Coal seam gas extraction method with integrated drilling and sealing

Abstract

The present invention discloses a coal seam gas extraction method with integrated through-layer drilling and isolation. Before grouting, slits are set in the coal rock layer around the sealing section so that each slit is connected to the sealing section of the gas extraction hole. The optimal number of slits is determined according to the following steps: the first step is to prepare a grouting coal rock sample; the second step is to perform CT scanning after loading; the third step is to obtain the fracture ratio by image processing; the fourth step is to obtain the relationship formula between permeability and the number of slits, i.e., Formula 2, and the relationship formula between fracture ratio and the number of slits, i.e., Formula 3, by curve fitting using Origin software; and the fifth step is to determine the optimal number of slits. The inventor has established a technical approach to determine the number of slits based on the relationship between the number of slits and the fracture ratio and permeability of the on-site grouting coal rock layer. By determining the appropriate number of slits in the drilling and grouting section, the grouting sealing effect and the structural strength of the grouting layer are taken into consideration to achieve the relatively best gas extraction effect.

Description

Technical Field

[0001] The present invention relates to the technical field of coal mine gas extraction. Background Art

[0002] In coal mining, there are often complex ore rock geology, deep burial, high gas content and low permeability, which are very likely to cause geological disasters such as coal and gas outbursts. In order to ensure the normal and safe production of coal mines in my country, gas extraction has become one of the most effective prevention and control measures. It is also an effective way to develop and utilize energy gas.

[0003] The effect of coal seam gas extraction is directly affected by the quality of the extraction borehole sealing. After grouting and sealing the rock formation cracks, as the coal seam is continuously mined, the borehole and surrounding rock mass are continuously subjected to mining stress and ground stress, which has a great impact on the extraction effect of coal seam gas.

[0004] Therefore, a more in-depth study of the mechanical characteristics and crack development of the grouting body after drilling and sealing can verify the rationality of the drilling and sealing process.

[0005] At present, many scholars have carried out in-depth research on drilling grouting sealing technology, and have developed drilling grouting sealing method structures such as "two blocking and one grouting", "three blocking and two-stage grouting" and "segmented and time-divided grouting". At the same time, they have also achieved relatively comprehensive results in the directions and contents of drilling grouting equipment, materials, etc., which has enabled grouting sealing technology to be widely used in mining and promoted the development of gas extraction work and its safety.

[0006] During the mine gas extraction process, the development pattern of the cracks outside the hole is not obvious enough during grouting and sealing, so the grouting slurry cannot completely penetrate into the gaps outside the hole to seal it, and stable drilling grouting pressure cannot be ensured, which affects the effect of gas extraction.

[0007] The use of slit grouting sealing technology can make the development of cracks more concentrated by slits in the drill hole, which can significantly improve the penetration and tightness of the grouting slurry in the cracks outside the hole, and make the pressure during grouting more stable, thereby significantly improving the gas extraction effect in the coal seam.

[0008] Based on the "two blocking and one injection" sealing technology, relevant scholars from the Chongqing Research Institute of China Coal Technology and Engineering Group have invented a method and structure for slit grouting with fixed-point pressurization and expansion of seepage, which uses an inflatable air bag for pressurization and expansion of seepage, thereby blocking the cracks around the borehole.

[0009] Bored hole slit grouting and sealing draws on the principles of integrated isolation and sealing. This innovative sealing process combines the traditional "two-blocking, one-injection" sealing process with ultra-high-pressure hydraulic slit grouting. It is suitable for environments with uneven coal seam thickness, low permeability, rapid gas attenuation, and insignificant permeability enhancement. The permeability of the rock grouting body is considered to determine the appropriate number of slits. In conjunction with borehole imaging equipment, the distribution of fractures under confining pressure is analyzed to determine the appropriate sealing length. A mechanical reamer slit grouting technique is then used to assist in sealing the hole.

[0010] To study the effect of the number of drilling slots on the grouting and gas extraction effects, the inventors conducted the following research:

[0011] 1. Triaxial seepage tests were conducted on coal rock samples with slot grouting. It was found that the average permeability increase would decrease exponentially with the increase in the number of slots.

