Experimental device and method for simulating breathing effect of shaft flow and formation fracture coupling

Abstract

The invention relates to an experimental device and method for simulating the breathing effect of wellbore flow and formation fracture coupling, and belongs to the technical field of oil and gas development. The experimental device comprises a rock core, a rock core holder, an air pressure output unit and a hydraulic pressure output unit, and the middle of the rock core is perforated in the axial direction to be connected into an axial liquid conveying pipeline of the rock core holder to simulate a wellbore; the middle part of the core is radially divided into two sections for simulating formation fractures; the air pressure output unit is used for providing core axial pressure; and the hydraulic output unit is used for providing core confining pressure and drilling fluid pumping pressure. A wellbore and a crack are simulated at the same time, a rock core is used for simulating a stratum, axial drilling is used for representing the wellbore, cutting perpendicular to the axial direction is used for representing the formation crack, a rock core axial through hole is used for simulating wellbore pressure through pressurization of a hydraulic pump, confining pressure of a rock core holder is used for simulating crack expansion pressure, and bottom air pressure is used for simulating crack closing pressure; the shaft stratum breathing effect is simulated by adjusting pressure changes, flow parameters and change characteristics of the shaft stratum breathing effect are analyzed, the structure is simple, and cost is saved.

Description

Technical Field

[0001] The present invention relates to an experimental device and method for simulating the breathing effect of the coupling of wellbore flow and formation fractures, and belongs to the technical field of oil and gas development. Background Art

[0002] With the gradual advancement of oil and gas development into deep water areas, oil and gas drilling has become increasingly challenging. A significant characteristic of deep water formations is their narrow safety density window. In particular, the complexity of special formation conditions and on-site situations will affect drilling operations and the safe and efficient drilling process. How to drill smoothly under a narrow safety density window is one of the key issues currently being concerned. When drilling in a narrow safety window, the breathing effect is likely to occur, that is, the equivalent circulating density (ECD) of the wellbore drilling fluid is easily exceeded the formation fracture pressure. At this time, a small amount of drilling fluid will be lost, and when the pump is stopped, as the ECD decreases, some of the fluid lost into the formation will flow back into the wellbore.

[0003] The occurrence of the breathing effect is fundamentally due to the opening and closing of natural fractures and induced fractures at the bottom of the well. That is, during the normal drilling cycle, due to factors such as fluctuating pressure and circulating pressure loss, the bottom hole pressure reaches or exceeds the formation fracture opening pressure, resulting in some drilling fluid leaking into the surrounding formation. Leakage is observed at the wellhead. After the circulation stops, due to the disappearance of the circulating pressure loss, the surrounding formation pressure is higher than the bottom hole pressure, causing the previously lost drilling fluid to return to the wellbore, resulting in the phenomenon of drilling fluid overflow at the wellhead.

[0004] The inducing causes of the breathing effect include bottom hole pressure fluctuations, encountering fractured formations, and a narrow safety density window in deep water. The breathing effect itself does not cause serious consequences to drilling, but during the formation breathing effect, there is breathing loss and backflow of drilling fluid, which to a certain extent has similar characteristics to well loss and overflow. When the breathing effect occurs during drilling, especially in formations with a narrow safety window and developed fractures, it is difficult to accurately identify the actual downhole conditions due to the complexity of the downhole situation. If the "sucking" process of the breathing effect is misidentified as leakage, the drilling fluid density will be wrongly reduced, thus inducing downhole overflow; and if the "blowing" process of the breathing effect is misidentified as an overflow, a kill operation will be wrongly carried out, resulting in the formation fractures being further opened and the formation being fractured by pressure, causing a large amount of drilling fluid to enter the formation interior. Incorrect identification and countermeasures will delay the drilling efficiency and even cause serious consequences. Therefore, it is necessary to further study and analyze the characteristics of the breathing effect in order to better study the differences between the breathing effect and well loss and overflow situations.

