A testing device and testing method for solubility of a coal cleat filler in a fracturing fluid

Abstract

The application discloses a kind of coal cutting filling in fracturing fluid solubility test method and device.The device includes: core test unit, including pressure reactor, coal sample fixing frame and stirring assembly, for containing coal sample block containing cutting filling and fracturing fluid;Working condition simulation unit, including pressure control system and temperature control system, for simulating the pressure and temperature conditions of downhole fracturing environment;Sampling analysis unit, including sampling filter assembly, component detection assembly and quality detection assembly, for extracting soaked fracturing fluid and detecting filling characteristic component concentration and undissolved residue quality;Liquid replenishment unit, for replenishing equal amount of fresh fracturing fluid after sampling.The application solves the technical problems that the solubility test method in the prior art cannot simulate downhole working condition, cannot maintain the actual occurrence state of filling, and cannot systematically evaluate the solubility of filling.

Description

Technical Field

[0001] This invention relates to the fields of coal mining and oil and gas well fracturing technology, and in particular to a testing device and method for testing the solubility of coal cleavage fillers in fracturing fluid. Background Technology

[0002] In coalbed methane extraction, underground coal gasification, and shale gas development, hydraulic fracturing technology is one of the key means to improve formation permeability and increase gas production efficiency. Coal, as a typical fractured reservoir, has a large number of cleavage fractures inside. Cleavage fractures are the main channels for coalbed methane migration and seepage, playing a decisive role in the permeability of the coal seam.

[0003] During geological history, coal seam fractures are often filled with minerals such as calcite (CaCO3), clay minerals (e.g., kaolinite, illite), and quartz. The presence of these fillers severely obstructs the flow space within the fractures, reducing the permeability of the coal seam. During hydraulic fracturing, when the fracturing fluid comes into contact with the fillers in the coal seam fractures, some of the fillers may dissolve or disperse, thereby improving the fracture conductivity. Therefore, accurately evaluating the solubility of coal seam fillers in fracturing fluid is of significant guiding importance for the selection of fracturing fluid systems, the optimization of fracturing fluid formulations, and the design of fracturing operation parameters.

[0004] However, existing solubility testing methods have the following main shortcomings: First, existing methods mostly test the dissolution of single solid particles (such as pure calcite powder) in solution. This involves peeling the filler material from the coal seam and grinding it into powder before testing, thus disrupting the original state of the filler material within the coal seam's fractures. In actual coal seams, the filler material is tightly bonded to the coal seam, has an irregular morphology, and its contact mode and dissolution behavior with fracturing fluid differ significantly from those of a single powder sample. Therefore, existing test results cannot accurately reflect the dissolution characteristics of the filler material under actual operating conditions.

[0005] Second, existing methods are typically tested under normal temperature and pressure conditions, failing to simulate the high-temperature and high-pressure environment of downhole fracturing operations. Coalbed methane wells generally have reservoir depths of 300–1500 m, corresponding to formation pressures of 3–15 MPa and formation temperatures of 30–60 °C; while deep coal seams or shale gas reservoirs can reach pressures of 20–30 MPa and temperatures of 60–150 °C. Temperature and pressure have significant effects on the thermodynamics and kinetics of the dissolution reaction, and test results under normal temperature and pressure conditions cannot accurately reflect the dissolution behavior of the filling material under actual downhole conditions.

[0006] Third, existing methods lack systematic multi-time-point sampling and multi-parameter joint detection methods, which cannot fully reveal the dissolution kinetics of the filling material as it changes with soaking time, and make it difficult to provide quantitative data support for the design of fracturing fluid soaking time and the optimization of fracturing operation cycle.

[0007] Therefore, there is an urgent need for a testing method and device that can accurately reflect the actual occurrence state of coal cleavage filling materials, simulate downhole fracturing conditions, and systematically evaluate the dissolution performance of filling materials. Summary of the Invention

[0008] The purpose of this invention is to provide a testing device and method for testing the solubility of coal cleavage fillers in fracturing fluid, so as to solve the technical problems in the prior art that the solubility testing methods cannot simulate downhole working conditions, cannot maintain the actual occurrence state of the fillers, and cannot systematically evaluate the solubility performance of the fillers.

