High-temperature delayed cross-linking type self-gel-breaking fracturing fluid as well as preparation method and application thereof

Abstract

The invention discloses a high-temperature delayed crosslinking type self-gel-breaking fracturing fluid as well as a preparation method and application thereof, and belongs to the technical field of fracturing fluids. The high-temperature delayed crosslinking type self-gel-breaking fracturing fluid is prepared from the following components in parts by weight: 100 parts of water, 0.2 to 0.5 part of a thickening agent, 0.5 to 1 part of a high-temperature delayed crosslinking agent, 1 to 2 parts of a cleanup additive, 0.5 to 1 part of an anti-swelling agent and 0.1 to 0.5 part of a sterilizing agent, the high-temperature delayed cross-linking agent is prepared by mixing a ligand solution prepared from potassium thiocyanate, sodium citrate, sodium oxalate and water with a ferric chloride solution and then reacting under acidic pH. The fracturing fluid has a dual regulation and control mechanism for realizing high-temperature delayed crosslinking and iron ion valence state conversion based on coordinated regulation and control of multistage ligands to realize a self-gel-breaking function.

Description

Technical Field

[0001] The present invention belongs to the technical field of fracturing fluids, and specifically relates to a high-temperature delayed cross-linking self-breaking fracturing fluid, a preparation method thereof, and an application thereof. Background Art

[0002] With the expansion of oil and gas exploration into deep and ultra-deep fields, traditional fracturing fluid systems are facing severe challenges. In deep fracturing operations, extremely long wellbores generate huge frictional resistance along the way. Experiments and field operations show that low-viscosity linear gels can effectively reduce frictional resistance, but the ultra-high temperature environment of the reservoir easily leads to system instability and insufficient sand-carrying capacity. Although high-strength gels can meet the requirements of temperature resistance and long-distance sand-carrying, it is difficult to reduce the wellbore frictional resistance. Therefore, two major contradictions have emerged in deep fracturing: "low viscosity and drag reduction in the wellbore" and "high viscosity and sand-carrying in the reservoir". The delayed cross-linking technology can effectively balance the two major contradictory requirements of the wellbore and the reservoir for the performance of the fracturing fluid by regulating the cross-linking time of the polymer and the cross-linking agent, so that the fracturing fluid remains in a low-viscosity or weakly cross-linked state in the wellbore and then cross-links into a high-viscosity and high-strength gel after entering the reservoir.

[0003] Existing delayed cross-linking technologies mainly rely on organic boron or organic transition metal cross-linking agents such as organic zirconium and organic titanium, and achieve delayed cross-linking through ligand complexation or pH regulation. Although a certain delayed cross-linking effect can be achieved, there are problems such as poor temperature sensitivity (for example, a cross-linking agent disclosed in a Chinese invention patent with publication number CN116332984A can only achieve controllable delayed cross-linking from room temperature to 60 °C), high environmental toxicity, and complex preparation processes, making it difficult to meet the requirements of high-temperature delayed cross-linking in deep reservoirs. In addition, conventional fracturing fluids rely on external gel breakers to achieve gel breaking and flowback, and there are problems such as gel breaker residues damaging the reservoir and the gel breaking temperature not matching the formation temperature, which affect the fracturing effect.

[0004] Therefore, to meet the requirements of efficient development of deep oil and gas reservoirs, there is an urgent need to develop a green and intelligent fracturing fluid system with both high-temperature delayed cross-linking and self-breaking functions, providing new technical support for the economic and efficient development of deep oil and gas resources. Summary of the Invention

[0005] In order to overcome the above-mentioned disadvantages of the prior art, the purpose of the present invention is to provide a high-temperature delayed cross-linking self-breaking fracturing fluid, a preparation method thereof, and an application thereof. The fracturing fluid has a dual regulation mechanism based on multi-level ligand cooperative regulation to achieve high-temperature delayed cross-linking and iron ion valence state conversion to achieve self-breaking function.

[0006] To achieve the above purpose, the present invention adopts the following technical solutions:

[0007] The present invention provides a high-temperature delayed cross-linking self-breaking fracturing fluid, which includes, by weight:

[0008] 100 parts of water, 0.2 - 0.5 parts of thickening agent, 0.5 - 1 part of high-temperature delayed crosslinking agent, 1 - 2 parts of flowback aid, 0.5 - 1 part of swelling inhibitor, 0.1 - 0.5 part of bactericide;

[0009] The high-temperature delayed crosslinking agent is prepared by reacting a ligand solution prepared from potassium thiocyanate, sodium citrate and sodium oxalate with water and a ferric chloride solution under acidic pH.

