Self-gel-breaking composite gel liquid bridge plug and preparation method thereof

Abstract

The invention provides a self-gel-breaking composite gel liquid bridge plug and a preparation method thereof, and belongs to necessary means in the development process of oil and gas fields. The preparation method comprises the following steps: tannic acid (TA), acrylamide (AM) and 2-acrylamide-2-methylpropanesulfonic acid (AMPS) are subjected to free radical polymerization to successfully prepare TA-PAM-AMPS gel, N, N '-methylenebisacrylamide (MBA) is used as a cross-linking agent, ammonium persulfate (APS) is used as an initiator, synthesis condition optimization is carried out on the total monomer addition amount, the TA addition amount and the MBA addition amount, the TA-PAM-AMPS gel is prepared, and the TA-PAM-AMPS gel is used for preparing the TA-PAM-AMPS gel. And the adhesion performance is improved by adding TA. In order to further improve the mechanical strength of the gel, natural plant polysaccharide konjac glucomannan and polymer gel are introduced to form composite gel, and the dosage of the konjac glucomannan and the dosage of the organic zirconium cross-linking agent are optimized. The self-breaking of the composite gel is realized by preparing the microcapsule gel breaker.

Description

Technical Field

[0001] The invention belongs to the field of wellbore plugging in oil and gas fields, and particularly relates to a wellbore plugging self-breaking composite gel liquid bridge plug and a preparation method thereof. Background Art

[0002] With the continued development of the global economy, especially the accelerated industrialization and urbanization in developing countries, the demand for energy continues to rise. As one of the world's most important energy sources, oil and gas resources play an indispensable role in numerous sectors, including power generation, transportation, and industrial production. Wellbore sealing is an essential component of oil and gas field development. Staged fracturing, temporary plugging and leak control, staged profile control and water shutoff, and abandoned well closure all require temporary or complete wellbore sealing. During well killing, completion, and workover operations, sealing non-productive zones and the annular space outside the casing ensures wellbore integrity and sealing, providing a reliable foundation for subsequent production operations. During workover operations, such as replacing downhole equipment and addressing well failures, wellbore sealing can isolate specific sections of the well, facilitating operations while preventing fluids from other zones from interfering with the workover. Effective wellbore sealing can reduce fluid leakage and crossflow within the wellbore, minimizing energy loss and equipment wear during production, improving the stability and reliability of the production system, and ultimately increasing production efficiency, costs, and economic benefits.

[0003] Wellbore plugging typically involves physical or chemical plugging of the wellbore casing. However, during long-term construction, casing can be easily deformed and damaged due to numerous factors, including formation stress, cementing quality, and casing quality. Conventional physical plugs are inadequate for these complex environments. In recent years, researchers have focused on developing chemical plugs, such as polymer gels, as an alternative to mechanical plugs, achieving significant success and applications. Before gelation, gels are typically liquids with excellent fluidity, enabling them to be pumped to the desired formation. Under certain conditions, they undergo polymerization and cross-linking reactions to form a solid gel with excellent mechanical properties and a certain pressure bearing capacity, meeting the requirements of wellbore casing plugging operations. Furthermore, after the gel is completed, the gel structure can be broken down through physical or chemical methods, facilitating flowback and restoring wellbore production. Furthermore, by selecting and modifying different polymer monomers, different functionalized gel materials can be obtained to withstand the complex conditions of high temperature and high salinity in oil and gas fields.

[0004] Gel bridge plugs offer excellent sealing capabilities when used for wellbore plugging operations and hold broad application prospects as a replacement for traditional mechanical bridge plugs. However, relatively little research has been conducted on the full application of gels for wellbore plugging, and several challenges remain. For one thing, the gel's adhesion and mechanical strength need to be improved. If the gel lacks sufficient adhesion to the wellbore casing, subsequent ingress of working fluids will facilitate the intrusion of other fluids through the gap between the gel and casing, accelerating gel slippage and resulting in suboptimal plugging effectiveness. Furthermore, if the gel's mechanical strength is low, it will be unable to withstand the pressure generated by hydraulic fracturing and other operations, leading to gel breakage and reduced pressure-bearing capacity. Therefore, research is needed to improve the gel's adhesion and mechanical strength. Furthermore, while its unique structure imparts a certain degree of pressure-bearing capacity after gelation, post-processing gel breakage often presents difficulties or incomplete breaking. This can not only cause wellbore blockage but also reservoir contamination, impacting the normal operation of oil and gas fields. Therefore, achieving gel self-breakage and flowback from the wellbore casing is a necessary process to restore the wellbore production capacity after liquid bridge plug construction. Summary of the Invention

[0005] In view of this, the purpose of the embodiments of the present application is to provide a self-breaking composite gel liquid bridge plug for wellbore plugging, as well as its preparation method and application, to address the common problems of the prior art, such as weak adhesion of gel in the wellbore, low mechanical strength, and easy breakage, and difficulty in breaking the gel after gelling. Furthermore, the self-breaking composite gel liquid bridge plug for wellbore plugging provided in the embodiments of the present application is simple to prepare and easy to implement.

