A method for determining the status of hydraulic fracturing tool strings in underground coal mines

Abstract

The present invention discloses a method for determining the status of a hydraulic fracturing tool string in an underground coal mine. The method comprises first deploying auxiliary equipment, and then determining the status of the tool string. The auxiliary equipment includes an inlet flow sensor, a pressure sensor, a tee, a linear relief valve, and an overflow flowmeter. The linear relief valve in the auxiliary equipment regulates the high-pressure water supply to the oil pipe, thereby regulating the system pressure. The pressure sensor, inlet flow sensor, and overflow flowmeter are used to sense parameters such as the fracturing pump pressure, total flow, and overflow flow in real time. The tool string status is determined by observing and comparing various bottom hole parameters. The present invention indicates the status of the fracturing tool string in a borehole in real time, quantitatively, and intelligently, thereby helping to improve the efficiency of hydraulic fracturing in underground coal mines, save construction costs, and ensure project quality.

Description

Technical Field

[0001] The present invention belongs to the technical field of coal mine safety, and relates to a method for optimizing hydraulic fracturing parameters in an underground coal mine, and in particular to a method for distinguishing the status of a hydraulic fracturing tool string in an underground coal mine. Background Art

[0002] Hydraulic fracturing is an emerging technology in the field of coal mine safety. This technology has a variety of application scenarios in the field of coal mine safety, such as coal seam hydraulic fracturing gas pre-extraction, hard roof weakening, coal seam roof water drainage and other projects. Hydraulic fracturing technology can be used.

[0003] Double packer drag fracturing technology is the most commonly used downhole staged hydraulic fracturing technology. It uses two packers to block the borehole, forming a closed space between the two packers. A choke is set between the two packers. The function of the choke is to open the water hole under a certain pressure for water injection fracturing. Its structure is as follows: Figure 1 As shown, the combination of packer and choke is collectively referred to as a "tool string".

[0004] During the fracturing process, the tool string is in the hole and cannot be seen by the naked eye. Therefore, the judgment of the status of the packer depends entirely on the operator's experience. For example, whether the packer is normally expanded and sealed, whether the choke is open, whether the tool string is intact, etc., can only be inferred based on observed phenomena (such as whether there is water return at the orifice, etc.). It is greatly affected by human factors and has low accuracy. In actual engineering, it leads to increased costs and reduced efficiency. For example: (1) When the tool string is intact, it is believed that the tool string is damaged based on the water return at the orifice. After the tool string is removed, it is found that the tool string is intact, resulting in the loss of the drilling rig team; (2) When the packer on the bottom of the hole is damaged, there is no water return at the orifice, and the tool string is mistakenly believed to be intact, and fracturing is continued, resulting in poor engineering results.

[0005] Therefore, there is an urgent need for a method and related equipment that can quantitatively and intelligently judge the status of the tool string to improve the efficiency of hydraulic fracturing in coal mines, save construction costs, and ensure project quality. Summary of the Invention

[0006] In view of the shortcomings of the existing technology, the purpose of the present invention is to provide a method for determining the status of a hydraulic fracturing tool string in a coal mine, so as to solve the problems in the existing technology such as the inability to accurately determine whether the packer is normally expanded and sealed, whether the throttle is open, and whether the tool string is intact.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] A method for determining the status of a hydraulic fracturing tool string in an underground coal mine, wherein the method first deploys auxiliary equipment and then determines the status of the tool string;

[0009] The auxiliary equipment includes an inlet flow sensor, a pressure sensor, a tee, a linear overflow valve and an overflow flowmeter; the inlet flow sensor is installed at the main inlet of the fracturing pump, the pressure sensor is installed on the fracturing pump, the first pipe of the tee is connected to the high-pressure water outlet of the fracturing pump, the second pipe supplies high-pressure water to the oil pipe for fracturing, and the linear overflow valve and overflow flowmeter are arranged on the third pipe; the tool string status is determined by adjusting the opening of the overflow valve and observing and comparing the bottom hole parameters.

[0010] The present invention also includes the following technical features:

[0011] Specifically, the linear relief valve has a natural logarithmic relationship between its local resistance coefficient and opening:

[0012]

[0013] Where, is the local resistance coefficient of the overflow valve, is the opening of the overflow valve; the high-pressure water supply of the oil pipe is adjusted by the linear overflow valve in the auxiliary equipment, thereby adjusting the system pressure, and the fracturing pump pressure, total flow and overflow flow are sensed in real time through the pressure sensor, inlet flow sensor and overflow flowmeter.

