Coalbed methane fracturing fluid system and multi-fracture equal propagation method

Abstract

This invention discloses a coalbed methane fracturing fluid system and a method for balanced propagation of multiple fractures, relating to the field of coalbed methane extraction technology. The coalbed methane fracturing fluid system includes a main fluid base as the transport medium, ceramic particles as proppant, coated heat-generating microcapsules, and a self-responsive diverting agent. The core material of the heat-generating microcapsules is a solid substance that releases heat upon contact with water. Its coating layer has temperature-responsive characteristics, softening and rupturing when a temperature threshold is reached, releasing heat to reduce the viscosity of the fracturing fluid. The self-responsive diverting agent has a core-shell structure; its outer shell hydrolyzes at specific temperatures and pH levels, causing the core elastomer to absorb water and expand. It can selectively and autonomously seal wide fractures according to the fracture width, guiding the fracturing fluid to narrower, less expanded fractures. The method includes: first, injecting a pre-fluid containing coated heat-generating microcapsules; then adding proppant and diverting agent and pumping sand-carrying fluid; finally, shutting in the well and controlling flowback. This optimizes fracture conductivity, promotes balanced propagation of multiple fractures, and improves extraction efficiency.

Description

Technical Field

[0001] This invention relates to the field of coalbed methane extraction technology, and in particular to a coalbed methane fracturing fluid system and a method for balanced propagation of multiple fractures. Background Technology

[0002] In coalbed methane extraction, hydraulic fracturing is a key technology for reservoir modification and increasing production capacity. However, conventional fracturing technology has significant shortcomings in low-temperature coal seams: on the one hand, the viscosity of conventional fracturing fluid increases sharply at low temperatures, leading to a significant decrease in proppant carrying capacity and insufficient proppant filling at the distal end of the fracture, affecting the fracture conductivity; on the other hand, artificial temporary plugging technology relies on external intervention methods such as ball dropping and fiber insertion, which results in a lag in the dynamic response to fracture propagation, easily leading to over-plugging or plugging failure, making it difficult to achieve balanced propagation of multiple fractures, thus restricting the efficiency of coalbed methane extraction. Summary of the Invention

[0003] The purpose of this invention is to provide a coalbed methane fracturing fluid system and a method for balanced propagation of multiple fractures, so as to solve the problems existing in the prior art, optimize the fracture conduction effect, achieve balanced propagation of multiple fractures, and improve mining efficiency.

[0004] To achieve the above objectives, the present invention provides the following solution:

[0005] This invention provides a coalbed methane fracturing fluid system, comprising a main liquid base, a proppant, encapsulated thermogenic microcapsules, and a self-responsive diverting agent; the main liquid base is an aqueous flow transport medium; and the main liquid base is used to fracture coal seams and form fractures within the coal seams; the proppant is ceramsite and is added to the main liquid base; the encapsulated thermogenic microcapsules are dispersed within the main liquid base; the core material of the encapsulated thermogenic microcapsules comprises a mixture of solid powdered reactive substances capable of generating an exothermic reaction in an aqueous environment; the encapsulation layer of the encapsulated thermogenic microcapsules is a temperature-responsive slow-release material, the encapsulation layer being in a hard state when the ambient temperature is below a preset temperature threshold, and the encapsulation layer being in a temperature-responsive slow-release state when the ambient temperature is below a preset temperature threshold. When the temperature reaches or exceeds a preset temperature threshold, the coating layer is in a molten and soft state. In this molten and soft state, the coating layer can rupture under the shear force of the flow or the collision with the crack wall to release the core material of the coated thermogenic microcapsule. The self-responsive deflecting agent is a core-shell structure particle, and the core layer of the self-responsive deflecting agent is a water-absorbing and swellable elastomer. The outer shell of the self-responsive deflecting agent can be hydrolyzed when the temperature and pH thresholds are exceeded, so that the core layer of the self-responsive deflecting agent is exposed to the main liquid base and absorbs water and swells. The outer dimensions of the ceramic particles are larger than the outer dimensions of the self-responsive deflecting agent, and the outer dimensions of the self-responsive deflecting agent are larger than the outer dimensions of the coated thermogenic microcapsule.

[0006] Preferably, the core material of the encapsulated heat-generating microcapsule further includes a reaction rate buffer; the mixed reactants include solid powdered ammonium nitrite and ammonium chloride; the molar ratio of ammonium nitrite to ammonium chloride is controlled at 1:(0.5~1.0); the reaction rate buffer is solid powdered boric acid; the amount of boric acid added is controlled at a mass fraction of 5%~15% of the main liquid base.

