Systematic multi-hole multi-stage coupling directional fracturing construction method for hard rock stratum

Abstract

The application discloses a kind of system porous multi-section coupling directional fracturing hard rock layer construction methods, steps are as follows: obtaining rock mechanics parameters and original rock stress of intended fracturing area;Obtain the pressure-maintaining hole pressure-maintaining threshold pressure;Drill hole interval is set;Several groups of hydraulic fracturing units are obtained by drilling hole in cut top plate construction, and hydraulic fracturing unit includes hydraulic fracturing hole and two pressure-maintaining holes;Pressure-maintaining hole is sealed in predetermined horizon and water injection pressure maintenance;Hydraulic fracturing hole is sealed and fractured in the same horizon with pressure-maintaining hole;After fracturing, hydraulic fracturing hole and pressure-maintaining hole are unsealed and retreat to next adjacent fracturing horizon to continue pressure maintenance and fracturing, repeat this step until all horizon fracturing of this group of hydraulic fracturing units is completed;Finally, double-way packer is withdrawn;Fracture to next group of hydraulic fracturing units, until all hole fracturing work is completed;After fracturing, a large number of vertical cracks can be generated in hard rock mass to connect each hydraulic fracturing hole, and directional fracturing effect on roof is realized.

Description

Technical Field

[0001] This invention relates to the field of coal mining technology, specifically to a systematic multi-pore, multi-segment coupled directional fracturing method for constructing hard rock strata. Background Technology

[0002] Many mining areas in my country are characterized by high roof strength and intact structure. Initial mining operations often result in roof collapse, and the large cyclic pressure intervals can easily lead to large-area roof overhangs, seriously threatening the safety of personnel and equipment. Relieving roof pressure can effectively solve the problem of large-area roof overhangs during mining operations.

[0003] Currently, commonly used decompression methods include blasting and hydraulic fracturing. These methods can alter the rock structure of the roof, reducing stress concentration and pressure intensity, but they often involve large-scale engineering and are costly. Blasting methods have lower safety; the simultaneous detonation of large amounts of explosives can produce toxic gases such as carbon monoxide and nitrogen oxides, and the decompression effect is difficult to control. Directional hydraulic fracturing guides the direction of hydraulic fracturing by cutting cracks around the borehole. This method primarily produces horizontal cracks, merely cutting the hard roof into multiple layers, resulting in poor crack development induction. Currently, all hydraulic fracturing decompression methods cannot achieve precise orientation along the tunnel direction or complete connection between boreholes to form a vertical fracture surface. Summary of the Invention

[0004] To address the aforementioned technical shortcomings, the purpose of this invention is to provide a systematic, multi-segment coupled, directional fracturing method for hard rock formations, which can generate a large number of vertical cracks and better achieve directional stress transfer in the roof and softening of the surrounding rock.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] This invention provides a method for constructing a multi-porous, multi-segment coupled directional fracturing method for hard rock strata, comprising the following steps:

[0007] S1. Obtain rock mechanics parameters and original rock stress in the suspected fracture zone through rock mechanics and geostress testing, specifically including: maximum original rock stress, minimum original rock stress, and rock tensile strength.

[0008] S2. A series of boreholes are drilled along the intended crack extension direction in the top plate of the cut eye to obtain several sets of hydraulic fracturing units. Each set of hydraulic fracturing units includes a hydraulic fracturing hole in the middle and two pressure-maintaining holes on both sides.

[0009] S3. Determine the pressure holding pressure P of the pressure holding hole;

[0010] S4. Seal the hydraulic fracturing holes and pressure-maintaining holes in the first set of hydraulic fracturing units at the layer closest to the bottom of the borehole.

[0011] S5. Inject water into the two pressure-holding holes until the water pressure reaches the pressure-holding pressure P determined in step S2, and maintain the water pressure to change the stress field around the hydraulically fractured hole so that the direction of the maximum principal stress is consistent with the direction of the series of boreholes.