[0010] In one embodiment, the preparation process of the high-temperature delayed crosslinking agent is as follows:

[0011] Dissolve ferric chloride in water to obtain a ferric chloride solution, dissolve potassium thiocyanate, sodium citrate and sodium oxalate in water to obtain a ligand solution, then mix the ferric chloride solution and the ligand solution and adjust the pH to 3.5 - 5.5, and react at 40 - 50 °C for 1 - 2 h to obtain the high-temperature delayed crosslinking agent.

[0012] In one embodiment, the molar ratio of potassium thiocyanate, sodium citrate and sodium oxalate is (2 - 8):(0.1 - 1):(0.1 - 1).

[0013] In one embodiment, the molar concentration of the ferric chloride solution is 1 mol / L; the total molar concentration of potassium thiocyanate, sodium citrate and sodium oxalate in the ligand solution is 2.2 - 10 mol / L.

[0014] In one embodiment, the volume ratio of the ferric chloride solution to the ligand solution is 1:1.

[0015] In one embodiment, the thickening agent is anionic polyacrylamide, the carboxylic acid group content of the anionic polyacrylamide is 15 - 25%, the sulfonic acid group content is 5 - 10%, the phenyl group content is 0.5 - 2%, and the molecular weight is 10 - 15 million.

[0016] In one embodiment, the flowback aid is fatty alcohol polyoxyethylene ether; the swelling inhibitor is potassium chloride or choline chloride; the bactericide is quaternary ammonium salt or formaldehyde.

[0017] The present invention also provides a preparation method of the above high-temperature delayed crosslinking self-breaking gel fracturing fluid, comprising the following steps:

[0018] Under the first stirring condition, add the thickening agent to water to completely dissolve it to obtain a thickening agent solution;

[0019] Add the flowback aid, swelling inhibitor, bactericide and high-temperature delayed crosslinking agent to the thickening agent solution and stir evenly under the second stirring condition to obtain a high-temperature delayed crosslinking self-breaking gel fracturing fluid.

[0020] In one embodiment, the first stirring condition is stirring for 30 to 50 minutes under the condition of 1000 to 2000 revolutions per minute; the second stirring condition is stirring for 3 to 5 minutes under the condition of 1000 to 2000 revolutions per minute.

[0021] The present invention also provides an application of the high-temperature delayed crosslinking self-breaking gel fracturing fluid as described above in the fracturing development process of deep oil and gas reservoirs.

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

[0023] The present invention provides a high-temperature delayed crosslinking self-breaking gel fracturing fluid. The high-temperature delayed crosslinking agent therein forms a multi-stage coordination structure with temperature-responsive characteristics through the hierarchical complexation of ferric chloride with three ligands, namely thiocyanate, citrate, and oxalate. Based on the synergistic effect of multiple ligands, a stable complex is formed under low-temperature conditions, and through the regulation of ligands, the controlled release of ferric ions from the complex under high-temperature conditions is effectively achieved. The released ferric ions coordinate and crosslink with active sites such as carboxyl groups / sulfonic acid groups in polyacrylamide in the thickening agent to form a three-dimensional crosslinked network structure, achieving the effect of high-temperature delayed crosslinking. As the fracturing fluid continuously acts in a high-temperature and highly reducing reservoir, ferric ions are gradually reduced to ferrous ions by formation reducing substances, and their crosslinking and coordination ability significantly decreases, thereby causing the crosslinked network to spontaneously disintegrate to achieve the effect of self-breaking gel. The fracturing fluid prepared by the present invention solves the three major technical problems of low-viscosity transportation of the fracturing fluid in the wellbore, high-viscosity sand carrying in the reservoir, and self-breaking gel of the flowback fluid through the design of multi-stage reactions of coordination-dissociation-reduction at the molecular level.

