Coal seam anti-reflection method based on liquid CO2 phase change induced hydraulic fracturing crack

Abstract

The invention provides a coal seam anti-reflection method based on liquid CO2 phase change induced hydraulic fracturing cracks. The method comprises the following steps that S1, construction equipment is prepared; s2, fracturing sections of the hydrofracture measure drill holes are divided, and the reasonable distance between the fracturing sections in the measure drill holes is determined when the guide drill holes are located on the left sides and the right sides of the measure drill holes; s3, determining the extension length range of the hydraulic fracturing fracture on the non-liquid CO2 phase change fracturing side of the measure drill hole; s4, drill hole arrangement; and S5, coal seam permeability increasing. According to the method, liquid CO2 phase change fracturing is carried out before hydraulic fracturing is carried out on the coal body, the expansion direction of hydraulic fracturing cracks can be induced, and the controllability of the hydraulic fracturing cracks is improved. According to the method, the reasonable distance between the fracturing sections in the measure drill hole is determined, and it can be guaranteed that the phenomenon of'seam crossing 'does not occur to the fracturing sections of the same measure drill hole in the fracturing process. And meanwhile, the expansion condition of the hydraulic fracturing crack on the non-liquid CO2 phase change fracturing side of the measure drilling hole is considered, and the permeability increasing range of the coal seam is guaranteed to the maximum degree.

Description

Technical Field

[0001] The invention belongs to the technical field of coal seam permeability enhancement, and specifically relates to a coal seam permeability enhancement method based on liquid CO2 phase change-induced hydraulic fracturing fractures. Background Art

[0002] Gas disasters have always been one of the main disasters that plague safe coal mining, especially for coal seams with coal and gas outburst hazards. Gas extraction, as one of the fundamental measures to control coal seam gas problems, has been widely used in the coal mining industry, especially the technical measure of pre-extraction of coal seam gas. As one of the main technical means to eliminate coal seam outbursts, it is the only technical measure for coal seam outburst elimination when the protective layer cannot be mined; and for non-outburst coal seams, pre-extraction of coal seam gas technology can also effectively reduce the amount of gas released during coal mining, thereby effectively preventing the occurrence of gas disasters. However, the different occurrence conditions of coal seams in different regions result in low permeability of some coal seams, which in turn causes poor or even ineffective coal seam gas pre-extraction effects. Therefore, in order to achieve better coal seam pre-extraction effects, it is necessary to increase the permeability of coal seams.

[0003] In recent years, in order to increase the permeability of coal seams and thus improve the pre-pumping effect of coal seams, hydraulic fracturing technology, hydraulic slitting technology, hydraulic punching technology, liquid CO2 phase change fracturing technology, deep hole blasting and other technologies have been proposed and applied. Among these coal seam permeability enhancement technologies, hydraulic fracturing technology is widely used because of its wide range of coal seam permeability enhancement. Coal seam hydraulic fracturing permeability enhancement technology can be divided into two fracturing methods: full-stage hydraulic fracturing and staged hydraulic fracturing. However, no matter which hydraulic fracturing method is used, it is impossible to effectively control the hydraulic fracturing cracks, which leads to uncontrollable hydraulic fracturing cracks inside the coal seam, and it is impossible to determine the effective permeability enhancement range inside the coal seam. Summary of the invention

[0004] Based on the deficiencies in the prior art, the purpose of the present invention is to improve the uncontrollable shortcomings of hydraulic fracturing cracks in the existing coal seam hydraulic fracturing technology, thereby improving the controllability of hydraulic fracturing cracks in coal seams, and proposing a coal seam permeability enhancement method based on liquid CO2 phase change-induced hydraulic fracturing cracks.

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

[0006] A method for increasing the permeability of coal seams based on liquid CO2 phase change-induced hydraulic fracturing fractures comprises the following steps:

[0007] S1. Preparation of construction equipment;

[0008] S2. Division of the fracturing sections of the hydraulic fracturing measure borehole, determining the reasonable spacing of the fracturing sections in the measure borehole when the pilot boreholes are located on the left and right sides of the measure borehole respectively;

[0009] S3, determining the extension length range of the hydraulic fracturing crack on the non-liquid CO2 phase change fracturing side of the measure borehole when the pilot borehole is located on the left and right sides of the measure borehole respectively;

[0010] S4, drilling arrangement: construct multiple groups of measure boreholes and guide boreholes in a row in the coal seam permeability enhancement area, in each group, the guide borehole is located on the left or right side of the measure borehole, and is arranged according to the side with a larger extension length range of the hydraulic fracturing crack in step S3, and the spacing between the non-liquid CO2 phase change fracturing side of the measure borehole and the guide borehole of the adjacent group is the maximum value of the extension length range of the hydraulic fracturing crack on the non-liquid CO2 phase change fracturing side of the measure borehole at that side;

[0011] S5. Coal seam permeability enhancement: According to the positions of the measure boreholes and the guide boreholes determined in step S4, each measure borehole and the guide borehole is subjected to liquid CO2 phase change fracturing with the fracturing section spacing on the corresponding side determined in step S2, and hydraulic fracturing is performed after each liquid CO2 phase change fracturing in the measure borehole.

[0012] Furthermore, the step S2 and step S3 specifically include the following steps:

[0013] S2. The fracturing section division of the hydraulic fracturing measure borehole is to determine the reasonable spacing of the fracturing sections in the measure borehole when the pilot borehole is located on the left and right sides of the measure borehole, specifically including the following steps:

[0014] S201. Construction of the first set of boreholes in the coal seam:

[0015] The first group of boreholes includes a measure borehole and a guide borehole. First, the spacing between the measure borehole and the guide borehole in the first group of boreholes is determined according to the actual situation and needs of the mine. Then, the first group of boreholes is constructed in the coal seam by using a drilling rig, and the guide borehole is located on the right side of the measure borehole.

[0016] S202, the first liquid CO2 phase change fracturing and hydraulic fracturing of the first group of boreholes:

[0017] The first liquid CO2 phase change fracturing is carried out at the bottom of the measure boreholes of the first group of boreholes using liquid CO2 phase change fracturing equipment, and the first liquid CO2 phase change fracturing is also carried out at the corresponding position in the pilot borehole. After the fracturing is completed, the liquid CO2 phase change fracturing equipment is recovered, and a sealing capsule and a hollow connecting pipe are placed in the pilot borehole. Then, hydraulic fracturing is carried out at the bottom of the measure borehole until water flows in the pilot borehole or the hollow connecting pipe.

[0018] S203, the second liquid CO2 phase change fracturing and hydraulic fracturing of the first group of boreholes:

[0019] Recover the sealing capsule and the hollow connecting tube, and perform a second liquid CO2 phase change fracturing in the measure borehole of the first group of boreholes. The distance between the second liquid CO2 phase change fracturing position and the first liquid CO2 phase change fracturing position, that is, the fracturing section distance, is L; at the same time, perform a second liquid CO2 phase change fracturing at the corresponding position in the pilot borehole. After the fracturing is completed, place a sealing capsule and a hollow connecting tube in the pilot borehole. The sealing capsule is located between the two adjacent liquid CO2 phase change fracturing positions. The sealing capsule is passed through the top of the hollow connecting tube. Then, hydraulic fracturing is performed at the second liquid CO2 phase change fracturing position in the measure borehole until water flows in the pilot borehole;

[0020] S204, judging whether a "seam-through" phenomenon occurs according to the water flow position in the pilot borehole;

[0021] (1) If the "seam-through" phenomenon does not occur in the pilot borehole, the sealing capsule and the hollow connecting tube are recovered, the interval between the fracturing sections in the measure borehole is shortened to (L-5) m, and the third liquid CO2 phase change fracturing is performed in the measure borehole. At the same time, the third liquid CO2 phase change fracturing is also performed at the corresponding position in the pilot borehole. The sealing capsule and the hollow connecting tube are placed in the pilot borehole. The sealing capsule is located between the two adjacent liquid CO2 phase change fracturing positions. The sealing capsule is passed through the hollow connecting tube. After that, hydraulic fracturing is performed at the third liquid CO2 phase change fracturing position in the measure borehole. Whether the "seam-through" phenomenon occurs is judged again based on the water flow position in the pilot borehole:

[0022] If the "seamless" phenomenon occurs, the construction is stopped. When it is determined that the pilot borehole is located on the right side of the measure borehole, the reasonable spacing of the fracturing sections in the measure borehole is L;

[0023] If the "seam-to-seam" phenomenon does not occur, the cycle is repeated, and the distance between the fracturing sections is shortened by 5m successively until the "seam-to-seam" phenomenon occurs in the pilot borehole at the nth liquid CO2 phase change fracturing position. In order to ensure that the "seam-to-seam" phenomenon does not occur, the distance between the n-1th liquid CO2 phase change fracturing position and the n-2nd liquid CO2 phase change fracturing position is used as the reasonable distance between the fracturing sections in the measure borehole;

[0024] (2) If a "seam" phenomenon occurs in the pilot borehole, a second group of boreholes is reconstructed at least 50 m to the right of the pilot borehole. The conditions of the measure boreholes and the pilot boreholes in the second group of boreholes are the same as those of the first group of boreholes. The second group of boreholes is subjected to the first liquid CO2 phase change fracturing and hydraulic fracturing according to step S202 until water flows in the pilot boreholes.

[0025] After completion, the second group of boreholes are subjected to a second liquid CO2 phase change fracturing and hydraulic fracturing according to step S203, wherein the spacing between the fracturing sections is increased to (L+10) m;

[0026] Again, determine whether the "seam" phenomenon occurs based on the water flow position in the pilot borehole:

[0027] If no "seam" phenomenon occurs in the pilot borehole, the construction is stopped. It is determined that the reasonable spacing of the fracturing sections when the pilot borehole is located on the right side of the measure borehole is (L+10) m.

