Visual evaluation system and method for pulverized coal retention-proppant failure in fracturing crack

Abstract

The invention discloses a visual evaluation system and method for pulverized coal retention-proppant failure in a fracturing fissure, and the system comprises a transparent fracturing fissure model which comprises a transparent box body and a piston, a containing cavity is formed in the transparent box body, the piston is installed in the containing cavity, the piston divides the containing cavity into a first cavity and a second cavity, and the first cavity is communicated with the second cavity; the coal rock slice is fixedly connected to the piston, the second cavity is filled with proppant particles, and the first injection assembly is configured to inject liquid, solid and gas into the second cavity to form gas-liquid-solid mixed liquid; the second injection assembly is configured to inject liquid into the first cavity so as to push the piston to move towards the second cavity; the three-phase separator is communicated with the third port and is configured to perform gas-liquid separation on the gas-liquid-solid mixed liquid in the second chamber; and the at least two optical microscopes are arranged on at least two end face sides of the transparent fracturing crack model and are configured to be used for observing flow field characteristics and propping agent behaviors of at least two end faces of the transparent fracturing crack model.

Description

Technical Field

[0001] The present invention relates to the technical field of flow field visualization, and in particular to a system and method for visualizing evaluation of coal dust retention and proppant failure in hydraulic fractures. Background Art

[0002] During coal-rock gas (CBM) reservoir development, fractures serve as the primary pathways for gas and water seepage, and their conductivity directly determines the effectiveness of reservoir stimulation. However, due to geological constraints such as low mechanical strength of coal rock and the development of endogenous fractures, CBM reservoirs are generally rich in pulverized coal particles. During the drainage process, pulverized coal easily migrates with the fluid and becomes trapped and blocked within the fractures. Furthermore, proppant particles within the fractures face three primary failure mechanisms under the coupled effects of stress and fluid flow: ① proppant migration and backflow caused by fluid shear; ② particle fragmentation and degradation due to stress concentration; and ③ wall embedment caused by interfacial contact. The synergistic deterioration of pulverized coal blockage and proppant failure leads to a gradual decline in fracture conductivity, becoming a key factor restricting the productivity of CBM wells. Therefore, scientifically elucidating the evolution of pulverized coal blockage and proppant failure within fractures and analyzing their coupled interactions are crucial for developing a pulverized coal blockage early warning system and a proppant failure prevention and control technology framework, thereby improving the long-term conductivity of fractures and enhancing the effectiveness and productivity of CBM well development.

[0003] Currently, most visualization evaluation devices for coal dust intrusion and retention and proppant failure behaviors within hydraulic fractures construct propped fractures using a glass plate-rock plate or glass plate-glass plate combination, and use microscopes or high-speed cameras to observe the solid-liquid / gas-liquid-solid multiphase flow field, coal dust retention and blockage, and the evolution of proppant failure within the fractures. However, the following limitations exist: existing technologies can only achieve visualization observation in a single direction (perpendicular to the fracture extension direction or parallel to the fracture extension direction), and cannot three-dimensionally capture the coal dust retention and blockage and proppant failure processes within the hydraulic fractures under multiphase flow fields; existing technologies can only perform visualization evaluations of the single behaviors of coal dust intrusion and retention or proppant failure within the hydraulic fractures, and cannot simultaneously evaluate the behaviors of coal dust intrusion and retention and proppant failure, and explore the coupling law between the two; under closed stress loading conditions, existing technologies mostly use precise displacement sensors to provide real-time feedback on the changes in the width of the hydraulic fractures, which has problems such as high complexity of the displacement control system, low loading accuracy, and interference from interface friction. Summary of the Invention

[0004] In response to the above-mentioned issues and needs, this solution proposes a visual evaluation system and method for coal powder retention and proppant failure in fractures. By adopting the following technical features, it can achieve the above-mentioned technical objectives and bring about multiple other technical effects.

[0005] One object of the present invention is to provide a visual evaluation system for coal dust retention and proppant failure in hydraulic fractures, comprising:

[0006] A transparent fracturing model comprises: a transparent box and a piston, wherein a receiving chamber is formed in the transparent box, a piston is installed in the receiving chamber, the piston divides the receiving chamber into a first chamber and a second chamber, a coal rock piece and proppant particles are arranged in the second chamber, the coal rock piece is fixedly connected to the piston, and the proppant particles are filled in the second chamber, wherein the transparent box is provided with a first port connected to the first chamber, and a second port, a third port, a fourth port, and a fifth port connected to the second chamber;

[0007] a first injection assembly, connected to the second port, configured to inject liquid and gas into the second chamber to form a gas-liquid mixture or to inject liquid, gas and solid to form a gas-liquid-solid mixture;

[0008] a second injection assembly, connected to the first port, configured to inject liquid into the first chamber to push the piston toward the second chamber, thereby applying a closing stress to the proppant in the second chamber;

[0009] a three-phase separator, connected to the third port, configured to separate the gas-liquid-solid mixture in the second chamber into gas and liquid;

[0010] At least two optical microscopes are respectively provided on at least two end surfaces of the transparent fracture model, and are configured to visually observe fluid flow, particle migration, and proppant failure behavior within the transparent fracture model from the two end surfaces, wherein the two end surfaces are respectively an end surface perpendicular to the fracture extension direction and an end surface parallel to the fracture extension direction;

[0011] A vacuum pump is connected to the third port and is configured to evacuate the second chamber.