[0024] Furthermore, the above high-temperature delayed crosslinking self-breaking gel fracturing fluid has a good high-temperature delayed crosslinking effect. Ferric ions can crosslink with crosslinking sites such as carboxyl groups / sulfonic acid groups in the polymer to form a strong crosslinked network. The synergistic effect of composite ligands such as potassium thiocyanate, sodium oxalate, and sodium citrate is used to form a low-temperature stable complex, and a high-temperature hierarchical dissociation mechanism is established through the synergistic regulation of multiple ligands to achieve the high-temperature delayed crosslinking effect. In addition, there is a breakthrough innovation in the self-breaking gel mechanism. No external gel breaker is required, reducing reservoir damage. Utilizing the valence state conversion of iron ions in the formation reduction environment (Fe3+→Fe2+) and the difference in crosslinking and coordination ability of iron ions in different valence states, the polymer crosslinked network automatically disintegrates to achieve the self-breaking gel function. Finally, it has good environmental protection and economic feasibility. Using iron-based and biodegradable ligands such as citric acid and oxalic acid as the crosslinking system to avoid formation pollution; the preparation process is simple, the raw materials are easy to obtain, and the single system integrates the functions of crosslinking and self-breaking gel, reducing the liquid preparation process and operation risks, and having good economic feasibility. Description of the Drawings

[0025] Figure 1The image after crosslinking of the high-temperature delayed crosslinking self-breaking gel fracturing fluid prepared in Example 1;

[0026] Figure 2 The image after gel breaking of the high-temperature delayed crosslinking self-breaking gel fracturing fluid prepared in Example 1. Detailed implementation manners

[0027] To enable those skilled in the art to understand the features and effects of the present invention, the following provides a general description and definition of the terms and phrases mentioned in the specification and claims. Unless otherwise specified, all technical and scientific terms used herein shall have the ordinary meaning understood by those skilled in the art for the present invention. In case of conflict, the definition in this specification shall prevail.

[0028] The theories or mechanisms described and disclosed herein, whether correct or incorrect, shall not in any way limit the scope of the present invention, that is, the content of the present invention can be implemented without being limited by any specific theory or mechanism.

[0029] In this article, all features defined in the form of numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are only for the sake of brevity and convenience. Accordingly, the description of numerical ranges or percentage ranges should be regarded as having covered and specifically disclosed all possible sub-ranges and individual numerical values (including integers and fractions) within the range.

[0030] In this article, unless otherwise specified, "comprising", "including", "containing", "having" or similar terms cover the meanings of "consisting of" and "consisting essentially of". For example, "A comprises a" covers the meanings of "A comprises a and others" and "A only comprises a".

[0031] In this article, for the sake of brevity of description, all possible combinations of all technical features in each embodiment or example are not described. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each embodiment or example can be combined arbitrarily, and all possible combinations should be considered as the scope described in this specification.

[0032] The present invention provides a high-temperature delayed crosslinking self-breaking gel fracturing fluid and its preparation method and application.

[0033] On the one hand, it provides a high-temperature delayed crosslinking self-breaking gel fracturing fluid, which, by weight, includes:

[0034] 100 parts of water, 0.2 - 0.5 parts of thickening agent, 0.5 - 1 part of high-temperature delayed crosslinking agent, 1 - 2 parts of flowback aid, 0.5 - 1 part of swelling inhibitor, 0.1 - 0.5 part of bactericide; the preparation raw materials of the high-temperature delayed crosslinking agent include ferric chloride, potassium thiocyanate, sodium citrate, and sodium oxalate.

[0035] The preparation process of the high-temperature delayed cross-linking agent is to react a ferric chloride solution with a ligand solution containing thiocyanate, citrate, and oxalate at pH 3.5-5.5 to construct a multi-ligand complex system based on Fe3+ to form a temperature-responsive delayed cross-linking agent.

[0036] The specific steps are as follows:

[0037] Ferric chloride is dissolved in water to obtain a ferric chloride solution, wherein the molar concentration of the ferric chloride solution is 1 mol / L; potassium thiocyanate, sodium citrate and sodium oxalate are dissolved in water to obtain a ligand solution, wherein the total molar concentration of the ligand solution is 2.2-10 mol / L; wherein the molar ratio of potassium thiocyanate, sodium citrate and sodium oxalate is (2-8):(0.1-1):(0.1-1); then the ferric chloride solution and the ligand solution are mixed in a volume ratio of 1:1, the pH value is adjusted to 3.5-5.5, and the mixture is reacted at 40-50°C for 1-2 hours to obtain a high-temperature delayed crosslinking agent.

