebisacrylamide plays the role of a cross-linking agent to form a three-dimensional network structure. At the same time, the added polyvinyl alcohol molecular chains are interspersed in the three-dimensional grid to form an interpenetrating network structure. Oxalic acid is loaded synchronously during the reaction process. Oxalic acid is loaded in large quantities by forming hydrogen bonds with carboxyl, hydroxyl, amide and other groups on the molecular chain. The prepared sustained-release capsule core has a large number of hydrophilic groups such as carboxyl, sulfonic acid, hydroxyl, and amide. The invention has excellent water absorption and swelling performance. During use, sufficient water absorption can make the sustained-release capsule core fully expand, thereby opening the internal channel to release the oxalic acid molecules loaded inside. After glutaraldehyde reacts with sodium carboxymethyl cellulose, it is added to the sustained-release capsule core and impregnated therein. After drying, a catalytic sustained-release capsule is obtained. Glutaraldehyde can react with sodium carboxymethyl cellulose to form a three-dimensional network structure, providing better mechanical properties. The prepared sustained-release capsule core is added to the cross-linked sodium carboxymethyl cellulose solution and fully immersed. A layer of carboxymethyl cellulose protective shell layer can be coated on the surface of the sustained-release capsule core. This protective layer can protect the internal capsule core from damage during storage and transportation, and at the same time prevent the capsule core from absorbing water and swelling too quickly when used in fracturing fluid, thereby achieving the effect of controlling the delayed release time of oxalic acid.

[0025] Finally, persulfate, sodium thiosulfate, sodium bisulfite formaldehyde solution, chelating agent, sodium nitrite, ammonium chloride, and catalytic sustained-release capsules are added to pure water and mixed evenly to prepare a high-efficiency and environmentally friendly low-temperature gel breaker for shallow oil and gas wells. Sodium thiosulfate and sodium bisulfite formaldehyde solution have good reducing properties under low temperature conditions. The synergistic effect of the two can activate the decomposition of persulfate to produce free radicals through the redox catalytic effect within the time required for construction. This process is completed within about 4 to 5 hours. The chelating agent glucose used can chelate the residual metals in the sodium bisulfite formaldehyde solution. ions to avoid affecting the reaction process, and can also serve as a reducing agent to assist in activating persulfate; in addition, sodium nitrite and ammonium chloride can undergo a self-heating reaction under acidic conditions, and the catalytic slow-release capsule is loaded with an acidic catalyst oxalic acid. When it is officially used, the catalytic slow-release capsule absorbs water and begins to expand. At the same time, due to the existence of the outer shell, it cannot fully expand at the first time, avoiding the rapid release of oxalic acid, which leads to a rapid viscosity reduction of the fracturing fluid and the failure of the fracturing construction. The subsequent slow-release capsule core expands after a period of sufficient water absorption, causing the outer sodium carboxymethyl cellulose layer to rupture, thereby The oxalic acid inside begins to be released. After being released, the oxalic acid begins to catalyze the self-heating system of sodium nitrite and ammonium chloride. In the formation, this self-heating system can rapidly heat the fracturing fluid to above 60°C. At this time, the added persulfate oxidant has been partially activated under the reduction catalysis of sodium thiosulfate, sodium bisulfite and formaldehyde. After the temperature rises, this activation system is rapidly accelerated. The persulfate releases a large number of free radicals and begins to attack the polymer chain segments of the thickener in the fracturing fluid, breaking them into small molecules to reduce the viscosity to facilitate flowback and form a slow-release capsule core. The polymer network will also be oxidized and degraded into small molecules under the action of persulfate, and then flowed back for recovery, avoiding staying in the formation and causing damage to the formation. In the process of sodium carboxymethyl cellulose shell rupture and slow-release capsule core releasing oxalic acid, the partially activated persulfate will also play its strong oxidizing role to accelerate this process. The two can cooperate with each other, so that both effects can be quickly exerted in a short time. At the same time, after the self-heating system heats up the formation, it can effectively reduce the viscosity of the formation oil storage, further improve the production capacity of subsequent oil fields, and achieve the effect of increased production. DETAILED DESCRIPTION

[0026] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0027] In all the following examples and comparative examples, ammonium persulfate was used as the persulfate.