[0012] In this technical solution, first, the second injection assembly injects liquid into the first chamber to push the piston toward the second chamber to apply pressure to the proppant in the second chamber. Then, the evaluation system is evacuated by a vacuum pump. Next, the first injection assembly injects liquid, gas and solid into the second chamber to form a mixed liquid. At the same time, at least two optical microscopes are used to observe the flow field characteristics and proppant behavior of at least one end face of the transparent fracture model. After the observation of the flow field characteristics and proppant behavior is completed, the gas-liquid-solid mixed liquid in the second chamber is separated by a three-phase separator.

[0013] In addition, the system and method for visually evaluating coal powder retention and proppant failure in hydraulic fractures according to the present invention may also have the following technical features:

[0014] In one example of the present invention, the first injection assembly includes: a first injection pump and a gas cylinder connected in parallel, and the first injection pump and the gas cylinder form a common node at the parallel node, and the common node is connected to the second port; wherein, an on-off assembly for controlling the flow of liquid and solid from the first injection pump toward the transparent fracturing model is provided between the first injection pump and the common node, and a flow controller for controlling the flow of gas toward the transparent fracturing model is provided between the gas cylinder and the common node.

[0015] In one example of the present invention, the on-off component includes: a first on-off valve and a stirring intermediate container connected in parallel between the first injection pump and the common node, the first on-off valve is configured to control the on-off between the first injection pump and the common node, and the stirring intermediate container is configured to stir the liquid and solid pumped in by the first injection pump.

[0016] In one example of the present invention, the on-off assembly further includes: a second on-off valve and a third on-off valve provided at both ends of the stirring intermediate container, wherein the second on-off valve is configured to control the on-off of the liquid from the first injection pump toward the stirring intermediate container, and the third on-off valve is configured to control the on-off of the liquid from the stirring intermediate container toward the transparent fracturing model.

[0017] In one example of the present invention, the on-off assembly further includes: a one-way valve, which is arranged between the gas flow controller and the common node and is configured to control the gas in the gas cylinder to flow only from the gas cylinder toward the transparent fracturing model.

[0018] In one example of the present invention, it also includes: a differential pressure sensor, which is connected in parallel between the fourth port and the fifth port of the transparent fracturing model and is configured to sense the pressure difference information at both ends of the proppant filling layer in the second chamber; wherein the fourth port and the fifth port are respectively arranged close to the second port and the third port.

[0019] In one example of the present invention, the present invention further includes: a vacuum pump connected to the third port and configured to evacuate the second chamber.

[0020] In one example of the present invention, the three-phase separator has a first interface, a second interface and a third interface, the first interface is connected to the third port, the second interface is connected to a first back pressure valve, and the third interface is connected to a second back pressure valve.

[0021] In one example of the present invention, the method further includes: a flow meter connected to the output port of the first back-pressure valve, configured to monitor the flow rate of the gas in the three-phase separator discharged through the second interface.

[0022] In one example of the present invention, a mounting groove is provided on the end surface of the piston located in the second chamber, and the coal rock piece is clamped in the mounting groove.

[0023] Another object of the present invention is to provide an evaluation method for the above-mentioned visual evaluation system of coal powder retention and proppant failure in fractures, comprising the following steps:

[0024] S10: Collect on-site coal samples, proppant, and produced formation water, prepare coal slices, grind the coal rock into fine particles, sieve out coal powder within a certain particle size range, pour the coal powder into the formation water, and prepare a coal powder suspension of a certain concentration;

[0025] S20: Mounting the prepared coal-rock slice on the piston, loading proppant particles into the second chamber according to a certain sand-laying concentration, injecting liquid into the first chamber by the second injection assembly to push the piston toward the second chamber to apply a certain closing stress to the proppant in the second chamber, placing the transparent fracture model vertically, and evacuating and saturating the fracture model with water by a vacuum pump;

[0026] S30: injecting gas and pulverized coal suspension into the second chamber at a certain gas flow rate and liquid flow rate by the first injection assembly to form a gas-liquid-solid mixture, monitoring the pressure difference across the fracture in real time, and calculating the gas permeability and liquid permeability based on Darcy's law;

[0027] S40: Simultaneously observe the gas-water-coal three-phase flow field and proppant breakage, embedding, and loss behaviors within the fracture using at least two microscopes, parallel to and perpendicular to the fracture extension direction. Combining the two-way observation information, the coal pulverized gas retention and blockage, as well as the proppant failure mechanism, are revealed in three dimensions. Changes in fracture width are also monitored along the fracture extension direction.

