Clean fracturing fluid for coal seam anti-reflection and preparation method thereof

Abstract

The invention relates to the technical field of coal bed gas reservoir reconstruction materials, and discloses a clean fracturing fluid for coal bed permeability increase and a preparation method thereof, and the preparation method comprises the following steps: constructing a gemini surfactant precursor solution; applying a specific shear rate to establish an anisotropic stretching flow field so as to expose micelle hydrophobic nuclei; controlling the mass flow rate in a shear field and continuously injecting an aromatic alcohol ether regulator to realize dynamic forced intercalation; a structure saturation end point is locked according to the torque change rate, the hydrodynamic effect is utilized to overcome the thermodynamic assembly obstacle of a dilute solution, the single-molecule compact filling of the modifier on a micelle interface is realized, a high-viscoelasticity wormlike micelle network is constructed under the extremely low active matter concentration, and the self-adaptive thixotropic gel breaking characteristic on hydrocarbon gas is achieved; and reservoir damage is reduced.

Description

Technical Field

[0001] This invention relates to a clean fracturing fluid for improving the permeability of coal seams and its preparation method, belonging to the technical field of coalbed methane reservoir modification materials. Background Technology

[0002] In current coalbed methane extraction operations, hydraulic fracturing is used to construct gas migration channels and increase single-well production. Clean fracturing fluids are the mainstream choice in the industry because they do not contain water-insoluble substances and cause less damage to the reservoir. This fracturing fluid relies on viscoelastic surfactant molecules to self-assemble into a worm-like micelle network in aqueous solution. It uses a three-dimensional entangled structure to provide the viscosity and elasticity required for fluid suspension proppant. Maintaining proppant carrying capacity usually requires the use of a high concentration of surfactant system.

[0003] Coal and rock have well-developed micropores and strong adsorption properties. High-concentration surfactant molecules easily adsorb and remain on the coal and rock surface in multiple layers, reducing coal seam permeability and exacerbating the water-locking effect. Reducing the concentration of surfactants can mitigate reservoir damage, but there are physicochemical barriers to constructing highly viscoelastic fluids under extremely low concentration conditions. In dilute solution systems, the number density of surfactant micelles decreases. Key structural modifiers, aromatic alcohol ethers, are hydrophobic. Under conventional stirring or simple mixing processes, modifier molecules have difficulty overcoming the diffusion barrier to enter the micelle palisade layer, easily stratifying or forming emulsions in the aqueous phase, and finding it difficult to penetrate into the micelles to construct a high-viscosity network. Limited by traditional preparation process concepts, existing technologies mostly focus on the compounding of chemical components, while neglecting the influence of the fluid dynamics environment on micro-coal. Observing the key influence of structural assembly, for example, Chinese invention patent CN102643637B discloses a clean fracturing fluid, which mainly prepares the fracturing fluid system by mixing and dissolving triethanolamine sulfosalicylate and dimethyloctadecylhydroxyethyl quaternary ammonium nitrate under stirring conditions. This type of conventional technology mainly relies on the Brownian motion and thermodynamic self-assembly mechanism of solute molecules. Under low concentration conditions, it is difficult for the additive molecules to overcome the energy barrier of the micelle interface to achieve high density filling. Therefore, in order to obtain the viscosity that can meet the engineering requirements, it is necessary to increase the overall amount of surfactant and increase the total amount of organic matter adsorbed on the coal and rock surface. It is difficult to fundamentally solve the contradiction between sand carrying performance and reservoir protection.

[0004] Therefore, how to establish a specific preparation mechanism to assemble the microstructure at extremely low surfactant concentrations and prepare fracturing fluids that combine low damage and high proppant carrying capacity has become the technical problem to be solved by this invention. Summary of the Invention

[0005] To address the problems mentioned in the background art, the technical solution of the present invention is as follows: A method for preparing a clean fracturing fluid for coal seam permeability enhancement, the method inducing kinetic forced intercalation of aromatic alcohol ether modifiers at the interface of gemini surfactant micelles through a shear flow field, comprising the following steps:

[0006] Step S1: Constructing the precursor base solution. Dissolve inorganic equilibrium salt in water to prepare a brine base solution with a mineralization of 1.0% to 2.0% by weight. Under conditions of 35°C to 40°C, add 0.3% to 0.6% by weight of long-chain alkylamidopropyl betaine gemini surfactant to the brine base solution and stir until a homogeneous and transparent precursor micelle solution is formed.

[0007] Step S2: Establish an anisotropic stretching flow field. Place the precursor micelle solution in a stirred container and apply a constant shear rate of 800 seconds to 1200 seconds to the precursor micelle solution. Under the action of this flow field, the spherical micelles of the Gemini surfactant will undergo anisotropic stretching deformation and expose the hydrophobic core.

[0008] Step S3: Perform kinetic forced intercalation. Under the condition of maintaining the shear rate, continuously inject aromatic alcohol ether modifier into the precursor micelle solution at a mass flow rate of 0.05 wt% / min to 0.1 wt% / min, so that the aromatic alcohol ether modifier is embedded in the fence layer of the stretched and exposed spherical micelles in a monomolecular diffused state.

[0009] Step S4: Lock the structural saturation endpoint, continue to perform the shearing operation in step S2 and the injection operation in step S3 until the rate of change of the output torque of the stirring container with time changes from a positive value to a zero or negative value, indicating that the micelle network structure has reached a saturated and closed state. At this time, stop the operation and let the obtained system stand in a closed environment to mature.

[0010] The final molar ratio of the long-chain alkylamidopropyl betaine gemini surfactant to the aromatic alcohol ether modifier is controlled to be 2.5:1 to 4.0:1.

