Sand-filled pipe slurry pollution experimental device and method

Abstract

The present invention discloses a sand-filled pipe mud pollution experimental device and method, belonging to the field of rock displacement experimental technology. The device includes an invasion unit and a backflow unit, which can simulate the influence of mud on artificial sand-filled fractures. Experiments were carried out using the device, and the invasion and backflow patterns of heavy mud in the sand-filled pipe were analyzed and obtained. The sand-filled pipe, a measuring cylinder, and an intermediate container are connected, and the regularity of low-density mud and heavy mud invasion can be obtained by changing the mud particle size. The intermediate container is connected to a pressure gauge for measuring its pressure; a high-pressure gas tank is used to provide nitrogen. The experimental device of the present invention has a simple structure and is easy to operate, and can quickly realize the measurement of rock parameters on site. The experimental method can relatively easily obtain various parameters: permeability, invasion distance, backflow rate, etc.; and the method can obtain the regularity of low-density mud and heavy mud invasion by changing the mud particle size.

Description

Technical Field

[0001] The invention belongs to the technical field of rock displacement experiments, and in particular relates to a sand-filled pipe mud pollution experimental device and method. Background Art

[0002] Workover operations in some wells of an ultra-deep, ultra-high-pressure, and high-temperature gas reservoir in a certain area after stimulation resulted in severe reservoir contamination, impacting individual well production. To address the issue of mud contamination in certain target areas, an experimental approach was conducted to investigate the intrusion and flowback of heavy mud in artificial sand-filled fractures, analyzing the impact of mud on the permeability of sand-filled fractures.

[0003] Since current research on mud pollution mainly focuses on the drilling process, with less consideration given to the working conditions of the fracturing process, for repeated reconstruction wells with mud pollution, sand-filled fractures are the main channels for mud loss and pollution. The existing technology lacks simulation experimental equipment and methods for the impact of mud on artificial sand-filled fractures, and cannot effectively derive the invasion and backflow rules of heavy mud in sand-filled pipes. Summary of the Invention

[0004] In order to overcome the shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide a sand-fill pipe mud pollution experimental device and method for simulating the influence of mud on artificial sand-filled cracks. Experiments were carried out using this device, and the intrusion and backflow patterns of heavy mud in sand-filled pipes were analyzed.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A sand-filled pipe mud pollution experimental device comprises: an invasion unit and a backflow unit; the invasion unit and the backflow unit are used alternately, and the invasion unit comprises a sand-filled pipe, the inlet end of the sand-filled pipe is connected to an intermediate container, the intermediate container is sequentially connected to a pressure gauge and a high-pressure gas storage tank, and the outlet end of the sand-filled pipe is connected to a measuring cylinder.

[0007] Preferably, the reverse discharge unit includes a six-way valve, which replaces the intermediate container, and the inlet and outlet ends of the sand filling pipe are swapped. The original outlet end is sequentially connected to the six-way valve, pressure gauge and high-pressure gas storage tank, and the original inlet end is connected to the measuring cylinder.

[0008] Preferably, nitrogen is stored in the high-pressure gas storage tank.

[0009] The present invention also discloses a sand-filled pipe slurry pollution experimental method, comprising the following steps:

[0010] Place the slurry in an intermediate container and connect the inlet end of the sand filling pipe to the intermediate container;

[0011] Adjust the high-pressure gas storage tank so that the pressure gauge reading reaches the set value and record the start time;

[0012] When there is no more liquid flowing out of the outlet of the sand filling tube, record the amount of liquid flowing out of the graduated cylinder and the end time;

[0013] Turn off the gas source, release the pressure at the inlet of the sand filling pipe and calculate the intrusion permeability;

[0014] Replace the inlet and outlet ends of the sand filling pipe, connect the reverse flow unit, repeat the above steps to simulate the reverse flow and end the experiment.

[0015] Preferably, the mud invasion distance is calculated based on the liquid output in the graduated cylinder and the void volume in the sand filling tube; the calculation method is to divide the liquid output by the void volume in the sand filling tube and then multiply by the length of the sand filling tube.

[0016] Preferably, the void volume in the sand-filled tube is calculated as follows:

[0017] Void volume = liquid output × (end time - start time).