[0007] Preferably, the temperature-responsive slow-release material is a composite of a phase change material and a polymer; the phase change material is paraffin or fatty acid; and the polymer is polystyrene, polyacrylate, or ethylene-vinyl acetate copolymer.

[0008] Preferably, the main liquid base is slickwater, which contains water, drag-reducing agent and anti-swelling agent; and the water content is above 98%.

[0009] Preferably, the original particle size of the self-responsive diverting agent before injection is 20-70 mesh; and the particle size after expansion is 1.5-2.5 times the original particle size.

[0010] Preferably, the content of the proppant is 5% to 15% of the main liquid base; and the amount of the self-responsive diverting agent added is 10% to 30% of the total mass of the proppant and the self-responsive diverting agent.

[0011] Preferably, the shell of the self-responsive deflector is a coal-rock affinity coating, and the surface of the coal-rock affinity coating is grafted with oxygen-containing functional groups.

[0012] The present invention also provides a method for balanced propagation of multiple fractures using a coalbed methane fracturing fluid system as described in any of the preceding claims, comprising the following steps:

[0013] S1: Pre-fluid Injection: The pre-fluid includes the main liquid base and the encapsulated thermogenic microcapsules, wherein the concentration of the encapsulated thermogenic microcapsules is 20~30 kg / m³. 3 ; with 10~16m 3 The pre-flush fluid is injected into the coal seam at a pumping rate of / min and an injection pressure lower than 85% to 95% of the formation fracturing pressure; the volume of the pre-flush fluid is 25% to 35% of the total fluid volume;

[0014] S2: Sand-carrying fluid injection: The proppant and the self-responsive diverting agent are added to the pre-filled fluid to form the sand-carrying fluid; the sand-carrying fluid is then continuously pumped into the coal seam.

[0015] S3: Shutdown and backflow: After stopping pumping, shut down the well for 1.5 to 3.0 hours, and then backflow at a rate ≤ 15% of the injection rate.

[0016] Preferably, in step S2, the concentration of the proppant is gradually increased to the designed maximum concentration in a stepwise manner; the self-responsive diverting agent is added by pulsed slug injection, with each slug accounting for 3% to 5% of the total liquid volume, and the mass of the self-responsive diverting agent in the slug is 15% to 25% of the mass of the proppant.

[0017] Preferably, in step S2, the bottom hole net pressure is monitored to determine the steering effect. When the bottom hole net pressure continues to rise by 5% to 15% after the injection of the slug containing the self-responsive steering agent, it indicates that the steering is successful.

[0018] The present invention achieves the following technical effects compared to the prior art:

[0019] The coalbed methane fracturing fluid system provided by this invention, after fracturing fluid with a main fluid base as a carrier enters the coal seam fracture, the coated heat-generating microcapsules in the system first play a role: when the coating layer reaches the coal seam ambient temperature threshold, it changes from a hard state to a molten and soft state, and ruptures under the action of fluid shear, releasing the solid powdery mixed reactive substances in the core material; these substances undergo an exothermic reaction in the water environment, and the heat generated effectively reduces the viscosity of the main fluid base, thereby significantly enhancing its ability to carry ceramic particles to the far end of the fracture, fundamentally solving the problem of high fracturing fluid viscosity and insufficient proppant delivery distance under low temperature conditions; and the generated gas pressure can help clean up the contamination in the near-wellbore area and assist in opening up a more complex fracture network, as the main fracture temporarily widens due to heat and pressure. Meanwhile, under the combined effects of downhole temperature and pH, the outer shell of the self-responsive diverting agent in the system gradually hydrolyzes, exposing the elastomer core. This elastomer expands upon absorbing water, and because the size design of the self-responsive diverting agent falls between that of encapsulated heat-generating microcapsules and ceramic particles, it can effectively seal over-expanded wide fractures. This forces the main fluid base carrying the ceramic particles to divert into narrower or secondary fracture networks that have not been sufficiently fractured. Compared to previous processes, proppant is less prone to near-wellbore accumulation, resulting in better support for the fractures. This heat-generating viscosity reduction and fracture diversion mechanism, triggered autonomously by material properties, achieves real-time response and adaptive control to fracture propagation dynamics. It not only overcomes the poor low-temperature performance of conventional fracturing fluids but also solves the industry pain points of slow response and difficulty in achieving balanced multi-fracture propagation in manual temporary plugging techniques.