[0028] If the "seam-through" phenomenon occurs, the cycle is repeated. In each group of re-constructed boreholes, the spacing of the fracturing sections in the measure boreholes increases by 10m compared with the previous group, until the nth group of re-constructed boreholes does not have the "seam-through" phenomenon. The spacing of the fracturing sections in the current nth group is the reasonable spacing of the fracturing sections when the pilot borehole is located on the right side of the measure borehole.

[0029] S205, using the same method to determine the reasonable spacing of the fracturing sections in the measure borehole when the pilot borehole is located on the left side of the measure borehole;

[0030] S3, determining the extension length range of the hydraulic fracturing crack on the non-liquid CO2 phase change fracturing side of the measure borehole when the pilot borehole is located on the left and right sides of the measure borehole respectively, specifically comprising the following steps:

[0031] S301, reconstructing a measure borehole and a guide borehole in the coal seam, the guide borehole is located on the right side of the measure borehole, and at the same time constructing an observation borehole on the left side of the measure borehole to observe the hydraulic fracturing crack expansion on the non-liquid CO2 phase change fracturing side of the measure borehole; wherein the spacing between the observation borehole and the measure borehole is half the spacing between the measure borehole and the guide borehole;

[0032] Multiple liquid CO2 phase change fracturing operations are carried out in sequence from the hole mouth to the bottom of the hole in the measure borehole, and the observation drilling depth is 10m-20m beyond the first liquid CO2 phase change fracturing position in the measure borehole;

[0033] S302, performing the first liquid CO2 phase change fracturing at the first liquid CO2 phase change fracturing position in the measure borehole, and at the same time performing the first liquid CO2 phase change fracturing at the corresponding position in the pilot borehole, after the fracturing is completed, recovering the liquid CO2 phase change fracturing equipment, and then performing hydraulic fracturing at the first liquid CO2 phase change fracturing position in the measure borehole;

[0034] S303. Observe which borehole, the observation borehole or the pilot borehole, has water flow first:

[0035] (1) If water flow occurs first in the observation borehole, it means that during the hydraulic fracturing process, the hydraulic fracturing crack on the right side of the measure borehole has not yet expanded to the pilot borehole, while the fracturing crack on the left side has expanded to the observation borehole position. Therefore, a second observation borehole is reconstructed 5m to the left of the observation borehole. The depth of the second observation borehole exceeds the second liquid CO2 phase change fracturing position in the measure borehole by 10m-20m, and the other conditions are the same as the first observation borehole;

[0036] A second liquid CO2 phase change fracturing is performed in the measure borehole, and the distance between the second liquid CO2 phase change fracturing position and the first liquid CO2 phase change fracturing position is a reasonable distance of the fracturing section when the pilot borehole determined in step S2 is located on the right side of the measure borehole; at the same time, a second liquid CO2 phase change fracturing is also performed at the corresponding position in the pilot borehole. After the fracturing is completed, the liquid CO2 phase change fracturing equipment is recovered, and then hydraulic fracturing is performed at the second liquid CO2 phase change fracturing position in the measure borehole, and it is observed which borehole, the second observation borehole or the pilot borehole, has water flow first;

[0037] If the second observation borehole still first experiences water flow, then step (1) is repeated, and the cycle is repeated until the pilot borehole first experiences water flow when the nth observation borehole is constructed, and it is determined that when the pilot borehole is located on the right side of the measure borehole, the hydraulic fracturing crack extension length range of the non-liquid CO2 phase change fracturing side of the measure borehole is from the distance between the n-1th observation borehole and the measure borehole to the distance between the nth observation borehole and the measure borehole;

[0038] (2) If water flow occurs first in the pilot borehole, a second observation borehole is constructed 5 m to the right of the first observation borehole. The depth of the second observation borehole is 10 m to 20 m beyond the second liquid CO2 phase change fracture position in the measure borehole. The other conditions are the same as those of the first observation borehole.

[0039] A second liquid CO2 phase change fracturing is performed in the measure borehole, and the spacing between the second liquid CO2 phase change fracturing position and the first liquid CO2 phase change fracturing position is a reasonable spacing of the fracturing sections in the measure borehole when the pilot borehole determined in step S2 is located on the right side of the measure borehole; at the same time, a second liquid CO2 phase change fracturing is also performed at the corresponding position in the pilot borehole. After the fracturing is completed, the liquid CO2 phase change fracturing equipment is recovered, and then hydraulic fracturing is performed at the second liquid CO2 phase change fracturing position in the measure borehole, and it is observed which borehole, the second observation borehole or the pilot borehole, has water flow first;

[0040] If the pilot borehole still has water flow first, the cycle continues until the nth observation borehole has water flow before the pilot borehole, and the hydraulic fracturing crack extension length range of the non-liquid CO2 phase change fracturing side of the measure borehole is determined when the pilot borehole is located on the right side of the measure borehole, which is the distance between the nth observation borehole and the measure borehole to the distance between the n-1th observation borehole and the measure borehole;

[0041] S304. Using the same method, determine the extension length range of the hydraulic fracturing crack on the non-liquid CO2 phase change fracturing side of the measure borehole when the pilot borehole is located on the left side of the measure borehole.

[0042] Furthermore, in step S2, in the first group of boreholes and the second group of boreholes, the spacing between the measure boreholes and the guide boreholes is 30m-50m; the depth of the guide borehole exceeds the depth of the measure borehole by 10m-15m, and at the same time, the depth of the measure borehole is less than the vertical distance between the coal mining face air intake lane and the coal mining face return air lane, which is 30m-50m; the apertures of the measure boreholes and the guide boreholes are determined according to the extraction requirements and the requirements of the coal seam permeability enhancement equipment.

[0043] Furthermore, in the step S2, a preliminary judgment is made as to whether a "seam-to-seam" phenomenon occurs based on the water flow position in the guide borehole. The judgment method is as follows: when the hydraulic fracturing crack and the communication position between the guide borehole are located above the sealing capsule, water will flow out of the hollow connecting tube, and a "seam-to-seam" phenomenon occurs; otherwise, water will flow out of the guide borehole, and a "seam-to-seam" phenomenon does not occur.

[0044] Furthermore, in step S202, the sealing capsule is located 2m-5m below the first liquid CO2 phase change fracturing position of the pilot borehole, and the sealing capsule is passed through the top of the hollow connecting pipe.

[0045] Furthermore, in step S203, the distance L between the fracturing sections is 20 m.

[0046] Furthermore, in step S301, the spacing between the measure drilling hole and the guide drilling hole is 30m-50m.

[0047] Furthermore, in the step S301, the first liquid CO2 phase change fracturing position in the borehole is 30m-50m away from the borehole mouth.

[0048] Furthermore, the specific steps of step S5 are as follows:

[0049] The first liquid CO2 phase change fracturing and hydraulic fracturing: The first liquid CO2 phase change fracturing is carried out at the bottom of the measure borehole using liquid CO2 phase change fracturing equipment, and at the same time, the first liquid CO2 phase change fracturing is also carried out at the corresponding position in the pilot borehole to induce the end of the hydraulic fracturing crack. After the fracturing is completed, the liquid CO2 phase change fracturing equipment is recovered, and then hydraulic fracturing is carried out at the bottom of the measure borehole until water flows in the pilot borehole;

[0050] Second liquid CO2 phase change fracturing and hydraulic fracturing: a second liquid CO2 phase change fracturing is performed in the measure borehole, and the distance between the second liquid CO2 phase change fracturing position and the first liquid CO2 phase change fracturing position is the distance between the fracturing sections in the measure borehole determined in step S2. At the same time, a second liquid CO2 phase change fracturing is also performed at the corresponding position in the pilot borehole 5. After the fracturing is completed, the liquid CO2 phase change fracturing equipment is recovered, and then hydraulic fracturing is performed at the second liquid CO2 phase change fracturing position in the measure borehole until water flows in the pilot borehole;

[0051] According to the above steps, liquid CO2 phase change fracturing and hydraulic fracturing are carried out in sequence from the bottom of the measure borehole and the pilot borehole to the hole mouth.

[0052] Furthermore, the fracturing direction of the phase change fracturing of the liquid CO2 in the measure borehole is perpendicular to the axial direction of the measure borehole and toward the guide borehole, and the fracturing direction of the phase change fracturing of the liquid CO2 in the guide borehole is perpendicular to the axial direction of the guide borehole and toward the measure borehole.

[0053] Compared with the prior art, the present invention has the following beneficial effects:

[0054] (1) The coal seam permeability enhancement method of the present invention first performs liquid CO2 phase change fracturing on the coal body before hydraulic fracturing the coal body, which can induce the expansion direction of the hydraulic fracturing cracks.

[0055] (2) Before hydraulic fracturing is performed in the measure borehole, the present invention first performs liquid CO2 phase change fracturing toward the guide borehole, so as to determine the cracking position of the cracks formed in the coal body on the side of the liquid CO2 phase change fracturing in the measure borehole caused by this hydraulic fracturing. At the same time, liquid CO2 phase change fracturing is performed at the same depth in the guide borehole toward the measure borehole, so as to determine the cracking position of the coal body cracks formed by this hydraulic fracturing reaching the guide borehole, that is, the starting position and the ending position of the main hydraulic fracturing cracks are clarified, thereby improving the controllability of the hydraulic fracturing cracks.