[0011] Preferably, the long-chain alkylamidopropyl betaine gemini surfactant is selected from one of erucamide propyl betaine gemini surfactant, oleamide propyl betaine gemini surfactant, and docosylamidopropyl betaine gemini surfactant; the molecular structure of the gemini surfactant includes two hydrophobic tail chains connected by linking groups and two hydrophilic head groups, and the number of carbon atoms in the hydrophobic tail chains is 18 to 22.

[0012] Preferably, the aromatic alcohol ether regulator is an alcohol ether compound with a monobenzene ring structure, selected from phenoxyethanol, benzyl alcohol, and phenylethylene glycol; the inorganic balance salt is selected from potassium chloride or ammonium chloride; and the pH of the precursor base solution is adjusted to between 6.5 and 7.5.

[0013] Preferably, in step S2, an anchor-type or ribbon-type agitator is provided in the stirring container to provide a macroscopically uniform shear field; the constant shear rate is applied continuously for 5 to 10 minutes, which allows the spherical micelles to complete the configurational transformation from an isotropic to a stretched orientation state before the aromatic alcohol ether modulator is injected.

[0014] Preferably, in step S3, the aromatic alcohol ether modifier is injected into the vortex center region of the stirring container through a microporous atomizing nozzle at a pressure of 0.2 MPa to 0.5 MPa; the set value of the mass flow rate is positively correlated with the shear rate. When the shear rate is set to the negative first power of 800 seconds, the mass flow rate is set to 0.05 wt% / min, and when the shear rate is set to the negative first power of 1200 seconds, the mass flow rate is set to 0.1 wt% / min.

[0015] Preferably, the static curing in step S4 is carried out in a constant temperature environment of 35°C to 40°C for 30 to 60 minutes. Static curing allows the micelle structure, which is in a high-energy metastable state due to shear stretching, to spontaneously evolve into a thermodynamically stable three-dimensional worm-like micelle entanglement network through configurational entropy relaxation. The resulting clean fracturing fluid has an apparent viscosity greater than 30 mPa·s at 25°C and a negative first-order shear rate of 170 seconds. The clean fracturing fluid obtained by the preparation method has adaptive deconstruction characteristics for hydrocarbon gases. When the clean fracturing fluid comes into contact with methane or liquid hydrocarbons with a volume fraction greater than 5%, the aromatic alcohol ether regulator is distributed and transferred from the palisade layer to the hydrocarbon phase, causing the worm-like micelle entanglement network to spontaneously collapse into spherical micelles. The system viscosity decreases to below 5 mPa·s within 5 minutes.

[0016] Preferably, the method utilizes fluid dynamics effects to overcome thermodynamic phase separation; the flow field intensity limited by the shear rate is greater than the droplet cohesion force of the aromatic alcohol ether regulator in a dilute solution system, and the mass transfer flux limited by the mass flow rate is lower than the critical supersaturation rate of the aromatic alcohol ether regulator in the aqueous phase. The synergistic effect of the two enables the aromatic alcohol ether regulator to complete the phase transfer from the free state to the interfacial bound state at the instant it enters the precursor micelle solution.

[0017] Preferably, in step S4, the determination of the structural saturation endpoint follows the following torque change rate relationship: ,in, This is a real-time measurement of the output torque of the stirring motor in the mixing vessel, which characterizes the rheological structural strength of the system. The duration after the aromatic alcohol ether modifier begins to be injected; when the calculated torque time derivative meets the above condition for three consecutive sampling periods, it is determined that the molar occupancy of the aromatic alcohol ether modifier at the micelle interface has reached the critical saturation value.

[0018] Preferably, the method does not introduce any oxidizing breaker, enzyme breaker, or polymer thickener; the preparation process of the clean fracturing fluid is carried out in a closed container at atmospheric pressure, and the temperature fluctuation throughout the preparation process is controlled within ±2℃ to limit the thermal motion amplitude of the Gemini surfactant molecular chains.

[0019] A clean fracturing fluid for improving the permeability of coal seams, the clean fracturing fluid being prepared by a method for preparing a clean fracturing fluid for improving the permeability of coal seams.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] 1. In the cleaning process for improving coal seam permeability, a high-density interface structure is constructed by overcoming the assembly barrier in dilute solutions. A specific interval shear flow field combined with a micro-continuous injection process is used to overcome the macroscopic phase separation or emulsion kinetic barrier of aromatic alcohol ether regulators in dilute solution systems. The fluid shear force causes anisotropic stretching deformation of the spherical micelles of the gemini surfactant, temporarily exposing the hydrophobic core. The controlled low-speed mass transfer flux enables the hydrophobic regulator molecules to be precisely embedded in the micelle fence layer in a single-molecule diffusion state. The kinetic forced assembly mechanism increases the molar occupancy of the regulator at the micelle interface, enabling the system to transition from spherical micelles to a long-range worm-like micelle network structure without increasing the total active ingredient concentration. This solves the technical problem of obtaining high viscoelastic fluid at low concentrations in traditional mixing processes.

[0022] 2. Achieving qualitative changes in rheological properties with low solid content to reduce reservoir damage: Based on improved micro-assembly efficiency, a three-dimensional viscoelastic network sufficient to suspend proppant is constructed using an extremely low concentration surfactant matrix. The interface topology optimization, rather than the material concentration stacking viscosity-enhancing mode, reduces the total amount of organic solid phase in the fracturing fluid, reduces the adsorption and retention sources of exogenous chemical substances in the micropores and cleavage surfaces of coal and rock, ensures fracture creation and proppant carrying performance, and the low adsorption characteristics maximize the preservation of the original permeability and surface wettability of the coal seam, avoiding reservoir damage caused by adsorption blockage of conventional high-concentration clean fracturing fluid.