[0014] Specifically, the tool string status determination includes: the first step, the tool string is lowered into the predetermined position in the hole, water is injected into the tool string using a fracturing pump, and the relief valve is opened to the full open state, that is, the relief valve opening is At this point, the tool string meets the following conditions at the same time, indicating that it is working normally:

[0015] Condition 1: Observe the overflow flow , slowly rising to , is the total flow of the fracturing pump;

[0016] Condition 2: Observe the pump pressure of the fracturing pump , Packer expansion pressure , throttle valve operating pressure :

[0017] .

[0018] Specifically, if the conditions in the first step are not met, the following questions are determined:

[0019] like ,observe After stabilization, it is always less than , indicating that there is a water leak in the tool string, and the tool string should be exited for inspection;

[0020] or , indicating that the overflow pipe is blocked or the overflow valve is improperly selected and should be cleaned or adjusted.

[0021] Specifically, the tool string status determination includes: a second step of adjusting and reducing the opening of the relief valve. At this time, the opening of the relief valve is calculated according to the following formula:

[0022]

[0023] Where: , is the total flow, is the overflow flow, is the relief valve radius, is the parameter related to the local resistance coefficient, is the radius of the fracturing tubing, is the friction coefficient of the oil pipe, is the total length of the pipeline from the fracturing pump to the fracturing point, is the density of the fracturing fluid, The vertical height of the fracturing point, i.e. the tool string position from the fracturing pump;

[0024] Adjusting and reducing the opening of the relief valve increases the local resistance of the relief pipeline, causing the pump pressure to rise, setting the packer and opening the choke. At this time, if the tool string meets any of the following conditions, it is in normal working condition:

[0025] Condition 1: ,at the same time At this time, the tool string packer is inflated and set, the choke is not opened, and the status is normal;

[0026] Condition 2: ,at the same time , indicating that the throttle is open.

[0027] Specifically, in the second step, the bottom hole pressure is calculated: Where, is the bottom hole pressure, For the fracturing pump pressure, is the total flow, is the overflow flow, is the radius of the fracturing tubing, is the friction coefficient of the oil pipe, is the total length of the pipeline from the fracturing pump to the fracturing point, is the density of the fracturing fluid, The vertical height of the fracturing point, i.e. the tool string position from the fracturing pump;

[0028] At this time, the bottom hole pressure Should not be greater than 10 Otherwise, check whether the oil pipe and fracturing pipeline are blocked.

[0029] Specifically, the tool string state determination includes: a third step of continuing to adjust and reduce the opening of the relief valve. At this time, the opening of the relief valve is calculated according to the following formula:

[0030]

[0031] Where: , is the total flow, is the overflow flow, is the relief valve radius, is the parameter related to the local resistance coefficient, is the radius of the fracturing tubing, is the friction coefficient of the oil pipe, is the total length of the pipeline from the fracturing pump to the fracturing point, is the density of the fracturing fluid, The vertical height of the fracturing point, i.e. the tool string position from the fracturing pump;

[0032] Observe the bottom hole pressure at this time:

[0033]

[0034] Where, is the bottom hole pressure, For the fracturing pump pressure, is the total flow, is the overflow flow, is the radius of the fracturing tubing, is the friction coefficient of the oil pipe, is the total length of the pipeline from the fracturing pump to the fracturing point, is the density of the fracturing fluid, The vertical height of the fracturing point, i.e. the tool string position from the fracturing pump;

[0035] As the opening of the overflow valve decreases, the pump pressure increases, and the bottom hole pressure increases accordingly. After the coal layer is fractured and fracturing is achieved, the bottom hole pressure and pump pressure both decrease. At this time, the bottom hole pressure is recorded. With pump pressure peak value.

[0036] Specifically, in the third step, during the normal fracturing process, the bottom hole pressure It remains basically stable. As the coal-rock layer continues to fracture, there will be a short period of sudden drop-sudden rise process. The pressure fluctuation time will not exceed 2 minutes. Therefore, if the following situations occur during the normal fracturing process, it is determined that the tool string is damaged, and the fracturing should be stopped and the tool string should be replaced: bottom hole pressure The decrease is more than 50% and lasts for more than 2 minutes.