[0012] Increasing the number of slits and then grouting the outer layer of the gas extraction section (the direction close to the surface or the tunnel is the outward direction, and the direction pointing deep into the coal rock is the inward direction) can better prevent external gas from reaching the gas extraction section through the grouting section. Therefore, increasing the number of slits and then grouting is a better sealing technology for gas extraction boreholes.

[0013] 2. At the same time, industrial CT scanning equipment and image analysis software were used to analyze grouting coal rock samples with different numbers of slits. It was found that with the increase in the number of slits, the crack rate of the sample after load failure also increased significantly, indicating that the increase in the number of slits will deteriorate the strength of the sample. After the strength of the grouting coal rock layer decreases, collapse, coal rock deformation and other phenomena may occur, destroying the normal environment for gas extraction, which is not conducive to normal gas extraction.

[0014] 3. 1 and 2 above indicate that increasing the number of slits can have both positive and negative effects on gas extraction (drill hole sealing). Without slits, the grouting area is significantly reduced, and the grouting fluid may not be able to enter certain coal rock fissures that are not directly connected to the gas extraction hole, thereby reducing the sealing effect during gas extraction. Excessive slits, resulting in excessive cracking of the grouting coal rock layer, can easily lead to hole collapse and coal rock deformation, disrupting the normal gas extraction environment. This can reduce gas extraction efficiency at best and render the borehole useless and ineffective.

[0015] Based on the above research, it is shown that in the engineering practice of drilling slits and grouting sealing, it is necessary to determine a reasonable number of slits to take into account the grouting sealing effect (the better the sealing effect, the better the extraction effect) and the structural strength of the grouting coal rock layer. The sealing effect and the structural strength of the grouting coal rock layer should be regarded as a whole, and the optimal number of slits should be found to make this whole most beneficial for gas extraction.

[0016] Therefore, the inventors have set up a technical idea of ​​determining the number of slits based on the relationship between the number of slits and the fracture rate and permeability of the on-site grouting coal rock layer (coal rock body). By determining the appropriate number of slits in the drilling grouting section, taking into account the grouting sealing effect and the structural strength of the grouting layer, the relatively best gas extraction effect can be achieved. Summary of the Invention

[0017] The purpose of the present invention is to provide an integrated coal seam gas extraction method with through-layer drilling and sealing, which regards the sealing effect and the structural strength of the grouting coal rock layer as a whole, and uses the number of slits that is most favorable for gas extraction as an operating parameter to achieve the relatively best gas extraction effect.

[0018] To achieve the above-mentioned objectives, the present invention provides a coal seam gas extraction method with integrated through-layer drilling and sealing, wherein a gas extraction hole is drilled into the coal seam from a roadway, a gas extraction pipe is inserted into the gas extraction hole from the roadway, the end of the gas extraction pipe is inserted into the coal seam, a sealing section is provided at the rear section of the gas extraction hole, and a section of the gas extraction hole from the end of the sealing section to the end of the gas extraction hole is the gas extraction section; grouting is injected into the coal rock layer around the drill hole in the sealing section, and a grouting area is formed after the slurry solidifies; the grouting area is used to form a barrier outside the coal seam to prevent air from flowing from areas outside the coal seam to the gas extraction section; finally, a negative pressure generating device is used to extract coal seam gas from the gas extraction hole through the gas extraction pipe.