[0009] To achieve the above objectives, the present invention provides a testing device for the solubility of coal fracturing filler in fracturing fluid, comprising: The core testing unit is used to contain coal samples containing cleavage filler and fracturing fluid; the core testing unit is the main reaction body of the entire device, providing a closed pressure-bearing space for the contact reaction between the coal samples and the fracturing fluid.

[0010] The working condition simulation unit, connected to the core testing unit, is used to simulate the pressure and temperature conditions in the downhole fracturing environment. The working condition simulation unit can independently control the pressure and temperature in the core testing unit, so that the temperature and pressure conditions during the test process are consistent with the actual formation conditions of the target coal seam, thereby realistically reproducing the reaction environment of the filling material and fracturing fluid during downhole fracturing.

[0011] The sampling and analysis unit, connected to the core testing unit, is used to extract fracturing fluid after soaking coal samples and to detect the concentration of characteristic components of the packing material and the mass of undissolved residue in the fracturing fluid. The sampling and analysis unit can extract fracturing fluid from the core testing unit without interrupting the testing process and perform solid-liquid separation and multi-parameter detection on the fracturing fluid after soaking coal samples, enabling real-time tracking of the packing material dissolution process.

[0012] Preferably, the core testing unit includes a pressure vessel, a coal sample holder placed inside the pressure vessel, and a stirring assembly. The pressure vessel is a sealed container capable of withstanding the pressure and temperature required for the test. The coal sample holder is used to fix coal sample blocks containing fracture fillers, exposing the fillers in the fractures of the coal sample blocks to the fracturing fluid, maintaining the original bonding state between the fillers and the coal body, and avoiding distortion caused by testing the fillers separately after removing them from the coal body. The stirring assembly includes a stirring rod and a variable-speed mixer, used to adjust the flow state of the fracturing fluid, simulating the flow effect of fracturing fluid in the fractures during downhole fracturing operations.

[0013] Preferably, the pressure vessel is a transparent pressure vessel, made of transparent material resistant to high temperature and high pressure. Operators can visually observe the reaction state inside the vessel through the vessel wall, including the color change of the fracturing fluid, the dissolution status of the filling material surface, and the generation of bubbles, which helps to detect abnormalities in a timely manner and adjust the test parameters.

[0014] Preferably, the coal sample holder can hold a block coal sample with a size of 50mm×50mm×50mm, and the structure of the coal sample holder is designed to be hollow or clamping, so as not to block the contact surface between the coal sample block's cleavage and the fracturing fluid, ensuring that the fracturing fluid can fully contact the cleavage in all directions of the coal sample block.

[0015] Preferably, the operating condition simulation unit includes a pressure control system and a temperature control system.

[0016] The pressure control system includes a gas source tank, a high-pressure gas pump, a high-pressure inlet pipe, an inlet valve, and a pressure gauge. The gas source tank stores the pressurized medium gas (such as methane gas to simulate the actual gas environment of a coalbed methane reservoir). The high-pressure gas pump pressurizes the gas and delivers it to the pressure vessel through the high-pressure inlet pipe. The inlet valve controls the on / off state of the inlet pipe and the gas flow rate. The pressure gauge monitors the pressure inside the vessel in real time. The pressure regulation range of the pressure control system is 0–30 MPa, covering the formation pressure range from shallow coal seams (3–15 MPa) to deep coal seams or shale gas reservoirs (20–30 MPa). The temperature control system includes a heating jacket and an insulation jacket. The heating jacket is installed on the outer wall of the pressure vessel to heat the vessel body; the insulation jacket is installed outside the heating jacket to reduce heat loss and maintain the stability of the temperature inside the vessel. The temperature regulation range of the temperature control system is 20–150°C. An electronic thermometer is also installed on the pressure vessel for real-time monitoring and display of the internal temperature.

[0017] Preferably, the sampling and analysis unit includes a sampling filter assembly, a sample collection bottle, a component detection assembly, and a quality detection assembly.