[0023] 3. Establish a thermodynamic metastable hydrocarbon adaptive breaking mechanism. The preparation process constructs a worm-like micelle network that relies on the intercalation support of aromatic alcohol ether molecules at the interface. The structure is in a process-locked thermodynamic metastable state. When the fracturing fluid is pumped into the formation and comes into contact with coalbed methane or condensate, hydrocarbon molecules competitively replace interface layer regulator molecules based on more favorable partition coefficients. This causes the interface curvature conditions required to maintain long micelles to fail, and the micelle network undergoes spontaneous morphological collapse and thixotropic viscosity reduction. The fluid returns to a state similar to the low viscosity of water. The self-responsive deconstruction mechanism eliminates the risk of uneven distribution of the breaker in the complex fracture network without the need for external chemical breaker, and achieves complete backflow after the fracturing fluid operation is completed. Attached Figure Description

[0024] Figure 1This is a flowchart of the shear flow field induced dynamics forced intercalation preparation process of the present invention;

[0025] Figure 2 This is a graph showing the evolution of micelle tensile deformation dynamics under different shear rates according to the present invention.

[0026] Figure 3 This diagram illustrates the evolution of micelle microstructure and intercalation assembly mechanism under shear flow field according to the present invention. Detailed Implementation

[0027] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are only used to explain the present invention and are not intended to limit the scope of protection of the present invention.

[0028] This invention proposes a clean fracturing fluid for coal seam permeability enhancement and its preparation method. It comprises a viscoelastic matrix formed by amphoteric gemini surfactants, a micelle structure modifier composed of aromatic alcohol ethers, inorganic equilibrium salts, and water. Utilizing specific fluid dynamics conditions, it overcomes the thermodynamic mass transfer barrier in dilute solutions, causing aromatic alcohol ether molecules to embed in the palisade layer of the gemini surfactant micelles in a monomolecular form. This induces a conformational transformation of spherical micelles into worm-like micelles at extremely low surfactant concentrations, forming a highly viscoelastic three-dimensional network structure in a process-locked thermodynamic metastable state. This is based on the fluid's spontaneous deconstruction response characteristics when encountering hydrocarbon environments. Furthermore, it addresses the technical challenges balancing reservoir damage and proppant carrying capacity in coalbed methane extraction. To address the technical contradiction, this embodiment constructs a low-concentration viscoelastic matrix and selects a long-chain alkylamidopropyl betaine gemini surfactant, the mass percentage of which is controlled within the range of 0.3% to 0.6%. The selection of this type of gemini surfactant is based on its special molecular topology, namely two hydrophilic head groups connected by linking groups and two long-chain hydrophobic tail groups. Compared with single-chain surfactants, gemini surfactants have a lower critical micelle concentration (CMC) and can form a micelle framework at low concentrations. In this embodiment, erucamide propyl betaine or oleamide propyl betaine is preferred, with the number of carbon atoms in its hydrophobic tail chain being 18 to 22. This long carbon chain structure provides the hydrophobic association force required to form a highly entangled network.

[0029] To address the issue that surfactants alone cannot generate sufficient viscosity in dilute solutions, this embodiment introduces an aromatic alcohol ether with a single benzene ring structure as a micelle structure modifier, preferably phenoxyethanol, benzyl alcohol, or phenylethylene glycol. The amount added is controlled within a specific ratio range of the surfactant's molar amount. The physical mechanism of introducing this modifier lies in utilizing the steric hindrance effect of its benzene ring structure and... - The stacking effect occurs when regulator molecules are precisely inserted into the palisade layer of the micelle interface. Their benzene ring structure expands the head-to-group spacing of the surfactant, leading to an increase in the critical stacking parameter (CPP) of the micelle interface and a decrease in the average curvature of the interface. This abrupt change in curvature is the thermodynamic driving force behind the fusion of spherical micelles and their longitudinal growth into ultra-long worm-like micelles. To ensure that the above micro-assembly process can occur in dilute solutions, this embodiment sets a component ratio window, that is, the molar ratio of the viscoelastic matrix agent to the micelle structure regulator is limited to 2.5:1 to 4.0:1. If this ratio... Below 2.5:1, the number of regulator molecules is insufficient to cover the micelle interface, failing to effectively shield the electrostatic repulsion between the head and base, and the system maintains a low-viscosity fluid state. If the ratio is higher than 4.0:1, the excessive alcohol-ether molecules will cause the mechanical properties of the micelle interface film to soften, disrupting the long-range continuity of the micelles, leading to a decrease in viscosity or macroscopic phase separation. In addition, 0.5% to 1.5% of inorganic equilibrium salts, such as potassium chloride or ammonium chloride, are added to the system to compress the double layer and shield electrostatic repulsion, assisting in the close arrangement of micelles, while also preventing the hydration and swelling of coal and clay minerals.