[0028] Example 1: A method for preparing a high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells, the method comprising the following steps:

[0029] (1) By weight, 20 parts of acrylic acid and 50 parts of 3 mol / L sodium hydroxide solution were mixed evenly, and then 2 parts of acrylamide were added. The mixture was stirred at 200 r / min at room temperature until the mixture was completely dissolved to prepare an acrylic acid mixed solution.

[0030] (2) By weight, 1 part of polyvinyl alcohol and 40 parts of pure water were mixed evenly, stirred at 85°C and 300 r / min for 60 min until dissolved, cooled to 50°C, added with 1 part of sodium lignin sulfonate and 0.2 part of ammonium persulfate, and continued to stir for 40 min, added with 4 parts of oxalic acid and 80 parts of acrylic acid mixed solution, and continued to stir for 40 min, added with 0.04 part of N,N'-methylenebisacrylamide, heated to 70°C, stirred at 300 r / min for 5 h, vacuum dried at 60°C for 20 h, soaked and washed with ethanol three times, vacuum dried at 60°C for 6 h, crushed and passed through a 40-mesh sieve to obtain a sustained-release capsule core;

[0031] (3) Add 2 parts of 1 wt% glutaraldehyde aqueous solution to 50 parts of 3 wt% sodium carboxymethyl cellulose aqueous solution at a rate of 2 drops per second in a 50°C water bath at 50 r / min. Continue stirring for 6 hours after the addition is complete. Cool naturally to room temperature. Add 2 parts of sustained-release capsule cores to the solution under stirring at 50 r / min and soak for 20 minutes. Take out and vacuum dry at 60°C for 18 hours to prepare catalytic sustained-release capsules.

[0032] (4) By weight, 0.4 parts of ammonium persulfate, 0.075 parts of sodium thiosulfate, 0.3 parts of sodium bisulfite formaldehyde solution, 0.003 parts of chelating agent, 8 parts of sodium nitrite, 6 parts of ammonium chloride, 0.8 parts of catalytic sustained-release capsules, and 40 parts of pure water were mixed evenly to prepare a high-efficiency and environmentally friendly low-temperature gel breaker for shallow oil and gas wells.

[0033] Example 2: A method for preparing a high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells, the method comprising the following steps:

[0034] (1) By weight, 25 parts of acrylic acid and 55 parts of 3 mol / L sodium hydroxide solution were mixed evenly, and then 2.5 parts of acrylamide were added. The mixture was stirred at 250 r / min at room temperature until it was completely dissolved to prepare an acrylic acid mixed solution;

[0035] (2) By weight, 1.1 parts of polyvinyl alcohol and 45 parts of pure water were mixed evenly, stirred at 85°C and 350 r / min for 55 minutes until dissolved, cooled to 55°C, 1.1 parts of sodium lignin sulfonate and 0.3 parts of ammonium persulfate were added, and stirring was continued for 35 minutes. A mixed solution of 5 parts of oxalic acid and 85 parts of acrylic acid was added, and stirring was continued for 35 minutes. 0.05 parts of N,N'-methylenebisacrylamide was added, and the temperature was raised to 70°C, and the mixture was stirred at 350 r / min for 4.5 hours. The mixture was vacuum dried at 65°C for 19 hours, soaked and washed with ethanol 4 times, and vacuum dried at 65°C for 5.5 hours. The mixture was crushed and passed through a 45-mesh sieve to obtain a sustained-release capsule core.

[0036] (3) By weight, 2.5 parts of 1 wt% glutaraldehyde aqueous solution were added to 55 parts of 3 wt% sodium carboxymethyl cellulose aqueous solution at a rate of 3 drops per second in a 55°C water bath at 75 r / min. After the addition was complete, stirring was continued for 5.5 hours. The mixture was naturally cooled to room temperature. 2.5 parts of sustained-release capsule cores were added thereto under stirring conditions of 75 r / min and immersed for 25 minutes. The mixture was taken out and vacuum-dried at 65°C for 17 hours to prepare catalytic sustained-release capsules.