[0018] The sampling and filtration assembly is located at the lower end of the pressure vessel and includes an outlet pipe, a sampling valve, and a filter funnel. One end of the outlet pipe is connected to the lower wall of the pressure vessel, and the other end is connected to the filter funnel via the sampling valve. During operation, opening the valve allows the fracturing fluid, after soaking the coal sample, to be discharged through the outlet pipe into the filter funnel under the pressure inside the pressure vessel. The filter funnel contains a filter membrane for solid-liquid separation of the fracturing fluid, obtaining filtrate containing dissolved components of the packing material and undissolved packing material residue.

[0019] Preferably, the filter membrane is a microporous filter membrane with a pore size of 0.22 μm, which can effectively retain undissolved fine filler particles and ensure that the filtrate contains only truly dissolved components.

[0020] The sample collection bottle is placed below the filter funnel to collect the filtrate after filtration, which is convenient for subsequent concentration detection.

[0021] The component detection assembly includes a detection solution delivery tube and an ion chromatograph. The detection solution delivery tube delivers the filtrate from the sample collection bottle to the ion chromatograph for analysis. The ion chromatograph can quantitatively determine the concentration of characteristic ions of the packing material in the filtrate, such as the Ca²⁺ concentration produced after the calcite packing material dissolves, and the Si concentration produced after the clay mineral packing material dissolves. 4 ⁺ concentration, etc.

[0022] The quality control component is an electronic scale (preferably with an accuracy of 0.0001g), used to weigh the mass of the packing material residue retained in the filter funnel after each sampling and filtration, after drying to constant weight. The residue mass data at each time point are corroborated with the concentration data of characteristic components in the filtrate, jointly reflecting the dissolution process of the packing material in the fracturing fluid.

[0023] Preferably, the testing apparatus further includes a liquid replenishment unit. The liquid replenishment unit is connected to the pressure vessel and includes a fracturing fluid tank, a fracturing fluid inlet pipe, and a replenishment valve. The fracturing fluid tank stores fresh fracturing fluid. One end of the fracturing fluid inlet pipe is connected to the fracturing fluid tank, and the other end is connected to the pressure vessel. The replenishment valve controls the amount and timing of fracturing fluid replenishment. After each sampling, the liquid replenishment unit replenishes the pressure vessel with an amount of fresh fracturing fluid equal to the sample volume, maintaining a stable fracturing fluid level in the vessel and ensuring that the coal sample remains fully submerged throughout the testing process, thus guaranteeing the comparability of sampling results at different time points.

[0024] In another aspect, the present invention provides a testing method for a testing apparatus for the solubility of coal cleavage fillers in fracturing fluid, comprising the following steps: Step S1, Sample Preparation: Select coal samples containing cleavage fillers and process them into coal sample blocks, ensuring that the cleavage joints of the coal sample blocks are exposed and the fillers are not destroyed. During the processing, avoid using methods such as water-cooled cutting or violent impact to prevent the fillers from falling off or dissolving during processing.

[0025] Preferably, the coal sample block has a size of 50mm×50mm×50mm. This size ensures that the coal sample block contains a sufficient number and area of ​​cut seams, and also facilitates fixing and handling on the coal sample holder.

[0026] The fracturing fluid should be prepared according to the fracturing fluid formula used in the actual project. The amount prepared should meet the total amount required for injection into the reactor and replenishment after multiple samplings during the test. A portion of the fracturing fluid is used to inject into the pressure reactor to immerse the coal sample block, and the remaining fracturing fluid is injected into the fracturing fluid tank of the liquid replenishment unit for equal replenishment after subsequent sampling.

[0027] Step S2, Operating Condition Settings: Secure the coal sample block to the sample holder inside the pressure vessel, ensuring the exposed fracture surface faces a direction conducive to contact with the fracturing fluid. Inject the prepared target fracturing fluid into the pressure vessel, completely submerging the coal sample block. Close the vessel and set the target pressure and temperature using the operational simulation unit. The target pressure and temperature are determined based on the actual formation conditions of the coal seam to be fractured. Start the agitator and adjust the agitation speed to the set value to simulate the flow environment of the fracturing fluid in the fractures during downhole fracturing operations. Once the pressure and temperature inside the vessel reach the set values ​​and stabilize, begin timing.