[0005] In the process of studying and analyzing the breathing effect, it is necessary to consider the wellbore flow characteristics and formation fractures in a coupled manner, which requires the use of a coupled theoretical model of wellbore flow and formation fracture deformation. At present, there have been many theoretical studies on the coupled analysis of wellbore and formation, but in order to verify the feasibility and accuracy of the theory in the case of the breathing effect, sufficient experiments are needed for effective verification. There is a lack of a simple and effective experimental device for simulating the breathing effect of wellbore flow and formation fracture coupling in the existing technology. Summary of the Invention

[0006] Aiming at the deficiencies of the existing technology, the present invention provides an experimental device for simulating the breathing effect of wellbore flow and formation fracture coupling. It simultaneously simulates the wellbore and fractures, uses a core to simulate the formation, an axial borehole to represent the wellbore, and a cut perpendicular to the axis to represent the formation fracture. The axial through-hole of the core is pressurized by a hydraulic pump to simulate the wellbore pressure, the confining pressure of the core holder simulates the fracture expansion pressure, and the bottom air pressure simulates the fracture closing pressure. This device simulates the breathing effect of the wellbore formation by adjusting the pressure change and analyzes its flow parameters and variation characteristics, and has a simple structure and low cost.

[0007] The present invention adopts the following technical solutions:

[0008] An experimental device for simulating the breathing effect of wellbore flow and formation fracture coupling, comprising a core, a core holder, a pneumatic output unit, and a hydraulic output unit. The middle part of the core is axially perforated and connected to the axial infusion pipeline of the core holder to simulate the wellbore, and the middle part of the core is radially divided into two sections to simulate the formation fracture;

[0009] The pneumatic output unit is used to provide the axial pressure of the core; the hydraulic output unit is used to provide the confining pressure of the core and the pump pressure of the drilling fluid.

[0010] Preferably, in order to explore the influence of the fracture surface roughness, different cutting methods are used to create fractures. The fracture creation method for simulating the formation fracture is diamond blade cutting or abrasive water jet cutting. Among them, the fracture surface cut by the diamond blade has better smoothness, while the fracture surface cut by the abrasive water jet is relatively rough.

[0011] Preferably, circular steel plates are placed between the upper and lower end faces of the core and the core holder as a sealing layer to ensure the sealing of the pressure boundary.

[0012] Preferably, the air pressure output unit includes a nitrogen gas cylinder. A pneumatic drive unit is provided at the bottom end of the core holder in contact with the core. The axial infusion pipeline of the core holder passes through the center of the pneumatic drive unit. The top of the pneumatic drive unit is in close contact with the core. An air cavity is formed between the bottom of the pneumatic drive unit and the core holder. The air cavity is connected to the nitrogen gas cylinder through pipeline A. A valve A, a back pressure valve and a pressure gauge are sequentially arranged on pipeline A. Different axial pressures can be set through the air pressure of the nitrogen gas cylinder to simulate different closure pressures of fractures.

[0013] Preferably, the hydraulic output unit includes a formation hydraulic pump and a drilling hydraulic pump;

[0014] An annulus pressure cavity is formed between the side wall of the core and the core holder. The formation hydraulic pump is connected to the annulus pressure cavity through pipeline B. A valve B and a hydraulic gauge A are arranged on pipeline B; Pipeline C is also connected to the annulus pressure cavity. A formation back pressure valve is arranged on pipeline C; Different formation annulus pressures can be set through the formation hydraulic pump to simulate different propagation pressures of fractures;

[0015] The top of the axial infusion pipeline of the core holder is respectively connected with pipeline D and pipeline E. Pipeline D is connected to the drilling hydraulic pump. A valve C and a hydraulic gauge B are sequentially arranged on pipeline D; A drilling back pressure valve is arranged on pipeline E. Different drilling fluid pumping pressures are set by the drilling hydraulic pump to simulate different drilling fluid circulation pressures during the drilling process.