[0056] (3) The present invention divides the fracturing sections of the hydraulic fracturing drill hole into reasonable spacings of the fracturing sections in the drill hole, and can ensure that the fracturing sections in the same drill hole will not have a "seamless crack" phenomenon during the fracturing process. At the same time, the expansion of the hydraulic fracturing cracks on the non-liquid CO2 phase change fracturing side of the drill hole is taken into consideration, and the drill holes and guide holes in the coal seam permeability enhancement area are reasonably arranged to reduce the number of holes and maximize the range of coal seam permeability enhancement. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 It is a schematic diagram of the arrangement of the measure drilling and guide drilling in the second embodiment;

[0058] Figure 2 Schematic diagram of the first liquid CO2 phase change fracturing at the bottom of the measure borehole and the guide borehole in Example 2;

[0059] Figure 3 A schematic diagram of hydraulic fracturing in a borehole after the first liquid CO2 phase change fracturing in Example 2;

[0060] Figure 4 A schematic diagram of the "seamless fracture" phenomenon after the second liquid CO2 phase change fracturing and hydraulic fracturing are carried out in the borehole with a fracturing section spacing of L in the second embodiment;

[0061] Figure 5 It is a schematic diagram of the phenomenon of no "seamless fracture" after the second liquid CO2 phase change fracturing and hydraulic fracturing with a fracturing section spacing of L in the borehole in the second embodiment;

[0062] Figure 6 It is a schematic diagram of the second group of boreholes constructed after the second liquid CO2 phase change fracturing and hydraulic fracturing with a fracturing section spacing of L in the measure borehole in Example 2, and the first liquid CO2 phase change fracturing, hydraulic fracturing and the second liquid CO2 phase change fracturing are performed when the fracturing interval in the measure borehole is increased to (L+10)m;

[0063] Figure 7 This is a schematic diagram of the arrangement of the measure drilling, the guide drilling and the observation and control in determining the hydraulic fracturing crack extension range of the non-liquid CO2 phase change fracturing side of the measure drilling in Example 2;

[0064] Figure 8 This is a schematic diagram of determining the extension length range of the hydraulic fracturing crack on the non-liquid CO2 phase change fracturing side of the measure borehole in Example 2, in which the pilot borehole is located on the right side of the measure borehole, and when the first observation borehole is constructed on the left side of the measure borehole, water flows first in the pilot borehole;

[0065] Fig. 9This is a schematic diagram of determining the extension length range of the hydraulic fracturing crack on the non-liquid CO2 phase change fracturing side of the measure borehole in Example 2, in which the pilot borehole is located on the right side of the measure borehole, and when the second observation borehole is constructed on the left side of the measure borehole, water flow first appears in the second observation borehole;

[0066] Fig.10 It is a schematic diagram of the arrangement of the measure drilling and the guide drilling in the drilling arrangement of the second embodiment.

[0067] Figure numerals: 1. Coal seam; 2. Return air lane of coal mining working face; 3. Air inlet lane of coal mining working face; 4. Measure borehole; 5. Guide borehole; 6. Hollow connecting pipe; 7. Sealing capsule; 8. Observation borehole; 9. Hydraulic fracturing crack. DETAILED DESCRIPTION

[0068] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0069] Embodiment 1

[0070] A coal seam permeability enhancement process based on liquid CO2 phase change-induced hydraulic fracturing fractures comprises the following steps:

[0071] S1. Preparation of construction equipment;

[0072] Drilling construction equipment, liquid CO2 phase change fracturing equipment and hydraulic fracturing equipment are arranged in the air intake tunnel 3 of the coal mining face, and corresponding warning signs are set; among them, drilling construction equipment, liquid CO2 phase change fracturing equipment and hydraulic fracturing equipment are all existing technologies and will not be described in detail here;

[0073] S2. Division of the fracturing sections of the hydraulic fracturing measure borehole 4, determining the reasonable spacing of the fracturing sections in the measure borehole 4 when the guide borehole 5 is located on the left and right sides of the measure borehole 4, wherein the measure borehole 4 is used to carry out liquid CO2 phase change fracturing and hydraulic fracturing measures, to carry out joint permeability enhancement of the coal body and to guide the hydraulic fracturing cracks 9 to a certain extent; the guide borehole 5 is used to carry out liquid CO2 phase change fracturing measures, to guide the hydraulic fracturing cracks 9 formed after the hydraulic fracturing of the measure borehole 4; divide the measure borehole 4 into as many fracturing sections as possible, so as to maximize the permeability enhancement range of the coal seam, and at the same time ensure that the hydraulic fracturing cracks 9 in each fracturing section will not have a "seamless" phenomenon, specifically including the following steps:

[0074] S201, constructing the first set of boreholes in coal seam 1 using drilling construction equipment:

[0075] The first group of boreholes includes a measure borehole 4 and a guide borehole 5. First, the spacing between the measure borehole 4 and the guide borehole 5 in the first group of boreholes is determined according to the actual situation and needs of the mine; then, a measure borehole 4 and a guide borehole 5 are constructed in the coal seam 1 by using a drilling rig, and the guide borehole 5 is located on the right side of the measure borehole 4;

[0076] In the first group of boreholes, the spacing between the measure borehole 4 and the guide borehole 5 is 30m-50m; the depth of the guide borehole 5 exceeds the depth of the measure borehole 4 by 10m-15m, and the depth of the measure borehole 4 is less than the vertical distance between the coal mining face air intake tunnel 3 and the coal mining face return air tunnel 2 by 30m-50m; the apertures of the measure borehole 4 and the guide borehole 5 are determined according to the extraction requirements and the requirements of the coal seam permeability enhancement equipment;

[0077] S202, the first liquid CO2 phase change fracturing and hydraulic fracturing of the first group of boreholes:

[0078] At the bottom of the measure borehole 4 of the first group of boreholes, liquid CO2 phase change fracturing equipment is used to perform the first liquid CO2 phase change fracturing, and the fracturing direction is perpendicular to the axial direction of the measure borehole 4 and toward the guide borehole 5. At the same time, the first liquid CO2 phase change fracturing is also performed at the corresponding position in the guide borehole 5 (corresponding to the first liquid CO2 phase change fracturing position of the measure borehole 4), and the fracturing direction is perpendicular to the axial direction of the guide borehole 5 and toward the measure borehole 4. After the fracturing is completed, the liquid CO2 phase change fracturing equipment is recovered, and a sealing capsule 7 and a hollow connecting pipe 6 are placed in the guide borehole 5. The sealing capsule 7 is located 2m-5m below the first liquid CO2 phase change fracturing position of the guide borehole 5, and the sealing capsule 7 is passed through the top of the hollow connecting pipe 6. Then, hydraulic fracturing is performed at the bottom of the measure borehole 4 until water flows in the guide borehole 5 or the hollow connecting pipe 6.

[0079] S203, the second liquid CO2 phase change fracturing and hydraulic fracturing of the first group of boreholes:

[0080] Recover the sealing capsule 7 and the hollow connecting tube 6, and perform a second liquid CO2 phase change fracturing in the measure borehole 4 of the first group of boreholes. The distance between the second liquid CO2 phase change fracturing position and the first liquid CO2 phase change fracturing position, that is, the fracturing section distance is L (L is 20m); at the same time, a second liquid CO2 phase change fracturing is also performed at the corresponding position in the guide borehole 5 (corresponding to the second liquid CO2 phase change fracturing position of the measure borehole 4). After the fracturing is completed, the sealing capsule 7 and the hollow connecting tube 6 are placed in the guide borehole 5. The sealing capsule 7 is located between the two adjacent liquid CO2 phase change fracturing positions. The sealing capsule 7 is passed through the top of the hollow connecting tube 6. Then, hydraulic fracturing is performed at the second liquid CO2 phase change fracturing position in the measure borehole 4 until water flows in the guide borehole 5;

[0081] S204, judging whether the "seam-to-seam" phenomenon occurs according to the water flow position in the guide borehole 5, the judging method is: when the hydraulic fracturing crack 9 and the guide borehole 5 communicate at a position above the sealing capsule 7, the water will flow out of the hollow connecting pipe 6, and the "seam-to-seam" phenomenon occurs; otherwise, the water will flow out of the guide borehole 5, and the "seam-to-seam" phenomenon does not occur;

[0082] (1) If the "seam-through" phenomenon does not occur in the pilot borehole 5, the sealing capsule 7 and the hollow connecting tube 6 are recovered, and the interval between the fracturing sections in the measure borehole 4 is shortened to (L-5) m, i.e., 15 m. The third liquid CO2 phase change fracturing is performed in the measure borehole 4, and the third liquid CO2 phase change fracturing is also performed at the corresponding position in the pilot borehole 5. The sealing capsule 7 and the hollow connecting tube 6 are placed in the pilot borehole 5. The sealing capsule 7 is located between the two adjacent liquid CO2 phase change fracturing positions, and the sealing capsule 7 is passed through the hollow connecting tube 6. Then, hydraulic fracturing is performed at the third liquid CO2 phase change fracturing position in the measure borehole 4, and whether the "seam-through" phenomenon occurs is judged again according to the water flow position in the pilot borehole 5:

[0083] If the "seamless" phenomenon occurs, the construction is stopped, and it is determined that the reasonable spacing of the fracturing sections in the measure borehole 4 is L when the pilot borehole 5 is located on the right side of the measure borehole 4;