[0020] This invention also provides a method for balanced propagation of multiple fractures. In the S1 pre-fracturing stage, a main liquid base containing a suitable concentration of encapsulated heat-generating microcapsules is injected at a specific flow rate and pressure. This safely and effectively opens the initial fracture network and smoothly delivers the heat-generating agent to the depths of the fractures, laying the foundation for subsequent heat-generating reactions. In the S2 proppant-carrying liquid injection stage, ceramsite and a self-responsive diverting agent are added on top of the preheated liquid. The heat continuously released by the encapsulated heat-generating microcapsules significantly reduces the proppant-carrying viscosity of the main liquid base, while the self-responsive diverting agent autonomously selects the sealing position based on the difference in fracture width. The two work together to ensure that the proppant is efficiently delivered and evenly distributed in the complex fracture network, including distal and secondary fractures, effectively solving the problems of insufficient proppant transport and uneven distribution in conventional fracturing. The final S3 well shut-in and flowback stage is crucial. Sufficient shut-in time provides the necessary conditions for the complete thermal reaction and full expansion of the diverting agent. Subsequent flowback at a strictly controlled rate effectively utilizes the residual heat of the reaction to further reduce fluid viscosity and any potential gas lift effect, significantly improving the flowback efficiency of the fracturing fluid and mitigating reservoir water lock damage. This method, through the coordinated control of these three stages, ultimately achieves comprehensive technical effects including balanced multi-fracture propagation in coalbed methane, enhanced conductivity, and efficient reservoir protection. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a flowchart illustrating the multi-crack balanced propagation method provided by the present invention.

[0023] Figure 2 This invention provides a comparative explanation of the effects of adding boric acid and not adding boric acid on heat generation in the encapsulated heat-generating microcapsules of the coalbed methane fracturing fluid system provided by the present invention.

[0024] Figure 3 A schematic diagram illustrating the failure and water absorption expansion mechanism of the self-responsive diverting agent shell in the coalbed methane fracturing fluid system provided by the present invention;

[0025] Figure 4 The cumulative release rate of coated heat-generating microcapsules versus temperature rise curve in the coalbed methane fracturing fluid system provided by this invention;

[0026] Figure 5 This is a schematic diagram illustrating the viscosity-temperature relationship test of fracturing fluid in the coalbed methane fracturing fluid system provided by the present invention.

[0027] Figure 6 This is a comparison diagram of the conductivity of the coalbed methane fracturing fluid system and the multi-fracture balanced propagation method provided by the present invention before and after plugging. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] The purpose of this invention is to provide a coalbed methane fracturing fluid system and a method for balanced propagation of multiple fractures, so as to solve the problems existing in the prior art, optimize the fracture conduction effect, achieve balanced propagation of multiple fractures, and improve mining efficiency.

[0030] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0031] Example 1

[0032] This embodiment provides a coalbed methane fracturing fluid system, such as... Figures 1-6 As shown, the device includes a main liquid base, a proppant, encapsulated thermogenic microcapsules, and a self-responsive diverting agent. The main liquid base is an aqueous flow transport medium used to fracture coal seams and create fractures within them. The proppant is ceramsite and is added to the main liquid base. The encapsulated thermogenic microcapsules are dispersed within the main liquid base. The core material of the encapsulated thermogenic microcapsules comprises a mixture of solid powdered reactive substances capable of generating an exothermic reaction in an aqueous environment. The encapsulation layer of the encapsulated thermogenic microcapsules is a temperature-responsive slow-release material. The encapsulation layer is in a hard state when the ambient temperature is below a preset temperature threshold, and in a molten state when the ambient temperature reaches or exceeds the preset temperature threshold. In a molten soft state, the coating layer can rupture under flowing shear force (which can break under slight shear force) or collision with the crack wall to release the core material of the encapsulated thermogenic microcapsule; the self-responsive diverting agent is a core-shell structure particle, and the core layer of the self-responsive diverting agent is a water-absorbing and swellable elastomer (such as nitrile rubber particles); the shell of the self-responsive diverting agent can hydrolyze when the temperature and pH thresholds are exceeded, so that the core layer of the self-responsive diverting agent is exposed to the main liquid base and absorbs water and swells; the size of the ceramic particles is larger than the size of the self-responsive diverting agent, and the size of the self-responsive diverting agent is larger than the size of the encapsulated thermogenic microcapsule.