[0012] S6. Then, water injection is performed on the hydraulic fracturing hole between the two pressure-holding holes. After the water injection pressure of the hydraulic fracturing hole reaches its peak and drops, water injection is stopped, and the sealing of the hydraulic fracturing hole is removed.

[0013] S7. Observe the pressure of the two pressure holding holes. If the pressure drops and water flows out from the middle fracturing hole, the hydraulic fracturing in the first hydraulic fracturing unit ends. Release the pressure holding hole seal and proceed to step S9.

[0014] S8. If the pressure in the two pressure-holding holes does not decrease, reseal the hydraulically fractured hole and repeat steps S6 and S7.

[0015] S9. Repeat steps S4 to S8 to perform fracturing at the next segment height until all segments are fracturing.

[0016] S10. Perform fracturing on the next set of hydraulic fracturing units until the fracturing work of all hydraulic fracturing units is completed.

[0017] Preferably, in step S3, the pressure holding pressure P of the pressure holding orifice is less than the maximum pressure holding threshold P. max And greater than 0.9P max Maximum holding pressure threshold P max The calculation formula is:

[0018] P max =3σ²-σ¹+σt

[0019] In the formula, σ t σ1 represents the tensile strength of the rock in the fractured section, MPa; σ2 represents the maximum original rock stress, MPa; and σ3 represents the minimum original rock stress, MPa.

[0020] Preferably, the hydraulic fracturing holes and pressure-holding holes in step S2 have the same diameter, and the distance between two adjacent holes is 15 to 20 times the hole diameter.

[0021] Preferably, the hydraulic fracturing hole and the pressure-holding hole in step S2 have the same length.

[0022] Preferably, the hydraulic fracturing holes in each hydraulic fracturing unit are subjected to simultaneous fracturing operations at the same stratum.

[0023] Preferably, in step S2, the hydraulic fracturing holes and pressure holding holes in each hydraulic fracturing unit are connected to independent fracturing pipelines of the same diameter to ensure that each fracturing pipeline can obtain the same water flow rate; the fracturing pipeline is equipped with a pressure gauge, a flow meter and a shut-off valve, and the water inflow of each hydraulic fracturing hole and pressure holding hole is controlled by adjusting the size of the shut-off valve during the fracturing process.

[0024] The beneficial effects of this invention are as follows:

[0025] 1. Unlike ordinary hydraulic fracturing and top-cutting pressure relief methods, the development direction and density of cracks in this invention are more controllable. By arranging a series of dense hydraulic fracturing holes and pressure-holding holes and using fracturing technology, the local geostress field around the hydraulic fracturing holes is changed, inducing the development direction of hydraulic fracturing cracks. Vertical cracks can be formed along the hole axis and the distribution direction of the series of boreholes, which is more conducive to the collapse of the hard roof.

[0026] 2. Compared with the blasting decompression method, this method is simple to manage, causes less disturbance, involves less engineering work, has a larger operating range, lower cost, and higher safety, greatly reducing dust hazards.

[0027] 3. By setting a central hydraulic fracturing hole and pressure-maintaining holes at both ends of each fracturing unit, the range of fracture development was controlled, and the fracturing effect within the fracturing range was enhanced.

[0028] 4. Segmented fracturing allows for targeted and intensive fracturing of key, intact, and hard rock strata, avoiding ineffective fracturing in fracture-developed areas and improving fracturing efficiency and effectiveness. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a schematic diagram of the construction of a hydraulically fractured hole provided in an embodiment of the present invention;

[0031] Figure 2 This is a diagram illustrating the situation where the minimum principal stress between holes is connected when the hole spacing is small, as provided in an embodiment of the present invention.

[0032] Among them, 1. First pressure holding hole; 2. Hydraulic fracturing hole; 3. Second pressure holding hole; 4. Hardened top plate; 5. Cut eye. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] like Figure 1 As shown, hydraulic fracturing is used in the cut 5 to weaken the hard roof 4, so as to achieve the effect of pre-fractured roof and ensure that the hard roof 4 in the goaf collapses smoothly during the working face advance.