[0018] Preferably, the intrusion permeability is calculated based on the start time, the end time, the liquid output and the pressure gauge reading. The calculation method of the intrusion permeability is as follows:

[0019]

[0020] Where: K1 is the invasion permeability; Δp is the pressure difference before and after the fluid passes through the sand body, Q1 is the fluid output through the sand body under the action of the invasion pressure difference, μ is the viscosity of the fluid passing through the sand body, L is the length of the sand body, and A is the cross-sectional area of ​​the sand body perpendicular to the flow direction.

[0021] Preferably, when the reverse flow unit is connected to simulate reverse flow, the six-way valve replaces the intermediate container, and the inlet and outlet ends of the sand filling pipe are swapped.

[0022] Preferably, turning on the reverse flow unit to simulate reverse flow specifically includes the following steps:

[0023] The inlet and outlet ends of the sand filling pipe are swapped, and the original outlet end is connected to the six-way valve, pressure gauge and high-pressure gas storage tank in sequence, and the original inlet end is connected to the measuring cylinder;

[0024] Adjust the high-pressure gas storage tank so that the pressure gauge reading reaches the set value and record the start time;

[0025] When no more liquid flows out of one end of the measuring cylinder, record the amount of liquid discharged and the end time; close the high-pressure gas storage tank and calculate the backflow rate.

[0026] Preferably, the backflow rate is calculated based on the liquid output before and after the inlet and outlet ends of the sand filling pipe are replaced. The calculation method is as follows:

[0027]

[0028] Where: K2 is the backflow rate; Q2 is the amount of liquid flowing through the sand body under the action of the backflow pressure difference; Q1 is the amount of liquid flowing through the sand body under the action of the intrusion pressure difference.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] The present invention discloses a sand filling pipe mud pollution experimental device, which comprises an invasion unit and a backflow unit, and can simulate the influence of mud on artificial sand filling cracks. Experiments are carried out by using the device, and the invasion and backflow rules of heavy mud in the sand filling pipe are analyzed and obtained. The sand filling pipe, a measuring cylinder and an intermediate container are connected, and the regularity of the invasion of low-density mud and heavy mud can be obtained by changing the mud particle size. The intermediate container is connected to a pressure gauge for measuring its pressure; a high-pressure gas storage tank is used to provide nitrogen. The experimental device of the present invention has a simple structure and is easy to operate, and can quickly realize the measurement of rock parameters on site.

[0031] The present invention discloses a sand-filled pipe mud pollution experimental method, which can relatively simply obtain various parameters: permeability, invasion distance, return flow rate, etc.; and the method can obtain regular conclusions on the invasion of low-density mud and heavy mud by changing the mud particle size. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 Schematic diagram of the intrusion unit structure of the experimental device of the present invention.

[0033] Figure 2 Schematic diagram of the reverse discharge unit structure of the experimental device of the present invention.

[0034] Figure 3 Schematic diagram of the relationship between the displacement pressure and the invasion depth of mud with different particle sizes in an embodiment of the present invention.

[0035] Figure 4 Schematic diagram of the flowback rate of mud with different particle sizes in an embodiment of the present invention.

[0036] Figure 5 Schematic diagram of dimensionless permeability relationship under different conditions in an embodiment of the present invention.

[0037] In the figure: 1- graduated cylinder, 2- sand filling tube, 3- pressure gauge, 41- intermediate container, 42- six-way valve, 5- high-pressure gas storage tank. DETAILED DESCRIPTION

[0038] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0039] It should be noted that the terms "first", "second", etc. in the description and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that the numbers used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0040] The present invention is described in further detail below with reference to the accompanying drawings:

[0041] Therefore, this study designed a sand-fill pipe mud pollution experimental device and experimental method to simulate the impact of mud on artificial sand-fill fractures. Experiments were carried out using this device, and the invasion and backflow patterns of heavy mud in sand-fill pipes were analyzed.