[0019] Before grouting, slots are set in the coal and rock layers around the sealed section so that each slot is connected to the sealed section of the gas extraction hole. The optimal number of slots is determined by the following steps:

[0020] The first step is to prepare the grouting coal rock sample;

[0021] Sampling is performed at a predetermined position of the coal rock layer, and the taken coal rock sample is divided into four cylinders of the same size. Each cylinder is drilled and grouting is performed to produce four grouting coal rock samples, which are numbered as grouting coal rock sample No. 0, grouting coal rock sample No. 1, grouting coal rock sample No. 2, and grouting coal rock sample No. 3, respectively; a hole is drilled at the center of grouting coal rock samples No. 0 to No. 3, and 0 slits are set around the drill hole on grouting coal rock sample No. 0, 1 slit is set on grouting coal rock sample No. 1, 2 slits are set on grouting coal rock sample No. 2, and 3 slits are set on grouting coal rock sample No. 3;

[0022] The second step is a post-loading CT scan;

[0023] First, the triaxial confining pressure of the true triaxial loading device is set according to Formula 1;

[0024] Formula 1 is: σ=γz;

[0025] In formula 1, σ is the stress at the sampling location in the coal rock layer, in MPa; γ is the weight of the coal rock per unit volume at the sampling location, in kN / m3; z is the burial depth of the sampling location in the coal rock layer, in meters; the triaxial confining pressure of the true triaxial loading device is σ;

[0026] Then triaxial compression tests were conducted;

[0027] A true triaxial loading device was used to apply a predetermined confining pressure to four grouting coal and rock samples for triaxial compression tests. After the triaxial compression tests, the four grouting coal and rock samples were scanned again with industrial CT to obtain scanned slice images of multiple locations of the loaded grouting coal and rock samples.

[0028] The third step is to process the image to obtain the crack rate;

[0029] Image analysis software ImageJ was used to analyze scanned slice images at multiple different positions of the grouting coal-rock specimens after loading. The scanned slice images at different positions were binarized to obtain the crack ratios of grouting coal-rock specimens No. 0 to No. 3 after loading. Specifically, for each scanned slice image, the crack ratio of the scanned slice image was obtained by calculating its crack area and the total area of ​​the scanned slice image. The average crack ratio of all scanned slice images of a grouting coal-rock specimen was taken as the crack ratio of the grouting coal-rock specimen.

[0030] The fourth step is to use Origin software to obtain the relationship formula between permeability and the number of slits (Formula 2), and the relationship formula between fracture rate and the number of slits (Formula 3) through curve fitting;

[0031] The first sub-step of the fourth step is to input the permeability of grouting coal rock samples No. 0 to No. 3 into the Origin software for curve fitting, obtain the dimensionless fitting coefficients y10, C11 and d11, and establish the relationship between permeability and the number of slits, that is, Formula 2:

[0032] y1=y10+C11×e (-x / d11 ) ;

[0033] In formula 2, y1 is the permeability, the unit is millidarcy or mD;

[0034] x is the number of slits, in strips;

[0035] y10, C11 and d11 are dimensionless fitting coefficients;

[0036] Formula 2 is used to predict the permeability of grouting coal rock layer when the number of cuts is greater than 3;

[0037] The second sub-step of the fourth step is to input the crack ratio of grouting coal rock samples No. 0 to No. 3 into the Origin software for curve fitting, obtain the dimensionless fitting coefficients y20, C21 and d21, and establish the relationship between the crack ratio and the number of slits, that is, Formula 3:

[0038] y2=y20+C21×e (-x / d21 ) ;

[0039] In formula 3, y2 is the crack ratio expressed in percentage;

[0040] x is the number of slits, in strips;

[0041] y20, C21 and d21 are dimensionless fitting coefficients;

[0042] Formula 2 is used to predict the fracture rate of grouting coal rock layer when the number of cuts is greater than 3;

[0043] The fifth step is to determine the optimal number of slits;

[0044] According to Formula 2 and Formula 3, with the number of slits as the horizontal coordinate and the permeability and fracture ratio as the vertical coordinates, a curve showing the relationship between fracture ratio and the number of slits, as well as a curve showing the relationship between permeability and the number of slits, are plotted in the same coordinate system. The integer adjacent to the horizontal coordinate corresponding to the intersection of the two curves is the optimal number of slits.