[0030] The core of this invention lies in its unique preparation process, employing a shear flow field-assisted kinetic forced intercalation method. This method overcomes the phase separation tendency of aromatic alcohol ethers in dilute solutions through fluid dynamics. The specific preparation procedure is as follows: First, the precursor base solution construction procedure is executed. In a mixing container with constant temperature control, the inorganic equilibrium salt is completely dissolved in water to prepare a brine base solution with a mineralization of 1.0 wt% to 2.0 wt%. The system temperature is maintained at 35°C to 40°C. A metered amount of long-chain alkylamidopropyl betaine gemini surfactant is added. Mechanical stirring is started at a speed of 200 rpm to 300 rpm and continued for 15 to 20 minutes until the system changes from an initial turbid state to a homogeneous transparent solution. At this point, the surfactant molecules self-assemble in water to form spherical micelle precursors with small particle sizes. Next, the anisotropic stretching flow field establishment procedure is executed. The above precursor micelle solution is placed in a reactor equipped with a precision variable frequency speed control motor and an anchor-type stirring paddle. The stirring paddle speed is adjusted to stabilize the average shear rate within the system. to Within this range, the shear rate range is set based on the fact that the flow field of this intensity is in a specific region where laminar flow transitions to turbulent flow. The resulting hydrodynamic stretching effect is sufficient to overcome the bending modulus of the micelle interface film, causing the spherical micelles to undergo ellipsoidal stretching deformation, thereby temporarily exposing the internal hydrophobic core. However, this shear intensity has not yet reached the mechanical degradation threshold of breaking the covalent bonds of surfactant molecules. Under the condition of maintaining the above shear flow field, a kinetic forced intercalation injection procedure is executed. Aromatic alcohol ether regulators are continuously injected into the strong vortex center region of the stirred tank in the form of submerged fine atomization using a precision metering pump. During this process, the mass flow rate of the regulator is controlled to be 0.05wt% / min to 0.1wt% / min of the total mass of the system. The purpose of setting this low injection flux is to ensure that the hydrophobic regulator molecules are torn apart and dispersed at the moment of contact with the aqueous phase interface, i.e., the high-shear fluid microparticles are torn apart and dispersed, so as to contact the stretched micelle interface in a monomolecular diffusion state rather than a droplet aggregation state. This monomolecular stretching interface contact mode reduces the diffusion barrier for the regulator to enter the micelle fence layer.

[0031] Finally, the rheological response endpoint locking and maturation procedure are executed. During the injection and shearing processes, a high-precision torque sensor is used to monitor the output torque value of the stirring motor in real time. The torque signal filtering and differentiation processing logic is as follows: The sampling frequency collects raw data of motor load torque, and the application window width is [missing information]. A moving average filter is used to remove high-frequency noise from mechanical vibrations; a five-point center difference algorithm is used to calculate the real-time torque time derivative. Set a numerical stability dead zone threshold. When the derivative is calculated, it satisfies And the duration exceeds During the steady-state confirmation window of each stirring cycle, a feed cutoff control signal is triggered to avoid false saturation misjudgments caused by turbulent flow pulsations. When the rate of change of output torque over time is monitored ( When the inflection point from positive to zero or negative, it is determined that the microscopic network structure within the system has reached a critical saturation and closure state. Injection and stirring are stopped, and the system, after shearing is stopped, is allowed to stand and mature under sealed conditions for 30 to 60 minutes. During this period, the micelle network, which is in a high-energy metastable state due to shear stretching, relaxes through configurational entropy and transforms into a thermodynamically stable, highly entangled worm-like micelle network, thus obtaining a clean fracturing fluid. The clean fracturing fluid obtained by this preparation method possesses unique hydrocarbon adaptive destructive properties. Because the stability of the worm-like micelle network is highly dependent on the fine topological support of aromatic alcohol ether molecules at the interface, this structure is essentially... The system is in a process-locked physical adsorption equilibrium state. When fracturing fluid is pumped into the coal seam and comes into contact with trace amounts of gas (methane) or coal seam condensate, hydrocarbon molecules, due to their stronger hydrophobic partition coefficient, will competitively displace alcohol and ether molecules in the interface layer. This displacement process causes the interface curvature conditions required to maintain long micelles to fail, and the micelle network will then spontaneously collapse, degenerating from a worm-like shape to a spherical shape. The viscosity of the system will rapidly decrease to a level close to that of water. This thixotropic complete disintegration mechanism, which can be achieved without the addition of external chemical disintegrators, solves the engineering problem that traditional fracturing fluids are prone to secondary damage due to incomplete disintegration in complex fracture networks of coal seams.

[0032] Example 1: In the fracturing operation scenario of deep high-rank coalbed methane wells, the reservoir depth exceeds 1200 meters, the formation temperature reaches 35℃ to 40℃, and the coal body structure is soft with extremely complex microfracture development. This condition poses a dual challenge to the proppant carrying capacity and reservoir protection capability of the fracturing fluid. Under the traditional technical approach, to meet the proppant carrying requirements, the surfactant concentration in the clean fracturing fluid is increased to above 2.0%. However, this leads to the adsorption and retention of a large amount of organic matter on the surface of coal and rock micropores, causing irreversible permeability damage. To address this contradiction, this example uses the clean fracturing fluid system prepared in the aforementioned specific implementation method. This fracturing fluid system achieves the pre-setting of the microstructure through the preparation process at the surface preparation station. 0.4% by mass of erucic acid amyl propyl betaine gemini surfactant is dissolved in 1.5% salinity potassium chloride brine. At a shear rate of 0.08 wt% / min, phenoxyethanol modifier is injected. This process utilizes hydrodynamic stretching to embed phenoxyethanol molecules in the palisade layer of the gemini surfactant micelles in a monomolecular form. This induces the fusion and longitudinal growth of spherical micelles at extremely low concentrations, constructing a highly viscoelastic worm-like micelle network. This network structure is based on the extremely high zero-shear viscosity of the fluid at low shear rates, enabling the effective suspension and delivery of high concentrations of quartz sand proppant into deep cracks.

[0033] When the fracturing fluid carrying proppant is pumped into the coal seam fracture, its unique adaptive deconstruction mechanism is immediately triggered. Because the worm-like micelle network formed during the preparation process is in a process-locked thermodynamic metastable state supported by phenoxyethanol molecules, when the fluid comes into contact with trace amounts of methane gas released from the coal seam, the methane molecules, with their stronger hydrophobic partition coefficient, competitively displace the phenoxyethanol molecules in the micelle interface layer. This microscopic displacement breaks the interface curvature balance required to maintain long micelles, causing the worm-like micelle network to rapidly undergo spontaneous morphological collapse and degenerate into spherical micelles. This thixotropic phase transition without the need for external breaker causes the fracturing fluid viscosity to drop significantly to near the level of water within minutes, so that it can be quickly returned to the surface through the wellbore after the operation, minimizing secondary damage to the coal seam's conductivity.