[0028] Step S3, Immersion and Sampling: Timing begins from the injection of fracturing fluid into the pressure vessel. At multiple preset soaking time points, fracturing fluid is extracted from soaked coal samples using a sampling and filtration assembly and filtered to obtain filtrate containing dissolved components of the packing material and undissolved packing material residue. After sampling, an equal volume of fresh fracturing fluid is replenished to the pressure vessel via a liquid replenishment unit to maintain a stable fracturing fluid volume within the pressure vessel.

[0029] The principle for setting multiple preset soaking time points is: the sampling interval is shorter in the early stage and gradually increases in the later stage, so as to better capture the changing pattern of the dissolution rate.

[0030] Preferably, the soaking time points include 1h, 3h, 6h, 12h, and 24h, for a total of 5 time points. Each sample volume is 50mL, and an equal amount of fresh fracturing fluid is added after sampling.

[0031] Step S4, Solubility test: The concentration of characteristic components of the packing material in the filtrate was determined using a component detection kit. Different characteristic component ions were selected as detection indicators for different types of packing materials: when the packing material is calcite-based, calcite (CaCO3) releases Ca2+ after dissolving in the fracturing fluid. 2+ Therefore, by detecting Ca in the filtrate 2+ The concentration is used to calculate the solubility of calcite; when the filling material is a clay mineral (such as kaolinite, illite, etc.), the dissolution of the clay mineral releases Si. 4+ Therefore, by detecting Si in the filtrate 4 + The concentration is used to calculate the solubility of clay minerals. When the filling material is a mixture of multiple minerals, the concentration of multiple characteristic component ions can be detected simultaneously, and the solubility of each type of mineral can be calculated separately.

[0032] After each sampling and filtration, the residue retained from the packing material was dried in an oven to constant weight, and then the mass of the residue was measured using a mass testing component. By comparing the trend of residue mass change at each time point, the dissolution process of the packing material can be reflected from a mass perspective, which can be corroborated with the results of the concentration method, thereby improving the reliability of the test data.

[0033] Step S5, Data Processing: Based on the concentration of characteristic components of the packing material and the mass of residue in the filtrate at different soaking times, the solubility and dissolution rate of the packing material in the fracturing fluid are calculated. Solubility reflects the cumulative amount of packing material dissolved at a given time point, while the dissolution rate reflects the rate of dissolution between adjacent time points.

[0034] A dissolution curve of the packing material in fracturing fluid was plotted with soaking time on the x-axis and solubility on the y-axis. The dissolution curve visually reflects the change in the amount of packing material dissolved over soaking time, allowing for the determination of the time point at which the dissolution reaches equilibrium. Based on the dissolution curve and dissolution rate data, fracturing fluid formulations (such as adjusting acid concentration and adding co-solvents) and fracturing operation parameters (such as fracturing fluid soaking time) can be optimized, providing quantitative data support for engineering practice.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows: First, the device can independently control the pressure and temperature of the reaction environment through the working condition simulation unit. The pressure adjustment range is 0-30MPa, and the temperature adjustment range is 20-150℃. It can cover the formation conditions from shallow coal seams to deep coal seams and shale gas reservoirs. The test conditions are highly consistent with the actual downhole environment, which significantly improves the accuracy of the test results and the engineering applicability.

[0036] Second, the test uses complete coal samples containing cleavage fillers. The fillers maintain their original bond with the coal body, which truly reflects the actual shape and contact mode of the fillers in the coal cleavage joints. This overcomes the problem of test result distortion caused by peeling and grinding the fillers into powder and then testing them separately.

[0037] Third, by taking samples at multiple time points (such as 1h, 3h, 6h, 12h, and 24h) and combining concentration detection and quality detection to verify the dissolution of the filler, the dissolution performance of the filler can be fully revealed, the trend of dissolution rate over time and the time point when dissolution reaches equilibrium can be clearly identified, providing direct quantitative data support for the optimization of fracturing fluid formulation and the adjustment of fracturing construction parameters.

[0038] Fourth, the testing device of the present invention integrates sample fixation, working condition simulation, stirring and flow, sampling and filtration, concentration detection and quality detection. The functional units work together in coordination. The liquid replenishment unit can automatically replenish an equal amount of fresh fracturing fluid after sampling to maintain the stability of the test conditions. The operation process is simple and the testing efficiency is high. Attached Figure Description

[0039] One or more embodiments are illustrated by way of example with reference numerals in the accompanying drawings. These illustrations do not constitute a limitation on the embodiments. Elements with the same reference numerals in the drawings are denoted as similar elements. Unless otherwise stated, the figures in the drawings are not to be limited by scale.