[0045] The specific position and length of the sealing section are determined by the following method:

[0046] Before inserting the gas extraction pipe into the gas extraction hole, Figure 2 As shown, a camera of a borehole imager is installed at the end of a measuring rod, and the camera is inserted into the gas extraction hole through the measuring rod, starting from the middle and rear part of the gas extraction hole, and the image taken by the camera is manually observed; there is a scale on the measuring rod, and the scale of the measuring rod indicates the depth of the camera inserted into the gas extraction hole; while manually observing the image taken by the camera, the camera is slowly pushed deeper into the gas extraction hole until it reaches the coal seam, that is, the end of the gas extraction hole; in this process, the picture is manually observed to determine the looseness of the coal and rock layers around the gas extraction hole, and the depth of the gas extraction hole corresponding to the position that needs to be reinforced is recorded, and the gas extraction hole section between the deepest and shallowest depths in the position that needs to be reinforced is used as the sealing section to ensure that the sealing section covers all positions that need to be reinforced.

[0047] The slit is an annular slit surrounding the gas extraction hole, which is used to make it easier for the grouting slurry to penetrate and diffuse into the cracks around the gas extraction hole under the action of the grouting pressure. After the grouting liquid solidifies, it forms a barrier to prevent external gas from entering the gas extraction section deep under the action of the extraction negative pressure, thereby improving the sealing effect.

[0048] Each grouting coal rock sample is a cylinder with a diameter of 50 mm and a length of 100 mm.

[0049] The present invention has the following advantages:

[0050] See also Figure 6 At the intersection of the two relationship curves, moving rightward indicates increasing the number of slits. The relationship curve between permeability and the number of slits exhibits a certain characteristic (independent of this embodiment): the greater the number of slits, the better the grouting effect and the lower the permeability. Furthermore, the fewer slits, the greater the impact of the number of slits on permeability, while the greater the number of slits, the smaller the impact. This is reflected in the curve: the curve becomes steeper the further to the left and flatter the further to the right. This characteristic of the curve being steep on the left and flatter on the right is very obvious.

[0051] Similarly, the curve relating fracture ratio to the number of slits also exhibits a certain characteristic (independent of this embodiment): the greater the number of slits, the higher the fracture ratio. However, unlike the curve relating permeability to the number of slits, the curve relating fracture ratio to the number of slits does not exhibit a distinct characteristic of steepening on the left and flattening on the right. As the number of slits increases, the fracture ratio continues to increase at a relatively rapid rate.

[0052] The inventors discovered the existence of an optimal number of slits based on the characteristics of two relationship curves, one with a very obvious characteristic of being steep on the left and flat on the right, and the other with no obvious characteristic of being steep on the left and flat on the right.

[0053] Based on the characteristics of the above-mentioned curves for the relationship between fracture ratio and number of slits, and the curve for the relationship between permeability and number of slits, the two curves inevitably intersect. The horizontal coordinate (nearby integer value) of the intersection of the two curves represents the optimal number of slits. If the number of slits is greater than the optimal value, the permeability decreases more slowly and smoothly as the number of slits increases (favoring gas extraction). At the same time, the fracture ratio increases more rapidly with the number of slits (disadvantaging gas extraction). In other words, as the intersection is to the right, the factors unfavorable to gas extraction increase faster than those favorable to gas extraction. To the right (with more slits than at the intersection), gas extraction becomes unfavorable.

[0054] Moving leftward at the intersection (reducing the number of slits) results in a rapid increase in permeability (steep on the left, flat on the right). This means that further reducing the number of slits at the intersection increases the factor unfavorable to gas extraction (increased permeability) more rapidly. Simultaneously, because the fracture ratio curve lacks the obvious steep-left, flat-right characteristic, further reducing the number of slits at the intersection results in a slower increase in the factor favorable to gas extraction (decreased fracture ratio) than the factor unfavorable to gas extraction (increased permeability). Combined with these two factors, moving the intersection leftward (fewer slits than at the intersection) also negatively impacts gas extraction.