[0006] The present application provides a method for preparing the above-mentioned wellbore plugging self-breaking composite gel liquid bridge plug, comprising the following steps:

[0007] S1. TA@PAM-AMPS gel was successfully prepared by free radical polymerization of tannic acid (TA), acrylamide (AM), and 2-acrylamido-2-methylpropanesulfonic acid (AMPS). N,N'-methylenebisacrylamide (MBA) was used as a cross-linker and ammonium persulfate (APS) was used as an initiator. The synthesis conditions were optimized by optimizing the total monomer addition, TA addition, and MBA addition. The resulting TA@PAM-AMPS gel had a tensile strength of 131.24 kPa and an adhesion strength of 23.42 kPa. The addition of TA improved the adhesion performance by 3.15 times compared to the gel without TA addition.

[0008] S2. introduces natural plant polysaccharide konjac glucomannan and polymer gel to form composite gel. By optimizing the dosage of konjac glucomannan and organic zirconium cross-linking agent, the tensile strength of the composite gel is 1687.46kPa when the dosage of konjac glucomannan is 4000mg / L and the dosage of organic zirconium cross-linking agent is 30wt%, and the adhesion strength is 30.48kPa, which is 12.86 times higher than that of the single TA@PAM-AMPS gel tensile strength. Micromorphology shows that the structure of the composite gel is more robust and dense, which enhances its mechanical properties. The wellbore plugging ability of the composite gel was tested, and its indoor simulated casing breakthrough pressure gradient reached 10.67MPa / m, and the addition of konjac glucomannan improved the temperature resistance of the gel.

[0009] S3. Self-breaking of the composite gel was achieved by preparing a microcapsule breaker. The microcapsule breaker uses toluene diisocyanate-propylene glycol as the shell and ammonium persulfate as the core. Interfacial polymerization was used to prepare core-shell polyurethane microcapsules capable of delayed release of ammonium persulfate. The microcapsule breaker's particle size ranged from 11 to 80 μm, with a median size of 32.70 μm. The microcapsules possessed numerous micropores on the surface, which allowed the shell to rupture upon water absorption and expansion. The delayed release time in water was 19.5 hours, and the encapsulation efficiency reached 47.29%. By adding 8 wt% of the microcapsule breaker, the composite gel broke within 4 days, with a breaker solution viscosity of 143.25 mPa·s. Indoor simulated casing plugging capacity experiments demonstrated that the addition of the microcapsule breaker did not affect the gelling properties and wellbore plugging capability of the composite gel, demonstrating its excellent adaptability.

[0010] Furthermore, the preparation method of the microcapsule breaker in step S3 includes the following steps:

[0011] S31. Prepare the oil phase using ethyl acetate as the organic solvent, toluene diisocyanate as the shell material for the polyurethane core-shell structure, and Tween 80 and Span 80 as emulsifiers. Dissolve 5 wt% of toluene diisocyanate and 2 wt% of the emulsifier (Tween 80:Span 80, mass ratio: 1:1) in ethyl acetate. Stir using a high-speed homogenizer at 12,000 rpm for 30 minutes.

[0012] S32. Prepare an aqueous phase, wherein ammonium persulfate is the microcapsule core, glycerol is the polyurethane core-shell shell material, and the solvent is pure water. Dissolve 25 wt% ammonium persulfate and 5 wt% glycerol in pure water and stir using a magnetic stirrer at 2000 rpm for 10 minutes.

[0013] S33. Transfer the oil phase solution to a three-necked flask and place it in a water bath. Add the aqueous phase dropwise to the oil phase. Set the reaction temperature to 40°C, the rotation speed to 2000 rpm, and the reaction time to 2 hours. After the reaction, wash the flask three times with ethyl acetate and then with pure water to remove unreacted monomers.

[0014] Furthermore, as described in step S1, 5 wt% of toluene diisocyanate and 2 wt% of emulsifier (Tween 80 and Span 80 in a mass ratio of 1:1) were dissolved in ethyl acetate.

[0015] Furthermore, the amount of the microcapsule breaker is 8% of the total mass of the monomers.

[0016] Furthermore, the amount of the initiator used in step S1 is 25% of the total mass of tannic acid (TA), acrylamide (AM), and 2-acrylamide-2-methylpropanesulfonic acid (AMPS) monomers.

[0017] Furthermore, the initiator in step S3 is ammonium persulfate.

[0018] Furthermore, 2wt%, 4wt%, 6wt%, 8wt% and 10wt% of microcapsule breaker are added respectively.

[0019] (1) Based on the above steps, a self-breaking composite gel liquid bridge plug for wellbore plugging was obtained. The addition of microcapsule breaker gave the composite gel good self-breaking ability.

[0020] The present application also provides an application of a self-breaking composite gel liquid bridge plug for wellbore plugging.

[0021] (2) Further, the polyurethane microcapsule breaker is mixed with the composite gel liquid bridge plug to produce a self-breaking tannic acid-acrylamide / konjac gum composite gel bridge plug. The polyurethane microcapsule breaker is filled into the composite gel liquid bridge plug. The addition of the microcapsule breaker does not affect the gelling properties and pressure bearing capacity of the composite gel. After the microcapsule core is released, the composite gel can be broken and the wellbore plugging ability is lost.

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

[0023] 1. The self-breaking composite gel liquid bridge plug for wellbore plugging provided in this application has significantly improved adhesion performance. The introduction of tannic acid (TA) containing catechol / pyrogallol increases the gel adhesion strength by 3.15 times to 23.42 kPa through biomimetic adhesion mechanisms (hydrogen bonding, metal coordination, etc.). After compounding with konjac gum, it is further increased to 30.48 kPa, reducing the risk of fluid intrusion along the interface.