[0028] S50: Repeat the above steps, but change the injection of gas and liquid in step S20 to analyze the failure behavior of the proppant and the induced conductivity attenuation under the condition of no coal powder invasion. By comparing the test results under the conditions of coal powder invasion and no coal powder invasion, the contribution of coal powder retention and proppant failure behavior to the damage of propped fracture conductivity and their coupling law are revealed.

[0029] S60: Repeat the above steps, change the gas flow rate, liquid flow rate and closure stress as single factors, and explore the control effect of the gas / liquid flow rate combination and closure stress on the coal powder retention and blockage and proppant failure behavior.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] (1) The present invention uses a fully transparent fracture model and arranges an optical microscope perpendicular to and parallel to the fracture extension direction to achieve two-way visual observation of the fracture, thereby three-dimensionally and comprehensively capturing the multiphase flow field characteristics within the fracture, as well as the coal powder retention and blockage, and proppant failure (loss, breakage, and embedding) processes;

[0032] (2) The present invention can realize the visualization observation of coal dust intrusion and retention and proppant failure process in the fracture separately or simultaneously, revealing the coupling effect of coal dust retention and proppant failure, and comprehensively evaluating the influence of the two on the fracture conductivity, providing a more comprehensive experimental analysis method for exploring the damage mechanism of the fracture conductivity of coalbed methane wells;

[0033] (3) The present invention adopts a fully transparent fracture model, which can accurately measure the fracture width at different positions parallel to the fracture extension direction through direct observation under a microscope. There is no need to configure a complex displacement control system, which improves the accuracy and intuitiveness of the fracture width measurement. The device structure is simpler, easier to operate, and the cost is lower.

[0034] Hereinafter, the best embodiment of the present invention will be described in more detail with reference to the accompanying drawings so that the features and advantages of the present invention can be easily understood. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings of the embodiments of the present invention. The drawings are only used to illustrate some embodiments of the present invention, but not to limit all embodiments of the present invention thereto.

[0036] Figure 1 Schematic diagram of the structure of a system for visually evaluating coal powder retention and proppant failure in hydraulic fractures according to an embodiment of the present invention;

[0037] Figure 2 is a front view of a transparent fracture model according to an embodiment of the present invention;

[0038] Figure 3 This is a left view of a transparent fracture model according to an embodiment of the present invention;

[0039] Figure 4 is a top view of a transparent fracture model according to an embodiment of the present invention;

[0040] Figure 5 Flowchart of a method for visually evaluating coal powder retention and proppant failure in hydraulic fractures according to an embodiment of the present invention.

[0041] List of reference numerals:

[0042] Evaluation system 100;

[0043] Transparent fracture model 10;

[0044] Transparent box 11;

[0045] cylindrical portion 111;

[0046] Cover body 112;

[0047] Base 113;

[0048] First chamber 114;

[0049] Second chamber 115;

[0050] Fastener 116;

[0051] Piston 12;

[0052] Mounting slot 121;

[0053] First port 13;

[0054] Second port 14;

[0055] The third port 15;

[0056] Observation area 16;

[0057] Fourth port 17;

[0058] Fifth port 18;

[0059] First injection assembly 20;

[0060] a first injection pump 21;

[0061] Gas cylinder 22;

[0062] On / off component 23;

[0063] First on-off valve 231;

[0064] A stirring intermediate container 232;

[0065] A second on-off valve 233;

[0066] a third on-off valve 234;

[0067] flow controller 24;

[0068] One-way valve 25;

[0069] Public node A;

[0070] A second injection assembly 30;

[0071] A second injection pump 31;

[0072] Fifth on-off valve 32;

[0073] Three-phase separator 40;

[0074] First interface 41;

[0075] Second interface 42;

[0076] The third interface 43;

[0077] a first back pressure valve 44;

[0078] Second back pressure valve 45;

[0079] Seventh on-off valve 46;

[0080] an eighth on-off valve 47;

[0081] Ninth on-off valve 48;

[0082] Optical microscope 50;

[0083] Vacuum pump 60;

[0084] Sixth on-off valve 61;

[0085] differential pressure sensor 70;

[0086] Fourth on-off valve 80;

[0087] Flow meter 90. DETAILED DESCRIPTION

[0088] In order to make the purpose, technical solution and advantages of the technical solution of the present invention clearer, the technical solution of the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings of specific embodiments of the present invention. The same figure marks in the drawings represent the same parts. It should be noted that the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the described embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0089] Unless otherwise defined, the technical or scientific terms used herein shall have the ordinary meaning understood by persons of ordinary skill in the field to which the invention belongs. The words "first", "second" and similar terms used in the patent application specification and claims of the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. Similarly, words such as "a" or "an" do not necessarily indicate a quantity limitation. Words such as "include" or "comprising" mean that the elements or objects preceding the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. Words such as "connected" or "connected" are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the object being described changes, the relative positional relationship may also change accordingly.