[0055] The present invention obtains the intersection point through experiments and formulas 2 and 3, determines the optimal number of slits, takes into account the permeability and fracture rate, and takes into account the sealing conditions required for extraction (to avoid drawing air outside the coal seam into the gas extraction pipe), that is, the sealing effect and the strength of the grouting coal rock layer, to create relatively optimal extraction conditions for gas extraction. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 This is the relationship line graph between the fracture ratio and the number of slits, and the relationship line graph between the permeability and the number of slits before curve fitting;

[0057] Figure 2 This is a schematic diagram of the working state structure when using a borehole imaging device to determine the position and length of the sealing section;

[0058] Figure 3 It is a schematic diagram of the structure of the gas extraction hole between the roadway and the coal seam;

[0059] Figure 4 It is the overall workflow diagram of the present invention;

[0060] Figure 5 This is a comparison chart of the crack ratio of No. 1-3 grouting coal rock specimens and No. 0 grouting coal rock specimen after loading;

[0061] Figure 6 is a curve diagram showing the relationship between the crack ratio and the number of slits and a curve diagram showing the relationship between the permeability and the number of slits in the fifth step after curve fitting;

[0062] Figure 7 This is a graph showing the effect of the number of slots on the permeability increase and the three-dimensional fracture rate increase of the grouting coal rock sample;

[0063] Figure 8 This is a comparison chart of gas concentration between the control group and the experimental group;

[0064] Figure 9 This is a comparison chart of the pure amount of gas extraction between the control group and the experimental group. DETAILED DESCRIPTION

[0065] like Figures 1 to 9As shown, the integrated coal seam gas extraction method of the present invention comprises the following steps: a gas extraction hole 4 is drilled into the coal seam from a roadway 5, a gas extraction pipe 15 is inserted into the gas extraction hole 4 from the roadway 5, the end of the gas extraction pipe 15 is inserted into the coal seam 10, a sealing section 12 is provided at the rear section of the gas extraction hole 4, and a section of the gas extraction hole 4 from the end of the sealing section 12 to the end of the gas extraction hole 4 is a gas extraction section 11 (the gas in the gas extraction section 11 is discharged by the gas extraction pipe 15). Extraction); Grouting is injected into the coal and rock layers around the borehole in the sealing section 12, and a grouting area is formed after the slurry solidifies; the grouting area is used to form a barrier outside the coal seam 10 to prevent air from flowing from the area outside the coal seam 10 to the gas extraction section 11; finally, a negative pressure generating device (such as an exhaust fan or a vacuum pump) is used to extract gas (mainly gas) from the coal seam 10 outside the gas extraction hole 4 through the gas extraction pipe 15; both ends of the sealing section 12 are sealed by a sealer 14 (or an air bag);

[0066] Figure 3 In the figure, number 6 indicates the crushing zone in the coal rock layer, number 7 indicates the plastic zone in the coal rock layer, number 8 indicates the elastic zone in the coal rock layer, and number 9 indicates the original rock stress zone in the coal rock layer.

[0067] Before grouting, slits 13 are provided in the coal and rock layers around the sealing section 12 so that each slit 13 is connected to the sealing section 12 of the gas extraction hole 4. The optimal number of slits 13 is determined by the following steps:

[0068] The first step is to prepare the grouting coal rock sample;

[0069] Sampling is performed at a predetermined position of the coal rock layer, and the taken coal rock sample is divided into four cylinders of the same size. Each cylinder is drilled and grouting is performed to produce four grouting coal rock samples, which are numbered as grouting coal rock sample No. 0, grouting coal rock sample No. 1, grouting coal rock sample No. 2 and grouting coal rock sample No. 3 respectively; a hole is drilled at the center of grouting coal rock samples No. 0 to No. 3, and 0 cutting slits 13 are set around the drill hole on grouting coal rock sample No. 0, 1 cutting slit 13 is set on grouting coal rock sample No. 1, 2 cutting slits 13 are set on grouting coal rock sample No. 2, and 3 cutting slits 13 are set on grouting coal rock sample No. 3; setting cutting slits around the hole in coal rock or in the sample is a conventional technique and will not be described in detail.

