Method for judging state of hydraulic fracturing tool string in underground coal mine

Abstract

The invention discloses a coal mine underground hydraulic fracturing tool string state distinguishing method which comprises the following steps: firstly, arranging auxiliary equipment, and then distinguishing the state of a tool string; the auxiliary equipment comprises an inlet flow sensor, a pressure sensor, a three-way pipe, a linear overflow valve and an overflow flow meter; the high-pressure water supply quantity of an oil pipe is adjusted through a linear overflow valve in the auxiliary equipment, so that the system pressure is adjusted, parameters such as the fracturing pump pressure, the total flow and the overflow flow are sensed in real time through a pressure sensor, an inlet flow sensor and an overflow flow meter, and the parameters of each hole bottom are observed and compared to achieve tool string state judgment. The state of the fracturing tool string in the drill hole is pointed out quantitatively and intelligently in real time, the underground hydraulic fracturing efficiency of a coal mine is improved, the construction cost is saved, and the engineering quality is guaranteed.

Description

Technical Field

[0001] The 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 state 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 various application scenarios in the field of coal mine safety. Hydraulic fracturing technology can be used in disaster management projects such as coal seam hydraulic fracturing gas pre-extraction, hard roof weakening, and coal seam roof water drainage.

[0003] The double packer drag fracturing technology is the most commonly used downhole segmented hydraulic fracturing technology. It uses two packers to block the borehole and form a closed space between the two packers. A throttle is set between the two packers. The function of the throttle 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 the packer and the 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 experience of the operator. For example, whether the packer expands and seals normally, whether the throttle is open, whether the tool string is intact, etc., can only be inferred based on observed phenomena (such as whether there is water backflow at the orifice, etc.). It is greatly affected by human factors and has low accuracy. In actual projects, 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 backflow at the orifice. After the tool string is taken out, it is found that the tool string is intact, resulting in loss of the drilling platform; (2) When the packer on the bottom side of the hole is damaged, there is no water backflow at the orifice, and it is mistakenly believed that the tool string is intact, and fracturing continues, 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 deficiencies in the prior art, the purpose of the present invention is to provide a method for determining the status of a hydraulic fracturing tool string in an underground coal mine, so as to solve the problem that the prior art cannot 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 to achieve the above problems:

[0008] A method for determining the status of a hydraulic fracturing tool string in an underground coal mine, the method firstly 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 flow meter; 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 pipeline of the tee is connected to the high-pressure water outlet of the fracturing pump, the second pipeline supplies high-pressure water to the oil pipe for fracturing, and the linear overflow valve and the overflow flow meter are arranged on the third pipeline; 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 local resistance coefficient and the opening of the linear relief valve are in a natural logarithmic relationship:

[0012]

[0013] In the formula, 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 of 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.

[0014] Specifically, the tool string status determination includes: the first step is to lower the tool string into a predetermined position in the hole, use a fracturing pump to inject water into the tool string, and open the overflow valve to a fully open state, that is, the overflow valve opening degree Deg vol =100%, the tool string meets the following conditions at the same time, indicating that its working status is normal:

[0015] Condition 1: Observe the overflow flow Q re , slowly rise to Q re =Q in , Q in is the total flow of the fracturing pump;

[0016] Condition 2: Observe the fracturing pump pressure P pm , Packer expansion pressure P ex , throttle valve operating pressure P op :

[0017] P pm <P ex <P op .

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

[0019] If P pm <P ex <P op , observe Q re After stabilization, it is always less than Qin , indicating that there is water leakage in the tool string, and the tool string should be withdrawn for inspection;

[0020] 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.

[0021] Specifically, the tool string state determination includes: the second step, adjusting and reducing the opening of the relief valve, at which time the opening of the relief valve is calculated according to the following formula:

[0022] Deg vol ∈[X-10%,X]

[0023] 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;

[0024] Due to the increase in local resistance of the overflow pipeline caused by adjusting and reducing the opening of the overflow valve, the pump pressure increases, causing the packer to be set and the throttle to open. At this time, the tool string meets any of the following conditions, indicating that its working state is normal:

[0025] 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 throttle is not opened, and the status is normal;

[0026] Condition 2: P ex <P op <P pm , while Q re in , indicating that the throttle is open.

[0027] Specifically, 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; ​

[0028] 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.