[0037] (4) By weight, 0.45 parts of ammonium persulfate, 0.096 parts of sodium thiosulfate, 0.338 parts of sodium bisulfite formaldehyde solution, 0.0052 parts of chelating agent, 8.5 parts of sodium nitrite, 6.5 parts of ammonium chloride, 0.9 parts of catalytic sustained-release capsules, and 45 parts of pure water were mixed evenly to prepare a high-efficiency and environmentally friendly low-temperature gel breaker for shallow oil and gas wells.

[0038] Example 3: A method for preparing a high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells, the method comprising the following steps:

[0039] (1) By weight, 30 parts of acrylic acid and 60 parts of 3 mol / L sodium hydroxide solution were mixed evenly, and then 3 parts of acrylamide were added. The mixture was stirred at 300 r / min at room temperature until it was completely dissolved to prepare an acrylic acid mixed solution;

[0040] (2) By weight, 1.2 parts of polyvinyl alcohol and 50 parts of pure water were mixed evenly, stirred at 90°C and 400 r / min for 50 minutes until dissolved, cooled to 60°C, 1.2 parts of sodium lignin sulfonate and 0.4 parts of ammonium persulfate were added, and stirring was continued for 30 minutes. A mixed solution of 6 parts of oxalic acid and 90 parts of acrylic acid was added, and stirring was continued for 30 minutes. 0.06 parts of N,N'-methylenebisacrylamide was added, and the temperature was raised to 75°C and stirred at 400 r / min for 4 hours. The mixture was vacuum dried at 70°C for 18 hours, soaked and washed with ethanol 4 times, and vacuum dried at 70°C for 5 hours. After crushing, the mixture was passed through a 50-mesh sieve to obtain a sustained-release capsule core.

[0041] (3) Add 3 parts of 1 wt% glutaraldehyde aqueous solution to 60 parts of 3 wt% sodium carboxymethyl cellulose aqueous solution at a rate of 3 drops per second in a 60°C water bath at 100 r / min. After the addition is complete, continue stirring for 5 hours, cool naturally to room temperature, add 3 parts of sustained-release capsule cores to the solution and soak for 30 minutes at 100 r / min. Take out and vacuum dry at 70°C for 16 hours to prepare catalytic sustained-release capsules.

[0042] (4) By weight, 0.5 parts of ammonium persulfate, 0.12 parts of sodium thiosulfate, 0.375 parts of sodium bisulfite formaldehyde solution, 0.0075 parts of chelating agent, 9 parts of sodium nitrite, 7 parts of ammonium chloride, 1 part of catalytic sustained-release capsules, and 50 parts of pure water were mixed evenly to prepare an efficient and environmentally friendly low-temperature gel breaker for shallow oil and gas wells.

[0043] Comparative Example 1:

[0044] The method for preparing a high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells in Comparative Example 1 differs from that in Example 2 in that step (3) is omitted and step (4) is modified as follows: 0.45 parts of ammonium persulfate, 0.096 parts of sodium thiosulfate, 0.338 parts of sodium bisulfite formaldehyde solution, 0.0052 parts of a chelating agent, 8.5 parts of sodium nitrite, 6.5 parts of ammonium chloride, 0.9 parts of a slow-release capsule core, and 45 parts of pure water are uniformly mixed, by weight, to prepare a high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells. The remaining steps are the same as in Example 2.

[0045] Comparative Example 2:

[0046] The preparation method of the high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells in Comparative Example 2 differs from that in Example 2 in that steps (1), (2), and (3) are not performed, and step (4) is modified as follows: 0.45 parts of ammonium persulfate, 0.096 parts of sodium thiosulfate, 0.338 parts of sodium bisulfite formaldehyde solution, 0.0052 parts of a chelating agent, 8.5 parts of sodium nitrite, 6.5 parts of ammonium chloride, 0.2 parts of oxalic acid, and 45 parts of pure water are uniformly mixed, by weight, to prepare a high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells. The remaining steps are the same as in Example 2.