[0084] If the "seamless fracture" phenomenon does not occur, the cycle is repeated, and the spacing between the fracturing sections is shortened by 5m successively until the "seamless fracture" phenomenon occurs in the guide borehole 5 at the nth liquid CO2 phase change fracturing position. In order to ensure that the "seamless fracture" phenomenon does not occur, the distance between the n-1th liquid CO2 phase change fracturing position and the n-2nd liquid CO2 phase change fracturing position is used as the reasonable spacing of the fracturing sections in the measure borehole; wherein, if the distance between the last liquid CO2 phase change fracturing position and the hole mouth in the measure borehole 4 of a group of boreholes is <30m, in order to avoid the hydraulic fracture being directly connected to the laneway due to the fracturing interval being too close to the laneway, a group of boreholes shall be reconstructed at least 50m to the right of the guide borehole 5 of the group of boreholes, and then the spacing of the fracturing sections at the last liquid CO2 phase change fracturing of the group of measure boreholes 4 shall be shortened by 5m, and liquid CO2 phase change fracturing and hydraulic fracturing shall be re-performed at the bottom of the re-constructed group of measure boreholes 4 and in the guide borehole 5, and the above operation shall be repeated;

[0085] (2) If a "seam" phenomenon occurs in the pilot borehole 5, a second group of boreholes is reconstructed at least 50 m to the right of the pilot borehole 5; the conditions of the measure boreholes 4 and the pilot boreholes 5 in the second group of boreholes are the same as those of the first group of boreholes, and the second group of boreholes are subjected to the first liquid CO2 phase change fracturing and hydraulic fracturing according to step S202 until water flows in the pilot borehole 5;

[0086] After completion, the second group of boreholes are subjected to a second liquid CO2 phase change fracturing and hydraulic fracturing according to step S203, wherein the spacing between the fracturing sections is increased to (L+10) m;

[0087] Again, determine whether the "seam" phenomenon occurs based on the water flow position in the pilot borehole 5:

[0088] If no "seam" phenomenon occurs in the pilot borehole 5, the construction is stopped. It is determined that when the pilot borehole 5 is located on the right side of the measure borehole 4, the reasonable spacing of the fracturing sections in the measure borehole 4 is (L+10) m.

[0089] If the "seam-through" phenomenon occurs, the cycle is repeated, and the spacing of the fracturing sections in the measure borehole 4 in each group of re-constructed boreholes is increased by 10m compared with the previous group, until the nth group of re-constructed boreholes does not have the "seam-through" phenomenon, and the spacing of the fracturing sections in the current nth group is the reasonable spacing of the fracturing sections in the measure borehole 4 when the pilot borehole 5 is located on the right side of the measure borehole 4;

[0090] S205, using the same method to determine the reasonable spacing of the fracturing sections in the measure borehole 4 when the pilot borehole 5 is located on the left side of the measure borehole 4;

[0091] S3. During the hydraulic fracturing process, the coal body on the non-liquid CO2 phase change fracturing side in the measure borehole 4 will also crack and expand. Therefore, it is necessary to master the expansion of the cracks on this side in order to arrange the extraction boreholes in the later stage. Determine the expansion length range of the hydraulic fracturing crack 9 on the non-liquid CO2 phase change fracturing side of the measure borehole 4 when the guide borehole 5 is located on the left and right sides of the measure borehole 4, specifically including the following steps:

[0092] S301, reconstructing a measure borehole 4 and a guide borehole 5 in the coal seam 1, the guide borehole 5 is located on the right side of the measure borehole 4, and at the same time constructing an observation borehole 8 on the left side of the measure borehole 4 to observe the expansion of the hydraulic fracturing crack 9 on the non-liquid CO2 phase change fracturing side of the measure borehole 4;

[0093] The distance between the measure borehole 4 and the pilot borehole 5 is 30m-50m; the distance between the observation borehole 8 and the measure borehole 4 is half of the distance between the measure borehole 4 and the pilot borehole 5, that is, 15m-25m;

[0094] Multiple liquid CO2 phase change fracturing operations are performed in sequence from the hole mouth to the hole bottom in the measure borehole 4. The first liquid CO2 phase change fracturing position in the measure borehole 4 is 30m-50m away from the hole mouth. The depth of the observation borehole 8 is 10m-20m beyond the first liquid CO2 phase change fracturing position in the measure borehole 4.

[0095] S302, performing the first liquid CO2 phase change fracturing at the first liquid CO2 phase change fracturing position in the measure borehole 4, with the fracturing direction being perpendicular to the axial direction of the measure borehole 4 and toward the guide borehole 5; at the same time, performing the first liquid CO2 phase change fracturing at the corresponding position in the guide borehole 5, with the fracturing direction being perpendicular to the axial direction of the guide borehole 5 and toward the measure borehole 4; after the fracturing is completed, recovering the liquid CO2 phase change fracturing equipment, and then performing hydraulic fracturing at the first liquid CO2 phase change fracturing position in the measure borehole 4;

[0096] S303, observe which borehole, the observation borehole 8 or the pilot borehole 5, has water flow first:

[0097] (1) If water flow occurs first in the observation borehole 8, it means that during the hydraulic fracturing process, the hydraulic fracturing crack 9 on the right side of the measure borehole 4 has not yet expanded to the pilot borehole 5, while the hydraulic fracturing crack 9 on the left side has expanded to the observation borehole 8. Therefore, a second observation borehole 8 is reconstructed 5m to the left of the observation borehole 8. The depth of the second observation borehole 8 exceeds the second liquid CO2 phase change fracturing position in the measure borehole 4 by 10m-20m. The other conditions are the same as those of the first observation borehole 8.

[0098] Then, the sealing capsule 7 and the hollow connecting tube 6 are recovered, and a second liquid CO2 phase change fracturing is performed in the measure borehole 4. The spacing between the second liquid CO2 phase change fracturing position and the first liquid CO2 phase change fracturing position is the reasonable spacing of the fracturing sections in the measure borehole 4 when the guide borehole 5 determined in step S2 is located on the right side of the measure borehole 4; at the same time, a second liquid CO2 phase change fracturing is also performed at the corresponding position in the guide borehole 5. After the fracturing is completed, the liquid CO2 phase change fracturing equipment is recovered, and then hydraulic fracturing is performed at the second liquid CO2 phase change fracturing position in the measure borehole 4, and it is observed which borehole, the second observation borehole 8 or the guide borehole 5, has water flow first;

[0099] If the second observation borehole 8 still first experiences water flow, then step (1) is repeated, and the cycle is repeated until the pilot borehole 5 first experiences water flow when the nth observation borehole 8 is constructed, and it is determined that when the pilot borehole 5 is located on the right side of the measure borehole 4, the extension length range of the hydraulic fracturing crack 9 on the non-liquid CO2 phase change fracturing side of the measure borehole 4 is from the distance between the n-1th observation borehole 8 and the measure borehole 4 to the distance between the nth observation borehole 8 and the measure borehole 4;

[0100] (2) If water flow occurs first in the pilot borehole 5, a second observation borehole 8 is reconstructed 5 m to the right of the first observation borehole 8. The depth of the second observation borehole 8 is 10 m to 20 m beyond the second liquid CO2 phase change fracture position in the measure borehole 4. The other conditions are the same as those of the first observation borehole 8.

[0101] Then, the sealing capsule 7 and the hollow connecting tube 6 are recovered, and a second liquid CO2 phase change fracturing is performed in the measure borehole 4. The distance between the second liquid CO2 phase change fracturing position and the first liquid CO2 phase change fracturing position is the reasonable distance of the fracturing section in the measure borehole 4 when the guide borehole 5 determined in step S2 is located on the right side of the measure borehole 4; at the same time, a second liquid CO2 phase change fracturing is also performed at the corresponding position in the guide borehole 5. After the fracturing is completed, the liquid CO2 phase change fracturing equipment is recovered, and then hydraulic fracturing is performed in the measure borehole 4, and it is observed which borehole, the second observation borehole 8 or the guide borehole 5, has water flow first;

[0102] If the pilot borehole 5 still has water flow first, the cycle continues until the nth observation borehole 8 has water flow before the pilot borehole 5, and it is determined that when the pilot borehole 5 is located on the right side of the measure borehole 4, the extension length range of the hydraulic fracturing crack 9 on the non-liquid CO2 phase change fracturing side of the measure borehole 4 is from the distance between the nth observation borehole 8 and the measure borehole 4 to the distance between the n-1th observation borehole 8 and the measure borehole 4;

[0103] S304, using the same method to determine the extension length range of the hydraulic fracturing crack 9 on the non-liquid CO2 phase change fracturing side of the measure borehole 4 when the pilot borehole 5 is located on the left side of the measure borehole 4 (in this step, the reasonable spacing of the fracturing sections in the measure borehole 4 is the reasonable spacing when the pilot borehole 5 is located on the left side of the measure borehole 4 determined in step S2);

[0104] S4. Drilling arrangement:

[0105] In the coal seam permeability enhancement area, multiple groups of measure boreholes 4 and guide boreholes 5 are constructed in a row. In each group, the guide borehole 5 is located on the left or right side of the measure borehole 4, and is arranged according to the side with a larger hydraulic fracturing crack extension length range in step S3, and the spacing between the non-liquid CO2 phase change fracturing side of the measure borehole 4 and the guide borehole 5 of the adjacent group is the maximum value of the hydraulic fracturing crack extension length range of the non-liquid CO2 phase change fracturing side of the measure borehole 4 at that side;

[0106] S5, coal seam permeability enhancement: according to the positions of the measure borehole 4 and the pilot borehole 5 determined in step S4, each measure borehole 4 and the pilot borehole 5 is subjected to liquid CO2 phase change fracturing at the fracturing section spacing of the corresponding side determined in step S2, and hydraulic fracturing is performed after each liquid CO2 phase change fracturing in the measure borehole 4;