[0090] According to the first aspect of the present invention, a system 100 for visually evaluating coal powder retention and proppant failure in a hydraulic fracture comprises:

[0091] A transparent fracturing model 10 includes a transparent housing 11 and a piston 12. The transparent housing 11 has a receiving chamber formed therein. The piston 12 is installed in the receiving chamber. The piston 12 divides the receiving chamber into a first chamber 114 and a second chamber 115. The second chamber 115 contains coal flakes and proppant particles. The coal flakes are fixedly connected to the piston 12. The proppant particles are filled into the second chamber 115. The transparent housing 11 is provided with a first port 13 communicating with the first chamber 114, and a second port 14, a third port 15, a fourth port 17, and a fifth port 18 communicating with the second chamber 115.

[0092] a first injection assembly 20 , in communication with the second port 14 , configured to inject liquid and gas into the second chamber 115 to form a gas-liquid mixture or to inject liquid, gas, and solid to form a gas-liquid-solid mixture;

[0093] The second injection assembly 30 is connected to the first port 13 and is configured to inject liquid into the first chamber 114 to push the piston 12 toward the second chamber 115 to apply closing stress to the proppant in the second chamber 115;

[0094] A three-phase separator 40 is connected to the third port 15 and is configured to separate the gas-liquid-solid mixture in the second chamber 115 into gas and liquid;

[0095] At least two optical microscopes 50 are respectively provided on at least two end surfaces of the transparent fracture model 10, and are configured to visually observe fluid flow, particle migration, and proppant failure behavior within the transparent fracture model 10 from the two end surfaces, wherein the two end surfaces are respectively an end surface perpendicular to the fracture extension direction and an end surface parallel to the fracture extension direction;

[0096] The working process of the evaluation system 100 is as follows: collecting on-site coal samples, proppant and produced formation water, and preparing coal rock slices with the same shape as the mounting groove 121 in the piston 12, grinding the coal rock into fine particles, screening out coal powder within a certain particle size range, pouring the coal powder into the formation water, and preparing a coal powder suspension of a certain concentration; loading the prepared coal rock slice into the mounting groove 121, and sealing the contact position between the coal rock slice and the mounting groove 121 with glue, and loading the proppant particles into the second chamber 115 in sequence according to a certain sand concentration, and then injecting liquid into the first chamber 114 by the second injection component 30 to push the piston toward the second chamber 115 to apply a certain closing stress to the proppant in the second chamber 115, placing the transparent fracturing model 10 vertically, and vacuuming and saturating the fracturing model with water by the vacuum pump 60; and injecting gas and coal powder suspension into the second chamber 115 by the first injection component 20 according to a certain gas phase flow rate and liquid phase flow rate to form a fracture model. A gas-liquid-solid mixture is formed, the pressure difference at both ends of the fracture is monitored in real time, and the gas and liquid permeabilities are calculated based on Darcy's law; along the direction parallel to the fracture extension and the direction perpendicular to the fracture extension, the gas-water-coal three-phase flow field and the proppant crushing, embedding and loss behavior in the fracture are simultaneously observed through at least two optical microscopes 50, and the two-way observation information is integrated to reveal the coal powder retention and blockage and proppant failure mechanism in three dimensions, and the crack width change is monitored along the direction parallel to the fracture extension; the above steps are repeated, and the above steps are changed to injecting gas and liquid, and the proppant failure behavior and the induced conductivity attenuation law under the condition of no coal powder invasion are analyzed, and by comparing the test results with and without coal powder conditions, the contribution of coal powder retention and proppant failure behavior to the damage to the proppant fracture conductivity and their coupling law are revealed; the above steps are repeated, and the gas phase flow rate, liquid phase flow rate and closure stress are changed as single factors to explore the control effect of the gas phase / liquid phase flow rate combination and closure stress on the coal powder retention and blockage and proppant failure behavior.

[0097] This system uses a fully transparent fracture model and arranges an optical microscope perpendicular to and parallel to the fracture extension direction to achieve two-way visual observation of the fracture. This allows for a three-dimensional and comprehensive capture of the multiphase flow field characteristics within the fracture, as well as the coal dust retention and blockage, and proppant failure (loss, breakage, and embedment).

[0098] This system can separately or simultaneously visualize the process of coal dust intrusion and proppant failure in fractures, revealing the coupling effect of coal dust retention and proppant failure, and comprehensively evaluating their impact on fracture conductivity, providing a more comprehensive experimental analysis method for exploring the damage mechanism of fracture conductivity in coalbed methane wells.

[0099] This system uses a fully transparent fracture model and can accurately measure the fracture width at different locations parallel to the fracture extension direction through direct observation under a microscope. It does not require the configuration of a complex displacement control system, which improves the accuracy and intuitiveness of fracture width measurement. The device structure is simpler, easier to operate, and more cost-effective.

