A vertical top cutting hydraulic fracturing method based on stress distribution characteristics of residual coal pillars

Abstract

This invention discloses a vertical roof-cutting hydraulic fracturing method based on the stress distribution characteristics of residual coal pillars, belonging to the field of coal mine roadway surrounding rock support technology. It includes the following steps: geological data investigation and analysis; determining the stress distribution characteristics of residual coal pillars; determining directional borehole layout parameters and performing fracturing; determining inclined and vertical borehole layout parameters and performing fracturing; and fracturing the roof in the peak stress region of the residual coal pillar. Directional boreholes penetrate vertically from the roadway sidewall through a spatial curve with a certain curvature, reaching the peak stress region of the residual coal pillar determined in S2. Hydraulic fracturing begins from the bottom of the borehole. Inclined and vertical boreholes are arranged from the roadway sidewall towards the roof above the spatial curve of the directional borehole in S3. Hydraulic fracturing also begins from the bottom of the borehole, with fracturing occurring every 3-5 meters. This method can cause premature instability and collapse of the coal pillar and upper roof, cutting off the transmission path of high dynamic and static loads on the high-level roof, and reducing the risk of multi-layered roof instability during face advancement.

Description

Technical Field

[0001] This invention belongs to the field of coal mine roadway surrounding rock support technology, specifically relating to a vertical roof cutting hydraulic fracturing method based on the stress distribution characteristics of residual coal pillars. Background Technology

[0002] Roof cutting and support collapse disasters, as a special type of mine geological hazard, require the simultaneous presence of certain causative factors to occur. Numerous practical engineering cases show that when mining the lower coal seams of the Jurassic coalfield in northern Shaanxi, it is necessary to extract various residual coal pillars from the overlying seams. This is often accompanied by severe mine pressure manifestation problems, among which the most common and harmful dynamic pressure disaster is roof cutting and support collapse disaster, which often causes huge economic and material losses to mining enterprises.

[0003] Residual coal pillars play a crucial role in preventing and controlling collapse disasters caused by coal pillar failures. Their main influencing factors are the pillar's shape (size, presence and characteristics of any empty roadway at its boundary) and its strength. These two factors primarily affect the pillar's ability to bear loads (i.e., "bearing the load from above"). The weaker the pillar's load-bearing capacity, the weaker its ability to transfer loads (i.e., "transferring the load from below"), thus making collapse disasters caused by coal pillar failures less likely to occur.

[0004] In the past, similar geological conditions have often been addressed by blasting to forcibly break up remaining coal pillars, causing the coal pillars and the upper roof to become unstable and collapse prematurely. This cuts off the transmission path of high dynamic and static loads on the upper roof, reducing the risk of multi-layered roof instability and stress during face advancement. However, blasting involves a huge amount of work and pollutes the underground air, posing a high safety risk and potentially leading to man-made accidents such as rockbursts and toxic gas leaks.

[0005] The invention patent CN113338925A describes a method for preventing dynamic load pressure on the lower working face by fracturing the coal pillars left over from the main roadway in multi-coal seam mining. This method involves sequentially drilling directional boreholes to hydraulically fracture the long, hard roof and then drilling horizontal boreholes to hydraulically fracture the remaining coal pillars. This eliminates the risk of dynamic load pressure on the lower working face caused by the coal pillars left over from the main roadway. However, although hydraulic fracturing is used, the fracturing boreholes are only directly facing the roof, resulting in too many boreholes and uncertain effectiveness, requiring further optimization. Summary of the Invention

[0006] The purpose of this invention is to provide a vertical roof-cutting hydraulic fracturing method based on the stress distribution characteristics of residual coal pillars, so as to cause the coal pillars and upper roof to become unstable and collapse in advance, cut off the transmission path of high dynamic and static loads on the upper roof, and reduce the risk of multi-layer roof instability during working face advancement. To achieve the above objective, this invention adopts the following technical solution:

[0007] A vertical roof-cutting hydraulic fracturing method based on the stress distribution characteristics of residual coal pillars includes the following steps:

[0008] S1: Geological data investigation and analysis;

[0009] S2: Determine the stress distribution characteristics of the remaining coal pillar support;

[0010] S3: Determine the directional drilling layout parameters and perform fracturing;

[0011] S4: Determine the layout parameters of inclined and straight boreholes and perform fracturing;

[0012] S5: Repeat S3 to S4 to induce cracking in the roof of the peak stress region on the other side of the remaining coal pillar.

[0013] S6: Inspection of the effect of fracturing and top cutting.

[0014] S1 includes the following: mine production data, which determines the distribution location and size parameters of the remaining coal pillars, as well as the lithology and thickness of the roof and floor strata based on the geological data within the mine; core sampling of the roof and floor strata of the remaining coal pillars for testing rock mechanical property parameters; and in-situ stress testing, which uses the hydraulic fracturing method to measure the direction and magnitude of the maximum principal stress, and can also obtain the pressure-time and flow-time curves of the hydraulic fracturing test in the fracturing area.