[0037] Specifically, the tool string status determination includes: Step 4, after the coal rock layer is fractured, observe for 10 minutes to 20 minutes at the current relief valve opening. If the fracturing process is normal, continue to reduce the relief valve opening and control the displacement according to project needs until the fracturing is completed.

[0038] Specifically, during the fourth step, if the bottom hole pressure If the drop exceeds 50% and lasts for more than 2 minutes, the tool string is considered damaged and fracturing should be stopped and the tool string should be replaced.

[0039] Compared with the prior art, the present invention has the following technical effects:

[0040] The present invention can indicate the status of the fracturing tool string in the borehole in real time, quantitatively and intelligently, which is helpful to improve the efficiency of hydraulic fracturing in coal mines, save construction costs and ensure project quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 Schematic diagram of the layout of dual packers and chokes in hydraulic fracturing.

[0042] Figure 2 This is a schematic diagram of the layout of auxiliary equipment.

[0043] Figure 3 This is a graph showing the change of bottom hole pressure over time under normal fracturing conditions in Example 1.

[0044] Figure 4 This is a graph showing the change of bottom hole pressure over time under abnormal fracturing conditions in Example 1.

[0045] Figure 5 This is the pressure curve of a certain fracturing operation.

[0046] Figure 6 Schematic diagram of tool string packer damage.

[0047] Figure 7 This is the pressure curve of a certain fracturing operation.

[0048] Figure 8 This is a schematic diagram of the tool string packer in good condition.

[0049] The meaning of each number in the figure is:

[0050] 11. Borehole, 12. Packer, 13. Choke, 14. Tubing; 21. Main inlet, 22. Inlet flow sensor, 23. Fracturing pump, 24. High-pressure water outlet, 25. Tee, 26. Linear relief valve, 27. Overflow flowmeter. DETAILED DESCRIPTION

[0051] The present invention provides a method for distinguishing the status of a hydraulic fracturing tool string in an underground coal mine. The method first arranges auxiliary equipment and then distinguishes the status of the tool string. The auxiliary equipment includes an inlet flow sensor 22, a pressure sensor, a tee pipe 25, a linear overflow valve 26 and an overflow flowmeter 27. The inlet flow sensor 22 is installed at the main inlet 21 of a fracturing pump 23, the pressure sensor is installed on the fracturing pump 23, a first pipe of the tee pipe 25 is connected to a high-pressure water outlet 24 of the fracturing pump 23, a second pipe supplies high-pressure water to an oil pipe 14 for fracturing, and a linear overflow valve 26 and an overflow flowmeter 27 are arranged on the third pipe. The tool string status is distinguished by adjusting the opening of the overflow valve and observing and comparing various bottom hole parameters.

[0052] The local resistance coefficient of the linear relief valve 26 and the opening are in a natural logarithmic relationship:

[0053]

[0054] Where, is the local resistance coefficient of the overflow valve, The linear overflow valve in the auxiliary equipment is used to adjust the high-pressure water supply in the oil pipe, thereby adjusting the system pressure, and the pressure sensor, inlet flow sensor 22, and overflow flowmeter 27 are used to sense the fracturing pump pressure, total flow, overflow flow and other parameters in real time.

[0055] Tool string status determination includes:

[0056] The first step is to lower the tool string into the predetermined position in the hole, use the fracturing pump 23 to inject water into the tool string, and open the relief valve to the full open state, that is, the relief valve opening At this point, the tool string meets the following conditions at the same time, indicating that it is working normally:

[0057] Condition 1: Observe the overflow flow , slowly rising to , is the total flow of the fracturing pump;

[0058] Condition 2: Observe the pump pressure of the fracturing pump , Packer expansion pressure , throttle valve operating pressure :

[0059] .

[0060] If the conditions in the first step are not met, then determine the following questions:

[0061] like ,observe After stabilization, it is always less than , indicating that there is a water leak in the tool string, and the tool string should be exited for inspection;

[0062] or , indicating that the overflow pipe is blocked or the overflow valve is improperly selected and should be cleaned or adjusted.