[0035] like Figure 2 As shown, when the hole spacing is small, the minimum principal stress between holes becomes connected, the minimum principal stress stress field is disturbed, and the ground stress value between boreholes is less than the original rock stress. This indicates that dense drilling has affected the stress field distribution characteristics of the surrounding rock of the roadway, and the borehole arrangement direction becomes the dominant direction of crack propagation.

[0036] This embodiment provides a method for constructing a multi-hole, multi-segment coupled directional fracturing method for hard rock strata, including the following steps:

[0037] S1. Obtain rock mechanics parameters and original rock stress in the suspected fracture zone through rock mechanics and geostress testing, specifically including: maximum original rock stress, minimum original rock stress, and rock tensile strength.

[0038] S2. A series of boreholes are drilled along the intended crack extension direction in the top plate of the cut eye to obtain several sets of hydraulic fracturing units. Each set of hydraulic fracturing units includes a hydraulic fracturing hole in the middle and two pressure-maintaining holes on both sides.

[0039] The hydraulic fracturing holes and pressure-maintaining holes have the same diameter, and the distance between two adjacent holes is 15 to 20 times the hole diameter.

[0040] The hydraulically fractured holes and the pressure-maintaining holes are of the same length.

[0041] Hydraulic fracturing holes within each hydraulic fracturing unit are simultaneously subjected to hydraulic fracturing operations at the same stratum.

[0042] S3. Determine the pressure holding pressure P of the pressure holding hole;

[0043] The pressure holding pressure P of the pressure holding orifice is less than the maximum pressure holding threshold P. max And greater than 0.9P max The maximum holding pressure threshold pressure P max The calculation formula is:

[0044] P max =3σ²-σ¹+σt

[0045] In the formula, σt σ1 represents the tensile strength of the rock in the fractured section, MPa; σ2 represents the maximum original rock stress, MPa; and σ3 represents the minimum original rock stress, MPa.

[0046] S4. Seal the hydraulic fracturing holes and pressure-maintaining holes in the first set of hydraulic fracturing units at the layer closest to the bottom of the borehole.

[0047] The hydraulic fracturing holes and pressure holding holes in each hydraulic fracturing unit are connected to independent fracturing pipelines of the same diameter to ensure that each fracturing pipeline can obtain the same water flow. The fracturing pipelines are equipped with pressure gauges, flow meters and shut-off valves. During the fracturing process, the water inflow of each hydraulic fracturing hole and pressure holding hole is controlled by adjusting the size of the shut-off valve.

[0048] The fracturing method in this embodiment is as follows:

[0049] See Figure 1 Each hydraulic fracturing unit includes one hydraulic fracturing hole 2 and two pressure-holding holes, namely the first pressure-holding hole 1 and the second pressure-holding hole 3. The diameter of both the hydraulic fracturing hole 2 and the pressure-holding holes is 85 mm, and the drilling spacing is 1500 mm.

[0050] The tensile strength of the drilled rock is 10 MPa, the maximum in-situ stress σ1 is 8 MPa, and the minimum in-situ stress σ2 is 5 MPa. Therefore, the maximum pressure holding threshold P for the pressure holding borehole is... max :

[0051] P max =3σ2-σ1+σt=3×5-8+10=17MPa

[0052] Therefore, the pressure P of the pressure holding hole needs to be less than 17MPa and greater than 0.9×17MPa=15.3MPa. Thus, the pressure of the pressure holding hole is determined to be 16MPa for this construction.

[0053] S5. Inject water into the first pressure-holding hole 1 and the second pressure-holding hole 3 until the water pressure reaches the pressure P determined in step S3, and maintain the water pressure to change the stress field around the hydraulically fractured hole so that the direction of the maximum principal stress is consistent with the direction of the series of boreholes.

[0054] S6. Then, water injection is performed on the hydraulic fracturing hole 2 between the first pressure holding hole 1 and the second pressure holding hole 3. After the water injection pressure of the hydraulic fracturing hole 2 reaches its peak and drops, water injection is stopped, and the sealing of the hydraulic fracturing hole 2 is removed.