[0015] In S2, numerical simulation methods using engineering software are employed to determine the stress distribution characteristics of the remaining coal pillar support. Flac3D software is frequently used to establish numerical calculation models. The specific process is as follows:

[0016] 1) Determine the size of the calculation model based on the actual situation of the coal pillar strata, establish a numerical calculation model, and assign the rock mechanical parameters of the corresponding strata to the model based on the test results of the rock mechanical property parameters in S1;

[0017] 2) Based on the geostress test results in S1, set the initial stress and boundary conditions in the calculation model;

[0018] 3) After solving until the model is in equilibrium, reset the model displacement and velocity to zero, then conduct an excavation simulation based on the mine production situation and post-process the simulation results;

[0019] 4) Derive the stress cloud map from the above simulation results. From the stress cloud map, the peak stress region of the roof strata supporting the residual coal pillar can be obtained. Combined with the relative position of the calculation model size and the size of the residual coal pillar, the actual peak region location and distribution characteristics of the residual coal pillar can be determined.

[0020] In S3, the directional borehole starts from the sidewall of the roadway and passes through a spatial curve with a certain curvature, then vertically penetrates the peak stress region of the remaining coal pillar determined in S2. The curvature of the spatial curve is determined by the location of the directional drilling rig and the performance of the rig. The borehole diameter is 90mm to 100mm. When performing hydraulic fracturing, it starts from the bottom of the hole and is fracturing once every 3 to 5m. The fracturing pressure and the time for each fracturing are determined by the pressure-time and flow-time curves of the hydraulic fracturing test in S1.

[0021] Because of the spatial curve in the directional borehole in S3, inclined and straight boreholes are arranged above the spatial curve to fracture and cut the top. The borehole diameter is 90mm to 100mm. When hydraulic fracturing is carried out, it starts from the bottom of the hole and is fracturing once every 3 to 5m. The spatial curve segment of the directional borehole is the non-fracture segment. The slope of this segment is determined by the drilling rig performance, drilling rig position and the location of the fracture segment. The reduction of the support stress of the remaining coal pillar is a manifestation of hydraulic fracturing and pressure relief, and has no direct relationship with the slope of the non-fracture segment of the directional borehole.

[0022] The fracturing pressure and the time for each fracturing test were determined by the pressure-time and flow-time curves of the hydraulic fracturing test in S2. The fracturing time was the time corresponding to the depressurization in the pressure-time curve obtained during the hydraulic fracturing test. For the hydraulic fracturing method, please refer to "Rock Mechanics and Engineering (Second Edition)" pp. 137-140, by Cai Meifeng.

[0023] The detection of the top cutting effect of S5 medium-pressure fracturing includes the use of borehole inspection instruments, borehole scanning, and acoustic detection of crack propagation depth.

[0024] Beneficial effects:

[0025] 1. This invention uses directional hydraulic fracturing technology, which is safe, reliable, and pollution-free.

[0026] 2. The method in this invention can cause the coal pillar and the upper roof to become unstable and collapse in advance, cut off the transmission path of high dynamic and static loads on the high-level roof, and reduce the risk of multi-layer roof combination instability and pressure frame during the advancement of the working face.

[0027] 3. The drilling layout of the present invention is more reasonable and the construction is more convenient. Attached Figure Description

[0028] Figure 1 This is a flowchart illustrating the implementation of the present invention;

[0029] Figure 2 This is a three-dimensional schematic diagram of the present invention;

[0030] Figure 3 This is a cross-sectional view of the present invention;

[0031] Figure 4 To determine the stress distribution cloud map for the support pressure distribution characteristics of this invention;

[0032] Figure 5 This is a fracturing diagram showing the reduction of support pressure and the peak stress region of the support on the other side in hydraulic fracturing according to the present invention.

[0033] Figure 6 This is a bar chart from an embodiment of the present invention;

[0034] Figure 7 To determine the stress distribution cloud map based on the support pressure distribution characteristics in the embodiments of the present invention;

[0035] Figure 8 This is a diagram showing the arrangement of high and low-level fracturing boreholes in an embodiment of the present invention;

[0036] Figure 9 This is a borehole view showing the cracking effect in an embodiment of the present invention.

[0037] Among them, 1-roadway, 2-residual coal pillar, 3-fracturing roof strata, 4-peak bearing stress, 5-slanted and vertical boreholes, 6-directional boreholes, 7-coal seam, 8-goaf, 9-bearing stress distribution, 10-reduction of bearing stress. Detailed Implementation

[0038] The specific solutions of the present invention will be further explained below with reference to the accompanying drawings.