[0063] The second step is to adjust and reduce the opening of the relief valve. At this time, the opening of the relief valve is calculated according to the following formula:

[0064]

[0065] Where: , is the total flow, is the overflow flow, is the relief valve radius, is the parameter related to the local resistance coefficient, is the radius of the fracturing tubing, is the friction coefficient of the oil pipe, is the total length of the pipeline from the fracturing pump to the fracturing point, is the density of the fracturing fluid, is the vertical height of the fracturing point, i.e. the tool string position from the fracturing pump; the derivation process of this formula is as follows:

[0066] Since the auxiliary equipment of the present invention is Figure 2 The linear relief valve 26 on the middle right side is connected in parallel with the high-pressure water supply outlet 24 to the oil pipe. According to the knowledge of fluid mechanics, the total pressure of the two (total pressure = static pressure + dynamic pressure) is equal.

[0067] Hydraulic fracturing construction process, hydraulic fracturing tool string ( Figure 1 ) can be used to analyze the fluid velocity. It is known that before the throttle valve is opened, the water flow in the pipeline is approximately in a static state. At the engineering level, the dynamic pressure can be regarded as 0, which means that the auxiliary equipment ( Figure 2 ) is equal to the static pressure in the high-pressure water supply line to the oil pipe, i.e., the pressure reflected by the pressure gauge. Therefore, based on a thorough understanding and knowledge of the auxiliary equipment and hydraulic fracturing process in this invention, we can deduce:

[0068] Local resistance coefficient of linear relief valve 26:

[0069] ;

[0070] From the local resistance formula, we can know the local pressure drop of the linear relief valve 26:

[0071] ;

[0072] It is also known that the outlet end of the linear relief valve 26 is open, and its overflow flow rate is small. It can be considered that the local pressure drop of the linear relief valve 26 is It can be directly measured by the pressure sensor on it.

[0073] Back calculation :

[0074] ;

[0075] exist Before (i.e. ), water flows through Figure 2 The high-pressure water supply pipeline in the middle of the oil pipe flows in the oil pipe 14 in the borehole 11, and the resistance caused by the flow It mainly consists of two parts: the resistance along the way and the gravitational potential, namely

[0076] ;

[0077] at this time,

[0078]

[0079] Bring in Calculation formula:

[0080] ;

[0081] In engineering practice, it is impossible to accurately control the valve opening to be exactly equal to Therefore, the required valve opening is controlled within a range of 10% below the value calculated by the above formula:

[0082] ;

[0083] ;

[0084] Due to the adjustment of the relief valve opening, the relief valve is closed, resulting in an increase in the local resistance of the relief pipeline, causing the pump pressure to rise, causing the packer 12 to be set and the choke 13 to open. At this time, if the tool string meets any of the following conditions, it is in normal working condition:

[0085] Condition 1: ,at the same time At this time, the tool string packer 12 is inflated and set, and the choke 13 is not opened, and the status is normal;

[0086] Condition 2: ,at the same time , indicating that the throttle 13 is open.

[0087] In the second step, the bottom hole pressure is calculated: Where, is the bottom hole pressure, For the fracturing pump pressure, is the total flow, is the overflow flow, is the radius of the fracturing tubing, is the friction coefficient of the oil pipe, is the total length of the pipeline from the fracturing pump to the fracturing point, is the density of the fracturing fluid, The vertical height of the fracturing point, i.e. the tool string position from the fracturing pump;

[0088] At this time, the bottom hole pressure Should not be greater than 10 Otherwise, check whether the oil pipe 14 and the fracturing pipeline are blocked.

[0089] The third step is to continue adjusting and reducing the opening of the relief valve. At this time, the opening of the relief valve is calculated according to the following formula:

[0090]

[0091] Where: , is the total flow, is the overflow flow, is the relief valve radius, is the parameter related to the local resistance coefficient, is the radius of the fracturing tubing, is the friction coefficient of the oil pipe, is the total length of the pipeline from the fracturing pump to the fracturing point, is the density of the fracturing fluid, The vertical height of the fracturing point, i.e. the tool string position from the fracturing pump;

[0092] Observe the bottom hole pressure at this time:

[0093]

[0094] Where, is the bottom hole pressure, For the fracturing pump pressure, is the total flow, is the overflow flow, is the radius of the fracturing tubing, is the friction coefficient of the oil pipe, is the total length of the pipeline from the fracturing pump to the fracturing point, is the density of the fracturing fluid, The vertical height of the fracturing point, i.e. the tool string position from the fracturing pump;

[0095] As the opening of the overflow valve decreases, the pump pressure increases, and the bottom hole pressure increases accordingly. After the coal layer is fractured and fracturing is achieved, the bottom hole pressure and pump pressure both decrease. At this time, the bottom hole pressure is recorded. With pump pressure peak value.