[0033] After fracturing fluid carrying a main liquid base enters the coal seam fracture, the encapsulated heat-generating microcapsules in the system first play their role: their coating layer changes from a hard state to a molten and soft state when the coal seam ambient temperature threshold is reached, and then ruptures under fluid shear, releasing the solid powdery mixed reactive substances in the core material; these substances undergo an exothermic reaction in the water environment, and the heat generated effectively reduces the viscosity of the main liquid base, thereby significantly enhancing its ability to carry ceramic particles to the far end of the fracture, fundamentally solving the problem of high fracturing fluid viscosity and insufficient proppant delivery distance under low temperature conditions; and the generated gas pressure can help clean up contamination in the near-wellbore zone and assist in opening up a more complex fracture network. As the main fracture temporarily widens due to heat and pressure (when the self-responsive diverting agent is activated, it can precisely seal the entrances of these treated main fractures), thus achieving modification and sealing. (and redirection). Simultaneously, under the combined influence of downhole temperature and pH, the outer shell of the self-responsive redirecting agent in the system gradually hydrolyzes, exposing the elastomer core. This elastomer expands in volume upon absorbing water, and because the size design of the self-responsive redirecting agent falls between that of encapsulated heat-generating microcapsules and ceramic particles, it can effectively seal over-expanded wide fractures. This forces the main fluid base carrying the ceramic particles to redirect and flow into narrower or secondary fracture networks that have not been sufficiently fractured. Compared to previous processes, proppant is less prone to near-wellbore accumulation, achieving better support for fracturing fractures. This heat-generating viscosity reduction and fracture redirection mechanism, triggered autonomously by material properties, achieves real-time response and adaptive control of fracture propagation dynamics. It not only overcomes the poor low-temperature performance of conventional fracturing fluids but also solves the industry pain points of delayed response and difficulty in achieving balanced multi-fracture propagation in manual temporary plugging technology.

[0034] The following are the settings instructions regarding the main liquid base:

[0035] In the optional solutions of this embodiment, the preferred option is that the main liquid base is slickwater, which contains water, drag-reducing agent and anti-swelling agent; and the water content is above 98%.

[0036] Specifically, the fracturing fluid is based on clean water (accounting for over 98%), with drag-reducing agents (such as polyacrylamide) as the core additive, supplemented by a small amount of clay stabilizers (anti-swelling agents), etc. The total amount of chemical additives (i.e., drag-reducing agents and anti-swelling agents, etc.) is only 0.5%~2%. Low-damage fracturing fluid is used. Here, low damage is a comprehensive design goal, which means that while the fracturing fluid performs its engineering functions (fracture creation and proppant transport), it minimizes the negative impact on the permeability of the reservoir matrix and the conductivity of the fractures.

[0037] The following are the instructions regarding the setting of the proppant:

[0038] Specifically, proppant is the main solid additive in fracturing fluid, making up the largest proportion. Its concentration varies greatly depending on the construction design, typically ranging from 5% to 15% (mass-volume ratio, i.e., 50 kg / m³). 3 ~150kg / m 3 Between ).

[0039] The following are the specifications regarding the encapsulated heat-generating microcapsules:

[0040] Specifically, the encapsulated thermogenic microcapsules have a particle size of 50-100 μm.

[0041] Among the optional solutions in this embodiment, the more preferred one is as follows: Figure 2 As shown, the core material of the encapsulated heat-generating microcapsule also includes a reaction rate buffer; the mixed reaction substances include solid powdered ammonium nitrite and ammonium chloride; the molar ratio of ammonium nitrite to ammonium chloride is controlled at 1: (0.5 ~ 1.0); the reaction rate buffer is solid powdered boric acid; the amount of boric acid added is controlled at 5% ~ 15% of the main liquid base by mass fraction.

[0042] Specifically, heat causes the micropores in the coal seam to expand, increasing the fracturing fluid filtration efficiency and increasing fracture branching.

[0043] Specifically, in the preparation of encapsulated heat-generating microcapsules, ammonium nitrite solid powder, ammonium chloride solid powder and boric acid solid powder are physically mixed evenly in a predetermined ratio, and then used as the whole core material, which is then encapsulated with a coating layer (such as paraffin / EVA composite material) with the same temperature control.

[0044] Specifically, boric acid is used as a reaction rate buffer to control the temperature rise gradient (target temperature rise of 20℃~30℃).