[0040] Figure 1 This is a schematic diagram of the structure of a testing device for the solubility of coal cleavage filler in fracturing fluid according to the present invention.

[0041] Figure 2 This is a dissolution curve of coal cleavage filler (calcite) in fracturing fluid in an embodiment of the present invention.

[0042] In the diagram: 1-Pressure reactor; 2-Coal sample holder; 3-Stirring rod; 4-Variable speed stirrer; 5-Heating jacket; 6-Insulation jacket; 7-Methane tank; 8-High-pressure inlet pipe; 9-High-pressure gas pump; 10-Inlet valve; 11-Pressure gauge; 12-Electronic thermometer; 13-Liquid outlet pipe; 14-Sampling valve; 15-Filter funnel; 16-Sample collection bottle; 17-Detection fluid delivery pipe; 18-Ion chromatograph; 19-Fracturing fluid tank; 20-Fracturing fluid input pipe; 21-Replenishment valve; 22-Electronic scale. Detailed Implementation

[0043] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Example

[0044] This embodiment provides a testing device for the solubility of coal cleavage filling materials in fracturing fluid, such as... Figure 1 As shown, it includes a core testing unit, a working condition simulation unit, a sampling and analysis unit, and a liquid replenishment unit.

[0045] The core testing unit includes a transparent pressure vessel 1, a coal sample holder 2, and a stirring assembly. The transparent pressure vessel 1 is made of high-temperature and high-pressure resistant borosilicate glass or transparent alloy material, capable of withstanding pressures of 0–30 MPa and temperatures of 20–150°C. Operators can visually observe the reaction state inside the pressure vessel 1 through its walls. The pressure vessel 1 contains a coal sample holder 2, made of corrosion-resistant stainless steel with a hollow clamping structure, capable of holding 50mm×50mm×50mm coal sample blocks containing fracturing filler without obstructing the contact between the fracturing fluid and the surface of the coal sample block. The stirring assembly includes a stirring rod 3 and a variable-speed mixer 4. The stirring rod 3 extends into the pressure vessel 1, and the variable-speed mixer 4 is installed at the top of the pressure vessel 1. Its speed can be continuously adjusted within the range of 0–300 r / min to regulate different flow states of the fracturing fluid in the fracture.

[0046] The operating condition simulation unit includes a pressure control system and a temperature control system.

[0047] The pressure control system includes a methane tank 7, a high-pressure inlet pipe 8, a high-pressure gas pump 9, an inlet valve 10, and a pressure gauge 11. The methane tank 7 provides the pressurizing medium; methane is chosen because the main component of coalbed methane is methane, and using methane as the pressurizing medium more realistically simulates the gas environment of a coalbed methane reservoir. The high-pressure gas pump 9 pressurizes the methane gas and delivers it to the pressure vessel 1 through the high-pressure inlet pipe 8. The inlet valve 10 is located on the high-pressure inlet pipe 8 to control the opening and closing of the gas path and the gas intake. The pressure gauge 11 displays the pressure inside the vessel in real time. The pressure adjustment range is 0–30 MPa.

[0048] The temperature control system includes a heating jacket 5 and an insulation jacket 6. The heating jacket 5 is an electric heating jacket, which is tightly attached to the outer wall of the pressure vessel 1. The insulation jacket 6 is located outside the heating jacket 5 and is made of heat-insulating material to reduce heat loss. The temperature sensing probe of the electronic thermometer 12 extends into the liquid inside the pressure vessel 1 through a sealed interface on the vessel wall, and the display end is located outside the pressure vessel 1, monitoring and displaying the temperature inside the pressure vessel 1 in real time.

[0049] The sampling and analysis unit includes a sampling and filtering component, a sample collection bottle 16, a component detection component, and a quality detection component.