[0096] In the third step, during normal fracturing, the bottom hole pressure It remains basically stable. As the coal layer continues to fracture, there will be a short period of sudden drop-sudden rise process, but the pressure fluctuation time is short and will not exceed 2 minutes. Therefore, if the following situations occur during the normal fracturing process, it is determined that the tool string is damaged, and the fracturing should be stopped and the tool string should be replaced: bottom hole pressure The decrease is more than 50% and lasts for more than 2 minutes.

[0097] Step 4: After the coal layer is fractured, observe the current relief valve opening for 10-20 minutes. If the fracturing process is normal, continue to reduce the relief valve opening and control the displacement according to the project needs. , until the fracturing is completed.

[0098] In the fourth step, if the bottom hole pressure If the drop exceeds 50% and lasts for more than 2 minutes, the tool string is considered damaged and fracturing should be stopped and the tool string should be replaced.

[0099] The parameters involved in the specific implementation of the present invention are shown in Table 1 below:

[0100] Table 1 Parameters, names and descriptions

[0101]

[0102] Specific embodiments of the present invention are given below. It should be noted that the present invention is not limited to the following specific embodiments, and all equivalent changes made on the basis of the technical solution of this application fall within the protection scope of the present invention.

[0103] Example 1:

[0104] This embodiment provides a method for determining the status of a hydraulic fracturing tool string in an underground coal mine. This embodiment is a hydraulic fracturing operation, and the parameters involved in the operation are shown in Table 2 below:

[0105] Table 2 Parameters related to Example 1

[0106]

[0107] This embodiment includes the following steps:

[0108] first step:

[0109] The tool string is lowered into the hole at a predetermined position, which is 30m vertically from the fracturing pump and the total length of the fracturing pipeline is 500m. Use the fracturing pump to inject water into the tool string and open the overflow valve to the full open state ( ), at this time, the total flow rate of the fracturing pump is observed to be , observe the overflow flow by Slowly rise to ; At the same time, observe the fracturing pump pressure slowly rising to If it stops rising, it means the tool string is in normal condition.

[0110] Other example 1: If the overflow flow rate does not rise to , and the pump pressure is stable at , indicating that water is always flowing in the fracturing pipeline when the throttle valve is not opened. This is an abnormal situation. There may be leakage in the fracturing pipeline and it needs to be checked.

[0111] Other example 2: When the relief valve is fully open, the pump pressure suddenly rises to The above indicates that the overflow valve is not selected properly or the pipeline is blocked, which needs to be checked.

[0112] Step 2:

[0113] Adjust the opening of the relief valve. The required opening can be calculated according to the following formula:

[0114]

[0115] Where:

[0116] ;

[0117] Bring in 、 、 、 、 、 、 、 、 ; Simultaneously design , put it into the formula, we can get , that is, the control valve opening

[0118]

[0119] The overflow valve closure causes an increase in local resistance in the overflow line, which in turn increases the pump pressure, causing the packer to set and the choke to open. At this point, the tool string meets the following conditions:

[0120] ,at the same time , indicating that the throttle is open and working normally. Calculate the bottom hole pressure at this time:

[0121]

[0122] The pump pressure was observed to be , calculate the bottom hole pressure , indicating that the fracturing tool string is operating normally.

[0123] Other examples: At this time, the pump pressure is observed to be , calculate the bottom hole pressure , greater than , check whether the oil pipe and fracturing pipeline are blocked.