[0034] Example 2: To systematically verify the rheological performance advantages and micro-assembly mechanism effectiveness of the clean fracturing fluid of the present invention at extremely low active ingredient concentrations, a comprehensive test platform was constructed, including precision rheological testing and simulated formation environment. Its core equipment is a high-precision rotational rheometer, equipped with a coaxial cylindrical measurement system, and the torque measurement accuracy is superior to... It can accurately capture the weak stress response of fluids at low shear rates; and a set of visualized high-pressure reactors, equipped with programmable stirring motors and micro-injection pumps, are used to accurately execute shear-induced intercalation processes. The base water sample used in the experiment is a potassium chloride salt aqueous solution with a salinity of 1.5 wt% prepared to simulate the produced water of coalbed methane wells. In order to simulate the non-ideal environment of real downhole fluids, random pressure fluctuation interference with a signal-to-noise ratio of 20 dB is additionally introduced into the base fluid to assess the anti-disturbance capability of the system assembly process.

[0035] During the setting of preparation process parameters, the shear rate ( As a core variable determining the degree of stretching at the micelle interface, its value follows the fluid dynamics decision-making logic. The setting of this parameter requires balancing the engineering contradiction between interfacial stretching efficiency and molecular chain mechanical stability: the shear rate must be high enough to allow the tensile stress of the fluid particles to overcome the bending modulus of the micelle interface, exposing hydrophobic sites; however, it must be below the covalent bond breaking threshold of surfactant molecules to prevent irreversible mechanical degradation. Characteristic relaxation time... Calibration procedure: Place the precursor micelle solution into the coaxial cylindrical measuring system of the rotational rheometer, and lock the temperature control accuracy to [value missing]. ;exist to Perform dynamic frequency scanning within the angular frequency range to obtain the energy storage modulus. With loss modulus Spectral data; using single-mode Maxwell constitutive equations ,in Nonlinear least squares fitting is performed on the platform modulus to extract the feature relaxation time. Set the lower limit of shear rate. To ensure Weissenberg number To construct the rheological basis for anisotropic tensile flow field, based on the pre-determined characteristic relaxation time of twin surfactant micelles ( According to the Weissenberg number ( The stretching criterion is greater than 1, and the results of flow field simulation at different rotational speeds are used to determine the... to To achieve an effective shear window for inducing anisotropic deformation, in the preferred embodiment of this experiment, the shear rate is locked at [value missing]. This is taken as the standard operating condition.

[0036] An experimental design included a verification scheme comprising the sample group of this invention and a multi-dimensional control group to objectively demonstrate the synergistic effect of the shear-induced process and a specific molar ratio window. In all sample groups, the concentration of the long-chain alkylamidopropyl betaine gemini surfactant was fixed at 0.4 wt%, and the inorganic salt concentration was fixed at 1.5 wt%. The only variables were the preparation process (whether shear was applied) and the molar ratio of the viscoelastic matrix agent to the phenoxyethanol modifier. The specific groupings are as follows: The sample group of this invention used a shear flow field-assisted kinetic forced intercalation method with a molar ratio of 3.0:1; Control group A used conventional magnetic stirring mixing (without high shear) with a molar ratio of 3.0:1; Control group B used the process of this invention, but with a molar ratio of 2.0:1 (below the lower limit); Control group C used the process of this invention, but with a molar ratio of 5.0:1 (above the upper limit). After preparation, each sample was allowed to stand at a constant temperature of 25°C for 60 minutes to mature, and its performance was tested. Apparent viscosity at shear rate and zero shear viscosity ( (See Table 1). The data clearly reveal the decisive influence of process and proportion on rheological properties.

[0037] Table 1. Comparison of Rheological Properties of Fracturing Fluids under Different Processes and Proportions:

[0038]

[0039] Data shows that, under the same formulation (sample group of this invention and control group A), by only changing the preparation process, the zero-shear viscosity of the system is achieved from... arrive The order-of-magnitude leap in viscosity directly confirms that shear-induced intercalation is a necessary condition for overcoming the thermodynamic barrier of dilute solutions and constructing high-viscosity networks, rather than an optional auxiliary means. Meanwhile, data from control groups B and C show that when the molar ratio deviates from the optimal window of 2.5:1 to 4.0:1, viscosity performance exhibits a non-linear decline. Control group B, due to insufficient regulator, cannot effectively shield electrostatic repulsion and fails to induce sufficient micelle growth; while control group C, due to excessive regulator, experiences a decrease in interfacial film rigidity and network disentanglement. This bell-shaped performance trend strongly supports the right to... The scientific validity and rationality of the molar ratio range limitation are sought, proving that this range is the optimal working region for achieving gemini-alcohol-ether topological synergy. To further verify the adaptive response characteristics of the system when encountering hydrocarbon environments, a methane contact sensitivity test was conducted on the sample group of this invention. The prepared fracturing fluid was placed in a sealed container, and methane gas with a purity of 99.9% was introduced and maintained at a pressure of 0.5 MPa to simulate the coalbed methane desorption environment. The change in system viscosity with contact time was monitored in real time using an online viscometer. The data showed that within 5.2 minutes after contact with methane, the apparent viscosity of the fluid changed from the initial... Sharply down to (The viscosity is close to that of water), and no increase in viscosity was observed during the subsequent 24-hour monitoring.