[0107] The specific steps are as follows: the first liquid CO2 phase change fracturing and hydraulic fracturing: the first liquid CO2 phase change fracturing is carried out at the bottom of the measure borehole 4 using liquid CO2 phase change fracturing equipment, and at the same time, the first liquid CO2 phase change fracturing is also carried out at the corresponding position in the pilot borehole 5 (corresponding to the first liquid CO2 phase change fracturing position of the measure borehole 4), so as to induce the end of the hydraulic fracturing crack. After the fracturing is completed, the liquid CO2 phase change fracturing equipment is recovered, and then hydraulic fracturing is carried out at the bottom of the measure borehole 4 using hydraulic fracturing equipment until water flows in the pilot borehole 5;

[0108] Second liquid CO2 phase change fracturing and hydraulic fracturing: a second liquid CO2 phase change fracturing is performed in the measure borehole 4, and the distance between the second liquid CO2 phase change fracturing position and the first liquid CO2 phase change fracturing position is the fracturing section distance in the measure borehole 4 determined in step S2. At the same time, a second liquid CO2 phase change fracturing is also performed at the corresponding position in the pilot borehole 5 (corresponding to the second liquid CO2 phase change fracturing position of the measure borehole 4). After the fracturing is completed, the liquid CO2 phase change fracturing equipment is recovered, and then hydraulic fracturing is performed at the second liquid CO2 phase change fracturing position in the measure borehole 4 until water flows in the pilot borehole 5;

[0109] According to the above steps, liquid CO2 phase change fracturing and hydraulic fracturing are carried out in sequence from the bottom of the measure borehole 4 and the pilot borehole 5 to the hole mouth. Among them, the distance between the last liquid CO2 phase change fracturing position in the measure borehole 4 and the hole mouth of the measure borehole 4 is not less than 30m.

[0110] Embodiment 2

[0111] Take a coal mine in Shanxi as an example. The inclined length of the working face is 150m, the strike length is 1500m, the thickness of coal seam 1 is 4.0m, the inclination angle of coal seam 1 is 6°, U-shaped ventilation method and top coal caving method are adopted. The gas content of coal seam 1 is 6.50-7.20m 3 / t, gas pressure is 0.52-0.68MPa, and the permeability coefficient of coal seam 1 is 0.01256-0.02365m 2 / (MPa 2 ·d), the coal seam permeability enhancement method of the present invention is applied to the working face to illustrate the same.

[0112] S1. Preparation of construction equipment;

[0113] Drilling construction equipment, liquid CO2 phase change fracturing equipment and hydraulic fracturing equipment are arranged in the air intake tunnel 3 of the coal mining face, and corresponding warning signs are set; among them, drilling construction equipment, liquid CO2 phase change fracturing equipment and hydraulic fracturing equipment are all existing technologies and will not be described in detail here;

[0114] S2, dividing the fracturing sections of the hydraulic fracturing measure borehole 4, determining the reasonable spacing of the fracturing sections in the measure borehole 4 when the pilot borehole 5 is located on the left and right sides of the measure borehole 4, specifically including the following steps:

[0115] S201, constructing the first set of boreholes in coal seam 1 using drilling construction equipment:

[0116] The first group of boreholes includes a measure borehole 4 and a guide borehole 5, wherein the measure borehole 4 is used to perform liquid CO2 phase change fracturing and hydraulic fracturing measures, to perform joint permeability enhancement on the coal body and to guide the hydraulic fracturing cracks 9; the guide borehole 5 is used to perform liquid CO2 phase change fracturing measures, to guide the hydraulic fracturing cracks 9 formed after the measure borehole 4 is hydraulically fractured;

[0117] First, according to the actual situation and needs of the mine, the spacing between the measure borehole 4 and the guide borehole 5 in the first group of boreholes is determined; then a measure borehole 4 and a guide borehole 5 are constructed in the coal seam 1 by using a drilling rig, and the guide borehole 5 is located on the right side of the measure borehole 4;

[0118] like Figure 1 As shown, according to the specific conditions of a certain mining face in a coal mine in Shanxi, the spacing between the measure borehole 4 and the guide borehole 5 is 30m, the vertical distance between the coal mining face air intake tunnel 3 and the coal mining face return air tunnel 2 is 120m, the depth of the measure borehole 4 is 90m, the depth of the guide borehole 5 is 100m, the apertures are both 94mm, the height of the borehole opening is 1.5m, and the borehole inclination is 6°;

[0119] S202, the first liquid CO2 phase change fracturing and hydraulic fracturing of the first group of boreholes:

[0120] like Figure 2 and Figure 3 As shown, the first liquid CO2 phase change fracturing is carried out at the bottom of the measure borehole 4 of the first group of boreholes, and the fracturing direction is perpendicular to the axial direction of the measure borehole 4 and toward the guide borehole 5. At the same time, the first liquid CO2 phase change fracturing is also carried out at the corresponding position in the guide borehole 5, and the fracturing direction is perpendicular to the axial direction of the guide borehole 5 and toward the measure borehole 4. After the fracturing is completed, the liquid CO2 phase change fracturing equipment is recovered, and a sealing capsule 7 and a hollow connecting pipe 6 are placed in the guide borehole 5. The sealing capsule 7 is located 2m-5m below the first liquid CO2 phase change fracturing position of the guide borehole 5, and the sealing capsule 7 is passed through the top of the hollow connecting pipe 6. Then, hydraulic fracturing is carried out at the bottom of the measure borehole 4 until water flows in the guide borehole 5 or the hollow connecting pipe 6.

[0121] S203, the second liquid CO2 phase change fracturing and hydraulic fracturing of the first group of boreholes:

[0122] Recover the sealing capsule 7 and the hollow connecting tube 6, and perform a second liquid CO2 phase change fracturing in the measure borehole 4 of the first group of boreholes. The distance between the second liquid CO2 phase change fracturing position and the first liquid CO2 phase change fracturing position, that is, the fracturing section distance is L, L=20m; at the same time, perform a second liquid CO2 phase change fracturing in the corresponding position in the pilot borehole 5. After the fracturing is completed, place the sealing capsule 7 and the hollow connecting tube 6 in the pilot borehole 5. The sealing capsule 7 is located between the two adjacent liquid CO2 phase change fracturing positions. The sealing capsule 7 is passed through the top of the hollow connecting tube 6. Then, hydraulic fracturing is performed at the second liquid CO2 phase change fracturing position in the measure borehole 4 until water flows in the pilot borehole 5;

[0123] S204, judging whether a "seam-through" phenomenon occurs according to the water flow position in the pilot borehole 5;

[0124] The judgment method is: when the hydraulic fracturing crack 9 and the guide drill hole 5 communicate at the position above the sealing capsule 7, the water will flow out of the hollow connecting pipe 6, and the "seam-through" phenomenon will occur (such as Figure 4 As shown), otherwise the water will flow out from the pilot borehole 5, and no "seam" phenomenon will occur (as shown). Figure 5 shown);

[0125] At this time, water is observed to flow out of the hollow connecting pipe 6, indicating that a "seam" phenomenon occurs in the pilot borehole 5, and a second group of boreholes is reconstructed at least 50m to the right of the pilot borehole 5; the conditions of the measure borehole 4 and the pilot borehole 5 in the second group of boreholes are the same as those of the first group of boreholes, and the second group of boreholes are subjected to the first liquid CO2 phase change fracturing and hydraulic fracturing according to step S202 until water flows in the pilot borehole 5;

[0126] After completion, the second group of boreholes are subjected to a second liquid CO2 phase change fracturing and hydraulic fracturing according to step S203, wherein the spacing between the fracturing sections is increased to (L+10) m, i.e., 30 m (e.g., Figure 6 shown);

[0127] Again, determine whether the "seam" phenomenon occurs based on the water flow position in the pilot borehole 5:

[0128] At this time, no "seam" phenomenon occurs in the guide borehole 5 (such as Figure 6 As shown), the construction is stopped. When it is determined that the pilot borehole 5 is located on the right side of the measure borehole 4, the reasonable spacing of the fracturing sections in the measure borehole 4 is 30m;

[0129] To ensure the accuracy of the experimental results, a third liquid CO2 phase change fracturing was carried out 30m outside the second liquid CO2 phase change fracturing in the measure borehole 4 in the second group of boreholes. Similarly, a third liquid CO2 phase change fracturing was carried out at the corresponding position in the guide borehole 5 in the same group of boreholes. After the fracturing was completed, the relevant equipment was recovered, and the sealing capsule 7 and the connecting pipe 6 were sent into the guide borehole 5 in the second group of boreholes, located in the middle position between the second liquid CO2 phase change fracturing and the third liquid CO2 phase change fracturing, and sealed. Then, hydraulic fracturing was carried out at the third liquid CO2 phase change fracturing in the measure borehole 4 in the second group of boreholes, and the water flow phenomenon in the guide borehole 5 was detected. The results showed that the water did not flow out of the hollow connecting pipe 6, that is, there was no water flow phenomenon. This once again verified that the fracturing section with a spacing of 30m can ensure that the hydraulic fracturing cracks 9 generated by the two adjacent fracturing sections will not have a "string seam" phenomenon.