[0045] Specifically, ammonium chloride hydrolysis provides an acidic environment:

[0046] NH4Cl→NH4 + + Cl - ;

[0047] NH4 + +H₂O→NH₃·H₂O+H + (The solution is weakly acidic)

[0048] Specifically, ammonium nitrite decomposes under acidic conditions (this is the main exothermic reaction):

[0049] NH4NO2+H + →N2↑+2H2O+H + (ΔH<<0, strongly exothermic).

[0050] In the optional solutions of this embodiment, the more preferred option is that the temperature-responsive slow-release material is a composite of a phase change material and a polymer; the phase change material is paraffin or fatty acid; and the polymer is polystyrene, polyacrylate, or ethylene-vinyl acetate copolymer.

[0051] Specifically, the polymer is dispersed within the coating layer to enhance the mechanical strength of the microcapsules (i.e., coated heat-generating microcapsules; all names used below have the same meaning and will not be annotated again) and prevent premature rupture during pumping.

[0052] 1. Stable during injection (temperature <40℃): At ground temperatures (typically below 30℃) and within the injection pipeline, the phase change material in the cladding layer is in a solid state, resulting in a rigid shell. This effectively resists the shear forces generated during pumping and completely isolates the internal reactants from the external fracturing fluid, ensuring absolute stability and preventing any reaction before reaching the target location.

[0053] 2. Release within fractures (temperature ≥ 40℃): When the microcapsules enter the deep formation with the fracturing fluid, and the ambient temperature gradually rises to above 40℃, the phase change material in the coating absorbs heat and undergoes a solid-liquid phase change, and the shell changes from a hard state to a molten and soft state.

[0054] 3. Release and reaction: In the molten state, the mechanical strength of the coating layer decreases significantly, and it can break under the shear force of fluid flow or collision with the crack wall.

[0055] The following are the settings instructions regarding self-responsive diverting agents:

[0056] Specifically, the response logic is as follows: when the fracture width is >3mm, the self-responsive diverting agent is embedded in the fracture wall under the action of shear force, and the elastomer absorbs water and expands (expansion rate 200%~300%), sealing the wide fracture and forcing the fracturing fluid to divert to the narrow fracture; when the fracture width is ≤2mm, the self-responsive diverting agent cannot be embedded and continues to advance with the fluid flow to the insufficiently expanded area.

[0057] In the optional schemes of this embodiment, it is more preferred that the original particle size of the self-responsive diverting agent before injection is 20 mesh to 70 mesh (approximately 850 μm to 212 μm; this particle size range ensures that the self-responsive diverting agent has good flowability during pumping and can be uniformly mixed with the proppant); and the particle size after expansion is 1.5 to 2.5 times the original particle size.

[0058] Specifically, for example, a particle with an initial diameter of 500 μm can expand to 750 μm to 1250 μm. This size is sufficient to form an effective bridging at the throat of a crack that has already been partially filled with proppant.

[0059] In the optional schemes of this embodiment, it is more preferred that the content of the proppant accounts for 5% to 15% of the main liquid base; and the amount of self-responsive diverting agent added is 10% to 30% of the total mass of the proppant and the self-responsive diverting agent.

[0060] In the optional embodiments of this example, the preferred option is that the shell of the self-responsive deflector is a coal-rock affinity coating, and the surface of the coal-rock affinity coating is grafted with oxygen-containing functional groups.

[0061] Regarding other related settings:

[0062] Specifically, the ratio of encapsulated heat-generating microcapsules to slickwater is not a fixed value, but is dynamically optimized based on geological conditions, construction design, and target temperature rise.

[0063] For example, raising the temperature of the local fluid within the crack by 20°C to 30°C can reduce the viscosity of the slickwater by 30% to 50%.

[0064] Thermodynamic calculations:

[0065] ①The specific heat capacity of slippery water is approximately 4.2 kJ / (kg·℃).

[0066] ② To raise the temperature of 1 cubic meter (approximately 1000 kg) of slickwater by 25°C, the required heat is: 1000 kg × 4.2 kJ / (kg·°C) × 25°C = 105,000 kJ.

[0067] ③The heat of reaction of our heat-generating agent (mainly the decomposition of NH4NO2) is approximately -330kJ / mol.

[0068] ④ To provide 105,000 kJ of heat, approximately 318 mol of NH4NO2 is required, with a mass of approximately 23 kg.