[0040] Example 3: This example combines Figures 1 to 3 This document describes a clean fracturing fluid for improving the permeability of coal seams and its preparation method. Figure 1 As shown, the process begins in step S1 to construct the precursor base solution, which involves stirring an inorganic equilibrium salt with long-chain alkylamidopropyl betaine at 35°C to 40°C until homogeneous and transparent to form a precursor micelle solution. Step S2 then establishes an anisotropic stretching flow field by applying an 800°C... Up to 1200 A constant shear rate induces the stretching deformation of spherical micelles and exposes their hydrophobic cores. Based on this, step S3, kinetic forced intercalation, is performed. The shear rate is maintained and aromatic alcohol ether modifiers are continuously injected at a rate of 0.05 wt% / min to 0.1 wt% / min, allowing the monomolecular diffuse state to embed into the fence layer exposed by the stretching of the spherical micelles. The structural saturation endpoint is then determined based on the torque change rate dT / dt. If the change rate is positive, it indicates that the system is not saturated and injection continues. If the change rate is zero or negative, it indicates that the system is saturated and proceeds to step S4, closed static curing. Shearing and injection are stopped, and a saturated closed micelle network structure is formed in the closed system, ultimately producing a clean fracturing fluid for coal seam permeability enhancement.

[0041] like Figure 2 As shown, the horizontal axis represents time (in minutes), and the vertical axis represents the dimensionless micelle stretching. The figure contains three evolution curves corresponding to different constant shear rates, representing shear rates of 800... 1000 And 1200 Under the operating conditions, as the shear rate increases, the time required for micelles to reach maximum tensile deformation from their initial state decreases, and the degree of tensile deformation at final stability shows a positive correlation with the increase of the shear rate; for example... Figure 3 As shown in the schematic diagram, the preparation process is divided into three continuous spatial stages: the precursor region, the shear intercalation region, and the network maturation region. In the precursor region, spherical micelles with a concentration of 0.3% to 0.6% are distributed. Their structure consists of an outer hydrophilic layer and an inner hydrophobic core (shown as dashed). As the fluid enters the shear intercalation region along the shear direction, the mixture is kept at a constant temperature of 35-40℃ and 800-1200℃ in the stirred reactor section. Under shearing action, spherical micelles transform into stretched and deformed micelles and expose hydrophobic cores. At this time, free aromatic alcohol ether modifiers (triangular diagram) are injected and transformed into modifiers embedded in the fence layer (solid triangle diagram). Finally, in the network maturity region, as the flow proceeds, the system evolves into a three-dimensional entangled network composed of worm-like micelles, and a physical cross-linked structure is formed at the micelle entanglement cross-linking points.

[0042] Example 4: This example provides supplementary mechanism verification and parameter calibration for the adaptive deconstruction mechanism of fracturing fluid systems in response to hydrocarbon gases, and the multiple synergistic functions of inorganic equilibrium salts in the system. This aims to eliminate potential mechanistic and parameter setting black boxes, ensuring the interpretability of the technical solution at the microscopic mechanism level and its reproducibility at the engineering application level. Regarding the microscopic mechanism of the sudden viscosity drop in fracturing fluid systems when encountering hydrocarbon environments, although it has been qualitatively described above as the partitioning of aromatic alcohols and ethers into the hydrocarbon phase leading to the disintegration of the metastable structure, this example designs a more intuitive and quantitative approach to provide more direct and quantitative evidence. A set of experiments based on ultraviolet-visible spectrophotometry (UV-Vis) to determine the partition coefficient was conducted. Phenoxyethanol was selected as the tracer, and a micellar solution of the same concentration as the sample group in Example 2 was prepared. This solution was mixed with an equal volume of n-heptane (simulating liquid hydrocarbon) in a closed separatory funnel and allowed to equilibrate for 24 hours. After separation, the aqueous phase was collected, centrifuged to remove oil, and its absorbance at 270 nm was measured. The remaining concentration of phenoxyethanol in the aqueous phase was calculated using the standard curve method. The experimental data showed that the partition coefficient of phenoxyethanol in the n-heptane / micelle aqueous solution system (…) The value is as high as 15.6, which means that in the presence of hydrocarbons, more than 90% of phenoxyethanol molecules will spontaneously migrate from the aqueous phase (including the micelle interface) to the oil phase. This thermodynamically driven mass transfer process directly confirms the authenticity of the interface replacement mechanism: that is, the key wedge that maintains the high curvature structure of worm-like micelles—phenoxyethanol molecules—is extracted away by hydrocarbons, resulting in the curvature of the micelle interface rising again, the structure degenerating into a spherical shape, and the viscosity being lost.

[0043] Secondly, regarding the setting of the concentration of inorganic equilibrium salts such as potassium chloride, the previous text gave a range of 1.0wt% to 2.0wt%, but did not elaborate on the trade-off logic between electrostatic shielding and anti-swelling mechanisms. Therefore, this embodiment constructs a gradient test system covering both viscosity growth and anti-swelling rate indicators, and prepares a series of fracturing fluid base fluids with different potassium chloride concentrations (0.5wt%, 1.0wt%, 1.5wt%, 2.0wt%, 2.5wt%), with the remaining components consistent with the sample group of the present invention in Example 2. On the one hand, the zero-shear viscosity of each sample group was tested. The results show that as the potassium chloride concentration increases from 0.5wt% to 1.5wt%, the viscosity of the system shows an upward trend. Increase to This is because inorganic salt ions compress the electric double layer of the gemini surfactant head group, weakening electrostatic repulsion and promoting the close arrangement and growth of micelles. However, when the concentration is further increased to 2.5 wt%, the viscosity increase tends to level off or even decrease slightly. The results indicate that the electrostatic shielding effect has reached saturation, and excessive salt may cause salting out, damaging micelle stability. On the other hand, the anti-swelling rate of each sample solution on coal and rock powder was determined using the industry-standard centrifugation method. The data showed that when the potassium chloride concentration was below 1.0 wt%, the anti-swelling rate was less than 85%, which could not effectively inhibit the hydration swelling of clay minerals such as montmorillonite. However, when the concentration reached 1.5 wt% or above, the anti-swelling rate stabilized at over 92%, meeting engineering requirements. Considering the nonlinear variation trend of the above viscosity-anti-swelling dual indicators, the engineering rationality of defining the preferred range of 1.0 wt% to 2.0 wt% is determined by the basic anti-swelling requirements and the electrostatic shielding threshold, while the upper limit (2.0 wt%) is limited by the saturation effect of viscosity growth and cost control.