[0024] 2. the wellbore plugging self-breaking glue composite gel liquid bridge plug provided by the application, mechanical strength and plugging ability are enhanced.Build tannic acid-acrylamide / konjac gum interpenetrating network gel, tensile strength reaches 1687.46kPa (12.86 times higher than single gel), indoor simulation casing breakthrough pressure gradient reaches 10.67MPa / m, and heat resistance improves (breakthrough pressure gradient is 2.48 times of single gel at 125 ° C). The gelling performance, mechanical properties and adhesion performance of gel at different temperatures and salinity, the results show that with the increase of temperature, the mechanical properties and adhesion properties of TA@PAM-AMPS gel are all decreased; with the increase of salinity, gel adhesion performance decreases but mechanical properties improve. In order to improve the mechanical strength of gel, interpenetrating network gel is constructed by introducing natural plant polysaccharide konjac gum and polymer gel.

[0025] 3. The self-destructing composite gel liquid bridge plug for wellbore plugging provided in this application has optimized environmental adaptability. Salt tolerance: At a salinity of 25,000 mg / L, the composite gel's tensile strength increased by 7.33%, while the decrease in adhesion strength (44.29%) was less than that of a single gel (60.93%). Temperature resistance: After aging for five days at 125°C, the composite gel maintained excellent stability, with a breakthrough pressure gradient decrease of only 8.2%.

[0026] 4. The self-breaking composite gel liquid bridge plug for wellbore plugging provided in this application demonstrates efficient and controllable self-breaking performance. Polyurethane microcapsules encapsulating ammonium persulfate were prepared, resulting in a delayed release time of 19.5 hours and an embedding efficiency of 47.29%. Adding 8 wt% of the microcapsules resulted in the composite gel breaking within 4 days, with a viscosity of 143.25 mPa·s, without compromising the initial plugging performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0028] Figure 1 (a) is the synthesis route of TA@PAM-AMPS prepared in the present invention, and (b) is the synthesis route of polyurethane capsule shell;

[0029] Figure 2 TGA / DTG curves; a: PAM-AMPS; b: TA@PAM-AMPS;

[0030] Figure 3 This is a schematic diagram of the preparation of the composite gel of the present invention;

[0031] Figure 4Infrared spectra of KGM and TA@PAM-AMPS / KGM gel;

[0032] Figure 5 Microscopic morphology of the gel; a: TA@PAM-AMPS; b: TA@PAM-AMPS / KGM composite gel;

[0033] Figure 6 is the adhesion performance of composite gel at different temperatures;

[0034] Figure 7 is the adhesion performance of composite gel at different mineralization degrees;

[0035] Figure 8 Mechanical properties of composite gel at different temperatures; a: stress-strain curve; b: tensile elastic modulus;

[0036] Figure 9 Mechanical properties of composite gels at different mineralization levels; a: stress-strain curve; b: tensile elastic modulus;

[0037] Figure 10 is the breakthrough pressure gradient of the composite gel at different temperatures;

[0038] Figure 11 The long-term stability of the composite gel at different aging temperatures;

[0039] Figure 12 The bearing capacity of composite gel wellbore under different salinities;

[0040] Figure 13 a is the curve of composite gel breakthrough pressure changing with bridge plug length under 25 mm casing diameter; Figure 13 b is the curve of composite gel breakthrough pressure changing with casing diameter when the bridge plug length is 30 cm;

[0041] Figure 14 This is the infrared spectrum of polyurethane microcapsules;

[0042] Figure 15 XRD patterns of microcapsules, microcapsules-encapsulated ammonium persulfate, and ammonium persulfate;

[0043] Figure 16 The microscopic morphology of polyurethane microcapsules; a: microcapsules; b: broken microcapsules;

[0044] Figure 17 is the particle size distribution diagram of polyurethane capsules;

[0045] Figure 18 is the release time of microcapsules at different temperatures;

[0046] Figure 19 The effect of microcapsule breaker on wellbore plugging ability. DETAILED DESCRIPTION

[0047] To make the objectives, technical solutions, and advantages of this application more clearly understood, this application is further described in detail below in conjunction with examples. The illustrative embodiments and descriptions of this application are intended only to explain this application and are not intended to limit this application. Any product identical or similar to the present application that is derived by anyone under the guidance of this application or by combining the features of this application with other prior arts shall fall within the scope of protection of this application.

[0048] Specific experimental steps or conditions not specified in the examples can be carried out according to the conventional experimental steps or conditions described in the prior art in the art. The reagents and other instruments used, for which the manufacturer is not specified, are all conventional reagent products that can be obtained commercially. In addition, the accompanying drawings are only schematic illustrations of the embodiments of the present application and are not necessarily drawn to scale. The same reference numerals in the figures represent the same or similar parts, and their repeated description will be omitted. Some block diagrams shown in the drawings are functional entities and do not necessarily correspond to physically or logically independent entities.

[0049] The present application provides a method for preparing a self-breaking composite gel liquid bridge plug for wellbore plugging, which specifically comprises the following steps:

[0050] S1. Preparation of tannic acid-acrylamide gel: To improve the adhesion of the gel system, tannic acid, a functional monomer containing catechol hydroxyl groups, was introduced. Optimal reaction conditions were achieved by optimizing the monomer, tannic acid, and crosslinker dosages. The gel's structure was characterized by FT-IR and SEM, and its thermal stability was analyzed by TGA. The gel's tensile strength and adhesion properties were tested, and the effects of temperature and mineralization on these properties were investigated.