[0039] like Figures 1-3 , Figure 5 As shown, a vertical roof-cutting hydraulic fracturing method based on the stress distribution characteristics of residual coal pillars includes the following steps:

[0040] S1: Geological data investigation and analysis; including mine production data, determining the distribution location and size parameters of the remaining coal pillars, as well as the lithology and thickness of the roof and floor strata based on the geological data within the mine; core sampling of the roof and floor strata of the remaining coal pillars for testing rock mechanical property parameters; in-situ stress testing, using the hydraulic fracturing method to measure the direction and magnitude of the maximum principal stress, and simultaneously obtaining pressure-time and flow-time curves of the hydraulic fracturing test in the fracturing area.

[0041] S2: Determine the stress distribution characteristics of the remaining coal pillar support; use engineering software numerical simulation methods to determine the stress distribution characteristics of the remaining coal pillar support, often using Flac3D software to establish a numerical calculation model. The specific process is as follows:

[0042] 1) Determine the size of the calculation model based on the actual situation of the coal pillar strata, establish a numerical calculation model, and assign the rock mechanical parameters of the corresponding strata to the model based on the test results of the rock mechanical property parameters in S1;

[0043] 2) Based on the geostress test results in S1, set the initial stress and boundary conditions in the calculation model;

[0044] 3) After solving until the model is in equilibrium, reset the model displacement and velocity to zero, then conduct an excavation simulation based on the mine production situation and post-process the simulation results;

[0045] 4) Derive the stress contour plot from the above simulation results, such as... Figure 4 As shown, the peak stress region of the roof strata supporting the residual coal pillar can be obtained from the stress cloud map. Combined with the relative position of the calculation model size and the size of the residual coal pillar, the actual peak region location and distribution characteristics of the residual coal pillar can be determined.

[0046] S3: Determine the directional drilling layout parameters and perform hydraulic fracturing; the directional borehole starts from the sidewall of the roadway, passes through a spatial curve with a certain curvature, and then vertically penetrates the peak stress region of the remaining coal pillar determined in S2. The curvature of the spatial curve is determined by the location of the directional drilling rig and the performance of the rig. The borehole diameter is 90mm to 100mm. When performing hydraulic fracturing, start from the bottom of the hole and perform fracturing once every 3 to 5m. The fracturing pressure and the time for each fracturing are determined by the pressure-time and flow-time curves of the hydraulic fracturing test in S1.

[0047] S4: Determine the layout parameters of the inclined and straight boreholes and perform hydraulic fracturing; there is a spatial curve in the directional borehole. Inclined and straight boreholes are arranged on the top plate above the spatial curve to fracture and cut the top. The borehole diameter is 90mm to 100mm. When performing hydraulic fracturing, start from the bottom of the hole and perform fracturing once every 3 to 5m. The fracturing pressure and the time for each fracturing are determined by the pressure-time and flow-time curves of the hydraulic fracturing test in S2.

[0048] S5: Repeat S3 to S4 to induce fracturing in the roof of the peak stress area on the other side of the remaining coal pillar; the effect of fracturing and roof cutting is detected by using borehole inspection instruments, borehole scanning, and sonic detection to detect the depth of crack propagation.

[0049] S6: Inspection of the effect of fracturing and top cutting.

[0050] There is a 50m wide coal pillar in the 2-2 coal seam above the 31107 working face of a certain mine. According to the extensive engineering practice experience of researchers in the Shendong mining area, the existence of the coal pillar causes the working face to frequently experience the high concentrated stress transmitted from the roof above through the coal pillar, which may cause strong mine pressure manifestation and roof collapse accidents.

[0051] Based on the core sampling results from the 31107 working face, its stratigraphic columnar section is as follows: Figure 6 The average mining thickness is 3.7m, the dip length is 240m, and the maximum horizontal principal stress σ in the mine was measured. H=6.29 MPa; the average initiation pressure of hydraulic fracturing in the roof was 18.3 MPa; rock samples taken from the geological core drilling of the 31107 working face were processed into standard specimens for testing. The tests included: apparent density, uniaxial compressive strength, tensile strength, shear strength, cohesion, internal friction angle, Poisson's ratio, and elastic modulus. The testing methods were in accordance with "GBT 23561 Methods for Determination of Physical and Mechanical Properties of Coal and Rock".

[0052] The test results are shown in the table below:

[0053] Rock physical and mechanical parameter test results

[0054]

[0055] First, the geological conditions of the II5 track uphill tunnel were modeled using FLAC3D software: the Extrusion modeling function was used to extrude the planar model into a three-dimensional model, resulting in the final numerical calculation model. The model is 90m high, 380m wide (dip), and 400m long (strike). The model includes the 3-1 coal seam, the 2-2 coal seam, inter-layer strata (including sandy mudstone, mudstone, and sandstone with a total thickness of 37m), and overlying and underlying strata (each 20m), eliminating boundary effects.