[0044] Example 5: Before large-scale fracturing operations in highly heterogeneous coal seams, a standardized on-site fluid performance calibration and pumping procedure fine-tuning process is implemented to address fluctuations in formation water salinity and differences in shear history during base fluid preparation. This procedure requires the extraction of formation produced water samples from the target well section at the operation site to determine its total dissolved solids (TDS) content and divalent cations (CFCs). , If the total mineralization measured on-site exceeds 1.5wt% of the benchmark formulation and the total concentration of divalent ions is higher than 500mg / L, the ion shielding compensation mechanism needs to be activated. That is, an additional amount of disodium ethylenediaminetetraacetate (EDTA-2Na) with the same molar concentration of divalent cations is added to the mixing tank as a chelating agent. At the same time, the amount of inorganic equilibrium salt (KCl) added is reduced to the range of 1.0wt% to 1.2wt% to maintain the relative stability of the total ionic strength of the system and prevent premature aging or phase separation of the micelle structure due to the salt sensitivity effect. In the continuous mixing process of fracturing fluid, in order to eliminate the influence of the difference in shear efficiency of different batches of stirring equipment on the formation of micro-network structure, online process correction based on rheological response feedback is implemented.

[0045] Specifically, an online viscosity monitoring unit is installed at the high-pressure discharge manifold of the blending vehicle, and a setting is implemented. Apparent viscosity at shear rate is a key control indicator. If the monitored reading is lower than the baseline set value for 30 consecutive seconds, for example... If the viscosity reaches 90%, it is determined that the current shear energy is insufficient to induce sufficient micelle growth. The control system automatically triggers a correction command: on the one hand, the stroke of the micelle structure modifier injection pump is increased by 5% to 10% to increase the local concentration of intercalated molecules; on the other hand, the speed of the mixer in the mixing tank is increased simultaneously or a backup shear pump is activated to increase the residence time of the fluid through the high-shear zone until the online viscosity reading recovers and stabilizes at the baseline value. Within the specified range, the wellhead main valve can be opened for formal fracturing operations. This procedure ensures that each cubic meter of fracturing fluid injected into the formation reaches the optimal rheological state designed for it, minimizing the uncertainty of project implementation.

[0046] Example 6: To address the differences in molecular weight distribution of long-chain alkylamidopropyl betaine gemini surfactant raw materials due to different production batches, and the sensitivity of fracturing fluid rheological properties to fluctuations in formation water salinity in the operating area, a standardized adaptive formulation calibration and quality control procedure was established. Based on a benchmark database of zero-shear viscosity response surfaces, representative surfactant raw materials with high, medium, and low molecular weight distributions were selected under laboratory conditions and formulated into a series of base solutions with potassium chloride brine at different concentration gradients (0.5wt% to 2.5wt%). Phenoxyethanol regulator was injected into each base solution according to a shear-induced intercalation process. The molar ratio of regulator to surfactant covered a wide range from 2.0:1 to 5.0:1. The zero-shear viscosity of each sample was measured using a rheometer. A multidimensional response surface model was constructed with raw material molecular weight, inorganic salt concentration, and regulator molar ratio as independent variables and zero-shear viscosity as the dependent variable. This model quantifies the nonlinear contribution and interaction of each key parameter to the system viscosity, and establishes the optimal formulation window under different raw material characteristics.

[0047] Before actual engineering application, a batch pre-inspection and dynamic fine-tuning procedure is performed. The molecular weight distribution of the surfactant raw materials in this batch is determined, and the salinity of the on-site water source is measured. These two measured data are used as input parameters and substituted into the response surface model of the above-mentioned benchmark database. The theoretical optimal regulator molar ratio and inorganic salt addition amount of this batch of raw materials under the current water source conditions are predicted by the interpolation algorithm. A small-scale test is conducted at the solution preparation station: 500 mL of sample solution is prepared according to the predicted formula, and the on-site shear rate is simulated ( Prepared under the following conditions, if the deviation between the measured zero-shear viscosity of the small-scale sample and the model prediction is within... If the deviation is within the specified range, the formula will be directly locked for large-scale production; if the deviation exceeds the specified range, the molar ratio of the regulator will be finely adjusted according to the gradient direction of the response surface model (adjustment step size is 0.1) until the measured viscosity returns to the target range.

[0048] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention.