[0130] S205, using the same method to determine the reasonable spacing of the fracturing sections in the measure borehole 4 when the pilot borehole 5 is located on the left side of the measure borehole 4;

[0131] In this embodiment, a pilot borehole 5 is reconstructed at a position of 50m to the right of the pilot borehole 5 of the second group of boreholes, and a measure borehole 4 is constructed at a position of 30m to the right of the pilot borehole 5, and the same method as above is used to complete the first liquid CO2 phase change fracturing and hydraulic fracturing, and then the second liquid CO2 phase change fracturing is performed at 30m outside the first liquid CO2 phase change fracturing of the measure borehole 4 and the pilot borehole 5 (i.e., the distance between the fracturing sections is 30m), and then the second liquid CO2 phase change fracturing is hydraulically fractured in the measure borehole 4, and the water flow phenomenon in the pilot borehole 5 is observed, and the results show that no "seam" phenomenon occurs;

[0132] Then, the third liquid CO2 phase change fracturing was carried out 25 m outside the second liquid CO2 phase change fracturing of the measure borehole 4 and the pilot borehole 5 (i.e., the fracturing interval was shortened by 5 m), and then hydraulic fracturing was carried out at the third liquid CO2 phase change fracturing in the measure borehole 4, and the water flow phenomenon in the pilot borehole 5 was observed. The results showed that the "seamless fracture" phenomenon occurred;

[0133] A pilot borehole 5 was reconstructed at 50m to the right of the measure borehole 4, and a measure borehole 4 was constructed at 30m to the right of the guide borehole 5. The same method as above was used to complete the first liquid CO2 phase change fracturing and hydraulic fracturing. Then, the second liquid CO2 phase change fracturing was performed 30m outside the first liquid CO2 phase change fracturing of the measure borehole 4 and the guide borehole 5. Then, hydraulic fracturing was performed on the second liquid CO2 phase change fracturing in the measure borehole 4, and the water flow phenomenon in the guide borehole 5 was observed. The results showed that no "seam-crossing" phenomenon occurred.

[0134] To ensure the accuracy of the experimental results, the third liquid CO2 phase change fracturing was carried out 30m outside the second liquid CO2 phase change fracturing of the measure borehole 4 and the guide borehole 5, respectively. Then, hydraulic fracturing was carried out on the third liquid CO2 phase change fracturing in the measure borehole 4 and the water flow phenomenon in the guide borehole 5 was observed. The results showed that the water did not flow out of the hollow connecting pipe 6, that is, there was no water flow phenomenon, which once again verified that the fracturing section with a spacing of 30m can ensure that the hydraulic fracturing cracks 9 generated by the two adjacent fracturing sections will not have the "string seam" phenomenon.

[0135] Therefore, no matter the guide borehole 5 is located on the left or right side of the measure borehole 4, when the distance between the two boreholes is 30m, when the liquid CO2 phase change fracturing induced hydraulic fracturing fracture 9 coal seam permeability enhancement process is adopted, it is more reasonable to take the distance between two adjacent fracturing positions in the measure borehole 4 as 30m.

[0136] S3, determining the extension length range of the hydraulic fracturing crack on the non-liquid CO2 phase change fracturing side of the measure borehole 4 when the pilot borehole 5 is located on the left and right sides of the measure borehole 4, specifically comprising the following steps:

[0137] like Figure 7 As shown, S301, a measure borehole 4 and a guide borehole 5 are reconstructed in the coal seam 1, and the guide borehole 5 is located on the right side of the measure borehole 4; the distance between the measure borehole 4 and the guide borehole 5 is 30m, and at the same time, an observation borehole 8 is constructed on the left side of the measure borehole 4 to observe the expansion of the hydraulic fracturing crack 9 on the non-liquid CO2 phase change fracturing side of the measure borehole 4; other conditions of the measure borehole 4 and the guide borehole 5 are the same as those of the first group of boreholes in step S201;

[0138] The distance between the observation borehole 8 and the measure borehole 4 is half of the distance between the measure borehole 4 and the pilot borehole 5, i.e., 15 m.

[0139] Multiple liquid CO2 phase change fracturing operations were carried out in sequence from the borehole to the bottom of the hole in the measure borehole 4. The first liquid CO2 phase change fracturing position was 30m away from the borehole, and the depth of the observation borehole 8 was 45m.

[0140] S302, performing the first liquid CO2 phase change fracturing at the first liquid CO2 phase change fracturing position in the measure borehole 4, with the fracturing direction being perpendicular to the axial direction of the measure borehole 4 and toward the guide borehole 5; at the same time, performing the first liquid CO2 phase change fracturing at the corresponding position in the guide borehole 5, with the fracturing direction being perpendicular to the axial direction of the guide borehole 5 and toward the measure borehole 4; after the fracturing is completed, recovering the liquid CO2 phase change fracturing equipment, and then performing hydraulic fracturing at the first liquid CO2 phase change fracturing position in the measure borehole 4;

[0141] S303, observe which borehole, the observation borehole 8 or the pilot borehole 5, has water flow first:

[0142] like Figure 8 As shown, the results show that water flow appears first in the pilot borehole 5, that is, when the hydraulic fracturing crack 9 penetrates the measure borehole 4 and the pilot borehole 5, the extension length of the hydraulic fracturing crack 9 on the non-liquid CO2 phase change fracturing side in the measure borehole 4 has not yet reached 15m in length;

[0143] Therefore, an observation borehole 8 is reconstructed at a position 5m to the right of the observation borehole 8, and the depth of the observation borehole 8 is 75m. Then, a second liquid CO2 phase change fracturing is carried out at a position 60m away from the orifice of the measure borehole 4 (the distance between two adjacent fracturing positions in the measure borehole 4 is 30m). At the same time, a second liquid CO2 phase change fracturing is also carried out at the corresponding position in the pilot borehole 5. Then, hydraulic fracturing is carried out at the second liquid CO2 phase change fracturing in the measure borehole 4 to observe which borehole, the observation borehole 8 or the pilot borehole 5, has water flow first.

[0144] like Fig. 9 As shown, the results show that water flow appears first in the observation borehole 8, so it is judged that when the pilot borehole 5 is located 30m to the right of the measure borehole 4, the extension length of the hydraulic fracture on the left side of the measure borehole 4 is between 10m and 15m;

[0145] S304, using the same method to determine the extension length range of the hydraulic fracturing crack 9 on the non-liquid CO2 phase change fracturing side of the measure borehole 4 when the pilot borehole 5 is located on the left side of the measure borehole 4:

[0146] A guide borehole 5 is reconstructed at a position 5m to the right of the guide borehole 5, and a measure borehole 4 is also reconstructed at a position 30m to the right of the guide borehole 5, and an observation borehole 8 is reconstructed at a position 15m to the right of the measure borehole 4. The depth of the observation borehole 8 is 45m. Liquid CO2 phase change fracturing is performed at the outermost end of the measure borehole 4 (i.e., at a position 30m away from the hole mouth), and the fracturing direction is perpendicular to the axial direction of the measure borehole 4 and toward the guide borehole 5; at the same time, liquid CO2 phase change fracturing is performed at the corresponding position in the guide borehole 5, and the fracturing direction is perpendicular to the axial direction of the guide borehole 5 and toward the measure borehole 4; then, hydraulic fracturing is performed on the liquid CO2 phase change fracturing position in the measure borehole 4 to examine which borehole, the observation borehole 8 or the guide borehole 5, has water flow first;

[0147] The results show that water flow occurs first in the observation borehole 8, that is, before the hydraulic fracturing crack 9 penetrates the measure borehole 4 and the guide borehole 5, the extension length of the hydraulic fracturing crack 9 on the non-liquid CO2 phase change fracturing side in the measure borehole 4 has reached 15m, so a second observation borehole 8 is constructed 5m to the right of the observation borehole 8 with a depth of 75m, and a second liquid CO2 phase change fracturing is performed at a distance of 60m from the borehole mouth in the measure borehole 8 toward the guide borehole 5, and at the same time, a second liquid CO2 phase change fracturing is performed at the corresponding position in the guide borehole 5 toward the measure borehole 4, and then hydraulic fracturing is performed at the second liquid CO2 phase change fracturing in the measure borehole 4 to observe which borehole, the second observation borehole 8 or the guide borehole 5, appears water flow first;

[0148] The results show that water flow occurs first in the pilot borehole 5, that is, when the hydraulic fracturing crack 9 penetrates the measure borehole 4 and the pilot borehole 5, the extension length of the hydraulic fracturing crack 9 on the non-liquid CO2 phase change fracturing side in the measure borehole 4 has not yet reached 20m. Therefore, it is judged that when the pilot borehole 5 is located 30m to the left of the measure borehole 4, the extension length of the hydraulic fracturing crack 9 on the left side of the measure borehole 4 is between 15m and 20m.

[0149] Therefore, when the guide borehole 5 is located 30m to the right of the measure borehole 4, the extension length range of the hydraulic fracturing crack 9 on the non-liquid CO2 phase change fracturing side in the measure borehole 4 is 10m-15m; when the guide borehole 5 is located 30m to the left of the measure borehole 4, the extension length range of the hydraulic fracturing crack 9 on the non-liquid CO2 phase change fracturing side in the measure borehole 4 is 15m-20m.