[0100] In one example of the present invention, the transparent box 11 includes: a transparent cylinder 111, a transparent cover 112 and a base 113. The base 113, the transparent cylinder 111 and the transparent cover 112 are fixed by fasteners 116. A accommodating chamber is formed in the transparent box 11, and a piston is installed in the accommodating chamber, and the piston divides the accommodating chamber into a first chamber and a second chamber. An observation area 16 is formed on at least one side of the box to facilitate microscopic observation of the flow characteristics of the gas-liquid-solid mixture inside the accommodating chamber; the first port 13 is located at the lower end of the cylinder 111, the second port 14 and the third port 15 are located on both sides of the cylinder 111, and the second port 14 and the third port 15 are arranged near the upper end of the accommodating chamber and are located in the second chamber 115. The fourth port 17 and the fifth port 18 are connected to the second chamber 115, and the fourth port 17 and the fifth port 18 are arranged near the second port 14 and the third port 15, respectively.

[0101] In one example of the present invention, the first injection assembly 20 includes: a first injection pump 21 and a gas cylinder 22 connected in parallel, and the first injection pump 21 and the gas cylinder 22 form a common node A at the parallel node, and the common node A is connected to the second port 14; wherein, an on-off assembly 23 for controlling the flow of liquid from the first injection pump 21 toward the transparent fracturing model 10 is provided between the first injection pump 21 and the common node A, and a flow controller 24 for controlling the flow of gas into the transparent fracturing model 10 is provided between the gas cylinder 22 and the common node A;

[0102] That is, the evaluation system 100 is evacuated by the vacuum pump 60, the on-off assembly 23 is opened to allow the solid suspension to flow from the first injection pump 21 toward the transparent fracture model 10, and at the same time, the flow controller 24 is opened to allow the gas to flow into the transparent fracture model 10, so that the gas, liquid and solid are combined in the second chamber 115 to form a gas-liquid-solid mixture, which is used to simulate the gas-liquid-solid three-phase flow field in the fracture.

[0103] In one example of the present invention, the on-off component 23 includes: a first on-off valve 231 and a stirring intermediate container 232 connected in parallel between the first injection pump 21 and the common node A, wherein the first on-off valve 231 is configured to control the on-off between the first injection pump 21 and the common node A, and the stirring intermediate container 232 is configured to stir the liquid and solid pumped into by the first injection pump 21;

[0104] That is to say, the first on-off valve 231 can be used to turn on and off the liquid from the first injection pump 21 toward the common node A, and the stirring intermediate container 232 is mainly used to mix and stir the liquid pumped in by the first injection pump 21 with the solid inside itself, so as to pump the mixed liquid to the common node A. By setting the on-off component 23, the liquid and solid can be mixed at the common node A to form a liquid-solid mixed liquid.

[0105] In one example of the present invention, the on-off assembly 23 further includes: a second on-off valve 233 and a third on-off valve 234 provided at both ends of the stirring intermediate container 232, wherein the second on-off valve 233 is configured to control the on-off flow of liquid from the first injection pump 21 toward the stirring intermediate container 232, and the third on-off valve 234 is configured to control the on-off flow of liquid from the stirring intermediate container 232 toward the transparent fracturing model 10;

[0106] A second on-off valve 233 and a third on-off valve 234 are respectively set at both ends of the stirring intermediate container 232 to control the stirring and mixing of the liquid and the solid and control the flow of the formed mixed liquid toward the common node A. For example, the second on-off valve 233 can be closed to prevent the solid from being stirred and mixed with the liquid; the second on-off valve 233 can be opened. Closing the second on-off valve 233 can allow the liquid and the solid to be fully stirred and mixed in the stirring intermediate container 232; opening the third on-off valve 234 and the second on-off valve 233 can allow the liquid-solid mixed liquid to be injected into the transparent fracturing model 10.

[0107] In one example of the present invention, the on-off component 23 further includes: a one-way valve 25, which is provided between the gas flow controller 24 and the common node A and is configured to control the gas in the gas cylinder 22 to flow only from the gas cylinder 22 toward the transparent fracturing model 10;

[0108] In short, by providing the one-way valve 25 , the gas can only flow from the gas cylinder 22 toward the transparent fracturing model 10 , thereby preventing the gas from flowing in the opposite direction.

[0109] In one example of the present invention, it also includes: a differential pressure sensor 70, which is connected in parallel between the fourth port 17 and the fifth port 18 of the transparent fracturing model 10, and is configured to sense the pressure difference information at both ends of the proppant filling layer in the second chamber 115; wherein the fourth port 17 and the fifth port 18 are respectively arranged close to the second port 14 and the third port 15.

[0110] The fourth port 17 is arranged close to the second port 14, and the fifth port 18 is arranged close to the third port 15. By respectively plugging the two sides of the differential pressure sensor 70 into the fourth port 17 and the fifth port 18 and arranging them close to the second chamber 115, it is convenient to more realistically measure the pressure difference information at both ends of the proppant filling layer in the second chamber 115 to obtain the dynamic characteristics of the gas-liquid-solid three-phase flow field in the second chamber 115.