[0069] ⑤ The encapsulated heat-generating microcapsules not only contain the heat-generating agent ammonium nitrite, but also ammonium chloride, boric acid buffer, and encapsulation layer.

[0070] ⑥ The active heat-generating agent (NH4NO2) accounts for approximately 50% to 60% of the total mass of the encapsulated heat-generating microcapsules.

[0071] ⑦ Therefore, to provide 23 kg of heat-generating agent, approximately 40 kg to 45 kg of encapsulated heat-generating microcapsules are needed.

[0072] ⑧ Taking into account factors such as heat loss (heating rock, proppant) and incomplete reaction, we appropriately increased the theoretical value and set an effective range, which yielded 10 kg / m 3 ~40kg / m 3 The addition range. Among them, 20kg / m³ 3 ~30kg / m3 (2% ~ 3%) is the "dessert zone" that achieves the desired effect and offers the best value for money.

[0073] Example 2

[0074] This embodiment provides a method for balanced propagation of multiple fractures using the coalbed methane fracturing fluid system of Embodiment 1, such as... Figure 1 As shown, it includes the following steps:

[0075] S1: Pre-fluid Injection: The pre-fluid consists of a main liquid base and encapsulated thermogenic microcapsules, wherein the concentration of the encapsulated thermogenic microcapsules is 20~30 kg / m³. 3 (This concentration is a "sweet spot" verified by thermodynamic calculations and experiments, ensuring an effective temperature rise of 20°C to 30°C within the crack without affecting the pumping performance of the slickwater); at 10~16m 3 A pumping rate of / min (this rate range is sufficient to initiate an effective initial fracture network in the target reservoir; too low a rate results in insufficient fracture extension; too high a rate may lead to overly simplistic near-wellbore fractures, hindering the formation of a complex fracture network) is used to inject pre-fluid into the coal seam at an injection pressure 85%–95% lower than the formation fracturing pressure (a "soft start" strategy is adopted during initial injection, with pressure gradually increasing to avoid severe impact on the rock; the injection pressure needs to be monitored in real time and ensured to be within the safe window of the formation fracturing pressure and the equipment's rated operating pressure) (the thermal reaction starts within 20 minutes); the volume of the pre-fluid is 25%–35% of the total fluid volume (ensuring sufficient fluid volume to create enough fracture volume to accommodate subsequent proppant and diverting agents).

[0076] S2: Sand-carrying fluid injection: Add proppant and self-responsive diverting agent to the pre-filled fluid to form sand-carrying fluid; continue pumping sand-carrying fluid into the coal seam;

[0077] S3: Well Shutdown and Runback: After stopping pumping, shut down the well for 1.5 to 3.0 hours (this time window is set based on experiments of thermal reaction kinetics and self-responsive diverting agent expansion kinetics. It ensures that: ① the encapsulated thermal microcapsules react fully and the heat is transferred to a wider area; ② the outer shell of the self-responsive diverting agent degrades fully and the core expands fully to reach the maximum sealing strength). Then, runback is carried out at a runback rate ≤ 15% of the injection rate (this is the key to utilizing thermal energy; slow runback allows more time for heat to reduce the viscosity of crude oil in the deep reservoir; small nozzles (such as 4mm to 8mm) are used for control, the initial runback rate is very low, and then the nozzle size and runback rate are gradually increased over 24-48 hours).

[0078] Specifically, in step S3, during the initial stage of the flowback, the temperature of the flowback liquid should be significantly higher than that of the injected liquid, which is direct evidence of a successful heat-generating reaction. Simultaneously, the nitrogen gas produced by the reaction increases the gas-liquid ratio in the early stages of the flowback, contributing to the gas lift effect and aiding in the liquid flowback.