[0150] S4. Drilling arrangement:

[0151] like Fig.10 As shown, in the coal seam permeability enhancement area, multiple groups of measure boreholes 4 and guide boreholes 5 are constructed in a row. According to the result that when the guide borehole 5 is located 30m to the left of the measure borehole 4 in step S3, the hydraulic fracturing cracks 9 on the non-liquid CO2 phase change fracturing side in the measure borehole 4 have a larger extension length range, in each group, the guide borehole 5 is arranged on the left side of the measure borehole 4 with a spacing of 30m, and the spacing between the non-liquid CO2 phase change fracturing side of the measure borehole 4 (i.e., the right side of the measure borehole 4) and the guide borehole 5 of the adjacent group is 20m;

[0152] S5, coal seam permeability enhancement: according to the position of the pilot borehole 5 determined in step S4, which is located on the left side of the measure borehole 4, and the fracturing section spacing of 30m in the measure borehole 4 determined in step S2, liquid CO2 phase change fracturing and hydraulic fracturing are performed on each measure borehole 4 and the pilot borehole 5;

[0153] The specific steps are as follows: the first liquid CO2 phase change fracturing and hydraulic fracturing: the first liquid CO2 phase change fracturing is carried out at the bottom of the measure borehole 4 using liquid CO2 phase change fracturing equipment, and at the same time, the first liquid CO2 phase change fracturing is also carried out at the corresponding position in the pilot borehole 5 (corresponding to the first liquid CO2 phase change fracturing position of the measure borehole 4) so ​​as to induce the end of the hydraulic fracturing crack 9. After the fracturing is completed, the liquid CO2 phase change fracturing equipment is recovered, and then hydraulic fracturing is carried out at the bottom of the measure borehole 4 using hydraulic fracturing equipment until water flows in the pilot borehole 5;

[0154] Second liquid CO2 phase change fracturing and hydraulic fracturing: The second liquid CO2 phase change fracturing is carried out in the measure borehole 4. The distance between the second liquid CO2 phase change fracturing position and the first liquid CO2 phase change fracturing position is 30m. At the same time, the second liquid CO2 phase change fracturing is also carried out at the corresponding position in the pilot borehole 5 (corresponding to the second liquid CO2 phase change fracturing position of the measure borehole 4). After the fracturing is completed, the liquid CO2 phase change fracturing equipment is recovered, and then hydraulic fracturing is carried out at the second liquid CO2 phase change fracturing position in the measure borehole 4 using hydraulic fracturing equipment until water flows in the pilot borehole 5;

[0155] The third liquid CO2 phase change fracturing and hydraulic fracturing: The third liquid CO2 phase change fracturing is carried out in the measure borehole 4. The distance between the third liquid CO2 phase change fracturing position and the second liquid CO2 phase change fracturing position is 30m. At the same time, the third liquid CO2 phase change fracturing is also carried out at the corresponding position in the pilot borehole 5 (corresponding to the third liquid CO2 phase change fracturing position of the measure borehole 4). After the fracturing is completed, the liquid CO2 phase change fracturing equipment is recovered, and then hydraulic fracturing is carried out at the third liquid CO2 phase change fracturing position in the measure borehole 4 using hydraulic fracturing equipment until water flows in the pilot borehole 5;

[0156] Since the distance between the third liquid CO2 phase change fracturing position and the orifice in the measure borehole 4 is 30m, the liquid CO2 phase change fracturing and hydraulic fracturing in the measure borehole 4 are completed. After the permeability enhancement measures of the coal seam 1 are completed, gas extraction boreholes are constructed in the coal seam permeability enhancement area according to actual needs. The extraction borehole spacing is generally 4-10m (depending on the actual situation), and the original guide borehole 5 and the measure borehole 4 are used as gas extraction boreholes. For example, the extraction borehole spacing between the liquid CO2 phase change fracturing side of the measure borehole 4 and the guide borehole 5 can be set to 6m, but between the measure borehole 4 and the guide borehole 5 on the non-liquid CO2 phase change fracturing side, since the crack extension length on this side fails to reach the guide borehole 5, the extraction boreholes in this area can be appropriately increased and can be set to 4m.

[0157] In Example 1 and Example 2, the fracturing direction of the phase change fracturing of liquid CO2 in the measure borehole 4 is perpendicular to the axial direction of the measure borehole 4 and toward the guide borehole 5, and the fracturing direction of the phase change fracturing of liquid CO2 in the guide borehole 5 is perpendicular to the axial direction of the guide borehole 5 and toward the measure borehole 4.

[0158] The principles and implementation methods of the present invention are described in this article using specific examples. The description of the above embodiments is only used to help understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, according to the idea of ​​the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A method for increasing the permeability of coal seams based on liquid CO2 phase change-induced hydraulic fracturing fractures, characterized in that: The following steps are involved: S1. Preparation of construction equipment; S2. Division of the fracturing sections of the hydraulic fracturing measure borehole, determining the reasonable spacing of the fracturing sections in the measure borehole when the pilot boreholes are located on the left and right sides of the measure borehole respectively; S3, determining the extension length range of the hydraulic fracturing crack on the non-liquid CO2 phase change fracturing side of the measure borehole when the pilot borehole is located on the left and right sides of the measure borehole respectively; S4, drilling arrangement: construct multiple groups of measure boreholes and guide boreholes in a row in the coal seam permeability enhancement area, and in each group, the guide borehole is located on the left or right side of the measure borehole, and is arranged according to the side with a larger hydraulic fracturing crack extension length range in step S3, and the spacing between the non-liquid CO2 phase change fracturing side of the measure borehole and the guide borehole of the adjacent group is the maximum value of the hydraulic fracturing crack extension length range on this side; S5. Coal seam permeability enhancement: According to the positions of the measure boreholes and guide boreholes in each group determined in step S4 and the fracturing section spacing on the corresponding side determined in step S2, liquid CO2 phase change fracturing is performed on each measure borehole and guide borehole, and hydraulic fracturing is performed after each liquid CO2 phase change fracturing in the measure borehole.