[0051] Tannic acid-acrylamide gels were prepared using an aqueous solution free radical polymerization method. AM, AMPS, TA, and the crosslinker MBA were dissolved in pure water in appropriate proportions. The mixture was heated to the set temperature in a water bath and nitrogen was flowed through for 10 minutes to remove oxygen from the system. The initiator APS was dissolved in pure water and preheated to the set temperature. A predetermined amount of the initiator was then added dropwise with stirring.

[0052] S2. Preparation of Tannic Acid-Acrylamide / Konjac Gum Composite Gel and Study of its Wellbore Plugging Capacity: To improve the gel strength of tannic acid-acrylamide gel, konjac gum was introduced to form a composite gel. Building on the previous section, the optimal reaction conditions were obtained by optimizing the dosage of konjac gum and the organic zirconium crosslinker. The structure was characterized by FT-IR and SEM, and the effects of temperature and salinity on the tensile strength and adhesion properties of the gel were investigated. Finally, the wellbore plugging capacity of the gel was tested.

[0053] Tannic acid-acrylamide / konjac gum composite gels were prepared using a free radical polymerization method. First, a certain amount of konjac gum was weighed and stirred for 1 hour to fully dissolve it in pure water. Then, according to the preferred TA@PAM-AMPS gel formula described in the previous section, AM, AMPS, TA, and MBA were weighed. After nitrogen was introduced and stirred for 10 minutes to remove oxygen, the initiator APS and the organic zirconium crosslinker were added. The reaction temperature was set to 40°C.

[0054] S3. Preparation of Microcapsule Breakers and Application Performance Study: The gel system exhibits a certain degree of stability after gelation. Using microencapsulated ammonium persulfate breaker can achieve self-breaking of the gel. By optimizing reaction conditions such as monomer ratio and emulsifier dosage, and characterizing the polyurethane capsules through FT-IR, SEM, and particle size analysis, the release performance of the polyurethane microcapsules was tested, and a self-breaking liquid bridge plug system was constructed.

[0055] The polyurethane microcapsule breaker was prepared by interfacial polymerization. The polyurethane microcapsules consist of toluene diisocyanate-glycerol as the shell and ammonium persulfate as the core. Polyurethane microcapsules have the advantages of simple synthesis, mild reaction conditions, and adjustable release properties.

[0056] First, an oil phase was prepared using ethyl acetate as the organic solvent, toluene diisocyanate as the shell material for the polyurethane core-shell structure, and Tween 80 and Span 80 as emulsifiers. 5 wt% toluene diisocyanate and 2 wt% emulsifier (Tween 80:Span 80, mass ratio 1:1) were dissolved in ethyl acetate and stirred for 30 minutes using a high-speed homogenizer at 12,000 rpm. Next, an aqueous phase was prepared using ammonium persulfate as the microcapsule core and glycerol as the shell material for the polyurethane core-shell structure. Purified water was used as the solvent. 25 wt% ammonium persulfate and 5 wt% glycerol were dissolved in pure water and stirred for 10 minutes using a magnetic stirrer at 2,000 rpm. The oil phase solution was transferred to a three-necked flask, placed in a water bath, and the aqueous phase was dripped dropwise into the oil phase. The reaction temperature was set at 40°C, the speed was set at 2,000 rpm, and the reaction time was 2 hours. After the reaction was completed, the mixture was washed three times with ethyl acetate and pure water respectively to remove unreacted monomers.

[0057] S4. The reaction product is thoroughly washed, preferably with anhydrous ethanol, to obtain a white colloidal substance. The white colloidal substance is dried, ground, and pulverized to obtain a finished wellbore plugging self-breaking composite gel liquid bridge plug.

[0058] The wellbore plugging self-breaking composite gel liquid bridge plug obtained through the above steps is obtained by polyurethane microcapsules with toluene diisocyanate-propylene glycol as the shell and ammonium persulfate as the core. Please refer to Figure 1As shown, the molecular chains in the product are connected to each other to form a stable three-dimensional network structure. The product morphology is shown in formula (1):

[0059] Formula (1) is:

[0060] Formula (1) only represents the molecular structure of the product. The crosslinking degree and thickness of the polyurethane microcapsule shells formed under different synthesis conditions vary. The thicker the microcapsule shell, the greater the force required to rupture it, which prolongs the release time of the core. Increasing the temperature increases the energy of the solution system, making the diffusion of water molecules more intense, which can accelerate the water absorption and expansion of the core, thereby accelerating the release time.

[0061] In addition, the construction of a self-breaking composite gel liquid bridge plug system and the study of its wellbore plugging ability were also carried out. When preparing the composite gel, 2wt%, 4wt%, 6wt%, 8wt% and 10wt% of microcapsule breakers were added respectively. After gelling, they were placed in a 75°C oven to test their breaking performance to explore the effect of the amount of microcapsule breaker added on the self-breaking performance of the composite gel liquid bridge plug. The composite gel was prepared according to the formula, and 8wt% of microcapsule breaker was added. After stirring evenly, it was placed in a simulated casing to test the breakthrough pressure gradient of the gel bridge plug. The simulated casing was placed in a 75°C oven for aging for 24 hours. The bridge plug length was 30 cm to explore the effect of the addition of microcapsule breakers on the wellbore plugging ability of the composite gel.