[0079] By injecting a main liquid base containing an appropriate concentration of encapsulated thermally generating microcapsules at a specific flow rate and pressure during the S1 pre-fracturing stage, the initial fracture network can be safely and effectively opened, and the thermally generating agent can be smoothly delivered to the depth of the fractures, laying the foundation for subsequent thermal reactions. After entering the S2 proppant-carrying liquid injection stage, ceramsite and a self-responsive diverting agent are added on top of the preheated liquid. The heat continuously released by the encapsulated thermally generating microcapsules significantly reduces the proppant-carrying viscosity of the main liquid base, while the self-responsive diverting agent autonomously selects the sealing position based on the difference in fracture width. The two work synergistically to ensure that the proppant is efficiently delivered and evenly distributed in the complex fracture network, including distal and secondary fractures, effectively solving the problems of insufficient proppant transport and uneven distribution in conventional fracturing. The final S3 well shut-in and flowback stage is crucial. Sufficient shut-in time provides the necessary conditions for the complete thermal reaction and full expansion of the diverting agent. Subsequent flowback at a strictly controlled rate effectively utilizes the residual heat of the reaction to further reduce fluid viscosity and any potential gas lift effect, significantly improving the flowback efficiency of the fracturing fluid and mitigating reservoir water lock damage. This method, through the coordinated control of these three stages, ultimately achieves comprehensive technical effects including balanced multi-fracture propagation in coalbed methane, enhanced conductivity, and efficient reservoir protection.

[0080] Specifically, the collaborative work process:

[0081] Phase 1 (Crack Creation and Heat Generation): Inject low-viscosity slippery water containing a heat-generating agent (i.e., encapsulated heat-generating microcapsules) to preferentially open natural cracks, and the heat generation reaction enhances the fluidity of the liquid.

[0082] Stage 2 (Self-direction and proppant carrying): Add self-responsive diverting agent and proppant (70 / 140 mesh ceramsite), the wide fracture is selectively sealed, and the fracturing fluid carries the proppant into the narrow fracture;

[0083] Phase 3 (Heat-Assisted Backflow): The heat-generating agent (i.e., encapsulated heat-generating microcapsules) continuously releases heat to reduce the viscosity of the liquid, making it easier for the liquid to flow back after fracturing and reducing water lock damage.

[0084] In the optional embodiments of this example, more preferably, in step S2, the concentration of the proppant is gradually increased to the designed maximum concentration in a stepwise manner (stepwise increase, from 100 kg / m³). 3 Up to 400 kg / m 3 The "stepwise concentration" method is used, with a low initial concentration (e.g., 5%, i.e., ~50 kg / m³). 3Based on the pressure response, the concentration is gradually increased to the design maximum (e.g., 12%, i.e., ~400 kg / m³). 3 This is industry standard practice, designed to ensure smooth sand laying; the self-responsive diverting agent is added via pulsed slug injection, with each slug containing 3% to 5% of the total volume (the diverting agent is not added continuously, but via pulsed slug injection; a small, controllable volume is injected each time, the pressure response is observed, and a decision is made on whether to proceed with the next diverting operation), and the mass of the self-responsive diverting agent in the slug is 15% to 25% of the proppant mass (in the "slug" requiring diverting, it is mixed with proppant in this proportion; for example, when a pressure drop indicates low fluid efficiency, a slug containing the self-responsive diverting agent is injected).

[0085] In the optional schemes of this embodiment, it is more preferred that in step S2, the bottom hole net pressure (bottom hole pressure - closure pressure; net pressure is the core indicator for judging whether the diversion is successful; when the slug containing the self-responsive diversion agent is injected, if the net pressure is observed to rise continuously and stably by 5% - 15%, it indicates that the diversion agent has effectively blocked the flow ahead, generating new flow resistance and forcing the fluid to change direction) is used to judge the diversion effect. When the bottom hole net pressure rises continuously by 5% to 15% after the slug containing the self-responsive diversion agent is injected, it indicates that the diversion is successful.

[0086] Specifically, regarding other relevant information:

[0087] Adaptive crack control: The self-responsive diverting agent autonomously decides the sealing location based on the crack width, without the need for manual intervention or complex monitoring;

[0088] Increased proppant carrying capacity: The heat-generating reaction reduces liquid viscosity, increasing proppant transport distance by more than 40%;

[0089] Environmentally friendly: The reaction products are NH4Cl and trace amounts of HNO2 (neutralized by pH buffer), with no heavy metal pollution;

[0090] Cost controllable: Encapsulated heat-generating microcapsules can be mass-produced through industrial encapsulation processes, with a lower cost than traditional smart temporary plugging agents.

[0091] Specific examples have been used to illustrate the principles and implementation methods of this invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of this invention. Furthermore, those skilled in the art will recognize that, based on the ideas of this invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this invention.