[0042] See also Figure 1 、 Figure 2 The present application discloses a sand-fill pipe mud pollution experimental device, characterized in that it includes: an invasion unit and a backflow unit; the invasion unit and the backflow unit are used alternately, and the invasion unit includes a sand-fill pipe 2, the inlet end of the sand-fill pipe 2 is connected to an intermediate container 41, the intermediate container 41 is sequentially connected to a pressure gauge 3 and a high-pressure gas storage tank 5, and the outlet end of the sand-fill pipe 2 is connected to a measuring cylinder 1. The use of the invasion unit and the backflow unit can simulate the influence of mud on artificial sand-fill fractures. Experiments were carried out using this device, and the invasion and backflow patterns of heavy mud in the sand-fill pipe were analyzed and obtained. The sand-fill pipe 2, the measuring cylinder 1 and the intermediate container 41 are connected, and the regularity of the invasion of low-density mud and heavy mud can be obtained by changing the mud particle size. The intermediate container 41 is connected to a pressure gauge 3 for measuring its pressure; the high-pressure gas storage tank 5 is used to provide nitrogen. The experimental device of the present invention has a simple structure and is easy to operate, and can quickly realize the measurement of rock parameters on site.

[0043] In some embodiments, the backflow unit includes a six-way valve 42, which replaces the intermediate container 41, and the inlet and outlet ends of the sand filling pipe 2 are swapped. The original outlet end is connected to the six-way valve 42, the pressure gauge 3 and the high-pressure gas storage tank 5 in sequence, and the original inlet end is connected to the measuring cylinder 1; after the six-way valve 42 replaces the intermediate container 41, it is connected to the sand filling pipe 2 and the measuring cylinder 1 to simulate the mud backflow situation. The regularity of the backflow of low-density mud and heavy mud can be obtained by changing the mud particle size.

[0044] Preferably, nitrogen is stored in the high-pressure gas storage tank 5 .

[0045] An experimental method for a sand-filled pipe slurry pollution experimental device comprises the following steps:

[0046] S1: Place the slurry in the intermediate container 41 and connect the inlet end of the sand filling pipe 2 to the intermediate container 41;

[0047] S2: Adjust the high-pressure gas storage tank 5 so that the pressure gauge 3 reading reaches the set value and record the start time;

[0048] S3: No more liquid flows out of the outlet of the sand-filled tube 2, and the amount of liquid discharged from the graduated cylinder 1 is recorded, and the end time is recorded;

[0049] S4: Turn off the gas source, release the pressure at the inlet of the sand filling pipe 2 and calculate the intrusion permeability;

[0050] S5: Replace the inlet and outlet ends of the sand filling pipe 2, connect the reverse flow unit, repeat the above steps to simulate the reverse flow, and then end the experiment.

[0051] This method can easily obtain various parameters: invasion permeability, invasion distance, return rate, etc.; and this method can obtain regular conclusions on the invasion of low-density mud and heavy mud by changing the mud particle size.

[0052] In some embodiments, the mud invasion distance is calculated based on the liquid output in the measuring cylinder 1 and the void volume in the sand filling tube; the calculation method is to divide the liquid output by the void volume in the sand filling tube and then multiply it by the length of the sand filling tube.

[0053] In some embodiments, the void volume in the sand-filled tube is calculated as follows:

[0054] Void volume = liquid output × end time - start time.

[0055] In some embodiments, the intrusion permeability is calculated based on the start time, the end time, the liquid output, and the pressure gauge reading. The calculation method of the intrusion permeability is as follows:

[0056]

[0057] Where: K1 is the invasion permeability; Δp is the pressure difference before and after the fluid passes through the sand body, Q1 is the fluid output through the sand body under the action of the invasion pressure difference, μ is the viscosity of the fluid passing through the sand body, L is the length of the sand body, and A is the cross-sectional area of ​​the sand body perpendicular to the flow direction.

[0058] In some embodiments, when the reverse flow unit is turned on to simulate reverse flow, the six-way valve 42 replaces the intermediate container 41, and the inlet and outlet ends of the sand filling pipe 2 are swapped.

[0059] In some embodiments, turning on the reverse flow unit to simulate reverse flow specifically includes the following steps:

[0060] S6: The inlet and outlet ends of the sand filling pipe 2 are swapped. The original outlet end is connected to the six-way valve 42, the pressure gauge 3 and the high-pressure gas storage tank 5 in sequence, and the original inlet end is connected to the graduated cylinder 1;

[0061] S7: Adjust the high-pressure gas storage tank 5 so that the pressure gauge 3 reading reaches the set value and record the start time;

[0062] S8: When no more liquid flows out from one end of the measuring cylinder 1, record the liquid output and the end time; close the high-pressure gas storage tank 5 and calculate the backflow rate.