[0062] In order to enable those skilled in the art to more clearly understand the present invention, the wellbore plugging self-breaking composite gel liquid bridge plug described in the present application will be described in detail below through examples, comparative examples and test examples.

[0063] Example 1: Preparation and structural characterization of tannic acid-acrylamide / konjac gum composite gel

[0064] Tannic acid-acrylamide gel was prepared using an aqueous solution free radical polymerization method. AM, AMPS, TA, and the crosslinker MBA were dissolved in pure water in appropriate proportions, heated to the set temperature in a water bath, and nitrogen was introduced for 10 minutes to remove oxygen from the system. The initiator APS was dissolved in pure water, preheated to the set temperature, and then a predetermined amount of the initiator was added dropwise with stirring. A predetermined amount of konjac gum was then weighed and stirred for 1 hour to fully dissolve in the pure water. The resulting TA@PAM-AMPS gel was then prepared by weighing AM, AMPS, TA, and MBA. N2 was introduced and stirred for 10 minutes to remove oxygen. The initiator APS and the organozirconium crosslinker were then added, and the reaction temperature was set at 40°C.

[0065] Through infrared spectroscopy and micromorphology observation of the composite gel, the results showed that the introduction of konjac gum can form a stable composite gel with TA@PAM-AMPS polymer gel, and an organic zirconium cross-linked structure appears; at the same time, the micromorphology found that the network structure in the formed composite gel is thicker and denser, indicating that the composite gel has better mechanical properties.

[0066] Example 2: Performance Analysis of Tannic Acid-Acrylamide / Konjac Gum Composite Gel

[0067] Composite gel forming properties

[0068] In order to explore the effect of reaction temperature on the gelling strength and time of composite gel, the prepared composite gel was placed at 30℃, 40℃, 50℃, 60℃, 70℃ and 80℃ for gelling, and the results are shown in the following table.

[0069]

[0070]

[0071] In order to explore the gelling properties of the composite gel in water with different mineralizations, the composite gel was gelled in water with different mineralizations, and its gelling time and gelling strength were tested. The reaction temperature was 40°C, and the results are shown in the following table.

[0072]

[0073] Composite gel adhesion properties

[0074] In order to explore the effect of temperature on the adhesion properties of composite gel, the adhesion properties of composite gel at different temperatures were tested.

[0075] In order to test the effect of mineralization on the adhesion performance of the composite gel, the prepared composite gel was immersed in 5000 mg / L, 10000 mg / L, 15000 mg / L, 20000 mg / L, and 25000 mg / L mineralization water for aging for 2 h before testing.

[0076] Mechanical properties of composite gel

[0077] In order to investigate the effect of temperature on the mechanical properties of the composite gel after gelation, the prepared gel was placed in a high-temperature and high-pressure reactor for aging for 24 hours, and the aging temperatures were set at 25°C, 50°C, 75°C, 100°C and 125°C respectively.

[0078] In order to explore the effect of mineralization on the mechanical properties of the composite gel, the prepared gel was placed in 0 mg / L, 5000 mg / L, 10000 mg / L, 15000 mg / L, 20000 mg / L, and 25000 mg / L mineral water for aging for 24 h, and the tensile strength of the gel was tested using an electronic universal testing machine.

[0079] The addition of KGM significantly improves the mechanical properties of the composite gel. On the one hand, the polymer chains formed by TA, AM, and AMPS under MBA crosslinking interpenetrate and entangle with KGM chains, forming an interpenetrating network structure. This strengthens the interaction between the polymer chains and KGM chains, thereby increasing the strength of the composite gel. On the other hand, the numerous hydroxyl groups present in the TA in the composite gel interact with other hydroxyl groups, amide groups, and sulfonic acid groups, forming intermolecular hydrogen bonds that act as physical crosslinks. Simultaneously, the organic zirconium crosslinker, while crosslinking KGM, coordinates with the hydroxyl, amino, and sulfonic acid groups in the polymer chains, making the structure more compact and complex. When subjected to external forces, adjacent crosslinking points effectively disperse stress, maintaining a certain level of structural stability and improving the tensile and compressive strength of the composite gel. The metal ions present in mineralized water, similar to the organic zirconium crosslinker, can bind to certain groups in the gel through electrostatic and coordination interactions, acting as ionic crosslinks and forming a tighter three-dimensional network. The presence of metal ions not only enhances the properties of TA@PAM-AMPS and konjac gum individually but also fosters a synergistic interaction between them. By linking TA@PAM-AMPS and konjac gum, the metal ions enable them to interact more effectively, forming a more stable and dense composite gel network. However, as the degree of cross-linking increases, while imparting higher tensile strength to the composite gel, the gel gradually transforms from an elastic to a brittle state, and the gel's toughness decreases.

[0080] Example 3: Study on the plugging capability of wellbore casing

[0081] In order to test the application effect of the composite gel, a sand-filled tube was used to simulate the wellbore casing. The plugging ability of the composite gel on the wellbore casing was studied through indoor physical model experiments. The wellbore plugging effect of the composite gel was tested under different temperatures, salinity and different bridge plug sizes.