[0049] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a clean fracturing fluid for improving the permeability of coal seams, characterized in that, The method induces kinetic forced intercalation of aromatic alcohol ether modifiers at the gemini surfactant micelle interface through a shear flow field, comprising the following steps: Step S1: Constructing the precursor base solution. Dissolve inorganic equilibrium salt in water to prepare a brine base solution with a mineralization of 1.0% to 2.0% by weight. Under conditions of 35°C to 40°C, add 0.3% to 0.6% by weight of long-chain alkylamidopropyl betaine gemini surfactant to the brine base solution and stir until a homogeneous and transparent precursor micelle solution is formed. Step S2: Establish an anisotropic stretching flow field. Place the precursor micelle solution in a stirred container and apply a constant shear rate of 800 seconds to 1200 seconds to the precursor micelle solution. Under the action of this flow field, the spherical micelles of the Gemini surfactant will undergo anisotropic stretching deformation and expose the hydrophobic core. Step S3: Perform kinetic forced intercalation. Under the condition of maintaining the shear rate, continuously inject aromatic alcohol ether modifier into the precursor micelle solution at a mass flow rate of 0.05 wt% / min to 0.1 wt% / min, so that the aromatic alcohol ether modifier is embedded in the fence layer of the stretched and exposed spherical micelles in a monomolecular diffuse state. Step S4: Lock the structural saturation endpoint, continue the shearing operation of step S2 and the injection operation of step S3 until the rate of change of the output torque of the stirring container with time changes from a positive value to a zero or negative value, indicating that the micelle network structure has reached a saturated and closed state. At this time, stop the operation and seal the obtained system for static aging. The final molar ratio of long-chain alkyl amide propyl betaine gemini surfactant to aromatic alcohol ether regulator is controlled to be 2.5:1 to 4.0:

1.

2. The method for preparing a clean fracturing fluid for improving coal seam permeability according to claim 1, characterized in that, The long-chain alkylamidopropyl betaine gemini surfactant is selected from one of the following: erucic acid amidopropyl betaine gemini surfactant, oleic acid amidopropyl betaine gemini surfactant, and docosylamidopropyl betaine gemini surfactant; the molecular structure of the gemini surfactant includes two hydrophobic tail chains connected by linking groups and two hydrophilic head groups, and the number of carbon atoms in the hydrophobic tail chains is 18 to 22.

3. The method for preparing a clean fracturing fluid for improving coal seam permeability according to claim 1, characterized in that, The aromatic alcohol ether regulator is an alcohol ether compound with a monobenzene ring structure, selected from phenoxyethanol, benzyl alcohol, and phenylethylene glycol; the inorganic balance salt is selected from potassium chloride or ammonium chloride; the pH of the precursor base solution is adjusted to between 6.5 and 7.

5.

4. The method for preparing a clean fracturing fluid for improving coal seam permeability according to claim 1, characterized in that, In step S2, an anchor-type or ribbon-type agitator is provided in the stirring container to provide a macroscopically uniform shear field; the constant shear rate is applied continuously for 5 to 10 minutes, which allows the spherical micelles to complete the configurational transformation from an isotropic to a stretched orientation state before the aromatic alcohol ether modulator is injected.

5. The method for preparing a clean fracturing fluid for improving coal seam permeability according to claim 1, characterized in that, In step S3, the aromatic alcohol ether modifier is injected into the vortex center region of the stirring vessel through a microporous atomizing nozzle at a pressure of 0.2 MPa to 0.5 MPa; the set value of the mass flow rate is positively correlated with the shear rate. When the shear rate is set to the negative first power of 800 seconds, the mass flow rate is set to 0.05 wt% / min, and when the shear rate is set to the negative first power of 1200 seconds, the mass flow rate is set to 0.1 wt% / min.

6. The method for preparing a clean fracturing fluid for improving coal seam permeability according to claim 1, characterized in that, The static curing in step S4 is carried out in a constant temperature environment of 35°C to 40°C for 30 to 60 minutes. Static curing allows the micelle structure, which is in a high-energy metastable state due to shear stretching, to spontaneously evolve into a thermodynamically stable three-dimensional worm-like micelle entanglement network through configurational entropy relaxation. The resulting clean fracturing fluid has an apparent viscosity greater than 30 mPa·s at 25°C and a negative first-order shear rate of 170 seconds. The clean fracturing fluid obtained by the preparation method has adaptive deconstruction characteristics for hydrocarbon gases. When the clean fracturing fluid comes into contact with methane or liquid hydrocarbons with a volume fraction greater than 5%, the aromatic alcohol ether regulator is distributed and transferred from the palisade layer to the hydrocarbon phase, causing the worm-like micelle entanglement network to spontaneously collapse into spherical micelles. The system viscosity decreases to below 5 mPa·s within 5 minutes.

7. The method for preparing a clean fracturing fluid for improving the permeability of coal seams according to claim 1, characterized in that, The method utilizes fluid dynamics effects to overcome thermodynamic phase separation; the flow field intensity limited by the shear rate is greater than the droplet coagulation force of aromatic alcohol ether modifiers in dilute solution systems, and the mass transfer flux limited by the mass flow rate is lower than the critical supersaturation rate of aromatic alcohol ether modifiers in aqueous phases. The synergistic effect of the two enables the aromatic alcohol ether modifiers to complete the phase transfer from the free state to the interfacial bound state at the instant they enter the precursor micelle solution.

8. The method for preparing a clean fracturing fluid for improving coal seam permeability according to claim 1, characterized in that, In step S4, the determination of the structural saturation endpoint follows the following torque change rate relationship: ,in, This is a real-time measurement of the output torque of the stirring motor in the mixing vessel, which characterizes the rheological structural strength of the system. The duration after the aromatic alcohol ether modifier begins to be injected; when the calculated torque time derivative meets the above condition for three consecutive sampling periods, it is determined that the molar occupancy of the aromatic alcohol ether modifier at the micelle interface has reached the critical saturation value.

9. A method for preparing a clean fracturing fluid for improving permeability in coal seams according to claim 1, characterized in that, The method does not introduce any oxidizing breaker, enzyme breaker, or polymer thickener; the preparation process of the clean fracturing fluid is carried out in a closed container under normal pressure, and the temperature fluctuation throughout the preparation process is controlled within ±2℃.

10. A clean fracturing fluid for improving the permeability of coal seams, characterized in that, The clean fracturing fluid is prepared by the method for preparing the clean fracturing fluid for coal seam permeability enhancement as described in claim 1.

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