[0016] Preferably, the formation hydraulic pump and the drilling hydraulic pump are respectively connected to a container through pipelines. Electronic balance A and electronic balance B are respectively placed at the bottom of the container. The ends of pipeline C and pipeline E are also connected to a container. Electronic balance C and electronic balance D are placed at the bottom of the container.

[0017] Preferably, the pressure gauge, hydraulic gauge A, hydraulic gauge B, electronic balance A, electronic balance B, electronic balance C and electronic balance D are all connected to the data acquisition system;

[0018] Relief valves A, B and C are respectively arranged on pipeline A, pipeline B and pipeline C.

[0019] A working method of the experimental device for simulating the breathing effect of the coupling of wellbore flow and formation fractures as described above. During the experiment, the drilling fluid can be injected into the wellbore by driving the intermediate container with the drilling hydraulic pump. The formation pressure environment can be simulated by controlling the formation hydraulic pump. When the axial pressure is greater than or equal to the drilling fluid back pressure, that is, when the pressure gauge is greater than the value of hydraulic gauge B, after the drilling fluid circulates normally, it is discharged from the wellbore, that is, corresponding to the situation where no breathing effect occurs;

[0020] When the axial pressure is less than the drilling fluid backpressure, the drilling fluid enters the wellbore, i.e., the axial perforation of the core, and gradually enters the formation along the radial cracks of the core to simulate the leakage caused by the breathing effect. At the same time, as the compression pressure of the axial pressure gas increases, when the formation pressure (i.e., the confining pressure) and the drilling fluid backpressure are balanced, the drilling fluid circulates normally, which corresponds to normal circulation drilling. If the drilling fluid circulation is stopped at this time and the drilling pump pressure is reduced, the axial pressure will compress the cracks again, and the drilling fluid that has entered the core cracks will flow back, which can simulate the situation of the corresponding backflow when the breathing effect stops pumping. The experiment records the relevant parameters of the coupled flow of the wellbore flow and the formation cracks during the breathing effect process through monitoring equipment such as electronic balances, pressure gauges, and pressure sensors during the simulation of the breathing effect process.

[0021] Preferably, the specific implementation process is as follows:

[0022] (1) Process the core: Drill the core axially to simulate the wellbore; create artificial fractures along the radial direction of the core, and two different cutting methods of diamond blade cutting and abrasive water jet cutting are used to characterize different roughnesses of the fractures;

[0023] (2) Prepare a set of cores. Considering that the rocks deep in the formation are in a saturated state, soak multiple sets of cores completely under the experimental pressure conditions to ensure that each set of cores is in a saturated state during the experiment, and avoid the situation of non-return caused by the experimental fluid entering the unsaturated voids of the matrix, which may affect the experimental results of the breathing effect. After the rock samples are saturated, return the pressure in the experimental barrel to zero to the initial state, and then the experiment can be started;

[0024] (3) Turn on the main power supply and the switches of each electronic balance, and check whether the pipeline and the data acquisition system are normal; open the valve A on the nitrogen cylinder to supply nitrogen and apply axial pressure to the core. When the pressure reaches the experimental axial pressure, close the valve A;

[0025] (4) Open the valve C and the valve B, adjust the pump speed of the formation hydraulic pump to about 50% of the experimental required pump speed, and pump in the experimental fluid until the connection state is reached, that is, fluid flows out from both the outlet of the formation backpressure valve and the outlet of the drilling backpressure valve;

[0026] (5) Close the formation backpressure valve to apply the formation backpressure until the hydraulic gauge A reaches stability, and collect the value of the hydraulic gauge A, which is the formation confining pressure. The circuit is in circulation, that is, only fluid flows out from the outlet of the drilling backpressure valve;

[0027] (6) Close the formation hydraulic pump, apply the drilling backpressure. When the hydraulic gauge B reaches the target pressure (the target pressure is the set pressure corresponding to the drilling cycle), close the drilling backpressure valve. At this time, no fluid flows out from the outlet of the drilling backpressure valve, and collect the value of the hydraulic gauge B as the initial wellbore pressure;