[0055] S7. If the pressure in the first pressure holding hole 1 and the second pressure holding hole 3 decreases and water flows out from the hydraulic fracturing hole 2, then the hydraulic fracturing in the first hydraulic fracturing unit ends, the pressure holding hole is unsealed, and the process proceeds to step S9.

[0056] S8. If the pressure in the first pressure-holding hole 1 and the second pressure-holding hole 3 does not decrease, then reseal the hydraulic fracturing hole 2 and repeat steps S6 and S7.

[0057] S9. Repeat steps S4 to S8 to perform fracturing at the next segment height until all segments are fracturing.

[0058] S10. Perform fracturing on the next set of hydraulic fracturing units until the fracturing work of all hydraulic fracturing units is completed.

[0059] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A system of multi-hole multi-stage coupling directional fracturing of hard rock strata construction method, characterized in that: The method comprises the following steps: S1, obtaining rock mechanics parameters and in-situ stress of a cracking region by rock mechanics and in-situ stress testing, specifically including maximum in-situ stress, minimum in-situ stress and rock tensile strength; S2, constructing a series of drill holes along the extension direction of the expected crack in the roof of the cut to obtain a plurality of groups of hydraulic cracking units, each group of hydraulic cracking units including a middle hydraulic cracking hole and two side pressure maintaining holes; S3, determining the pressure maintaining pressure P of the pressure maintaining holes; S4, sealing the hydraulic cracking hole and the pressure maintaining holes in the first group of hydraulic cracking units at the horizon closest to the drill hole bottom; S5, injecting water into the two pressure maintaining holes until the water pressure reaches the pressure maintaining pressure P determined in step S2, and maintaining the water pressure, changing the stress field of the region around the hydraulic cracking hole to make the maximum principal stress direction consistent with the arrangement direction of the series of drill holes; S6, then injecting water into the hydraulic cracking hole between the two pressure maintaining holes for hydraulic fracturing, stopping the water injection after the water injection pressure of the hydraulic cracking hole reaches the peak value and drops, and unsealing the hydraulic cracking hole; S7, observing the pressure of the two pressure maintaining holes, if the pressure drops and water flows out of the middle hydraulic cracking hole, the hydraulic cracking in the first group of hydraulic cracking units is completed, the pressure maintaining holes are unsealed, and step S9 is entered; S8, if the pressure of the two pressure maintaining holes does not drop, the hydraulic cracking hole is resealed, and steps S6 and S7 are repeated; S9, repeating steps S4-S8 to fracture at the next segmented height until all the segmented fracturing is completed; S10, fracturing the next group of hydraulic cracking units until the fracturing work of all the hydraulic cracking units is completed; The holding pressure P in the holding hole in step S3 is less than a maximum holding threshold pressure P max , and greater than 0.9P max , the maximum holding threshold pressure P max is calculated by the following formula: ； wherein is the tensile strength of the rock in the fractured section, MPa; is the maximum in-situ stress, MPa; is the minimum in-situ stress, MPa; In step S2, the hydraulic cracking hole and the pressure maintaining hole in each group of hydraulic cracking units are respectively connected to independent and same-diameter fracturing pipelines to ensure that each fracturing pipeline can obtain the same water flow; a pressure gauge, a flow meter and a stop valve are arranged on the fracturing pipeline, and the water inflow of each hydraulic cracking hole and pressure maintaining hole is controlled by adjusting the size of the stop valve during the fracturing process.

2. The method according to claim 1, wherein the method is characterized by, In step S2, the hydraulic cracking hole and the pressure maintaining hole have the same diameter, and the spacing between the adjacent two drill holes is 15-20 times the diameter of the drill hole.

3. The method according to claim 1, wherein the method is characterized by, In step S2, the hydraulic cracking hole and the pressure maintaining hole have the same length.

4. The system porous multi-stage coupling directional fracturing hard rock formation construction method according to claim 1, characterized in that, The hydraulic cracking holes in each group of hydraulic cracking units are simultaneously fractured at the same horizon.

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