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

[0030] Deg vol ∈[X-10%,X]

[0031] 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;

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

[0033]

[0034] 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;

[0035] As the opening of the overflow valve decreases, the pump pressure increases, and the bottom hole pressure increases accordingly. After the coal rock 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.

[0036] Specifically, in the third step, during normal fracturing, the bottom hole pressure P bt It basically remains stable. As the coal-rock layer continues to fracture, a short-term sudden drop-sudden rise process occurs. The pressure fluctuation time will not exceed 2 minutes. Therefore, if the following situations occur during normal fracturing, the tool string is judged to be damaged, and fracturing should be stopped and the tool string should be replaced: Bottom hole pressure P bt 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 10min-20min at the current overflow valve opening. If the fracturing process is normal, continue to reduce the overflow valve opening and control the displacement according to engineering needs until the fracturing is completed.

[0038] Specifically, 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.

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

[0040] The present invention points out the status of the fracturing tool string in the borehole in real time, quantitatively and intelligently, which is helpful to improve the hydraulic fracturing efficiency 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 variation of bottom hole pressure over time under normal fracturing conditions in Example 1.

[0044] Figure 4 This is a graph showing the variation 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. Oil pipe; 21. Main inlet, 22. Inlet flow sensor, 23. Fracturing pump, 24. High-pressure water outlet, 25. Tee pipe, 26. Linear overflow valve, 27. Overflow flow meter. DETAILED DESCRIPTION

[0051] The present invention provides a method for distinguishing the state of a hydraulic fracturing tool string in an underground coal mine. The method comprises firstly arranging auxiliary equipment and then distinguishing the state of the tool string. The auxiliary equipment comprises an inlet flow sensor 22, a pressure sensor, a tee pipe 25, a linear overflow valve 26 and an overflow flow meter 27. The inlet flow sensor 22 is installed at a total inlet 21 of a fracturing pump 23, the pressure sensor is installed on the fracturing pump 23, a first pipeline of the tee pipe 25 is connected to a high-pressure water outlet 24 of the fracturing pump 23, a second pipeline supplies high-pressure water to an oil pipe 14 for fracturing, and a linear overflow valve 26 and an overflow flow meter 27 are arranged on the third pipeline. The state of the tool string 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 natural logarithmic relationship:

[0053]

[0054] In the formula, 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 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 parameters such as the fracturing pump pressure, total flow, and overflow flow 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 overflow valve to the full open state, that is, the overflow valve opening Deg vol =100%, the tool string meets the following conditions at the same time, indicating that its working status is normal:

[0057] Condition 1: Observe the overflow flow Q re , slowly rise to Q re =Q in , Q in is the total flow of the fracturing pump;

[0058] Condition 2: Observe the fracturing pump pressure P pm , Packer expansion pressure P ex , throttle valve operating pressure P op :

[0059] P pm <P ex <P op .

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

[0061] If P pm <Pex <P op , observe Q re After stabilization, it is always less than Q in , indicating that there is water leakage in the tool string, and the tool string should be withdrawn for inspection;

[0062] 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.

[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] Deg vol ∈[X-10%,X]

[0065] 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; the derivation process of this formula is described as follows:

[0066] Since the auxiliary equipment of the present invention is Figure 2 The linear overflow valve 26 on the middle right side is in parallel with the outlet 24 for supplying high-pressure water to the oil pipe. From the knowledge of fluid mechanics, it can be known that the total pressures of the two (total pressure = static pressure + dynamic pressure) are 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 pipeline to the oil pipe, that is, the pressure reflected by the pressure gauge. Therefore, based on the full understanding and recognition of the auxiliary equipment in the present invention and the hydraulic fracturing process, it can be deduced:

[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 overflow valve 26:

[0071]

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

[0073] Back calculation Deg vol :

[0074]

[0075] In Q re =Q in Before (i.e. Q re in When Figure 2 The high-pressure water pipeline is supplied to the oil pipe in the middle, and flows in the oil pipe 14 in the borehole 11. The resistance P caused by the flow in It is mainly composed of two parts: the resistance along the way and the gravitational potential, namely

[0076]

[0077] at this time,

[0078] P vol =P in

[0079] Bring in Deg vol Calculation formula:

[0080]

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

[0082] Deg vol ∈[X-10%,X];

[0083]

[0084] Since the overflow valve is closed due to the adjustment of the opening of the overflow valve, the local resistance of the overflow pipeline increases, causing the pump pressure to rise, so that the packer 12 is set and the throttle 13 is opened; at this time, the tool string meets any of the following conditions, indicating that its working state is normal:

[0085] Condition 1: P ex <P pm <P op , while Q re =Q in At this time, the tool string packer 12 is inflated and set, and the throttle 13 is not opened, and the state is normal;

[0086] Condition 2: P ex <P​op <R pm , while Q re in , indicating that the throttle 13 is open.