[0111] In one example of the present invention, the system further includes a vacuum pump 60 , which is connected to the third port 15 and configured to evacuate the second chamber 115 .

[0112] In one example of the present invention, the three-phase separator 40 has a first interface 41, a second interface 42 and a third interface 43, wherein the first interface 41 is connected to the third port 15, the second interface 42 is connected to a first back pressure valve 44, and the third interface 43 is connected to a second back pressure valve 45;

[0113] That is to say, specifically, after the gas, liquid and solid three phases in the three-phase separator 40 are separated, the gas and liquid continue to accumulate in the three-phase separator 40. When the pressure of the second interface 42 connected to the first back-pressure valve 44 reaches a certain threshold, the first back-pressure valve 44 is turned on to allow the gas to flow to the exhaust gas collector; when the pressure of the third interface 43 connected to the second back-pressure valve 45 reaches a certain threshold, the second back-pressure valve 45 is turned on to allow the liquid to flow to the liquid storage tank; that is, the first back-pressure valve 44 and the second back-pressure valve 45 provide pressure for the entire system to simulate the real pressure environment underground.

[0114] In one example of the present invention, the system further includes a flow meter 90 connected to the output port of the first back pressure valve 44 and configured to monitor the flow of the gas in the three-phase separator 40 discharged through the second interface 42 .

[0115] In one example of the present invention, it further includes: a fourth on-off valve 80, which is provided between the common node A and the second end point and is configured to control the on-off of the gas-liquid-solid mixture from the common node A toward the second end point.

[0116] In one example of the present invention, the second injection assembly 30 includes:

[0117] a second injection pump 31 , which is in communication with the first port 13 and is configured to inject liquid into the first chamber 114 ;

[0118] The fifth on-off valve 32 is disposed between the second injection pump 31 and the first port 13 , and is configured to control the on-off flow of liquid from the second injection pump 31 toward the first chamber 114 .

[0119] In one example of the present invention, it further includes: a sixth on-off valve 61 , which is provided between the vacuum pump 60 and the third port 15 and is configured to control the on-off of the airflow generated by the vacuum pump 60 between the second chamber 115 and the vacuum pump 60 .

[0120] In one example of the present invention, the seventh on-off valve 46, the eighth on-off valve 47 and the ninth on-off valve 48 are further included.

[0121] The seventh on-off valve 46 is provided between the third port 15 and the first interface 41 and is configured to control the on-off of the gas-liquid-solid mixture between the third port 15 and the first interface 41;

[0122] The eighth on-off valve 47 is provided at the second interface 42 and is configured to control the on-off of the gas at the second interface 42;

[0123] The ninth on-off valve 48 is provided at the third interface 43 and is configured to control the on-off of the liquid at the third interface 43 .

[0124] In one example of the present invention, a mounting groove 121 is formed on the end surface of the piston 12 located in the second chamber 115 , and the coal rock piece is clamped in the mounting groove 121 .

[0125] The installation groove 121 is provided in the piston 12 to avoid direct contact between the coal rock piece and the wall of the accommodating cavity in the transparent box 11, thereby preventing the movement of the piston 12 from causing the glue seal between the coal rock piece and the wall of the accommodating cavity to fail.

[0126] It is understood that when the coal rock slice is prepared, its shape is formed to be consistent with the mounting groove 121 in the piston 12.

[0127] According to the second aspect of the present invention, an evaluation method of the above-mentioned visual evaluation system 100 for coal powder retention and proppant failure in fractures is as follows: Figure 5 As shown, the following steps are included:

[0128] S10: Collect on-site coal samples, proppant, and produced formation water, and prepare coal rock slices (for example, the shape of the coal rock slices is consistent with the formation of the mounting groove 121 in the piston 12), grind the coal rock into fine particles, screen out coal powder within a certain particle size range, pour the coal powder into the formation water, and prepare a coal powder suspension of a certain concentration;

[0129] For example, a thin coal rock slice having the same shape as the installation groove 121 is prepared by wire cutting with a thickness of 0.5 to 1.0 cm, and the coal sample is ground into fine particles by mechanical grinding, and coal powder within a certain particle size range is screened out, and the coal powder is poured into formation water to prepare a coal powder suspension of a certain concentration;

[0130] S20: The prepared coal rock slice is mounted on the piston 12, and proppant particles are loaded into the second chamber 115 according to a certain sand concentration, and the transparent cover 112 is sealed, and then the second injection assembly 30 injects liquid into the first chamber 114 to push the piston 12 toward the second chamber 115 to apply a certain closing stress to the proppant in the second chamber 115, and the transparent fracturing model 10 is placed vertically, and the fracturing model is vacuumed and saturated with water by the vacuum pump 60; for example, when installing the coal rock slice, the prepared coal rock slice is loaded into the installation groove 121, and the contact position between the coal rock slice and the installation groove 121 is sealed with glue.