[0050] The sampling and filtration assembly is located at the lower end of the pressure vessel 1 and includes an outlet pipe 13, a sampling valve 14, and a filter funnel 15. One end of the outlet pipe 13 is connected to the lower wall of the pressure vessel 1, and the other end is connected to the filter funnel 15 through the sampling valve 14. The filter funnel 15 is filled with a microporous filter membrane with a pore size of 0.22 μm, which is used to perform solid-liquid separation on the extracted fracturing fluid, retaining undissolved fine particles of the packing material, and allowing the filtrate containing dissolved components of the packing material to pass through.

[0051] The sample collection bottle 16 is placed below the filter funnel 15 to collect the filtrate after filtration.

[0052] The component detection assembly includes a detection solution delivery tube 17 and an ion chromatograph 18. The detection solution delivery tube 17 delivers the filtrate from the sample collection bottle 16 to the ion chromatograph 18 for ion concentration analysis.

[0053] The quality inspection component is an electronic scale 22 with an accuracy of 0.0001g, used to weigh the mass of the dried filler residue retained in the filter funnel 15.

[0054] The fluid replenishment unit includes a fracturing fluid tank 19, a fracturing fluid inlet pipe 20, and a replenishment valve 21. The fracturing fluid tank 19 stores fresh target fracturing fluid. One end of the fracturing fluid inlet pipe 20 is connected to the fracturing fluid tank 19, and the other end is connected to the pressure vessel 1. After each sampling, the replenishment valve 21 is opened, and an amount of fresh fracturing fluid equal to the sampled volume is replenished into the pressure vessel 1 via the fracturing fluid inlet pipe 20 to maintain a stable fluid level within the vessel. Example

[0055] This embodiment provides a method for testing the solubility of coal cleavage fillers in fracturing fluid using the method described in Example 1, including the following steps: I. Experimental Preparation 1. Coal Sample Selection: Coal samples from the No. 3 coal seam of a certain coal mine were selected. In the laboratory, the coal samples were cut into 50mm × 50mm × 50mm blocks using a dry-cutting method to avoid erosion by water cooling fluid, which could cause the filling material to fall off. After cutting, the coal samples were visually inspected to ensure that the cleavage fractures were exposed and that the filling material within the fractures was not damaged. X-ray diffraction (XRD) analysis showed that the filling material in the cleavage fractures of the coal samples mainly consisted of calcite (approximately 75%) and a small amount of clay minerals.

[0056] 2. Preparation of standard samples of filling material: Carefully peel about 5g of filling material from the cleavage of the prepared coal sample block, grind it into powder in a mortar, sieve it through an 80-mesh standard sieve (sieve aperture 0.18mm), collect the powder under the sieve, and dry it in an oven at 105℃ until constant weight (the difference between two consecutive weighings is less than 0.0002g) to obtain the standard sample of filling material.

[0057] 3. Fracturing fluid preparation: Prepare 750 mL of fracturing fluid according to the fracturing fluid formula used in the actual construction of the coal seam. 500 mL of the fracturing fluid is injected into the pressure reactor to immerse the coal sample block, and the remaining 250 mL is injected into the fracturing fluid tank 19 of the liquid replenishment unit for equal replenishment after sampling at each subsequent time point.

[0058] II. Testing Process 1. Fix the coal sample block on the coal sample holder 2 inside the pressure vessel 1, ensuring that the main cleavage surface of the coal sample block faces upward and is exposed to the fracturing fluid contact direction. Inject the prepared fracturing fluid into the vessel, and after confirming that the fracturing fluid completely submerges the coal sample block, close the vessel.

[0059] 2. Set operating parameters: Increase the pressure inside the vessel to 21 MPa (simulating the actual formation pressure of the coal seam) using the pressure control system, and increase the temperature inside the vessel to 70℃ (simulating the actual formation temperature of the coal seam) using the temperature control system. Start the stirring assembly and set the stirring speed to 120 r / min. After the pressure and temperature have both reached the set values ​​and stabilized for 5 minutes, start timing.

[0060] 3. Immersion Sampling: Samples were taken at 1h, 3h, 6h, 12h, and 24h of immersion. Each time, the sampling valve 14 of the sampling filter assembly was opened, and 50mL of the immersed fracturing fluid was discharged through the outlet pipe 13 into the filter funnel 15 under the pressure inside the pressurized reactor 1. After filtration through a 0.22μm microporous membrane, the filtrate was collected in the sample collection bottle 16, and the packing material residue was retained on the filter membrane. After sampling, the sampling valve 14 was closed, and the replenishment valve 21 of the liquid replenishment unit was opened to replenish 50mL of fresh fracturing fluid into the reactor through the fracturing fluid inlet pipe 20. The replenishment valve 21 was then closed.