[0028] (7) Increase the pump speed of the drilling hydraulic pump to the target pump speed and wait until it stabilizes, i.e., the flow rate at the outlet of the drilling backpressure valve stabilizes (stabilization means that the flow rate does not change or changes very little within a short period, which can be judged according to the actual situation). Collect the data of pressure and flow rate changes through the data acquisition system;

[0029] (8) Control the sizes of the drilling hydraulic pump and the drilling backpressure valve so that the axial pressure is the initial wellbore pressure in step (6), and gradually reduce the pump speed of the well hydraulic pump, and collect the data of pressure and flow rate changes;

[0030] (9) When the simulation of the breathing effect process ends, stop the pump, close valve A of the nitrogen cylinder, and slowly open all pressure relief valves,

[0031] until all the experimental fluid is emptied;

[0032] (10) Empty the nitrogen, turn off the switches of each electronic balance, turn off the switch of the data acquisition system, and turn off the main power supply, and the experiment ends.

[0033] Preferably, in step (2), the experimental pressure conditions are an axial pressure of 10 MPa, a confining pressure of 10 MPa, and it is preferably immersed in the experimental liquid for more than 3 h.

[0034] For those parts not elaborated in the present invention, reference can be made to the prior art.

[0035] The beneficial effects of the present invention are as follows:

[0036] The present invention simultaneously simulates the wellbore and fractures, uses a core to simulate the formation, an axial borehole represents the wellbore, and cutting perpendicular to the axis represents the formation fractures. The axial through-hole of the core is pressurized by a hydraulic pump to simulate the wellbore pressure, the confining pressure of the core holder simulates the fracture expansion pressure, and the bottom air pressure simulates the fracture closing pressure. This device simulates the breathing effect of the wellbore formation by adjusting the pressure change, analyzes its flow parameters and change characteristics, and has a simple structure and low cost.

[0037] The present invention can simulate the breathing effect of the wellbore formation, comprehensively consider the flow characteristics of the wellbore and the situation of formation fractures; and can obtain the flow parameters and change characteristics of the breathing effect, which are used to study the mechanism of the breathing effect and provide support for better distinguishing and identifying the situations of lost circulation and overflow. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The schematic diagrams in the specification forming a part of this application are used to provide a further understanding of this application. The illustrative embodiments of this application and their descriptions are used to explain this application and do not constitute an improper limitation to this application.

[0039] Figure 1 It is a schematic diagram of the experimental device for simulating the breathing effect of the coupling of wellbore flow and formation fractures of the present invention;

[0040] Figure 2Schematic diagram of the model's breathing leakage process;

[0041] Figure 3 Schematic diagram of the simulated breathing return process;

[0042] In the figure, 1 - core holder, 2 - core, 3 - and electronic balance B, 4 - electronic balance D, 5 - electronic balance A, 6 - electronic balance, 7 - valve A, 8 - valve C, 9 - valve B, 10 - nitrogen cylinder, 11 - back pressure valve, 12 - pressure gauge, 13 - drilling hydraulic pump, 14 - drilling back pressure valve, 15 - formation hydraulic pump, 16 - formation back pressure valve, 17 - hydraulic gauge B, 18 - hydraulic gauge A, 19 - pressure relief valve C, 20 - pressure relief valve B, 21 - pressure relief valve A, 22 - pneumatic drive unit, 23 - data acquisition system. Specific implementation mode

[0043] In order to enable those skilled in the art to better understand the technical solutions in this specification, the following combines the accompanying drawings in the implementation of this specification to clearly and completely describe the technical solutions in the embodiments of the present invention, but not limited to this. For those not elaborated in the present invention, they are all in accordance with the conventional technologies in the field.

[0044] Example 1

[0045] An experimental device for simulating the breathing effect of the coupling of wellbore flow and formation fractures, as Figure 1 shown, includes a core 2, a core holder 1, a pneumatic output unit and a hydraulic output unit. The middle part of the core 2 is axially perforated and connected to the axial liquid delivery pipeline of the core holder 1 to simulate the wellbore, and the middle part of the core 2 is radially divided into two sections to simulate the formation fracture;

[0046] The pneumatic output unit is used to provide the axial pressure of the core; the hydraulic output unit is used to provide the confining pressure of the core and the pump pressure of the drilling fluid.