[0082] Wellbore casing plugging pressure test method

[0083] After stirring the composite gel for a period of time to thoroughly mix it, it is transferred to a simulated casing, sealed at both ends, and placed in an oven to allow the gel to gel. After gelation, the plugs at both ends of the simulated casing are removed, and one end is connected to a six-way valve via a pipeline, while the other end remains open. A horizontal pump then injects pure water or mineralized water from the intermediate container into the simulated casing through the six-way valve. The horizontal pump flow rate is set to 0.5 mL / min. Pressure changes during the displacement process are recorded using a pressure sensor and pressure output device. The maximum pressure during the displacement process is the breakthrough pressure of the composite gel, which is used to indicate the compressive strength of the composite gel liquid bridge plug. The breakthrough pressure gradient is the ratio of the breakthrough pressure to the bridge plug strength.

[0084] Effect of temperature on wellbore casing plugging ability

[0085] The sealing pressure bearing capacity of the gel bridge plug on the simulated casing was tested under different temperature conditions. After the gel was gelled in the simulated casing, it was placed in an oven at 25℃, 50℃, 75℃, 100℃ and 125℃ for aging for 24 hours, and then its breakthrough pressure was tested.

[0086] Effect of mineralization on wellbore plugging ability

[0087] During the wellbore casing bridge plugging process, the intrusion of formation water or other working fluids will have a certain impact on the performance of the composite gel. Since the composite gel can improve its mechanical strength under the influence of metal ions, in order to explore the effect of mineralization on the wellbore plugging ability of the composite gel, the composite gel was first prepared and added to the simulated casing at 3 / 4 the length of the bridge plug. After it was gelled, 1 / 4 of 0mg / L, 5000mg / L, 10000mg / L, 15000mg / L, 20000mg / L, and 25000mg / L of mineralized water were added respectively. After sealing both ends, it was placed in an oven for aging for 24 hours and then its breakthrough pressure was tested. The aging temperature was 75℃.

[0088] Effect of liquid bridge plugs on wellbore sealing ability under different bridge plug sizes

[0089] The composite gel liquid bridge plug was tested in simulated casing diameters of 25mm, 50mm, 75mm, and 100mm, and the designed bridge plug lengths were 30cm, 60cm, 90cm, and 100cm respectively.

[0090] Example 4: Preparation of polyurethane microcapsule breaker

[0091] First, an oil phase was prepared using ethyl acetate as the organic solvent, toluene diisocyanate as the shell material for the polyurethane core-shell structure, and Tween 80 and Span 80 as emulsifiers. 5 wt% toluene diisocyanate and 2 wt% emulsifier (Tween 80:Span 80, mass ratio 1:1) were dissolved in ethyl acetate and stirred for 30 minutes using a high-speed homogenizer at 12,000 rpm. Next, an aqueous phase was prepared using ammonium persulfate as the microcapsule core and glycerol as the shell material for the polyurethane core-shell structure. Purified water was used as the solvent. 25 wt% ammonium persulfate and 5 wt% glycerol were dissolved in pure water and stirred for 10 minutes using a magnetic stirrer at 2,000 rpm. The oil phase solution was transferred to a three-necked flask, placed in a water bath, and the aqueous phase was dripped dropwise into the oil phase. The reaction temperature was set at 40°C, the speed was set at 2,000 rpm, and the reaction time was 2 hours. After the reaction was completed, the mixture was washed three times with ethyl acetate and pure water respectively to remove unreacted monomers.

[0092] Example 5: Characterization of polyurethane microcapsule breaker

[0093] The prepared microcapsule breaker was thoroughly mixed with potassium bromide and ground into a tablet, which was then placed in a Fourier transform infrared spectrometer with a scanning wavelength of 400-4000 cm -1 , testing the chemical structure of polyurethane microcapsule breaker, infrared spectrum of polyurethane microcapsule. -1 The NH stretching vibration peaks are at 3059, 2921, and 2861 cm -1 The two peaks are the CH stretching vibration absorption peaks on the benzene ring and the CH in the methyl and methylene groups, 1707 cm -1 The C=O stretching vibration peak in carbamate is 1400-1600 cm -1 The stretching vibration peak of the carbon-carbon double bond of the benzene ring skeleton appears at 1050-1250cm -1 There is a CO absorption peak between the two, and at 2270 cm -1 There is no characteristic absorption peak of isocyanate group -NCO nearby, indicating that -NCO fully participates in the reaction to form urethane bond. According to the infrared spectrum results, the polyurethane microcapsules were successfully synthesized.

[0094] The microscopic morphology of the polyurethane microcapsule breaker was observed using a field emission scanning electron microscope. The prepared microcapsule breaker was bonded to a sample stage with conductive adhesive and then sprayed with gold for observation.

[0095] The prepared microcapsule breaker was ultrasonically dispersed in water, and the particle size of the polyurethane microcapsule breaker was measured using a laser particle size analyzer. The particle size of the prepared polyurethane microcapsules mainly ranged from 11 to 80 μm, accounting for 92.57% of the volume fraction. The minimum particle size was 7.10 μm, the maximum particle size was 355.65 μm, and the median particle size was 32.70 μm. During the microcapsule preparation process, the amount of emulsifier added and the emulsification time had a certain influence on the particle size of the microcapsules. A suitable emulsion ensured a relatively uniform particle size dispersion of the prepared microcapsules, which was sufficient to fully encapsulate the ammonium persulfate core.