[0070] The second step is a post-loading CT scan;

[0071] First, the triaxial confining pressure of the true triaxial loading device is set according to Formula 1;

[0072] Formula 1 is: σ=γz;

[0073] In formula 1, σ is the stress at the sampling location in the coal rock layer (depending on the burial depth, 20 MPa in this embodiment), with the unit of MPa; γ is the weight per unit volume of the coal rock at the sampling location (bulk density), with the unit of kN / m3 (kN•m -3 ); z is the burial depth of the coal stratum sampling location (i.e. the vertical distance between the sampling location and the coal stratum surface), in meters (m); the triaxial confining pressure of the true triaxial loading device is set to σ;

[0074] Then triaxial compression (loading) experiments were conducted;

[0075] A triaxial compression test was conducted on four grouting coal-rock specimens using a true triaxial loading device, applying a predetermined confining pressure (i.e., σ MPa). After the triaxial compression test, the four grouting coal-rock specimens were again subjected to industrial CT scanning to obtain scanned slice images of multiple different positions of the grouting coal-rock specimens after loading (1,200 different positions were taken in this embodiment). The scanned slice images of the grouting coal-rock specimens after loading express the internal crack development of the four grouting coal-rock specimens under loading.

[0076] The true triaxial loading device can be the RLW-500G true triaxial loading system independently developed by Henan Polytechnic University, or other models of true triaxial loading devices can be used. Using a true triaxial loading device to perform triaxial compression tests on specimens is a conventional technique and will not be described in detail.

[0077] In this embodiment, whether it is a CT scan before or after loading, the number of scanned slice images of each grouting coal rock sample is 1200, the scanning accuracy is 55-57 μm, and the scanning voltage is 140-150 Kv; the more scans, the greater the voltage, and the higher the accuracy (the smaller the accuracy value), the higher the quality of the obtained CT slice image, and the longer the corresponding scanning time.

[0078] The third step is to process the image to obtain the crack rate;

[0079] The image analysis software ImageJ was used to analyze the scanned slice images of multiple different positions of the grouting coal rock specimens after loading. The scanned slice images at different positions were binarized to obtain the crack ratios of the grouting coal rock specimens No. 0 to No. 3 after loading. Figure 5 Specifically, for each scan slice image, the crack area and the total area of ​​the scan slice image are calculated to obtain the crack ratio of the scan slice image, and the average crack ratio of all scan slice images of a grouting coal rock sample (the sum of the crack ratios of each scan slice image divided by the number of scan slice images) is used as the crack ratio of the grouting coal rock sample;

[0080] The fourth step is to use Origin software to obtain the relationship formula between permeability and the number of slits 13, namely Formula 2, and the relationship formula between fracture rate and the number of slits, namely Formula 3, through curve fitting.

[0081] The first sub-step of the fourth step is to input the permeability of grouting coal rock samples No. 0 to No. 3 into the Origin software for curve fitting (obtain Figure 6 The left side of the curve is higher than the right side), and the dimensionless fitting coefficients y10, C11 and d11 are obtained. The relationship between the permeability and the number of slits 13 is established, that is, Formula 2:

[0082] y1=y10+C11×e (-x / d11 ) ;

[0083] In formula 2, y1 is the permeability, the unit is millidarcy or mD;

[0084] x is the number of slits 13, in strips;

[0085] y10, C11 and d11 are dimensionless fitting coefficients;

[0086] Formula 2 is used to predict the permeability of the grouting coal rock layer when the number of cuts 13 is greater than 3;

[0087] The second sub-step of the fourth step is to input the crack ratio of grouting coal rock samples No. 0 to No. 3 into the Origin software for curve fitting (obtain Figure 6 The dimensionless fitting coefficients y20, C21 and d21 are obtained, and the relationship between the crack rate and the number of slits 13 is established, that is, Formula 3:

[0088] y2=y20+C21×e (-x / d21 ) ;

[0089] In formula 3, y2 is the crack ratio expressed in percentage;

[0090] x is the number of slits 13, in units of strips;

[0091] y20, C21 and d21 are dimensionless fitting coefficients;

[0092] Formula 2 is used to predict the crack rate of the grouting coal rock layer when the number of cuts 13 is greater than 3;

[0093] In this embodiment, the values ​​of the dimensionless fitting coefficients in Formula 2 and Formula 3 obtained by the Origin software are shown in Table 1, which is a table of the values ​​of the dimensionless fitting coefficients in Formula 2 and Formula 3.