[0047] Example 2

[0048] An experimental device for simulating the breathing effect of the coupling of wellbore flow and formation fractures, as described in Example 1. The difference is that in order to explore the influence of the fracture surface roughness, different cutting methods are used to create fractures. The fracture creation method for simulating the formation fracture adopts diamond blade cutting or abrasive water jet cutting. Among them, the smoothness of the fracture surface cut by the diamond blade is better, while the fracture surface cut by the abrasive water jet is relatively rough.

[0049] Example 3

[0050] An experimental device for simulating the breathing effect of the coupling of wellbore flow and formation fractures, as described in Example 2. The difference is that circular steel plates are placed between the upper and lower end faces of the core and the core holder as a sealing layer to ensure the sealing of the pressure boundary.

[0051] Example 4

[0052] An experimental device for simulating the breathing effect of the coupling between wellbore flow and formation fractures is as described in Example 3. The difference is that the air pressure output unit includes a nitrogen cylinder 10. A pneumatic driving unit 22 is provided at the bottom end where the core holder 1 contacts the core 2. The axial liquid delivery pipeline of the core holder 1 passes through the center of the pneumatic driving unit. The top of the pneumatic driving unit 22 is in close contact with the core, and an air cavity is formed between the bottom of the pneumatic driving unit 22 and the core holder. The air cavity is connected to the nitrogen cylinder 10 through pipeline A. A valve A 7, a back pressure valve 11, and a pressure gauge 12 are sequentially arranged on pipeline A. Different axial pressures can be set through the nitrogen cylinder 10 to simulate different closure pressures of the fractures.

[0053] In this embodiment, the pneumatic driving unit 22 is T-shaped, similar to a piston. As nitrogen is filled into the air cavity, the pressure in the air cavity increases and will push the pneumatic driving unit to make a small movement, applying an axial pressure to the core.

[0054] Example 5

[0055] An experimental device for simulating the breathing effect of the coupling between wellbore flow and formation fractures is as described in Example 4. The difference is that the hydraulic output unit includes a formation hydraulic pump 15 and a drilling hydraulic pump 13;

[0056] An annulus pressure cavity is formed between the side wall of the core 2 and the core holder 1. The formation hydraulic pump 15 is connected to the annulus pressure cavity through pipeline B. A valve B 9 and a hydraulic gauge A 18 are arranged on pipeline B; the annulus pressure cavity is also connected to pipeline C, and a formation back pressure valve 16 is arranged on pipeline C; different formation annulus pressures are set through the formation hydraulic pump 15 to simulate different fracture propagation pressures;

[0057] The top of the axial liquid delivery pipeline of the core holder 1 is respectively connected to pipeline D and pipeline E. Pipeline D is connected to the drilling hydraulic pump 13. A valve C 8 and a hydraulic gauge B 17 are sequentially arranged on pipeline D; a drilling back pressure valve 14 is arranged on pipeline E. Different pumping pressures of the drilling fluid are set by the drilling hydraulic pump to simulate different drilling fluid circulation pressures during the drilling process.

[0058] The formation hydraulic pump 15 and the drilling hydraulic pump 13 are respectively connected to a container through pipelines. An electronic balance A 5 and an electronic balance B 3 are placed at the bottom of the container. The ends of pipeline C and pipeline E are also connected to a container, and an electronic balance C 6 and an electronic balance D 4 are placed at the bottom of the container.

[0059] The pressure gauge 12, the hydraulic gauge A 18, the hydraulic gauge B 17, the electronic balance A 5, the electronic balance B 3, the electronic balance C 6, and the electronic balance D 4 are all connected to the data acquisition system 23;

[0060] Pressure relief valves A 21, B 20 and C 19 are respectively provided on pipeline A, pipeline B and pipeline C.