[0096] The crystal structure of the polyurethane microcapsules was tested using an X-ray diffractometer. Microcapsules encapsulating ammonium persulfate and microcapsules without ammonium persulfate were prepared, and the crystal structure of ammonium persulfate was tested at test angles of 5° to 85°. The XRD pattern of ammonium persulfate showed distinct diffraction peaks, which were relatively sharp, indicating that ammonium persulfate had a high degree of crystallinity and a distinct crystalline structure. For the empty microcapsules prepared without an ammonium persulfate core, no sharp diffraction peaks appeared, indicating that the prepared polyurethane microcapsules were a polymer without a crystalline structure and were amorphous materials. The diffraction peaks of the polyurethane microcapsules encapsulating the persulfate core were similar to those of the empty microcapsules, but diffraction peaks of a certain intensity appeared at some diffraction angles. However, these peaks were lower than the diffraction intensity at the corresponding angles of ammonium persulfate. This may be because some microcapsules were broken during sample preparation and grinding, exposing the ammonium persulfate core and showing diffraction peaks. Overall, the polyurethane microcapsules were able to effectively encapsulate ammonium persulfate.

[0097] Example 6: Performance test of polyurethane microcapsule breaker

[0098] Release effect test

[0099] The sustained-release effect of polyurethane microcapsules was tested using a conductivity test method. First, standard ammonium persulfate solutions of varying concentrations were prepared, and the conductivity of the standard solutions at different concentrations was measured to generate a concentration-conductivity curve for ammonium persulfate. Next, the conductivity of 100 mL of pure water was measured as an initial value. 0.5 g of polyurethane microcapsules were then ultrasonically dispersed in pure water. The conductivity of the dispersion was measured at regular intervals and compared to the standard curve to determine the ammonium persulfate concentration in the dispersion. When the conductivity of the dispersion no longer changed over time, the ammonium persulfate release from the polyurethane microcapsules was complete.

[0100] Effect of temperature on the release of polyurethane microcapsule breaker

[0101] To investigate the effect of ambient temperature on the release of polyurethane microcapsules from the core material, polyurethane microcapsules were dispersed in water at different temperatures, and their conductivity was measured over time. As the ambient temperature increased, the initial release time of the microcapsules gradually decreased. At 30°C, the release time was 20 hours, and complete core release took 2.0 hours. At 90°C, the initial release time was 2 hours, and complete core release took 0.5 hours. While increasing temperature shortened the initial release time of the microcapsules, it also shortened the time it took for the core material to fully release. On the one hand, increasing temperature intensifies molecular thermal motion, accelerating the diffusion of water molecules into the microcapsule interior. Simultaneously, the core material molecules move faster within the microcapsule, gaining more energy to overcome the constraints of the wall material, making it easier for them to diffuse into the external environment through the wall material's pores or defects. On the other hand, as the temperature rises, the wall material expands, potentially increasing internal stress, leading to cracks or increased pores, which in turn promotes core material release.

[0102] Example 7: Construction of a self-breaking composite gel liquid bridge plug system and study of its wellbore plugging capability

[0103] Determination of the dosage of polyurethane microcapsule breaker

[0104] To investigate the effect of microcapsule breaker dosage on the self-breaking performance of composite gel liquid bridge plugs, 2wt%, 4wt%, 6wt%, 8wt%, and 10wt% microcapsule breaker were added to the composite gels, respectively. After gel formation, the gels were placed in a 75°C oven to test their breaking performance. The table below shows the breaking time and post-breaking viscosity of the composite gels at different microcapsule breaker dosages. When the microcapsule breaker dosage was 2wt%, the composite gel broke within 8.5 days, but the breaking was incomplete, with some gel blocks still present. The viscosity of the gel solution after breaking was 1368.47 mPa·s. As the amount of microcapsule breaker increased, the breaking time of the composite gel gradually shortened, and the viscosity of the broken solution also decreased significantly. When the amount of microcapsule breaker was 8 wt%, the composite gel broke within 4 days, and the viscosity of the broken liquid was 143.25 mPa·s, which showed good fluidity. When the amount of microcapsule breaker was further increased, the viscosity of the composite gel broken liquid decreased slightly.

[0105]

[0106] Study on the wellbore isolation capability of self-breaking composite gel liquid bridge plug

[0107] To investigate the effect of microcapsule breaker addition on the wellbore plugging capability of composite gels, a composite gel was prepared according to the formula and added with 8 wt% microcapsule breaker. After uniform mixing, the gel was placed in a simulated casing and aged for 24 h in a 75°C oven. The bridge plug length was 30 cm. The plugging capability of the composite gels with and without the addition of 8 wt% microcapsule breaker was evaluated. The results showed that before the release of the ammonium persulfate core by the microcapsule breaker, the breakthrough pressure gradient of the composite gel remained essentially unchanged, and the composite gel maintained good plugging pressure resistance. Subsequently, with the release of the ammonium persulfate core, the composite gel began to gradually break. After 4 days of aging, the breakthrough pressure gradient of the composite gel was only 3.16 MPa / m, indicating partial gel breakage, resulting in a significant decrease in the composite gel's plugging pressure resistance. After 5 days of aging at 75°C, the breakthrough pressure gradient of the composite gel was 0.42 MPa / m, indicating complete gel breakage and a substantial loss of plugging capability. With the addition of microcapsule breaker, the composite gel has good self-breaking ability.