[0038] The thickener is anionic polyacrylamide, the carboxylic acid content of anionic polyacrylamide is 15-25%, the sulfonic acid content is 5-10%, the phenyl content is 0.5-2%, and the molecular weight is 10-15 million. The drainage aid is fatty alcohol polyoxyethylene ether; the anti-swelling agent is potassium chloride or choline chloride; the bactericide is quaternary ammonium salt or formaldehyde.

[0039] The second aspect provides a method for preparing a high-temperature delayed cross-linking self-destructive fracturing fluid, as follows:

[0040] The thickener is added into water under stirring condition to make it completely dissolved to obtain a thickener solution; the drainage agent, anti-swelling agent, bactericide and high temperature delayed crosslinking agent are added into the thickener solution and stirred evenly to obtain a high temperature delayed crosslinking self-destructive fracturing fluid.

[0041] Furthermore, the stirring conditions for preparing the thickener solution are stirring at 1000-2000 rpm for 30-50 minutes; the stirring conditions for preparing the high-temperature delayed cross-linking self-degrading fracturing fluid are stirring at 1000-2000 rpm for 3-5 minutes.

[0042] Thirdly, the high-temperature delayed cross-linking self-destructive fracturing fluid prepared by the present invention is mainly used in the fracturing development of deep oil and gas reservoirs.

[0043] The present invention forms a temperature-responsive dissociation design of Fe3+ through the synergistic complexation of multiple ligands to achieve high-temperature delayed crosslinking; at the same time, the reservoir environment is used to reduce Fe3+ to Fe2+, and the crosslinking coordination ability differences of iron ions with different valence states are utilized to dissociate the polymer crosslinking network, thereby achieving the self-breaking gel effect. The present invention does not require an external breaker, and has the advantages of high-temperature delayed crosslinking, self-breaking gel, environmental friendliness and economic feasibility, providing an innovative solution for the efficient development of deep oil and gas reservoirs.

[0044] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0045] Conventional instrument equipment in the art is used in the following embodiments. The experimental methods without specific conditions noted in the following embodiments are usually carried out under conventional conditions or according to the conditions recommended by the manufacturer. Various raw materials are used in the following embodiments. Unless otherwise stated, commercially available products are used, and their specifications are conventional specifications in the art. In the specification of the present invention and the following embodiments, unless otherwise specified, "%" represents weight percentage, "parts" represents weight parts, and the ratio represents weight ratio.

[0046] Example 1

[0047] This example provides a high-temperature delayed crosslinking type self-breaking gel fracturing fluid, which includes, by weight parts:

[0048] 100 parts of water, 0.2 parts of thickening agent, 0.5 parts of high-temperature delayed crosslinking agent, 1 part of flowback aid, 0.5 parts of swelling inhibitor, 0.1 part of bactericide.

[0049] The preparation process of the above high-temperature delayed crosslinking agent is as follows:

[0050] Dissolve 1 mol of ferric chloride in 1 L of water to obtain a ferric chloride solution, dissolve 2 mol of potassium thiocyanate, 0.1 mol of sodium citrate, and 0.1 mol of sodium oxalate in 1 L of water to obtain a ligand solution, mix the two solutions in a volume ratio of 1:1 and adjust the pH to 3.5, and react at 40 °C for 1 h to obtain the high-temperature delayed crosslinking agent.

[0051] On the other hand, this example provides a preparation method of a high-temperature delayed crosslinking type self-breaking gel fracturing fluid, and the specific steps are as follows:

[0052] Under the stirring condition of 1000 revolutions per minute, 2 g of polyacrylamide was added to 1000 g of water and stirred for 30 minutes to completely dissolve it, obtaining a polyacrylamide solution. Then, 10 g of AEO-9, 5 g of choline chloride, 1 g of quaternary ammonium salt, and 5 g of the above-prepared high-temperature delayed crosslinking agent were added to the polyacrylamide solution, and stirred at 1000 revolutions per minute for 3 minutes to make it evenly dispersed, obtaining a high-temperature delayed crosslinking self-breaking gel fracturing fluid.