[0047] Comparative Example 3:

[0048] The preparation method of the high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells in Comparative Example 3 differs from that in Example 2 in that step (4) is modified as follows: 0.45 parts of ammonium persulfate, 0.0052 parts of a chelating agent, 8.5 parts of sodium nitrite, 6.5 parts of ammonium chloride, 0.9 parts of a catalytic sustained-release capsule, and 45 parts of pure water are uniformly mixed, by weight, to prepare a high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells. The remaining steps are the same as in Example 2.

[0049] Comparative Example 4:

[0050] The preparation method of the high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells in Comparative Example 4 differs from that in Example 2 in that step (4) is modified as follows: 0.45 parts of ammonium persulfate, 0.338 parts of sodium bisulfite formaldehyde solution, 0.0052 parts of a chelating agent, 8.5 parts of sodium nitrite, 6.5 parts of ammonium chloride, 0.9 parts of a catalytic sustained-release capsule, and 45 parts of pure water are uniformly mixed, by weight, to prepare a high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells. The remaining steps are the same as in Example 2.

[0051] Comparative Example 5:

[0052] The preparation method of the high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells in Comparative Example 5 differs from that in Example 2 in that step (4) is modified as follows: 0.45 parts of ammonium persulfate, 0.096 parts of sodium thiosulfate, 0.0052 parts of a chelating agent, 8.5 parts of sodium nitrite, 6.5 parts of ammonium chloride, 0.9 parts of a catalytic sustained-release capsule, and 45 parts of pure water are uniformly mixed, by weight, to prepare a high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells. The remaining steps are the same as in Example 2.

[0053] Comparative Example 6:

[0054] The method for preparing a high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells in Comparative Example 6 differs from that in Example 2 in that steps (1), (2), and (3) are omitted, and step (4) is modified as follows: 0.45 parts of ammonium persulfate, 0.096 parts of sodium thiosulfate, 0.338 parts of sodium bisulfite formaldehyde solution, 0.0052 parts of a chelating agent, and 45 parts of pure water are uniformly mixed, by weight, to prepare a high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells. The remaining steps are the same as in Example 2.

[0055] Test Example 1:

[0056] Gel breaking performance test: The prepared high-efficiency and environmentally friendly low-temperature gel breaker for shallow oil and gas wells was tested for its viscosity reducing effect on the fracturing fluid to evaluate its gel breaking effect. The polymer fracturing fluid formula is as follows: by mass fraction, 0.5% anionic polyacrylamide, 5% high-efficiency and environmentally friendly low-temperature gel breaker for shallow oil and gas wells, 0.5% potassium chloride, and the balance is water. 100 ml of fracturing fluid was prepared. The viscosity was measured by a rotational viscometer and was between 18 and 21 mPa·s. The fracturing fluid was placed in a constant temperature water bath at 20°C, and the viscosity was measured at 1 h, 2 h, 4 h, 6 h, and 8 h. Gel breaking was considered complete when the viscosity was ≤5 mPa·s. At the same time, the temperature in the system was measured and recorded. Each group of parallel experiments was repeated 5 times. The viscosity was recorded as the maximum and minimum values, and the temperature was recorded as the average temperature when gel breaking was completed.

[0057] The anionic polyacrylamide used was purchased from Henan Yushuifeng Environmental Protection Technology Co., Ltd.

[0058] The results are shown in Table 1.

[0059] Table 1

[0060]

[0061] From the comparison of the experimental data of Examples 1 to 3 and Comparative Examples 1 to 6 in Table 1, it can be found that the high-efficiency and environmentally friendly low-temperature gel breaker for shallow oil and gas wells prepared by the present invention has the advantages of self-heating, thorough gel breaking, no heavy metal ion pollution, and no impact on fracturing construction.

[0062] By comparing the data in the table, it is shown that the reducing properties of sodium thiosulfate and sodium bisulfite formaldehyde have a synergistic effect, which together produce a catalytic effect on the decomposition of ammonium persulfate and promote the gel breaking. The self-heating system of sodium nitrite and ammonium chloride also plays a good role. From the temperature test, both have reached the ideal temperature. At this temperature, the viscosity of the solution can be reduced, and the decomposition of ammonium persulfate and the gel breaking are promoted. At the same time, the addition of the self-heating system alone or the use of the reduction catalytic system of sodium thiosulfate and sodium bisulfite formaldehyde alone are both poor, and the gel breaking time is long. The simultaneous addition of both shortens the gel breaking time and accelerates the gel breaking efficiency. At the same time, the loading of oxalic acid on the sustained-release capsule core in the catalytic sustained-release capsule and the wrapping of the sodium carboxymethyl cellulose shell effectively delay the gel breaking time, ensuring the maintenance of the viscosity of the fracturing fluid during the fracturing construction.