[0124] Step 3:

[0125] Continue to adjust the opening of the relief valve. The required opening can be calculated according to the following formula:

[0126]

[0127] Where:

[0128] ;

[0129] Bring in 、 、 、 、 、 、 、 、 ; Simultaneously design , put it into the formula, we can get , that is, the control valve opening

[0130]

[0131] As the relief valve pressure decreases, the pump pressure increases, and the bottom hole pressure increases accordingly. After the coal layer is fractured and fracturing is achieved, the bottom hole pressure and pump pressure both decrease. The peak pump pressure at this time is , calculate the bottom hole pressure Peak:

[0132]

[0133] After reaching the peak and dropping slightly, the bottom hole pressure It basically remains stable. As the coal layer continues to fracture, there may be a short period of sudden drop-sudden rise process, but the pressure fluctuation time is short and will not exceed 2 minutes. Figure 3 shown.

[0134] Other examples: Bottom hole pressure The decrease is more than 50% and lasts for more than 2 minutes, such as Figure 4 shown.

[0135] Step 4:

[0136] After the coal layer is broken, observe for 10-20 minutes at the current relief valve opening. If the fracturing process is normal, continue to reduce the relief valve opening and control the displacement according to the project needs ( ), generally controlled at ,like When controlling During this process, if the bottom hole pressure If the drop exceeds 50% and lasts for more than 2 minutes, the tool string is considered damaged and fracturing should be stopped and the tool string should be replaced.

[0137] The method of the present invention is applied to hydraulic fracturing construction in a certain mine. During the construction, the method is used to judge the status of the tool string, which greatly improves the accuracy of the judgment of the tool string status and increases the success rate of the fracturing construction.

[0138] like Figure 5 It is the pressure curve of a certain fracturing operation. The operation is carried out according to the steps in the present invention, and the tool string is judged to be damaged during the fracturing process, such as Figure 6 After stopping fracturing, the tool string was pulled out, proving that the judgment was correct.

[0139] like Figure 7 It is a pressure curve of a certain fracturing operation. The method of the present invention is used to judge that the working state of the tool string is normal. After the fracturing is completed, the tool string is taken out to prove that the packer is intact. Figure 8 , which verifies the effectiveness and practicality of the present invention.

[0140] The preferred embodiments of the present invention are described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the scope of protection of the present invention.

[0141] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner without contradiction. To avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0142] In addition, the various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A method for determining the status of a hydraulic fracturing tool string in an underground coal mine, characterized in that: This method first deploys auxiliary equipment and then determines the tool string status; The auxiliary equipment includes an inlet flow sensor, a pressure sensor, a tee, a linear overflow valve, and an overflow flowmeter; the inlet flow sensor is installed at the main inlet of the fracturing pump, the pressure sensor is installed on the fracturing pump, the first pipe of the tee is connected to the high-pressure water outlet of the fracturing pump, the second pipe supplies high-pressure water to the oil pipe for fracturing, and the third pipe is arranged with a linear overflow valve and an overflow flowmeter; the tool string status is determined by adjusting the opening of the overflow valve and observing and comparing various bottom hole parameters; The linear relief valve has a natural logarithmic relationship between its local resistance coefficient and opening: Where k vol is the local resistance coefficient of the overflow valve, Deg vol is the opening of the overflow valve; the linear overflow valve in the auxiliary equipment is used to adjust the high-pressure water supply to the oil pipe, thereby adjusting the system pressure, and the pressure sensor, inlet flow sensor, and overflow flowmeter are used to sense the pump pressure, total flow, and overflow flow of the fracturing pump in real time; The tool string status determination includes: the first step is to lower the tool string into the predetermined position in the hole, use the fracturing pump to inject water into the tool string, and open the relief valve to the full open state, that is, the relief valve opening degree Deg vol =100%, the tool string meets the following conditions at the same time, indicating that it is working normally: Condition 1: Observe the overflow flow Q re , slowly rising to Q re =Q in , Q in is the total flow of the fracturing pump; Condition 2: Observe the fracturing pump pressure P pm , Packer expansion pressure P ex , throttle valve operating pressure P op : P pm <P ex <P op ; If the conditions in the first step are not met, then determine the following questions: If P pm <P ex <P op , observe Q re After stabilization, it is always less than Q in , indicating that there is a water leak in the tool string, and the tool string should be exited for inspection; P pm >P ex or P pm >P op , indicating that the overflow pipe is blocked or the overflow valve is improperly selected and should be cleaned or adjusted; The tool string status determination includes: a second step of adjusting and reducing the opening of the relief valve. At this time, the opening of the relief valve is calculated according to the following formula: Deg vol ∈[X-10%,X] Where: Q in is the total flow, Q re is the overflow flow rate, R is the radius of the overflow valve, α is the parameter related to the local resistance coefficient, r is the radius of the fracturing tubing, λ is the friction resistance coefficient of the tubing, L is the total length of the pipeline from the fracturing pump to the fracturing point, ρ is the density of the fracturing fluid, and h is the vertical height from the fracturing point, i.e., the tool string position, to the fracturing pump; Adjusting and reducing the opening of the relief valve increases the local resistance of the relief pipeline, causing the pump pressure to rise, setting the packer and opening the choke. At this time, if the tool string meets any of the following conditions, it is in normal working condition: Condition 1: P ex <P pm <P op , while Q re =Q in At this time, the tool string packer is inflated and set, the choke is not opened, and the status is normal; Condition 2: P ex <P op <P pm , while Q re in , indicating that the throttle is open.​ 2. The method for determining the status of a hydraulic fracturing tool string in an underground coal mine according to claim 1, wherein: In the second step, the bottom hole pressure is calculated: Where, P bt is the bottom hole pressure, P pm is the fracturing pump pressure, Q in is the total flow, Q re is the overflow flow rate, r is the radius of the fracturing tubing, λ is the friction coefficient of the tubing, L is the total length of the pipeline from the fracturing pump to the fracturing point, ρ is the density of the fracturing fluid, and h is the vertical height from the fracturing point, i.e., the tool string position, to the fracturing pump; At this time, the bottom hole pressure P bt It should not be greater than 10MPa, otherwise check whether the oil pipe and fracturing pipeline are blocked.