[0056] The model is fixed in the x and y directions, and at the bottom in the z direction. An equivalent overburden self-weight stress of 5.0 MPa is applied at the upper boundary. The constitutive model for the coal seam adopts the strain softening criterion, while the Mohr-Coulomb model is used for other rock strata.

[0057] The simulated excavation scheme is as follows: Model ground stress balance → excavation of the 2-2 coal seam, forming strike and dip-side coal pillars, and monitoring of floor and coal pillar stress and plastic zone distribution → excavation of the 31107 working face and the 3-1 coal seam roadway → excavation of the 31108 working face, conducting stress concentration and plastic zone analysis to determine the main fracturing areas. Figure 7 As can be seen, after the excavation of the 2-2 coal seam, the vertical stress of the remaining coal pillar showed a trend of first increasing and then decreasing, with local stress concentration on both sides of the coal pillar. The maximum vertical stress was 13.7 MPa, located at approximately 12-14 m from the coal pillar boundary (i.e., the fracturing zone).

[0058] Fracture-inducing borehole arrangement as follows Figure 8 As shown.

[0059] like Figure 9 The cracks inside the borehole are generally distributed in a linear pattern, and most of them extend along the borehole axis. The crack width is between 0.5mm and 3mm, and some cracks show good cracking performance.

[0060] Through construction, the risk of the structure being crushed was released in advance, ensuring the safety of the coal mine.

Claims

1. A vertical roof-cutting hydraulic fracturing method based on the stress distribution characteristics of residual coal pillars, characterized by comprising the following steps: S1: Geological data investigation and analysis; S2: Determine the stress distribution characteristics of the remaining coal pillar support; S3: Determine the directional drilling layout parameters and perform fracturing; S4: Determine the layout parameters of inclined and straight boreholes and perform fracturing; S5: Repeat S3~S4 to induce fracturing in the roof of the remaining coal pillar support stress peak area; S6: Inspection of the effectiveness of hydraulic fracturing and top cutting; The S2 method uses engineering software numerical simulation to determine the stress distribution characteristics of the remaining coal pillar support. A numerical calculation model is established using Flac3D software for calculation. The specific process is as follows:

1. Determine the size of the calculation model based on the actual strata of the remaining coal pillars, establish a numerical calculation model, and assign the rock mechanical parameters of the corresponding strata to the model based on the test results of the rock mechanical property parameters in S1; 2. Based on the geostress test results in S1, set the initial stress and boundary conditions in the calculation model; 3. After solving until the model is in equilibrium, reset the model displacement and velocity to zero, then conduct an excavation simulation based on the mine production situation and post-process the simulation results; 4. Derive the stress cloud map from the above simulation results. From the stress cloud map, the peak stress region of the roof strata supporting the residual coal pillar can be obtained. Combined with the relative position of the calculation model size and the size of the residual coal pillar, the actual peak region location and distribution characteristics of the residual coal pillar can be determined. In S3, the directional borehole starts from the sidewall of the roadway with a certain curvature, passes through a spatial curve, and then vertically penetrates the peak stress region of the residual coal pillar determined in S2. When hydraulic fracturing is carried out, it starts from the bottom of the borehole and fracturing is performed every 3 to 5 meters. The inclined straight borehole in S4 is arranged on the top plate above a section of spatial curve from the side of the roadway toward the directional borehole in S3. When hydraulic fracturing is performed, it starts from the bottom of the borehole and is fracturing once every 3 to 5 meters.

2. The vertical roof-cutting hydraulic fracturing method based on the stress distribution characteristics of residual coal pillars according to claim 1, characterized in that, The geological data in S1 includes the following: mine production data, which determines the distribution location and size parameters of the remaining coal pillars, as well as the lithology and thickness of the top and bottom rock strata based on the geological data within the mine; core sampling of the top and bottom rock strata of the remaining coal pillars for testing rock mechanical property parameters; and in-situ stress testing, which uses the hydraulic fracturing method to measure the direction and magnitude of the maximum principal stress and obtains the pressure-time and flow-time curves of the hydraulic fracturing test in the fracturing area.

3. The vertical roof-cutting hydraulic fracturing method based on the stress distribution characteristics of the remaining coal pillar as described in claim 2, characterized in that, The fracturing time is determined by the pressure-time and flow-time curves of the hydraulic fracturing test.

4. The vertical roof-cutting hydraulic fracturing method based on the stress distribution characteristics of residual coal pillars according to claim 1, characterized in that, The borehole diameter in S3 and S4 is 90mm~100mm.

5. The vertical roof-cutting hydraulic fracturing method based on the stress distribution characteristics of residual coal pillars according to claim 1, characterized in that, The detection of the fracturing and top cutting effect in S6 includes the use of borehole inspection instruments, borehole scanning, and acoustic detection of crack propagation depth.

Images