[0053] Example 2

[0054] This example provides a high-temperature delayed crosslinking type self-breaking gel fracturing fluid, which includes, by weight:

[0055] 100 parts of water, 0.2 part of thickening agent, 0.5 part of high-temperature delayed crosslinking agent, 1 part of flowback aid, 0.5 part of swelling inhibitor, and 0.1 part of bactericide.

[0056] The preparation process of the above high-temperature delayed crosslinking agent is as follows:

[0057] 1 mol of ferric chloride was dissolved in 1 L of water to obtain a ferric chloride solution, 8 mol of potassium thiocyanate, 1 mol of sodium citrate, and 1 mol of sodium oxalate were dissolved in 1 L of water to obtain a ligand solution. The two solutions were mixed in a volume ratio of 1:1 and the pH was adjusted to 5.5, and reacted at 50 °C for 2 h to obtain the high-temperature delayed crosslinking agent.

[0058] On the other hand, this example provides a preparation method of a high-temperature delayed crosslinking type self-breaking gel fracturing fluid. The specific steps are as follows:

[0059] Under the stirring condition of 1000 revolutions per minute, 2 g of polyacrylamide was added to 1000 g of water and stirred for 30 minutes to completely dissolve it, obtaining a polyacrylamide solution. Then, 10 g of AEO-9, 5 g of potassium chloride, 1 g of quaternary ammonium salt, and 5 g of the above-prepared high-temperature delayed crosslinking agent were added to the polyacrylamide solution, and stirred at 1000 revolutions per minute for 3 minutes to make it evenly dispersed, obtaining a high-temperature delayed crosslinking self-breaking gel fracturing fluid.

[0060] Example 3

[0061] This example provides a high-temperature delayed crosslinking type self-breaking gel fracturing fluid, which includes, by weight:

[0062] 100 parts of water, 0.5 part of thickening agent, 1 part of high-temperature delayed crosslinking agent, 2 parts of flowback aid, 1 part of swelling inhibitor, and 0.5 part of bactericide.

[0063] The preparation process of the above high-temperature delayed crosslinking agent is as follows:

[0064] Dissolve 1 mol of ferric chloride in 1 L of water to obtain an iron solution, dissolve 2 mol of potassium thiocyanate, 0.1 mol of sodium citrate, and 0.1 mol of sodium oxalate in 1 L of water to obtain a ligand solution. Mix the two solutions in a volume ratio of 1:1 and adjust the pH to 4, and react at 40 °C for 1 h to obtain a high-temperature delayed crosslinking agent.

[0065] On the other hand, this example provides a preparation method of a high-temperature delayed crosslinking self-breaking gel fracturing fluid, and the specific steps are as follows:

[0066] Under the stirring condition of 2000 revolutions per minute, add 5 g of polyacrylamide to 1000 g of water and stir for 50 minutes to completely dissolve it to obtain a polyacrylamide solution. Then add 20 g of AEO-9, 10 g of potassium chloride, 5 g of quaternary ammonium salt, and 10 g of the high-temperature delayed crosslinking agent prepared above to the polyacrylamide solution, and stir for 3 minutes under the condition of 1000 revolutions per minute to disperse it evenly to obtain a high-temperature delayed crosslinking self-breaking gel fracturing fluid.

[0067] Example 4

[0068] This example provides a high-temperature delayed crosslinking self-breaking gel fracturing fluid, which includes, by weight:

[0069] 100 parts of water, 0.4 part of thickening agent, 0.7 part of high-temperature delayed crosslinking agent, 1.5 parts of flowback aid, 0.7 part of swelling inhibitor, and 0.3 part of bactericide.

[0070] The preparation process of the above high-temperature delayed crosslinking agent is as follows:

[0071] Dissolve 1 mol of ferric chloride in 1 L of water to obtain an iron solution, dissolve 2 mol of potassium thiocyanate, 0.1 mol of sodium citrate, and 0.1 mol of sodium oxalate in 1 L of water to obtain a ligand solution. Mix the two solutions in a volume ratio of 1:1 and adjust the pH to 4, and react at 40 °C for 1 h to obtain a high-temperature delayed crosslinking agent.