[0131] S30: The first back-pressure valve 44 and the second back-pressure valve 45 are set to the same back-pressure value. Then, the first injection assembly 20 injects gas and coal powder suspension into the second chamber 115 at a certain gas flow rate and liquid flow rate to form a gas-liquid-solid mixture. The differential pressure sensor 70 monitors the pressure difference across the fracture in real time, and the gas and liquid permeabilities are calculated based on Darcy's law.

[0132] S40: Simultaneously observing the gas-water-coal powder three-phase flow field and proppant breakage, embedding, and loss behaviors within the fracture using at least two optical microscopes 50 along a direction parallel to and perpendicular to the fracture extension direction. Combining the two-way observation information, the coal powder retention and blockage and proppant failure mechanisms are revealed in a three-dimensional manner, and changes in the fracture width are monitored along the direction parallel to the fracture extension direction.

[0133] S50: Repeat the above steps, but change the injection of gas and liquid in step S20 to analyze the failure behavior of the proppant and the induced conductivity attenuation under the condition of no coal powder invasion. By comparing the test results under the conditions of coal powder invasion and no coal powder invasion, the contribution of coal powder retention and proppant failure behavior to the damage of propped fracture conductivity and their coupling law are revealed.

[0134] S60: Repeat the above steps, change the gas flow rate, liquid flow rate and closure stress as single factors, and explore the control effect of the gas / liquid flow rate combination and closure stress on the coal powder retention and blockage and proppant failure behavior.

[0135] This method uses a fully transparent fracture model and arranges an optical microscope perpendicular to and parallel to the fracture extension direction to achieve two-way visual observation of the fracture. This allows for a three-dimensional and comprehensive capture of the multiphase flow field characteristics within the fracture, as well as the coal dust retention and blockage, and proppant failure (loss, breakage, and embedding) processes.

[0136] This method can achieve separate or simultaneous visualization of coal dust intrusion and proppant failure processes within fractures, revealing the coupling effect of coal dust retention and proppant failure, and comprehensively evaluating their impact on fracture conductivity, providing a more comprehensive experimental analysis method for exploring the damage mechanism of fracture conductivity in coalbed methane wells.

[0137] This method uses a fully transparent fracture model and can accurately measure the fracture width at different locations parallel to the fracture extension direction through direct observation under a microscope. It does not require the configuration of a complex displacement control system, which improves the accuracy and intuitiveness of fracture width measurement. The device structure is simpler, easier to operate, and more cost-effective.

[0138] The exemplary implementation scheme of the system 100 and method for visually evaluating coal powder retention and proppant failure in fractures proposed by the present invention is described in detail above with reference to preferred embodiments. However, it will be understood by those skilled in the art that, without departing from the concept of the present invention, various modifications and variations can be made to the above-mentioned specific embodiments, and various technical features and structures proposed by the present invention can be combined in various ways without exceeding the scope of protection of the present invention, which is determined by the appended claims.

Claims

1. A visual evaluation system for coal dust retention and proppant failure in hydraulic fractures, characterized by: include: A transparent fracturing model (10) comprises: a transparent box (11) and a piston (12); a receiving chamber is formed in the transparent box (11); a piston (12) is installed in the receiving chamber; the piston (12) divides the receiving chamber into a first chamber (114) and a second chamber (115); a coal rock sheet and proppant particles are arranged in the second chamber (115); the coal rock sheet is fixedly connected to the piston (12); the proppant particles are filled in the second chamber (115); wherein the transparent box (11) is provided with a first port (13) connected to the first chamber (114); and a second port (14), a third port (15), a fourth port (17), and a fifth port (18) connected to the second chamber (115); a first injection assembly (20) in communication with the second port (14) and configured to inject liquid and gas into the second chamber (115) to form a gas-liquid mixture or to inject liquid, gas and solid to form a gas-liquid-solid mixture; a second injection assembly (30), connected to the first port (13), configured to inject liquid into the first chamber (114) to push the piston (12) toward the second chamber (115), thereby applying a closing stress to the proppant in the second chamber (115); a three-phase separator (40) in communication with the third port (15) and configured to separate the gas-liquid-solid mixture in the second chamber (115); At least two optical microscopes (50) are respectively provided on at least two end faces of the transparent fracture model (10), and are configured to visually observe fluid flow, particle migration, and proppant failure behavior within the transparent fracture model (10) from the two end faces, wherein the two end faces are respectively an end face perpendicular to the fracture extension direction and an end face parallel to the fracture extension direction; A vacuum pump (60) is connected to the third port (15) and is configured to evacuate the second chamber (115).

2. The visual evaluation system for coal dust retention and proppant failure in hydraulic fractures according to claim 1, characterized in that: The first injection assembly (20) comprises: a first injection pump (21) and a gas cylinder (22) connected in parallel with each other, and a common node (A) is formed by the first injection pump (21) and the gas cylinder (22) at a parallel node, and the common node (A) is connected to the second port (14); wherein, an on-off assembly (23) for controlling the flow of liquid and solid from the first injection pump (21) toward the transparent fracture model (10) is provided between the first injection pump (21) and the common node (A), and a flow controller (24) for controlling the flow of gas toward the transparent fracture model (10) is provided between the gas cylinder (22) and the common node (A).