[0087] 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;

[0088] At this time, the bottom hole pressure P bt It should not be greater than 10MPa, otherwise it should be checked whether the oil pipe 14 and the fracturing pipeline are blocked.

[0089] The third step is to continue 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:

[0090] Deg vol ∈[X-10%,X]

[0091] 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;

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

[0093]

[0094] Where P bt is the bottom hole pressure, P pm is the fracturing pump pressure, Q ib 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;

[0095] As the opening of the overflow valve decreases, the pump pressure increases, and the bottom hole pressure increases accordingly. After the coal rock 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.​

[0096] In the third step, during normal fracturing, the bottom hole pressure P bt It basically remains stable. As the coal-rock layer continues to fracture, there is a short-term 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 fracturing should be stopped and the tool string should be replaced: Bottom hole pressure P bt The decrease is more than 50% and lasts for more than 2 minutes.

[0097] Step 4: After the coal-rock 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 Q according to the project needs. in -Q re , until the fracturing is completed.

[0098] In 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.

[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] Embodiment 1:

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

[0105] Table 2 Related parameters of Example 1

[0106]

[0107] This embodiment includes the following steps:

[0108] first step:

[0109] The tool string is lowered into the predetermined position in the hole. The vertical height of the position from the fracturing pump is 30m, 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 (Deg vol=100%), at which point the total flow rate of the fracturing pump is observed to be Q in =36m 3 / h, observe the overflow flow rate from Q re =10m 3 / h slowly rise to Q re =36m 3 / h; at the same time, observe the fracturing pump pressure slowly rise to P pm =1.5MPa and then it stops rising, indicating that the tool string is in normal condition.

[0110] Example 1 of other cases: If the overflow flow rate cannot rise to Q for a long time re =36m 3 / h, and the pump pressure is stable at P pm =1.5MPa, 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, which needs to be checked.

[0111] Other cases example 2: When the relief valve is fully open, the pump pressure suddenly rises to P pm =3MPa or above, indicating that the overflow valve is not properly selected or the pipeline is blocked, and 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] Deg vol ∈[X-10%,X]

[0115] Where:

[0116]

[0117] Bring in R = 36mm, r = 36mm, ρ = 1000kg / m 3 , Q in =36m 3 / h, L = 500m, h = 30m, g = 9.8m / s 2 , α=120, λ=0.01Pa / s; design Q re =18n 3 / h, put it into the formula, we can get X = 85%, that is, the control valve opening

[0118] Deg vol ∈[75%,85%]

[0119] The overflow valve is closed, which increases the local resistance of the overflow pipeline, causing the pump pressure to rise, which can cause the packer to be set and the throttle to open. At this time, the tool string meets the following conditions:

[0120] P pm >3.0MPa, Q re =18m 3 / h in , indicating that the throttle is open and working normally. At this time, calculate the bottom hole pressure:

[0121]

[0122] At this time, the pump pressure is observed to be 5MPa, and the bottom hole pressure P is calculated. bt =4.7MPa, indicating that the fracturing tool string operates normally.

[0123] Other examples: At this time, the pump pressure is observed to be 15MPa, and the bottom hole pressure P is calculated. bt =14.7MPa, greater than 10MPa, 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] Deg vol ∈[X-10%,X]

[0127] Where:

[0128]

[0129] Bring in R = 36mm, r = 36mm, ρ = 1000kg / m 3 , Q in =36m 3 / h, L = 500m, h = 30m, g = 9.8m / s 2 , α=120, λ=0.01Pa / s; design Q re =9m 3 / h, put it into the formula, we can get X = 39%, that is, the control valve opening

[0130] Deg vol ∈[29%,39%]

[0131] As the relief valve pressure decreases, the pump pressure increases, and the bottom hole pressure increases accordingly. After the coal rock layer is fractured and fracturing is achieved, the bottom hole pressure and pump pressure both decrease. Assuming that the peak pump pressure at this time is 25MPa, the bottom hole pressure P is calculated. bt Peak:

[0132]

[0133] After reaching the peak and dropping slightly, the bottom hole pressure P bt ​It basically remains stable. As the coal-rock layer continues to break, there may be a short-term 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 P bt 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-rock 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 (Q in -Q re ), generally controlled at Q re =Q in ×80%, such as Q in =30m 3 / h, control Q re =24m 3 / h until the fracturing is completed. During this process, 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.