[0072] On the other hand, this example provides a preparation method of a high-temperature delayed crosslinking self-breaking gel fracturing fluid, and the specific steps are as follows:

[0073] Under the stirring condition of 1000 revolutions per minute, add 4 g of polyacrylamide to 1000 g of water and stir for 30 minutes to completely dissolve it to obtain a polyacrylamide solution. Then add 15 g of AEO-9, 7 g of potassium chloride, 3 g of quaternary ammonium salt, and 7 g of the high-temperature delayed crosslinking agent prepared above to the polyacrylamide solution, and stir for 5 minutes under the condition of 2000 revolutions per minute to disperse it evenly to obtain a high-temperature delayed crosslinking self-breaking gel fracturing fluid.

[0074] Example 5

[0075] This embodiment provides a high-temperature delayed crosslinking self-breaking gel fracturing fluid, which, by weight, includes:

[0076] 100 parts of water, 0.4 parts of thickening agent, 0.7 parts of high-temperature delayed crosslinking agent, 1.5 parts of flowback aid, 0.7 parts of swelling inhibitor, and 0.3 parts of bactericide.

[0077] The preparation process of the above high-temperature delayed crosslinking agent is as follows:

[0078] Dissolve 1 mol of ferric chloride in 1 L of water to obtain an iron solution, dissolve 5 mol of potassium thiocyanate, 0.5 mol of sodium citrate, and 0.5 mol of sodium oxalate in 1 L of water to obtain a ligand solution. Mix the two solutions in a volume ratio of 1:1 and adjust the pH to 4, and react at 40 °C for 1 h to obtain the high-temperature delayed crosslinking agent.

[0079] On the other hand, this example provides a preparation method of a high-temperature delayed crosslinking self-breaking gel fracturing fluid. The specific steps are as follows:

[0080] Under the stirring condition of 1000 revolutions per minute, add 4 g of polyacrylamide to 1000 g of water and stir for 30 minutes to completely dissolve it to obtain a polyacrylamide solution. Then add 15 g of AEO-9, 7 g of potassium chloride, 3 g of formaldehyde, and 7 g of the above-prepared high-temperature delayed crosslinking agent to the polyacrylamide solution, and stir at 2000 revolutions per minute for 5 minutes to make it disperse evenly to obtain a high-temperature delayed crosslinking self-breaking gel fracturing fluid.

[0081] Evaluation of high-temperature delayed crosslinking performance

[0082] In order to characterize the high-temperature delayed crosslinking performance of a high-temperature delayed crosslinking self-breaking gel fracturing fluid prepared by the present invention, a high-temperature rheometer (HAAKE MARS 40 / 60 type, Thermo Fisher Scientific) was used to measure the apparent viscosity change of the fracturing fluids prepared in Examples 1-5 in the temperature range of 30-120 °C at a constant shear rate of 170 s-1.

[0083]

[0084] It can be seen from the experimental results shown in the above table that in the low-temperature stage (≤70 °C), the viscosities of all examples show a slight downward trend with the increase of temperature, indicating that the crosslinking reaction in the system is not triggered at this time, meeting the low-viscosity transportation requirements in the wellbore; after the temperature rises to 80 °C, the viscosities of Examples 1, 3, and 5 increase significantly, indicating that Fe3+ is gradually released from the complex at this time and crosslinks with the carboxyl / sulfonic acid groups of polyacrylamide to form a three-dimensional network structure. By comparing Example 1 and Example 2, it can be found that increasing the proportion of thiocyanate ions can delay the crosslinking starting temperature, indicating that the precise design of the crosslinking temperature window can be achieved by regulating the ligand ratio, realizing high-temperature controllable delayed crosslinking.

[0085] Evaluation of High Temperature Stability and Self - Breaking Gel Performance

[0086] To characterize the high - temperature delayed cross - linking performance of a high - temperature delayed cross - linking self - breaking gel fracturing fluid prepared by the present invention, the fracturing fluid of the above - mentioned fracturing fluid examples was sheared at 120 °C and 170 s -1 for 1 h, and then the viscosity was measured. Then, it was placed at a constant temperature of 130 °C for 8 h for self - breaking gel, and the viscosity changes before and after gel - breaking were determined.