[0063] In some embodiments, the backflow rate is calculated based on the liquid output before and after the inlet and outlet ends of the sand filling tube 2 are replaced. The calculation method is as follows:

[0064]

[0065] Where: K2 is the backflow rate; Q2 is the amount of liquid flowing through the sand body under the action of the backflow pressure difference; Q1 is the amount of liquid flowing through the sand body under the action of the intrusion pressure difference.

[0066] [Example]

[0067] like Figure 1 and Figure 2 As shown, a sand-filled pipe mud pollution experimental device includes a sand-filled pipe 2, the outlet end of the sand-filled pipe 2 is connected to a measuring cylinder 1, and the inlet end is connected to an intermediate container 41 or a six-way valve 42; the intermediate container 41 or the six-way valve 42 is connected to a pressure gauge 3 for measuring its pressure; and also includes a high-pressure gas storage tank 5 for providing nitrogen.

[0068] When simulating mud invasion, the inlet end of the sand filling pipe 2 is connected to the intermediate container 41, the intermediate container 41 is connected in sequence to the pressure gauge 3 and the high-pressure gas storage tank 5, and the outlet end of the sand filling pipe 2 is connected to the measuring cylinder 1;

[0069] When simulating mud backflow, the inlet and outlet ends of the sand filling pipe 2 are swapped, the six-way valve 42 replaces the intermediate container 41, the original outlet end is connected to the six-way valve 42, the pressure gauge 3 and the high-pressure gas storage tank 5 in sequence, and the original inlet end is connected to the measuring cylinder 1.

[0070] The experimental method using the above-mentioned sand-filled pipe mud pollution experimental device includes the following steps:

[0071] Step 1: Connect the inlet end of the sand filling pipe 2 to the intermediate container 41 and place the slurry in the intermediate container 41;

[0072] Step 2: Adjust the high-pressure gas storage tank 5 so that the pressure gauge 3 reading reaches the set value and start recording the time;

[0073] Step 3: When no more liquid flows out of the outlet, record the liquid volume at the outlet and the time at which this occurs;

[0074] Step 4: Turn off the gas source and release the pressure at the inlet end;

[0075] Step 5: Clean the graduated cylinder 1, swap the inlet and outlet ends of the sand filling tube 2, and connect the inlet end to the six-way valve 42; the inlet end in step 1 is used as the outlet end to connect to the graduated cylinder 1;

[0076] Step 6: Adjust the high-pressure gas storage tank 5 so that the pressure gauge 3 reading reaches the set value;

[0077] Step 7: When no more liquid flows out of the outlet, record the amount of liquid flowing out; close the high-pressure gas storage tank 5, and the experiment is complete. After the experiment is completed, release the pressure at the inlet, disassemble the pipeline, and remove and clean the sand filling tube 2 and measuring cylinder 1.

[0078] The void volume in the sand filling tube 2 is calculated based on the pressure gauge readings, the liquid volume at the outlet and the corresponding time obtained in steps 1 to 4; the calculation method is flow rate multiplied by time, that is, the volume corresponding to the liquid volume at the outlet.

[0079] Calculate the permeability K based on the pressure gauge readings, outlet liquid volume, and corresponding time obtained in steps 1 to 4; the calculation method is as follows:

[0080]

[0081] Where: Δp is the pressure difference before and after the fluid passes through the sand body (this pressure difference is the pressure value measured by pressure gauge 3), Q is the flow rate through the sand body under the action of the pressure difference, μ is the viscosity of the fluid passing through the sand body, L is the length of the sand body, and A is the cross-sectional area of ​​the sand body perpendicular to the flow direction.

[0082] The permeability test is conducted according to Darcy's law. When water passes through the sand-filled pipe 2, the flow rate Q is proportional to the cross-sectional area of ​​the sand body in the sand-filled pipe 2 and the inlet and outlet pressure difference, and inversely proportional to the length of the sand body.