[0061] 4. Solubility test: The filtrate in the sample collection bottle 16 is transferred to the ion chromatograph 18 through the detection solution delivery tube 17 to determine the Ca²⁺ concentration in the filtrate. The residue of the packing material retained on the filter membrane is dried in an oven at 105℃ until constant weight, and then the mass of the residue is weighed using an electronic balance 22 (accuracy 0.0001g).

[0062] III. Test Results and Analysis Theoretical conversion formula for calcite solubility: The molar mass of calcite (CaCO3) = 40.08 (Ca) + 12.01 (C) + 16.00 × 3 (O) = 100.09 g / mol Ca²⁺ molar mass = 40.08 g / mol The mass ratio of calcite to Ca²⁺ is: 100.09 / 40.08≈2.497 40.08 / 100.09≈2.497 Therefore, calcite concentration (g / L) = Ca²⁺ concentration (g / L) × 2.497 The detection results at each time point are as follows: When soaked for 1 hour, the Ca²⁺ concentration is 0.85 g / L. According to the theoretical conversion formula of calcite solubility, the corresponding calcite solubility is 2.12 g / L.

[0063] After soaking for 3 hours, the Ca²⁺ concentration was 1.38 g / L, corresponding to a calcite solubility of 3.45 g / L.

[0064] After soaking for 6 hours, the Ca²⁺ concentration was 1.82 g / L, corresponding to a calcite solubility of 4.54 g / L.

[0065] After soaking for 12 hours, the Ca²⁺ concentration was 2.18 g / L, corresponding to a calcite solubility of 5.44 g / L.

[0066] After soaking for 24 hours, the Ca²⁺ concentration remained stable at 2.32 g / L, corresponding to a calcite solubility of 5.79 g / L.

[0067] A melting curve was plotted using the above data (reference). Figure 2 It can be concluded that: (1) The calcite dissolution rate is relatively fast in the first 6 hours, with an average dissolution rate of about 0.85 g / (L·h). This is because the surface of the filling material is fresh in the initial stage and reacts actively with the fracturing fluid.

[0068] (2) After 6 hours, the dissolution rate slowed down significantly, with the average dissolution rate dropping to about 0.12 g / (L·h). The reasons are: first, the active components in the fracturing fluid that react with the packing material are gradually consumed; second, a layer of dissolution products forms on the surface of the packing material, which hinders the further penetration of the fracturing fluid into the packing material.

[0069] (3) After 24 hours, the Ca²⁺ concentration tended to stabilize, indicating that the dissolution of the filling material had basically reached a dynamic equilibrium.

[0070] (4) Based on the above test results, the fracturing fluid formulation of the coal seam can be optimized. For example, the pH value of the fracturing fluid can be appropriately increased to 9.0 to enhance the dissolution capacity of calcite. At the same time, it is recommended to control the soaking time of the fracturing fluid in the coal seam fracture to 12-24h to ensure that the filling material is fully dissolved and to maximize the fracture conductivity.

[0071] 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 embodiments of the present invention have been described in detail, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of protection of the claims of the present invention.

Claims

1. A testing device for the solubility of coal cleavage packing in fracturing fluid, characterized in that, include: The core testing unit is used to contain coal sample blocks containing cleavage filler and fracturing fluid; The working condition simulation unit is connected to the core testing unit and is used to simulate the pressure and temperature conditions in the downhole fracturing environment. The sampling and analysis unit, connected to the core testing unit, is used to extract fracturing fluid after soaking coal samples and to detect the concentration of characteristic components of the filling material and the mass of undissolved residue in the fracturing fluid after soaking coal samples.

2. The testing device for the solubility of coal cleavage filling material in fracturing fluid according to claim 1, characterized in that, The core testing unit includes a pressure vessel (1), a coal sample holder (2) installed inside the pressure vessel (1), and a stirring assembly; The coal sample holder (2) is used to fix coal sample blocks containing cleavage filler, the filler in the cleavage of the coal sample block being exposed to the fracturing fluid; The stirring assembly includes a stirring rod (3) and a variable speed mixer (4) for adjusting the flow state of the fracturing fluid.