[0094] Table 1

[0095]

[0096] The fifth step is to determine the optimal number of slits 13;

[0097] According to Formula 2 and Formula 3, with the number of slits 13 as the horizontal coordinate and the permeability and crack ratio as the vertical coordinate, a curve of the relationship between the crack ratio and the number of slits 13, as well as a curve of the relationship between the permeability and the number of slits 13, are drawn in the same coordinate system; the integer adjacent to the horizontal coordinate corresponding to the intersection of the two relationship curves is the optimal number of slits 13. Figure 6 The optimal number of slits 13 in this embodiment is shown to be 1.

[0098] In this embodiment, since the optimal number of slits is 1, there is no need to use Formulas 2 and 3 to infer the fracture rate and permeability of the grouting coal rock layer when the number of slits 13 is greater than 3. However, the optimal number of slits is not necessarily less than or equal to 3 in every extraction scenario (due to varying stress conditions and coal rock properties). With the present invention, since Formulas 2 and 3 are generated, there is no need to produce additional samples. A sufficiently long relationship curve can be drawn from Formulas 2 and 3 until the two relationship curves intersect, significantly reducing the experimental workload and time.

[0099] After finding the optimal number of slits 13, refer to Figure 6 At the intersection of the two relationship curves, moving rightward indicates increasing the number of slits 13. The relationship curve between permeability and the number of slits 13 exhibits a certain characteristic (independent of this embodiment): the greater the number of slits 13, the better the grouting effect and the lower the permeability. Furthermore, the fewer slits 13, the greater the impact of the number of slits 13 on permeability, while the greater the number of slits 13, the smaller the impact. This is reflected in the curve: the curve becomes steeper the further to the left and flatter the further to the right. This characteristic of the curve being steep on the left and flatter on the right is very obvious.

[0100] Similarly, the curve showing the relationship between the fracture ratio and the number of slits 13 also exhibits a certain characteristic (independent of this embodiment): the greater the number of slits 13, the higher the fracture ratio. However, unlike the curve showing the relationship between the permeability and the number of slits 13, the curve showing the relationship between the fracture ratio and the number of slits 13 does not exhibit a distinct characteristic of steepening on the left and flattening on the right. As the number of slits 13 increases, the fracture ratio continues to increase at a relatively rapid rate.

[0101] Based on the characteristics of the two relationship curves, one with a very obvious characteristic of being steep on the left and flat on the right, and the other with no obvious characteristic of being steep on the left and flat on the right, the inventors discovered that the optimal number of slits is 13.

[0102] Based on the characteristics of the curves showing the relationship between fracture ratio and the number of 13 slits, and the curve showing the relationship between permeability and the number of 13 slits, the two curves inevitably intersect. The horizontal coordinate (nearby integer value) of the intersection of the two curves represents the optimal number of slits. If the number of 13 slits is greater than the optimal value, the permeability decreases more slowly and smoothly as the number of 13 slits increases (favoring gas extraction). At the same time, the fracture ratio increases more rapidly with the number of 13 slits (disadvantaging gas extraction). In other words, as the intersection is to the right, the factors unfavorable to gas extraction increase faster than those favorable to gas extraction. To the right (with more 13 slits than the intersection), gas extraction becomes unfavorable.

[0103] Moving leftward at the intersection (reducing the number of slits 13) results in a rapid increase in permeability (steep on the left and flat on the right). This means that further reducing the number of slits 13 at the intersection increases the factor unfavorable to gas extraction (increased permeability) more rapidly. Furthermore, because the fracture ratio curve lacks a clear steep-left-and-flat-right characteristic, further reducing the number of slits 13 at the intersection results in a slower increase in the factor favorable to gas extraction (decreased fracture ratio) than the factor unfavorable to gas extraction (increased permeability). Combined with these two factors, moving the intersection leftward (fewer slits 13 than at the intersection) also negatively impacts gas extraction.