2. A method for increasing the permeability of coal seams based on liquid CO2 phase change-induced hydraulic fracturing cracks as claimed in claim 1, characterized in that: The steps S2 and S3 specifically include the following steps: S2. The fracturing section division of the hydraulic fracturing measure borehole is to determine the reasonable spacing of the fracturing sections in the measure borehole when the pilot borehole is located on the left and right sides of the measure borehole, specifically including the following steps: S201. Construction of the first set of boreholes in the coal seam: The first group of boreholes includes a measure borehole and a guide borehole. First, the spacing between the measure borehole and the guide borehole in the first group of boreholes is determined according to the actual situation and needs of the mine. Then, the first group of boreholes is constructed in the coal seam by using a drilling rig, and the guide borehole is located on the right side of the measure borehole. S202, the first liquid CO2 phase change fracturing and hydraulic fracturing of the first group of boreholes: The first liquid CO2 phase change fracturing is carried out at the bottom of the measure borehole of the first group of boreholes, and the first liquid CO2 phase change fracturing is also carried out at the corresponding position in the pilot borehole. After the fracturing is completed, the liquid CO2 phase change fracturing equipment is recovered, and the sealing capsule and the hollow connecting pipe are placed in the pilot borehole. Then, hydraulic fracturing is carried out at the bottom of the measure borehole until water flows in the pilot borehole or the hollow connecting pipe; S203, the second liquid CO2 phase change fracturing and hydraulic fracturing of the first group of boreholes: Recover the sealing capsule and the hollow connecting tube, and perform a second liquid CO2 phase change fracturing in the measure borehole of the first group of boreholes. The distance between the second liquid CO2 phase change fracturing position and the first liquid CO2 phase change fracturing position, that is, the fracturing section distance, is L; at the same time, perform a second liquid CO2 phase change fracturing at the corresponding position in the pilot borehole. After the fracturing is completed, place a sealing capsule and a hollow connecting tube in the pilot borehole. The sealing capsule is located between the two adjacent liquid CO2 phase change fracturing positions. The sealing capsule is passed through the top of the hollow connecting tube. Then, hydraulic fracturing is performed at the second liquid CO2 phase change fracturing position in the measure borehole until water flows in the pilot borehole; S204, judging whether a "seam-through" phenomenon occurs according to the water flow position in the pilot borehole; (1) If the "seam-through" phenomenon does not occur in the pilot borehole, the sealing capsule and the hollow connecting tube are recovered, the interval between the fracturing sections in the measure borehole is shortened to (L-5) m, and the third liquid CO2 phase change fracturing is performed in the measure borehole. At the same time, the third liquid CO2 phase change fracturing is also performed at the corresponding position in the pilot borehole. The sealing capsule and the hollow connecting tube are placed in the pilot borehole. The sealing capsule is located between the two adjacent liquid CO2 phase change fracturing positions. The sealing capsule is passed through the hollow connecting tube. After that, hydraulic fracturing is performed at the third liquid CO2 phase change fracturing position in the measure borehole. Whether the "seam-through" phenomenon occurs is judged again based on the water flow position in the pilot borehole: If the "seamless" phenomenon occurs, the construction is stopped. When it is determined that the pilot borehole is located on the right side of the measure borehole, the reasonable spacing of the fracturing sections in the measure borehole is L; If the "seam-to-seam" phenomenon does not occur, the cycle is repeated, and the distance between the fracturing sections is shortened by 5m successively until the "seam-to-seam" phenomenon occurs in the pilot borehole at the nth liquid CO2 phase change fracturing position. In order to ensure that the "seam-to-seam" phenomenon does not occur, the distance between the n-1th liquid CO2 phase change fracturing position and the n-2nd liquid CO2 phase change fracturing position is used as the reasonable distance between the fracturing sections in the measure borehole; (2) If a "seamless fracturing" phenomenon occurs in the pilot borehole, a second group of boreholes is reconstructed at least 50 m to the right of the pilot borehole. The conditions of the measure boreholes and the pilot boreholes in the second group of boreholes are the same as those of the first group of boreholes. The second group of boreholes is subjected to the first liquid CO2 phase change fracturing and hydraulic fracturing according to step S202 until water flows in the pilot boreholes. After completion, the second group of boreholes are subjected to a second liquid CO2 phase change fracturing and hydraulic fracturing according to step S203, wherein the spacing between the fracturing sections is increased to (L+10) m; Again, determine whether the "seam" phenomenon occurs based on the water flow position in the pilot borehole: If there is no "seam" phenomenon in the pilot borehole, the construction is stopped. It is judged that the reasonable spacing of the fracturing sections when the pilot borehole is located on the right side of the measure borehole is (L+10)m; If the "seam-through" phenomenon occurs, the cycle is repeated. In each group of re-constructed boreholes, the spacing of the fracturing sections in the measure boreholes increases by 10m compared with the previous group, until the nth group of re-constructed boreholes does not have the "seam-through" phenomenon. The spacing of the fracturing sections in the current nth group is the reasonable spacing of the fracturing sections in the measure borehole when the pilot borehole is located on the right side of the measure borehole. S205, using the same method to determine the reasonable spacing of the fracturing sections in the measure borehole when the pilot borehole is located on the left side of the measure borehole; S3, determining the extension length range of the hydraulic fracturing crack on the non-liquid CO2 phase change fracturing side of the measure borehole when the pilot borehole is located on the left and right sides of the measure borehole respectively, specifically comprising the following steps: S301, reconstructing a measure borehole and a guide borehole in the coal seam, the guide borehole is located on the right side of the measure borehole, and at the same time constructing an observation borehole on the left side of the measure borehole to observe the hydraulic fracturing crack expansion on the non-liquid CO2 phase change fracturing side of the measure borehole; wherein the spacing between the observation borehole and the measure borehole is half the spacing between the measure borehole and the guide borehole; Multiple liquid CO2 phase change fracturing operations are carried out in sequence from the hole mouth to the bottom of the hole in the measure borehole, and the observation drilling depth is 10m-20m beyond the first liquid CO2 phase change fracturing position in the measure borehole; S302, performing the first liquid CO2 phase change fracturing at the first liquid CO2 phase change fracturing position in the measure borehole, and at the same time performing the first liquid CO2 phase change fracturing at the corresponding position in the pilot borehole, after the fracturing is completed, recovering the liquid CO2 phase change fracturing equipment, and then performing hydraulic fracturing at the first liquid CO2 phase change fracturing position in the measure borehole; S303. Observe which borehole, the observation borehole or the pilot borehole, has water flow first: (1) If water flow occurs first in the observation borehole, it means that during the hydraulic fracturing process, the hydraulic fracturing cracks on the right side of the measure borehole have not yet expanded to the pilot borehole, while the hydraulic fracturing cracks on the left side have expanded to the observation borehole position. Therefore, a second observation borehole is reconstructed 5m to the left of the observation borehole. The depth of the second observation borehole exceeds the second liquid CO2 phase change fracturing position in the measure borehole by 10m-20m, and the other conditions are the same as the first observation borehole; A second liquid CO2 phase change fracturing is performed in the measure borehole, and the spacing between the second liquid CO2 phase change fracturing position and the first liquid CO2 phase change fracturing position is a reasonable spacing of the fracturing sections in the measure borehole when the pilot borehole determined in step S2 is located on the right side of the measure borehole; at the same time, a second liquid CO2 phase change fracturing is also performed at the corresponding position in the pilot borehole. After the fracturing is completed, the liquid CO2 phase change fracturing equipment is recovered, and then hydraulic fracturing is performed at the second liquid CO2 phase change fracturing position in the measure borehole, and it is observed which borehole, the second observation borehole or the pilot borehole, has water flow first; If the second observation borehole still first experiences water flow, then step (1) is repeated, and the cycle is repeated until the pilot borehole first experiences water flow when the nth observation borehole is constructed, and it is determined that when the pilot borehole is located on the right side of the measure borehole, the hydraulic fracturing crack extension length range of the non-liquid CO2 phase change fracturing side of the measure borehole is from the distance between the n-1th observation borehole and the measure borehole to the distance between the nth observation borehole and the measure borehole; (2) If water flow occurs first in the pilot borehole, a second observation borehole is constructed 5 m to the right of the first observation borehole. The depth of the second observation borehole is 10 m to 20 m beyond the second liquid CO2 phase change fracture position in the measure borehole. The other conditions are the same as those of the first observation borehole. A second liquid CO2 phase change fracturing is performed in the measure borehole, and the distance between the second liquid CO2 phase change fracturing position and the first liquid CO2 phase change fracturing position is a reasonable distance between the fracturing sections in the measure borehole when the pilot borehole determined in step S2 is located on the right side of the measure borehole; at the same time, a second liquid CO2 phase change fracturing is also performed at the corresponding position in the pilot borehole. After the fracturing is completed, the liquid CO2 phase change fracturing equipment is recovered, and then hydraulic fracturing is performed at the second liquid CO2 phase change fracturing position in the measure borehole, and it is observed which borehole, the second observation borehole or the pilot borehole, has water flow first; If the pilot borehole still has water flow first, the cycle continues until the nth observation borehole has water flow before the pilot borehole, and the hydraulic fracturing crack extension length range of the non-liquid CO2 phase change fracturing side of the measure borehole is determined when the pilot borehole is located on the right side of the measure borehole; the length range is from the distance between the nth observation borehole and the measure borehole to the distance between the n-1th observation borehole and the measure borehole; S304. Using the same method, determine the extension length range of the hydraulic fracturing crack on the non-liquid CO2 phase change fracturing side of the measure borehole when the pilot borehole is located on the left side of the measure borehole.

3. A method for increasing the permeability of coal seams based on liquid CO2 phase change-induced hydraulic fracturing cracks as claimed in claim 2, characterized in that: In step S2, in the first group of boreholes and the second group of boreholes, the spacing between the measure boreholes and the guide boreholes is 30m-50m; the depth of the guide borehole exceeds the depth of the measure borehole by 10m-15m, and at the same time, the depth of the measure borehole is less than the vertical distance between the coal mining face air intake tunnel and the coal mining face return air tunnel of 30m-50m; the apertures of the measure boreholes and the guide boreholes are determined according to the extraction requirements and the requirements of the coal seam permeability enhancement equipment.

4. A method for increasing the permeability of coal seams based on liquid CO2 phase change-induced hydraulic fracturing cracks as claimed in claim 2, characterized in that: In the step S2, a preliminary judgment is made as to whether a "seam-to-seam" phenomenon occurs based on the water flow position in the guide borehole. The judgment method is as follows: when the hydraulic fracturing crack and the communication position of the guide borehole are located above the sealing capsule, water will flow out of the hollow connecting tube, and a "seam-to-seam" phenomenon occurs; otherwise, water will flow out of the guide borehole, and a "seam-to-seam" phenomenon does not occur.

5. A method for increasing the permeability of coal seams based on liquid CO2 phase change-induced hydraulic fracturing cracks as claimed in claim 2, characterized in that: In the step S202, the sealing capsule is located 2m-5m below the first liquid CO2 phase change fracturing position of the pilot borehole, and the sealing capsule is passed through the top of the hollow connecting pipe.

6. A method for increasing the permeability of coal seams based on liquid CO2 phase change-induced hydraulic fracturing cracks as claimed in claim 2, characterized in that: In step S203, the distance L between the fracturing sections is 20 m.

7. A method for increasing coal seam permeability based on liquid CO2 phase change-induced hydraulic fracturing cracks as claimed in claim 2, characterized in that: In step S301, the spacing between the measure drilling hole and the guide drilling hole is 30m-50m.

8. A method for increasing coal seam permeability based on liquid CO2 phase change-induced hydraulic fracturing cracks as claimed in claim 2, characterized in that: In the step S301, the first liquid CO2 phase change fracturing position in the borehole is 30m-50m away from the borehole mouth.

9. A method for increasing the permeability of coal seams based on liquid CO2 phase change-induced hydraulic fracturing cracks as claimed in claim 1, characterized in that: The specific steps of step S5 are as follows: The first liquid CO2 phase change fracturing and hydraulic fracturing: The first liquid CO2 phase change fracturing is carried out at the bottom of the measure borehole using liquid CO2 phase change fracturing equipment, and at the same time, the first liquid CO2 phase change fracturing is also carried out at the corresponding position in the pilot borehole to induce the end of the hydraulic fracturing crack. After the fracturing is completed, the liquid CO2 phase change fracturing equipment is recovered, and then hydraulic fracturing is carried out at the bottom of the measure borehole until water flows in the pilot borehole; Second liquid CO2 phase change fracturing and hydraulic fracturing: a second liquid CO2 phase change fracturing is performed in the measure borehole, and the distance between the second liquid CO2 phase change fracturing position and the first liquid CO2 phase change fracturing position is the distance between the fracturing sections in the measure borehole determined in step S2. At the same time, a second liquid CO2 phase change fracturing is also performed at the corresponding position in the pilot borehole. After the fracturing is completed, the liquid CO2 phase change fracturing equipment is recovered, and then hydraulic fracturing is performed at the second liquid CO2 phase change fracturing position in the measure borehole until water flows in the pilot borehole; According to the above method, liquid CO2 phase change fracturing and hydraulic fracturing are carried out in sequence from the bottom of the measure borehole and the guide borehole to the hole mouth.

10. A method for increasing the permeability of coal seams based on liquid CO2 phase change-induced hydraulic fracturing cracks as claimed in claim 2 or 9, characterized in that: The fracturing direction of the phase change fracturing of the liquid CO2 in the measure borehole is perpendicular to the axial direction of the measure borehole and toward the guide borehole, and the fracturing direction of the phase change fracturing of the liquid CO2 in the guide borehole is perpendicular to the axial direction of the guide borehole and toward the measure borehole.

Images