Claims

1. A coalbed methane fracturing fluid system, characterized in that: Including main liquid base, proppant, encapsulated thermogenic microcapsules and self-responsive diverting agent; The main liquid base is a water-containing flowing transport medium; and the main liquid base is used to fracturing coal seams and forming fractures within the coal seams; The proppant is ceramsite and is used to be added to the main liquid base; The encapsulated thermogenic microcapsules are dispersed within the main liquid base; the core material of the encapsulated thermogenic microcapsules comprises a mixture of solid powdered reactive substances capable of generating an exothermic reaction in an aqueous environment; the encapsulation layer of the encapsulated thermogenic microcapsules is a temperature-responsive slow-release material, which is in a hard state when the ambient temperature is below a preset temperature threshold, and in a molten and soft state when the ambient temperature reaches or exceeds the preset temperature threshold. The encapsulation layer in the molten and soft state can rupture under the shear force of flow or collision with the crack wall to release the core material of the encapsulated thermogenic microcapsules. The self-responsive diverting agent is a core-shell structured particle, and the core layer of the self-responsive diverting agent is a water-absorbing and swellable elastomer; the outer shell of the self-responsive diverting agent can be hydrolyzed when the temperature threshold and pH threshold are exceeded, so that the core layer of the self-responsive diverting agent is exposed to the main liquid base and absorbs water and swells. The outer dimensions of the ceramsite are larger than those of the self-responsive diverting agent, and the outer dimensions of the self-responsive diverting agent are larger than those of the encapsulated thermogenic microcapsule. The core material of the encapsulated thermogenic microcapsule also includes a reaction rate buffer; The mixed reactants include solid powdered ammonium nitrite and ammonium chloride; the molar ratio of ammonium nitrite to ammonium chloride is controlled at 1: (0.5 ~ 1.0); The reaction rate buffer is boric acid in solid powder form.

2. The coalbed methane fracturing fluid system according to claim 1, characterized in that: The temperature-responsive slow-release material is a composite of phase change material and polymer; The phase change material is a paraffin or fatty acid; The polymer is polystyrene, polyacrylate, or ethylene-vinyl acetate copolymer.

3. The coalbed methane fracturing fluid system according to claim 1, characterized in that: The main liquid base is slickwater, which contains water, drag-reducing agents, and anti-swelling agents; and the water content is above 98%.

4. The coalbed methane fracturing fluid system according to claim 1, characterized in that: The self-responsive deflecting agent has an original particle size of 20-70 mesh before injection and use; and the particle size after expansion is 1.5-2.5 times the original particle size.

5. The coalbed methane fracturing fluid system according to claim 1, characterized in that: The content of the proppant is 5% to 15% of the main liquid base; and the amount of the self-responsive diverting agent added is 10% to 30% of the total mass of the proppant and the self-responsive diverting agent.

6. The coalbed methane fracturing fluid system according to claim 1, characterized in that: The shell of the self-responsive deflector is a coal-rock affinity coating, and the surface of the coal-rock affinity coating is grafted with oxygen-containing functional groups.

7. A method for balanced propagation of multiple fractures using the coalbed methane fracturing fluid system as described in any one of claims 1 to 6, characterized in that: Includes the following steps: S1: Pre-fluid Injection: The pre-fluid includes the main liquid base and the encapsulated thermogenic microcapsules, wherein the concentration of the encapsulated thermogenic microcapsules is 20~30 kg / m³. 3 ; with 10~16m 3 The pre-flush fluid is injected into the coal seam at a pumping rate of / min and an injection pressure lower than 85% to 95% of the formation fracturing pressure; the volume of the pre-flush fluid is 25% to 35% of the total fluid volume; S2: Sand-carrying fluid injection: The proppant and the self-responsive diverting agent are added to the pre-filled fluid to form the sand-carrying fluid; the sand-carrying fluid is then continuously pumped into the coal seam. S3: Shutdown and backflow: After stopping pumping, shut down the well for 1.5 to 3.0 hours, and then backflow at a rate ≤ 15% of the injection rate.

8. The method for balanced propagation of multiple cracks according to claim 7, characterized in that: In step S2, the concentration of the proppant is gradually increased to the designed maximum concentration in a stepwise manner; The self-responsive diverting agent is added via pulsed slug injection, with each slug accounting for 3% to 5% of the total liquid volume, and the mass of the self-responsive diverting agent in the slug being 15% to 25% of the mass of the proppant.

9. The method for balanced propagation of multiple cracks according to claim 8, characterized in that: In step S2, the bottom hole net pressure is monitored to determine the steering effect. When the bottom hole net pressure continues to rise by 5% to 15% after the slug containing the self-responsive steering agent is injected, it indicates that the steering is successful.

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