[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 improves the success rate of the fracturing construction.

[0138] like Figure 5 is a 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, which proves that the packer is intact. Figure 8 , verifying the effectiveness and practicability 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, a variety of simple modifications can be made to the technical solution of the present invention, and these simple modifications all belong to the protection scope 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 suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0142] In addition, 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 a coal mine, characterized in that: This method first deploys auxiliary equipment and then determines the status of the tool string; The auxiliary equipment includes an inlet flow sensor, a pressure sensor, a tee, a linear overflow valve and an overflow flow meter; 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 pipeline of the tee is connected to the high-pressure water outlet of the fracturing pump, the second pipeline supplies high-pressure water to the oil pipe for fracturing, and the linear overflow valve and the overflow flow meter are arranged on the third pipeline; the tool string status is determined by adjusting the opening of the overflow valve and observing and comparing the bottom hole parameters.

2. The method for determining the state of a hydraulic fracturing tool string in an underground coal mine according to claim 1, characterized in that: The linear relief valve has a natural logarithmic relationship between its local resistance coefficient and opening: In the formula, 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 of 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.

3. The method for determining the state of a hydraulic fracturing tool string in an underground coal mine according to claim 1, characterized in that: The tool string status determination includes: the first step is to lower the tool string into a predetermined position in the hole, use a fracturing pump to inject water into the tool string, and open the overflow valve to a fully open state, that is, the overflow valve opening degree Deg vol =100%, the tool string meets the following conditions at the same time, indicating that its working status is normal: Condition 1: Observe the overflow flow Q re , slowly rise 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 。 4. The method for determining the state of a hydraulic fracturing tool string in an underground coal mine according to claim 3, characterized in that: If the conditions in the first step are not met, the following questions are determined: If P pm <P ex <P op , observe Q re After stabilization, it is always less than Q in , indicating that there is water leakage in the tool string, and the tool string should be withdrawn 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.

5. The method for determining the state of a hydraulic fracturing tool string in an underground coal mine according to claim 3, characterized in that: The tool string state determination includes: the second step, adjusting and reducing the opening of the relief valve, at which 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; Due to the increase in local resistance of the overflow pipeline caused by adjusting and reducing the opening of the overflow valve, the pump pressure increases, causing the packer to be set and the throttle to open. At this time, the tool string meets any of the following conditions, indicating that its working state is normal: 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 throttle 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.​ 6. The method for determining the state of a hydraulic fracturing tool string in an underground coal mine according to claim 5, characterized in that: 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.

7. The method for determining the state of a hydraulic fracturing tool string in an underground coal mine according to claim 5, characterized in that: The tool string state determination includes: the third step, continuing to adjust and reduce the opening of the relief valve, at which 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 overflow valve decreases, the pump pressure increases, and the bottom hole pressure increases accordingly. After the coal rock 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.

8. The method for determining the state of a hydraulic fracturing tool string in an underground coal mine according to claim 7, characterized in that: In the third step, during normal fracturing, the bottom hole pressure P bt It basically remains stable. As the coal and rock layers continue to break, there will be a short-term sudden drop-sudden rise process, 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, fracturing should be stopped, and the tool string should be replaced: Bottom hole pressure P bt The decrease is more than 50% and lasts for more than 2 minutes.

9. The method for determining the state of a hydraulic fracturing tool string in an underground coal mine according to claim 7, characterized in that: The tool string status determination includes: the fourth step, after the coal rock layer is broken, observe for 10min-20min at the current overflow valve opening. If the fracturing process is normal, continue to reduce the overflow valve opening and control the displacement according to engineering needs until the fracturing is completed.

10. The method for determining the state of a hydraulic fracturing tool string in an underground coal mine according to claim 9, characterized in that: 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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