[0061] Example 6

[0062] A working method of an experimental device for simulating the breathing effect of the coupling of wellbore flow and formation fractures in Example 5. During the experiment, the drilling fluid can be injected into the wellbore by driving the intermediate container through the drilling hydraulic pump, and the formation pressure environment can be simulated by controlling the formation hydraulic pump. When the axial pressure is greater than or equal to the drilling fluid back pressure, that is, when the air pressure gauge is greater than the value of hydraulic pressure gauge B, after the drilling fluid circulates normally, it is discharged from the wellbore, which corresponds to the situation where no breathing effect occurs;

[0063] When the axial pressure is less than the drilling fluid back pressure, the drilling fluid enters the wellbore, that is, the axial perforation of the core, and gradually enters the formation along the radial fractures of the core to simulate the situation of leakage caused by the breathing effect. At the same time, as the compression pressure of the axial pressure gas increases, when the formation pressure (i.e., confining pressure) and the drilling fluid back pressure are balanced, the drilling fluid circulates normally again, which corresponds to normal circulation drilling. If the drilling fluid circulation is stopped at this time and the drilling pump pressure is reduced, the axial pressure will compress the fractures again, and the drilling fluid that has entered the core fractures will be retched, which can simulate the situation of the corresponding return when the breathing effect stops the pump. The experiment records the relevant parameters of the coupling flow of wellbore flow and formation fractures during the breathing effect obtained by monitoring equipment such as electronic balances, pressure gauges and pressure sensors through simulating the breathing effect process.

[0064] Example 7

[0065] A working method of an experimental device for simulating the breathing effect of the coupling of wellbore flow and formation fractures is as described in Example 6. The difference is that the specific implementation process is as follows:

[0066] (1) Process the core: Drill the core 2 axially to simulate the wellbore; create artificial fractures along the radial direction of the core. Two different cutting methods, diamond blade cutting and abrasive water jet cutting, are used to characterize different roughnesses of the fractures;

[0067] (2) Prepare the paired cores. Considering that the rocks deep in the formation are in a saturated state, soak multiple groups of cores completely under the conditions of axial pressure of 10 MPa and confining pressure of 10 MPa for 3 h to ensure that each group of cores is in a saturated state during the experiment, and avoid the situation where the experimental fluid enters the unsaturated voids of the matrix and cannot be returned, which will affect the experimental results of the breathing effect. After the rock samples are saturated, zero the pressure in the experimental barrel to the initial state, and then the experiment can be started;

[0068] (3) Turn on the main power supply and the switches of each electronic balance, and check whether the pipelines and the data acquisition system are normal; open the valve A 7 on the nitrogen cylinder to supply nitrogen and apply axial pressure to the core. After the pressure reaches the experimental axial pressure, close the valve A 7;

[0069] (4) Open valve C 8 and valve B 9, adjust the pump speed of the formation hydraulic pump 15 to about 50% of the required pump speed for the experiment, and pump in the experimental fluid until the connection state is reached, that is, fluid flows out from both the outlet of the formation backpressure valve 16 and the outlet of the drilling backpressure valve 14;

[0070] (5) Close the formation backpressure valve 16 to apply formation backpressure until the hydraulic gauge A 18 stabilizes. Collect the value of the hydraulic gauge A18, which is the formation confining pressure. The circuit is in circulation, that is, only fluid flows out from the outlet of the drilling backpressure valve;

[0071] (6) Close the formation hydraulic pump 15, apply drilling backpressure. After the hydraulic gauge B17 reaches the target pressure (the target pressure is the corresponding pressure set for the drilling cycle), close the drilling backpressure valve. At this time, no fluid flows out from the outlet of the drilling backpressure valve. Collect the value of the hydraulic gauge B17, which is the initial wellbore pressure;