[0087]

[0088] It can be seen from the above experimental results that after all the examples were sheared at a high temperature of 120 °C and 170 s⁻¹ for 1 h, the retention value of the system viscosity was higher than 90 mPa·s, meeting the viscosity requirements in the fractures of deep reservoirs. Under the condition of no external gel - breaking agent intervention, after standing at 130 °C for 8 h, the viscosity of the gel - broken fluid of the system dropped below 5 mPa·s, and the viscosity of the gel - broken fluid met the industry's flow - back requirements. As Figure 1 and Figure 2 shown, through the visual comparison before and after gel - breaking of Example 1, the system presented a typical elastic gel form before gel - breaking and transformed into a transparent low - viscosity liquid after gel - breaking, confirming its excellent cross - linking and efficient self - breaking gel performance.

[0089] The above content is only to illustrate the technical idea of the present invention and cannot be used to limit the protection scope of the present invention. Any modification made on the basis of the technical solution according to the technical idea proposed by the present invention falls within the protection scope of the claims of the present invention.

Claims

1. A high temperature delayed cross-linking self-destructive fracturing fluid, characterized in that: By weight, it includes: 100 parts of water, 0.2-0.5 parts of thickener, 0.5-1 parts of high temperature delayed crosslinking agent, 1-2 parts of drainage aid, 0.5-1 parts of anti-swelling agent, 0.1-0.5 parts of bactericide; The high temperature delayed cross-linking agent is prepared by mixing a ligand solution prepared with potassium thiocyanate, sodium citrate and sodium oxalate and water with a ferric chloride solution and reacting the mixture under acidic pH.

2. The high temperature delayed cross-linking self-destructive fracturing fluid according to claim 1, characterized in that: The preparation process of the high temperature delayed crosslinking agent is as follows: Ferric chloride is dissolved in water to obtain a ferric chloride solution, potassium thiocyanate, sodium citrate and sodium oxalate are dissolved in water to obtain a ligand solution, and then the ferric chloride solution and the ligand solution are mixed, and the pH is adjusted to 3.5-5.5, and the mixture is reacted at 40-50° C. for 1-2 hours to obtain a high-temperature delayed crosslinking agent.

3. The high temperature delayed cross-linking self-destructive fracturing fluid according to claim 1 or 2, characterized in that: The molar ratio of potassium thiocyanate, sodium citrate and sodium oxalate is (2-8): (0.1-1): (0.1-1).

4. The high temperature delayed cross-linking self-destructive fracturing fluid according to claim 1 or 2, characterized in that: The molar concentration of the ferric chloride solution is 1 mol / L; the total molar concentration of potassium thiocyanate, sodium citrate and sodium oxalate in the ligand solution is 2.2-10 mol / L.

5. The high temperature delayed cross-linking self-destructive fracturing fluid according to claim 1 or 2, characterized in that: The volume ratio of the ferric chloride solution to the ligand solution is 1:

1.

6. The high temperature delayed cross-linking self-destructive fracturing fluid according to claim 1, characterized in that: The thickener is anionic polyacrylamide, the carboxylic acid content of the anionic polyacrylamide is 15-25%, the sulfonic acid content is 5-10%, the phenyl content is 0.5-2%, and the molecular weight is 10 million to 15 million.

7. The high temperature delayed cross-linking self-destructive fracturing fluid according to claim 1, characterized in that: The drainage aid is fatty alcohol polyoxyethylene ether; the anti-swelling agent is potassium chloride or choline chloride; and the bactericide is quaternary ammonium salt or formaldehyde.

8. A method for preparing a high temperature delayed cross-linking self-destructive fracturing fluid according to any one of claims 1 to 7, characterized in that: The following steps are involved: adding a thickener into water under a first stirring condition to completely dissolve the thickener, thereby obtaining a thickener solution; The drainage aid, the anti-swelling agent, the bactericide and the high-temperature delayed cross-linking agent are added into the thickener solution and stirred evenly under the second stirring condition to obtain a high-temperature delayed cross-linking self-destructive fracturing fluid.

9. The method for preparing the high temperature delayed cross-linking self-destructive fracturing fluid according to claim 8, characterized in that: The first stirring condition is stirring at 1000-2000 rpm for 30-50 minutes; the second stirring condition is stirring at 1000-2000 rpm for 3-5 minutes.

10. Use of the high temperature delayed cross-linking self-destructive fracturing fluid according to any one of claims 1 to 7 in the fracturing development process of deep oil and gas reservoirs.

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