3. The visual evaluation system for coal dust retention and proppant failure in hydraulic fractures according to claim 2, characterized in that: The on-off assembly (23) comprises: a first on-off valve (231) and a stirring intermediate container (232) connected in parallel between the first injection pump (21) and the common node (A); the first on-off valve (231) is configured to control the on-off between the first injection pump (21) and the common node (A); and the stirring intermediate container (232) is configured to stir the liquid and solid pumped in by the first injection pump (21).

4. The system for visualizing coal powder retention and proppant failure evaluation in fractures according to claim 3, characterized in that: The on-off assembly (23) further comprises: a second on-off valve (233) and a third on-off valve (234) provided at both ends of the stirring intermediate container (232), wherein the second on-off valve (233) is configured to control the on-off of liquid from the first injection pump (21) toward the stirring intermediate container (232), and the third on-off valve (234) is configured to control the on-off of liquid from the stirring intermediate container (232) toward the transparent fracturing model (10).

5. The system for visualizing coal powder retention and proppant failure evaluation in fractures according to claim 3, characterized in that: The on-off assembly (23) further includes a one-way valve (25), which is arranged between the gas flow controller (24) and the common node (A) and is configured to control the gas in the gas cylinder (22) to flow only from the gas cylinder (22) toward the transparent fracturing model (10).

6. The system for visualizing coal dust retention and proppant failure evaluation in fractures according to claim 1, characterized in that: The invention also includes: a differential pressure sensor (70), which is connected in parallel between the fourth port (17) and the fifth port (18) of the transparent fracturing model (10) and is configured to sense the pressure difference information at both ends of the proppant filling layer in the second chamber (115); wherein the fourth port (17) and the fifth port (18) are respectively arranged close to the second port (14) and the third port (15).

7. The system for visualizing coal dust retention and proppant failure evaluation in fractures according to claim 1, characterized in that: The three-phase separator (40) has a first interface (41), a second interface (42) and a third interface (43), wherein the first interface (41) is connected to the third port (15), the second interface (42) is connected to a first back-pressure valve (44), and the third interface (43) is connected to a second back-pressure valve (45).

8. The system for visualizing coal dust retention and proppant failure evaluation in hydraulic fractures according to claim 7, characterized in that: It also includes a flow meter (90) connected to the output port of the first back pressure valve (44) and configured to monitor the flow of gas discharged from the three-phase separator (40) via the second interface (42).

9. The system for visualizing coal powder retention and proppant failure evaluation in fractures according to claim 1, characterized in that: An installation groove (121) is provided on the end surface of the piston (12) located in the second chamber (115), and the coal rock piece is clamped in the installation groove (121).

10. An evaluation method for a visual evaluation system for coal powder retention and proppant failure in a hydraulic fracture according to any one of claims 1 to 9, characterized in that: The steps include: S10: Collect on-site coal samples, proppant, and produced formation water, prepare coal slices, grind the coal rock into fine particles, sieve out coal powder within a certain particle size range, pour the coal powder into the formation water, and prepare a coal powder suspension of a certain concentration; S20: The prepared coal rock slice is mounted on the piston (12), and proppant particles are sequentially loaded into the second chamber (115) according to a certain sand concentration. Then, a second injection assembly (30) injects liquid into the first chamber (114) to push the piston toward the second chamber (115) to apply a certain closing stress to the proppant in the second chamber (115). The transparent fracture model (10) is placed vertically, and the fracture model is vacuumed and saturated with water by a vacuum pump (60); S30: injecting gas and coal powder suspension into the second chamber (115) at a certain gas phase flow rate and liquid phase flow rate by the first injection assembly (20) to form a gas-liquid-solid mixed liquid, monitoring the pressure difference at both ends of the fracture in real time, and calculating the gas phase permeability and liquid phase permeability based on Darcy's law; S40: along the direction parallel to the fracture extension and the direction perpendicular to the fracture extension, simultaneously observing the gas-water-coal powder three-phase flow field and the proppant crushing, embedding and loss behavior in the fracture by at least two optical microscopes (50), integrating the two-way observation information to reveal the coal powder retention and blockage and proppant failure mechanism in a three-dimensional manner, and monitoring the fracture width change along the direction parallel to the fracture extension; S50: Repeat the above steps, but change the injection of gas and liquid in step S20 to analyze the failure behavior of the proppant and the induced conductivity attenuation under the condition of no coal powder invasion. By comparing the test results under the conditions of coal powder invasion and no coal powder invasion, the contribution of coal powder retention and proppant failure behavior to the damage of propped fracture conductivity and their coupling law are revealed. S60: Repeat the above steps, change the gas flow rate, liquid flow rate and closure stress as single factors, and explore the control effect of the gas / liquid flow rate combination and closure stress on the coal powder retention and blockage and proppant failure behavior.

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