3. The method for determining the status of a hydraulic fracturing tool string in an underground coal mine according to claim 1, wherein: The tool string state determination includes: a third step of continuing to adjust and reduce the opening of the relief valve. At this time, the opening of the relief valve is calculated according to the following formula: Deg vol ∈[X-10%,X] Where: Q in is the total flow, Q re is the overflow flow rate, R is the radius of the overflow valve, α is the parameter related to the local resistance coefficient, r is the radius of the fracturing tubing, λ is the friction resistance coefficient of the tubing, L is the total length of the pipeline from the fracturing pump to the fracturing point, ρ is the density of the fracturing fluid, and h is the vertical height from the fracturing point, i.e., the tool string position, to the fracturing pump; Observe the bottom hole pressure at this time: Where, P bt is the bottom hole pressure, P pm is the fracturing pump pressure, Q in is the total flow, Q re is the overflow flow rate, r is the radius of the fracturing tubing, λ is the friction coefficient of the tubing, L is the total length of the pipeline from the fracturing pump to the fracturing point, ρ is the density of the fracturing fluid, and h is the vertical height from the fracturing point, i.e., the tool string position, to the fracturing pump; As the opening of the relief valve decreases, the pump pressure increases, and the bottom hole pressure increases accordingly. After the coal layer is fractured and fracturing is achieved, the bottom hole pressure and pump pressure both decrease. At this time, the bottom hole pressure P is recorded. bt With pump pressure P pm peak value.

4. The method for determining the status of a hydraulic fracturing tool string in an underground coal mine according to claim 3, wherein: In the third step, during the normal fracturing process, the bottom hole pressure P bt Maintain stability. As the coal layer continues to fracture, a short period of sudden drop-sudden rise process occurs, and the pressure fluctuation time will not exceed 2 minutes; Therefore, if the following conditions occur during normal fracturing, the tool string is considered damaged and fracturing should be stopped and the tool string replaced: bottom hole pressure P bt A decrease of more than 50% and lasting for more than 2 minutes.

5. The method for determining the status of a hydraulic fracturing tool string in an underground coal mine according to claim 3, wherein: The tool string status determination includes: Step 4, after the coal rock layer is fractured, observe for 10-20 minutes at the current relief valve opening. If the fracturing process is normal, continue to reduce the relief valve opening and control the displacement according to engineering needs until the fracturing is completed.

6. The method for determining the status of a hydraulic fracturing tool string in an underground coal mine according to claim 5, wherein: During the fourth step, if the bottom hole pressure P bt If the drop exceeds 50% and lasts for more than 2 minutes, the tool string is considered damaged and fracturing should be stopped and the tool string should be replaced.

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