[0072] (7) Increase the pump speed of the drilling hydraulic pump 13 to the target pump speed and wait until it stabilizes, that is, the flow rate at the outlet of the drilling backpressure valve 14 stabilizes (stable means that the flow rate does not change or changes very little within a short time, which can be judged according to the actual situation). This process represents the process of the drilling cycle starting to breathe and leak as Figure 2 , and collect the pressure and flow rate change data through the data acquisition system 23;

[0073] (8) Control the sizes of the drilling hydraulic pump 13 and the drilling backpressure valve 14 so that the axial pressure is the initial wellbore pressure in step (6), and gradually reduce the pump speed of the well hydraulic pump. This process represents the process of the drilling cycle stopping to breathe and flowing back as Figure 3 , and collect the pressure and flow rate change data;

[0074] (9) After the simulation of the breathing effect process is completed, stop the pump, close the valve A 7 of the nitrogen cylinder, and slowly open all the pressure relief valves until all the experimental fluid is emptied;

[0075] (10) Empty the nitrogen, turn off the switches of each electronic balance, turn off the switch of the data acquisition system, and turn off the main power supply. The experiment is over.

[0076] The above is the preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle described in the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. An experimental device for simulating the breathing effect of coupling of wellbore flow and formation fractures, characterized in that: It includes a core, a core holder, a pneumatic output unit and a hydraulic output unit. The middle of the core is axially perforated to connect the axial fluid delivery pipeline of the core holder to simulate a wellbore. The middle of the core is radially divided into two sections to simulate formation fractures. The air pressure output unit is used to provide the core axial pressure; the hydraulic pressure output unit is used to provide the core confining pressure and the drilling fluid pump pressure.

2. The experimental device for simulating the breathing effect of coupling of wellbore flow and formation fractures according to claim 1, characterized in that: The method of creating fractures to simulate stratum fractures is to use diamond blade cutting or abrasive water jet to create fractures.

3. The experimental device for simulating the breathing effect of coupling of wellbore flow and formation fractures according to claim 2, characterized in that: A circular steel plate is placed between the upper and lower end surfaces of the core and the core holder as a sealing layer.

4. The experimental device for simulating the breathing effect of coupling of wellbore flow and formation fractures according to claim 3, characterized in that: The air pressure output unit comprises a nitrogen bottle, a pneumatic driving unit is arranged at the bottom of the core holder in contact with the core, the top of the pneumatic driving unit is in close contact with the core, an air cavity is formed between the bottom of the pneumatic driving unit and the core holder, the air cavity is connected with the nitrogen bottle through a pipeline A, and a valve A, a back pressure valve and a pressure gauge are arranged on the pipeline A in sequence.

5. The experimental device for simulating the breathing effect of coupling of wellbore flow and formation fractures according to claim 4, characterized in that: The hydraulic output unit includes a formation hydraulic pump and a drilling hydraulic pump; A confining pressure cavity is formed between the core sidewall and the core holder, and the formation hydraulic pump is connected to the confining pressure cavity via a pipeline B, on which a valve B and a hydraulic gauge A are provided; the confining pressure cavity is also connected to a pipeline C, on which a formation back pressure valve is provided; The top of the axial liquid delivery pipeline of the core clamp is respectively connected with pipeline D and pipeline E, pipeline D is connected with a drilling hydraulic pump, valve C and hydraulic gauge B are arranged on pipeline D in sequence; pipeline E is provided with a drilling back pressure valve.

6. The experimental device for simulating the breathing effect of coupling of wellbore flow and formation fractures according to claim 5, characterized in that: The formation hydraulic pump and the drilling hydraulic pump are connected to a container through pipelines respectively, and an electronic balance A and an electronic balance B are placed at the bottom of the container respectively. The ends of the pipeline C and the pipeline E are also connected to a container, and an electronic balance C and an electronic balance D are placed at the bottom of the container.

7. The experimental device for simulating the breathing effect of coupling of wellbore flow and formation fractures according to claim 5, characterized in that: The air pressure gauge, hydraulic pressure gauge A, hydraulic pressure gauge B, electronic balance A, electronic balance B, electronic balance C and electronic balance D are all connected to the data acquisition system; The pipeline A, pipeline B and pipeline C are respectively provided with a pressure relief valve A, a pressure relief valve B and a pressure relief valve C.