[0063] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A high-efficiency and environmentally friendly low-temperature gel breaker for shallow oil and gas wells, characterized by: The high-efficiency and environmentally friendly low-temperature gel breaker for shallow oil and gas wells is prepared by uniformly mixing, by weight, 0.4-0.5 parts of persulfate, 0.075-0.12 parts of sodium thiosulfate, 0.3-0.375 parts of sodium bisulfite formaldehyde solution, 0.003-0.0075 parts of chelating agent, 8-9 parts of sodium nitrite, 6-7 parts of ammonium chloride, 0.8-1 parts of catalytic sustained-release capsules, and 40-50 parts of pure water. The chelating agent is glucose; The catalytic sustained-release capsule is prepared by reacting glutaraldehyde with sodium carboxymethyl cellulose, adding the reacted glutaraldehyde to the sustained-release capsule core, impregnating the core with the reacted glutaraldehyde, and drying the resulting capsule. The sustained-release capsule core is prepared according to the following process: 1-1.2 parts of polyvinyl alcohol and 40-50 parts of pure water are mixed uniformly by mass, stirred at 85-90° C. and 300-400 r / min for 50-60 min until dissolved, cooled to 50-60° C., 1-1.2 parts of sodium lignin sulfonate and 0.2-0.4 parts of ammonium persulfate are added, and stirring is continued for 30-40 min, 4-6 parts of oxalic acid and 80-90 parts of acrylic acid mixed solution are added, and stirring is continued for 30-40 min, 0.04-0.06 parts of N,N'-methylenebisacrylamide are added, the temperature is raised to 70-75° C., stirred at 300-400 r / min for reaction for 4-5 h, vacuum dried at 60-70° C. for 18-20 h, soaked and washed with ethanol 3-4 times, vacuum dried at 60-70° C. for 5-6 h, crushed and passed through a 40-50 mesh sieve to prepare the sustained-release capsule core; The acrylic acid mixed solution is prepared by uniformly mixing 20-30 parts of acrylic acid and 50-60 parts of 3 mol / L sodium hydroxide solution, adding 2-3 parts of acrylamide, and stirring at 200-300 r / min at room temperature until the mixture is completely dissolved.

2. The high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells according to claim 1, characterized in that: The persulfate is one of ammonium persulfate, potassium persulfate and sodium persulfate.

3. The high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells according to claim 1, characterized in that: The catalytic sustained-release capsules are prepared according to the following process: by weight, 2 to 3 parts of a 1 wt% glutaraldehyde aqueous solution are added dropwise to 50 to 60 parts of a 3 wt% sodium carboxymethyl cellulose aqueous solution in a water bath at 50 to 60° C. and 50 to 100 r / min at a rate of 2 to 3 drops per second; after the addition is complete, stirring is continued for 5 to 6 hours; the solution is naturally cooled to room temperature; 2 to 3 parts of sustained-release capsule cores are added thereto while stirring at 50 to 100 r / min and immersed for 20 to 30 minutes; the solution is taken out and vacuum-dried at 60 to 70° C. for 16 to 18 hours to prepare the catalytic sustained-release capsules.

4. The high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells according to claim 1, characterized in that: The mass fraction of the sodium bisulphite formaldehyde solution is 35%.

5. A method for using the high-efficiency, environmentally friendly, low-temperature gel breaker for shallow oil and gas wells in petroleum fracturing fluid according to any one of claims 1 to 4, characterized in that: The high-efficiency and environmentally friendly low-temperature gel breaker for shallow oil and gas wells is added to fracturing fluid at a mass fraction of 1% to 10% and is used at a temperature of 20 to 50°C.

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