[0083] The mud invasion distance is calculated based on the liquid volume at the outlet and the void volume in the sand filling tube 2 obtained in step 3; the calculation method is to divide the liquid volume at the outlet by the void volume in the sand filling tube 2 and then multiply it by the length of the sand filling tube 2.

[0084] The flowback rate is calculated based on the liquid volume at the outlet obtained in step 3 and the liquid volume at the outlet obtained in step 7. The calculation method is that the flowback rate is the ratio of the liquid volume at the outlet obtained in step 7 to the liquid volume at the outlet obtained in step 3.

[0085] The experimental method of the present invention is used to carry out invasion experiments of ceramsite with different particle sizes in low-density mud and ceramsite with different particle sizes in high-density mud under different displacement pressures; the particle sizes of the ceramsite are selected to be 20 / 40, 30 / 50, and 40 / 70 meshes respectively.

[0086] The penetration depth is calculated according to the above method. The results are as follows: Figure 3 As shown in the figure, the invasion depth of low-density mud increases significantly with increasing displacement pressure at 20 / 40 and 30 / 50 particle sizes. Furthermore, the greatest increase in invasion depth occurs at displacement pressures between 3 MPa and 5 MPa. The invasion depth of high-density mud remains largely unchanged with displacement pressure. As proppant particle size increases, the pore space within the sandpack expands, providing more room for fluid flow, making it easier for mud to flow within the sandpack and increasing the invasion depth.

[0087] The mud flowback rate is calculated according to the above method, and the results are as follows: Figure 4 As shown in the figure, the high-density mud + 20 / 40 mesh ceramsite combination exhibited continuous mud outflow from the sandfill, making it impossible to calculate the penetration depth. The low-density mud + 30 / 50 mesh ceramsite and low-density mud + 20 / 40 mesh ceramsite combinations passed through the sandfill at displacement pressures of 5 MPa and 3 MPa, respectively. However, liquid outflow occurred only within a few seconds after the pressure was increased.

[0088] As can be seen from the figure, when the proppant particle size is larger, the mud flow resistance in the sand pack is reduced, the heavy mud intrusion is greater, and it is difficult to expel the mud in the sand pack during nitrogen displacement. According to statistics, the mud return rate in sand packs filled with 20 / 40 mesh and 30 / 50 mesh ceramsite is 15% to 30%. Large amounts of mud are retained in the fractures, causing serious damage to the permeability of the sand pack.

[0089] According to the above method, the dimensionless permeability under different conditions is calculated, and the results are as follows: Figure 5 As shown in the figure, at a particle size of 30 / 50, the residual permeability after mud contamination is 2.2% to 3.0% of the original permeability, and the mud damages the permeability of the sandfill pipe by 97%. Compared with high-density mud, low-density mud has less damage, but the difference is only 0.3% to 0.7%.

[0090] Table 1 shows the change in permeability of the sand-filled pipe before and after mud contamination, and Table 2 shows the mud invasion volume and flowback rate. The above-mentioned change pattern can also be seen from the following tables.

[0091] Table 1. Changes in permeability of sand-filled pipes before and after mud pollution

[0092]

[0093] Table 2. Mud invasion volume and flowback rate

[0094]

[0095] The experimental device of the present invention has a simple structure and is easy to operate. It can quickly measure rock parameters on site. The experimental method can relatively easily obtain various parameters: permeability, invasion distance, return flow rate, etc. From the above embodiments, it can be seen that at 20 / 40 and 30 / 50 particle sizes, the invasion depth of low-density mud increases significantly with the increase of displacement pressure. At the same time, the invasion depth increases the most at displacement pressures of 3MPa to 5MPa. The invasion depth of high-density mud basically does not change with the change of displacement pressure. As the proppant particle size increases, the pore space in the sand filling tube increases, the space for fluid flow increases, the mud is easier to flow in the sand filling tube, and the mud invasion depth increases. When the proppant particle size is larger, the flow resistance of the mud in the sand filling tube is smaller, the invasion amount of heavy mud is larger, and it is difficult to remove the mud invading the sand filling tube during nitrogen reverse displacement. The mud flowback rate in sandfill pipes filled with 20 / 40 and 30 / 50 mesh ceramsite is 15%-30%. Large amounts of mud are retained in the cracks, severely damaging the permeability of the sandfill pipe. With the 30 / 50 mesh size, the residual permeability after mud contamination is 2.2%-3.0% of the original permeability, resulting in a 97% permeability loss. Compared to high-density mud, low-density mud has less damage, but the difference is only 0.3%-0.7%.