8. A method for operating the experimental device for simulating the breathing effect of coupling of wellbore flow and formation fractures as claimed in claim 7, characterized in that: When the axial pressure is greater than or equal to the drilling fluid back pressure, that is, when the air pressure gauge is greater than the value of the hydraulic pressure gauge B, the drilling fluid will be discharged from the wellbore after normal circulation, which corresponds to the situation where there is no breathing effect. When the axial pressure is less than the drilling fluid back pressure, the drilling fluid enters the wellbore, that is, the axial perforation of the core, and gradually enters the formation along the radial cracks of the core to simulate the leakage caused by the breathing effect. At the same time, as the axial pressure gas is compressed and the pressure increases, when the formation pressure and the drilling fluid back pressure are balanced, the drilling fluid circulates normally again, which corresponds to normal circulation drilling. If the drilling fluid circulation is stopped at this time and the drilling pump pressure is reduced, the axial pressure will compress the cracks again, and the drilling fluid entering the core cracks will be spit out, which can simulate the corresponding return situation of stopping the pump due to the breathing effect.

9. The working method of the experimental device for simulating the breathing effect of coupling of wellbore flow and formation fractures according to claim 8, characterized in that: The specific implementation process is: (1) Core processing: Drill holes in the core axial direction to simulate the wellbore; artificially create fractures in the core radial direction. Two different cutting methods, diamond blade cutting and abrasive water jet cutting, are used to characterize the different roughness of fractures; (2) preparing pairs of cores and completely immersing multiple groups of cores under experimental pressure conditions to ensure that each group of cores is in a saturated state during the experiment; (3) Turn on the main power supply and the switches of each electronic balance, and check whether the pipeline and data acquisition system are normal; open valve A on the nitrogen bottle to supply nitrogen, apply axial pressure to the core, and close valve A when the pressure reaches the experimental axial pressure; (4) Open valve C and valve B, adjust the pump speed of the formation hydraulic pump to 50% of the pump speed required by the experiment, and pump the experimental fluid until a connected state is achieved, that is, fluid flows out of the outlet of the formation back pressure valve and the outlet of the drilling back pressure valve; (5) Close the formation back pressure valve to apply formation back pressure, so that the hydraulic gauge A reaches stability, collect the value of the hydraulic gauge A, i.e., the formation confining pressure, and the circuit is flowing, i.e., only the drilling back pressure valve outlet flows out fluid; (6) Turn off the formation hydraulic pump and apply drilling back pressure. When the hydraulic gauge B reaches the target pressure, close the drilling back pressure valve. At this time, no fluid flows out of the drilling back pressure valve outlet. Collect the value of the hydraulic gauge B, which is the initial wellbore pressure. (7) Raise the pump speed of the drilling hydraulic pump to the target pump speed, wait until it reaches stability, that is, the flow rate at the outlet of the drilling back pressure valve is stable, and collect pressure and flow change data through the data acquisition system; (8) Control the drilling hydraulic pump and the drilling back pressure valve so that the axial pressure is the initial wellbore pressure of step (6), and gradually reduce the pump speed of the well hydraulic pump, and collect pressure and flow change data; (9) After the simulation of the breathing effect process is completed, stop the pump, close valve A of the nitrogen bottle, and open all pressure relief valves until all the experimental fluid is emptied; (10) Release the nitrogen, turn off the switches of each electronic balance, turn off the switch of the data acquisition system, turn off the main power supply, and the experiment is over.

10. The working method of the experimental device for simulating the breathing effect of the coupling of wellbore flow and formation fractures according to claim 9, characterized in that: In step (2), the experimental pressure conditions are an axial pressure of 10 MPa and a confining pressure of 10 MPa, and the immersion in the experimental liquid is preferably for more than 3 hours.

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