Claims

1. The method for preparing the self-breaking composite gel liquid bridge plug for wellbore plugging according to claim 1, characterized in that: The following steps are involved: S1. TA@PAM-AMPS gel was successfully prepared by free radical polymerization of tannic acid (TA), acrylamide (AM), and 2-acrylamido-2-methylpropanesulfonic acid (AMPS). N,N'-methylenebisacrylamide (MBA) was used as a cross-linker and ammonium persulfate (APS) was used as an initiator. The synthesis conditions were optimized by optimizing the total monomer addition, TA addition, and MBA addition. The resulting TA@PAM-AMPS gel had a tensile strength of 131.24 kPa and an adhesion strength of 23.42 kPa. The addition of TA improved the adhesion performance by 3.15 times compared to the gel without TA addition. S2. introduces natural plant polysaccharide konjac glucomannan and polymer gel to form composite gel. By optimizing the dosage of konjac glucomannan and organic zirconium cross-linking agent, the tensile strength of the composite gel is 1687.46kPa when the dosage of konjac glucomannan is 4000mg / L and the dosage of organic zirconium cross-linking agent is 30wt%, and the adhesion strength is 30.48kPa, which is 12.86 times higher than that of the single TA@PAM-AMPS gel tensile strength. Micromorphology shows that the structure of the composite gel is more robust and dense, which enhances its mechanical properties. The wellbore plugging ability of the composite gel was tested, and its indoor simulated casing breakthrough pressure gradient reached 10.67MPa / m, and the addition of konjac glucomannan improved the temperature resistance of the gel. S3. Self-breaking of the composite gel was achieved by preparing a microcapsule breaker. The microcapsule breaker uses toluene diisocyanate-propylene glycol as the shell and ammonium persulfate as the core. Interfacial polymerization was used to prepare core-shell polyurethane microcapsules capable of delayed release of ammonium persulfate. The microcapsule breaker's particle size ranged from 11 to 80 μm, with a median size of 32.70 μm. The microcapsules possessed numerous micropores on the surface, which allowed the shell to rupture upon water absorption and expansion. The delayed release time in water was 19.5 hours, and the encapsulation efficiency reached 47.29%. By adding 8 wt% of the microcapsule breaker, the composite gel broke within 4 days, with a breaker solution viscosity of 143.25 mPa·s. Indoor simulated casing plugging capacity experiments demonstrated that the addition of the microcapsule breaker did not affect the gelling properties and wellbore plugging capability of the composite gel, demonstrating its excellent adaptability.

2. The method for preparing the self-breaking composite gel liquid bridge plug for wellbore plugging according to claim 2, characterized in that: The preparation method of the microcapsule breaker in step S3 comprises the following steps: S31. Prepare the oil phase using ethyl acetate as the organic solvent, toluene diisocyanate as the shell material for the polyurethane core-shell structure, and Tween 80 and Span 80 as emulsifiers. Dissolve 5 wt% of toluene diisocyanate and 2 wt% of the emulsifier (Tween 80:Span 80, mass ratio: 1:1) in ethyl acetate. Stir using a high-speed homogenizer at 12,000 rpm for 30 minutes. S32. Prepare an aqueous phase, wherein ammonium persulfate is the microcapsule core, glycerol is the polyurethane core-shell shell material, and the solvent is pure water. Dissolve 25 wt% ammonium persulfate and 5 wt% glycerol in pure water and stir using a magnetic stirrer at 2000 rpm for 10 minutes. S33. Transfer the oil phase solution to a three-necked flask and place it in a water bath. Add the aqueous phase dropwise to the oil phase. Set the reaction temperature to 40°C, the rotation speed to 2000 rpm, and the reaction time to 2 hours. After the reaction, wash the flask three times with ethyl acetate and then with pure water to remove unreacted monomers.

3. The method for preparing the wellbore plugging self-breaking composite gel liquid bridge plug according to claim 2, characterized in that: As described in step S1, 5 wt% toluene diisocyanate and 2 wt% emulsifier (Tween 80 and Span 80 mass ratio is 1:1) are dissolved in ethyl acetate.

4. The method for preparing the wellbore plugging self-breaking composite gel liquid bridge plug according to claim 2, characterized in that: The amount of the microcapsule breaker is 8% of the total weight of the monomers.

5. The method for preparing the self-breaking composite gel liquid bridge plug for wellbore plugging according to claim 2, characterized in that: The amount of the initiator used in step S1 is 25% of the total mass of tannic acid (TA), acrylamide (AM), and 2-acrylamide-2-methylpropanesulfonic acid (AMPS) monomers.

6. The method for preparing the self-breaking composite gel liquid bridge plug for wellbore plugging according to claim 2, characterized in that: The initiator in step S3 is ammonium persulfate.

7. The method for preparing the self-breaking composite gel liquid bridge plug for wellbore plugging according to claim 7, characterized in that: The microcapsule breaker is added in amounts of 2wt%, 4wt%, 6wt%, 8wt% and 10wt% respectively.

8. An application of the self-breaking composite gel liquid bridge plug for wellbore plugging according to claim 8.

9. The use of the self-breaking composite gel liquid bridge plug for wellbore plugging according to claim 9, characterized in that: The microcapsule breaker is mixed with a composite gel liquid bridge plug to prepare a self-breaking tannic acid-acrylamide / konjac gum composite gel bridge plug.

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