[0096] In summary, the sand filling pipe mud pollution experimental device disclosed in the present application includes an invasion unit and a backflow unit, which can simulate the influence of mud on artificial sand filling cracks. Experiments were carried out through the device, and the invasion and backflow rules of heavy mud in the sand filling pipe were analyzed. The sand filling pipe, measuring cylinder and intermediate container are connected, and the regularity of low-density mud and heavy mud invasion can be obtained by changing the mud particle size. The intermediate container is connected to a pressure gauge for measuring its pressure; the high-pressure gas tank is used to provide nitrogen. The experimental device of the present invention has a simple structure and is easy to operate, and can quickly realize the measurement of rock parameters on site.

[0097] The present invention discloses a sand-filled pipe mud pollution experimental method, which can relatively simply obtain various parameters: permeability, invasion distance, return flow rate, etc.; and the method can obtain regular conclusions on the invasion of low-density mud and heavy mud by changing the mud particle size.

[0098] The above content is only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. An experimental method based on a sand-filled pipe mud pollution experimental device, characterized in that: The experimental device comprises an intrusion unit and a backflow unit; the intrusion unit and the backflow unit are used alternately, and the intrusion unit comprises a sand filling pipe (2), the inlet end of the sand filling pipe (2) is connected to an intermediate container (41), the intermediate container (41) is connected in sequence to a pressure gauge (3) and a high-pressure gas storage tank (5), and the outlet end of the sand filling pipe (2) is connected to a measuring cylinder (1); the backflow unit comprises a six-way valve (42); the method comprises the following steps: The slurry is placed in an intermediate container (41) and the inlet end of the sand filling pipe (2) is connected to the intermediate container (41); Adjust the high-pressure gas storage tank (5) so that the pressure gauge (3) reading reaches the set value and record the start time; When no more liquid flows out of the outlet of the sand filling pipe (2), the liquid output in the measuring cylinder (1) and the end time are recorded; the mud invasion distance is calculated based on the liquid output in the measuring cylinder (1) and the void volume in the sand filling pipe; the calculation method is to divide the liquid output by the void volume in the sand filling pipe and then multiply it by the length of the sand filling pipe; the void volume in the sand filling pipe is calculated as follows: Void volume = liquid output (End time - Start time) The gas source is turned off, the pressure at the inlet end of the sand filling pipe (2) is released, and the intrusion permeability is calculated; the intrusion permeability is calculated based on the start time, end time, liquid output and pressure gauge reading. The calculation method of the intrusion permeability is as follows: Where: is the intrusion permeability; is the pressure difference before and after the fluid passes through the sand body, is the amount of liquid discharged through the sand body under the action of the intrusion pressure difference, is the viscosity of the fluid passing through the sand body, L is the length of the sand body, is the cross-sectional area of ​​the sand body perpendicular to the flow direction; Replace the inlet and outlet of the sand filling pipe (2) and connect the reverse discharge unit. Connecting the reverse discharge unit to simulate reverse discharge specifically includes the following steps: The inlet and outlet ends of the sand filling pipe (2) are swapped, with the original outlet end connected to the six-way valve (42), the pressure gauge (3) and the high-pressure gas storage tank (5) in sequence, and the original inlet end connected to the measuring cylinder (1); Adjust the high-pressure gas storage tank (5) so that the pressure gauge (3) reading reaches the set value and record the start time; When no more liquid flows out of one end of the measuring cylinder (1), record the amount of liquid discharged and the end time; close the high-pressure gas storage tank (5) and calculate the backflow rate; The backflow rate is calculated based on the liquid output before and after the inlet and outlet of the sand filling pipe (2) are replaced. The calculation method is as follows: Where: is the back discharge rate; is the amount of liquid discharged through the sand body under the action of the backflow pressure difference, It is the amount of liquid discharged through the sand body under the action of the intrusion pressure difference.

2. The experimental method based on the sand-filled pipe mud pollution experimental device according to claim 1 is characterized in that: Nitrogen is stored in the high-pressure gas storage tank (5).

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