3. The testing device for the solubility of coal cleavage filling material in fracturing fluid according to claim 2, characterized in that, The pressure vessel (1) is a transparent pressure vessel (1) made of transparent material that is resistant to high temperature and high pressure.

4. The testing device for the solubility of coal cleavage filling material in fracturing fluid according to claim 1, characterized in that, The operating condition simulation unit includes: The pressure control system includes a gas source tank, a high-pressure gas pump (9), a high-pressure gas inlet pipe (8), a gas inlet valve (10), and a pressure gauge (11), which are used to regulate the pressure inside the pressure vessel (1); The temperature control system includes a heating jacket (5) and an insulation jacket (6) for regulating the temperature inside the pressure vessel (1).

5. The testing device for the solubility of coal cleavage filling material in fracturing fluid according to claim 4, characterized in that, The reactor is also equipped with a temperature sensor for real-time monitoring of the temperature inside the reactor.

6. The apparatus for testing the solubility of coal cleavage filling material in fracturing fluid according to claim 1, characterized in that, The sampling and analysis unit includes: The sampling and filtration assembly is located at the lower end of the pressure reactor (1) and is used to extract the fracturing fluid after soaking the coal sample block and filter it to obtain filtrate containing dissolved components of the filling material and undissolved filling material residue. Sample collection bottle for collecting the filtrate; A component detection component is used to determine the concentration of characteristic components of the filler in the filtrate; A quality inspection component is used to weigh the mass of the filling material residue after drying.

7. The testing device for the solubility of coal cleavage filling material in fracturing fluid according to claim 1, characterized in that, Also includes: The liquid replenishment unit is connected to the pressure vessel (1) and is used to replenish the fracturing fluid into the pressure vessel (1) after sampling to maintain the amount of fracturing fluid in the pressure vessel (1).

8. A test method using the testing apparatus for the solubility of coal fracturing filler in fracturing fluid as described in any one of claims 1 to 7, characterized in that, Includes the following steps: S1. Select coal samples containing cleavage fillers, process them into coal sample blocks, ensuring that the cleavage fractures of the coal sample blocks are exposed and the fillers are not destroyed; prepare fracturing fluid according to engineering requirements; S2. Fix the coal sample block on the coal sample holder (2) inside the pressure reactor (1), inject fracturing fluid into the pressure reactor (1) to immerse the coal sample block; set the pressure and temperature values ​​through the working condition simulation unit, start the stirring assembly, and simulate the downhole fracturing environment; S3. Starting from the injection of the fracturing fluid into the pressure reactor (1), at multiple preset soaking time points, the fracturing fluid after soaking the coal sample block is extracted by the sampling and filtering assembly and filtered to obtain filtrate containing dissolved components of the filling material and undissolved filling material residue; after sampling, fresh fracturing fluid is replenished through the liquid replenishment unit to maintain the stability of the fracturing fluid volume in the pressure reactor (1); S4. The concentration of characteristic components of the packing material in the filtrate is determined using a component detection component; the packing material residue is dried to constant weight and then weighed using a mass detection device. S5. Based on the concentration of characteristic components of the packing material and the mass of residue in the filtrate at different soaking times, calculate the solubility and dissolution rate of the packing material in the fracturing fluid, plot the dissolution curve, and complete the solubility test.

9. The test method for the test apparatus for the solubility of coal cleavage filling material in fracturing fluid according to claim 8, characterized in that, In step S3, the preset soaking time nodes include 1h, 3h, 6h, 12h and 24h; the fracturing fluid sampling volume after each soaking of the coal sample block is 50mL, and an equal amount of fresh fracturing fluid is added after sampling.

10. The test method for the apparatus for testing the solubility of coal cleavage filling material in fracturing fluid according to claim 8, characterized in that, In step S4, when the packing material is calcite-based, the Ca content in the filtrate is detected. 2+ Concentration calculation of solubility; when the filling material is clay mineral-based, the amount of Si in the filtrate is measured. 4+ Concentration calculation of dissolved amount.

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