[0104] The present invention obtains the intersection point through experiments and formulas 2 and 3 to determine the optimal number of slits 13, taking into account both the permeability and the fracture rate, as well as the sealing conditions required for extraction (to prevent air outside the coal seam 10 from being drawn into the gas extraction pipe 15), namely the sealing effect and the strength of the grouting coal rock layer, thereby creating relatively optimal extraction conditions for gas extraction.

[0105] The specific position and sealing length of the sealing section 12 are determined by the following method:

[0106] Before extending the gas extraction pipe 15 into the gas extraction hole 4, Figure 2 As shown, a camera 2 of a borehole imager 1 is mounted at the end of a measuring rod 3, and the camera 2 is extended into the gas extraction hole 4 through the measuring rod 3, starting from the middle and rear part of the gas extraction hole 4 (the rear direction refers to the direction from the mouth of the gas extraction hole to the deep bottom of the hole), and the image captured by the camera 2 is manually observed; there is a scale on the measuring rod 3, and the scale of the measuring rod 3 indicates the depth of the camera 2 inserted into the gas extraction hole 4; while observing the image captured by the camera 2, the person slowly pushes the camera 2 deeper into the gas extraction hole 4 until it reaches the coal seam 10, i.e., the end of the gas extraction hole 4; the standard of "slowly" is to ensure that the person has sufficient observation time. As long as the person can carefully observe the image, the speed of the camera 2 penetrating into the gas extraction hole 4 can be increased as much as possible.

[0107] During this process, the looseness of the coal and rock layers around the gas extraction hole 4 is determined by manual observation of the pictures, and the depth of the gas extraction hole 4 corresponding to the position that needs to be reinforced is recorded. The section of the gas extraction hole 4 between the deepest and shallowest depths in the position that needs to be reinforced is used as the sealing section 12 to ensure that the sealing section 12 covers all positions that need to be reinforced.

[0108] Among them, the drilling imager 1 adopts the CXK6 mining intrinsically safe drilling imager 1 system produced by Wuhan Tianchen Geophysical Exploration Technology Co., Ltd., and its camera 2 is a panoramic camera 2.

[0109] The slit 13 is an annular slit surrounding the gas extraction hole 4, which is used to make it easier for the grouting slurry to penetrate and diffuse into the cracks around the gas extraction hole 4 under the action of the grouting pressure. After the grouting liquid solidifies, it forms a barrier to prevent external gas from entering the gas extraction section 11 deep under the action of the extraction negative pressure, thereby improving the sealing effect.

[0110] Each grouting coal rock sample is a cylinder with a diameter of 50 mm and a length of 100 mm.

[0111] A control experiment was conducted using the gas extraction hole of the present invention and the gas extraction hole not using the present invention. The experimental results are as follows: Figure 8 and Figure 9 As shown, by adopting the present invention to determine the optimal number of slits, compared with randomly determining the number of slits, the gas extraction concentration and the pure amount of gas extraction are significantly increased.

[0112] The above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the present invention can still be modified or replaced by equivalents. Any modification or partial replacement that does not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A coal seam gas extraction method with integrated through-layer drilling and sealing: a gas extraction hole is drilled from the roadway into the coal seam, a gas extraction pipe is inserted from the roadway into the gas extraction hole, and the end of the gas extraction pipe extends into the coal seam. A sealing section is provided at the rear of the gas extraction hole, and the section of the gas extraction hole from the end of the sealing section to the end of the gas extraction hole is the gas extraction section. Grouting is injected into the coal and rock strata surrounding the borehole in the sealing section, and a grouting area is formed after the slurry solidifies. The grouting area is used to form a barrier outside the coal seam to prevent air from flowing from areas outside the coal seam to the gas extraction section. Finally, a negative pressure generating device is used to extract coal seam gas from the gas extraction hole through the gas extraction pipe. Its characteristics are: Before grouting, slots are set in the coal and rock layer around the sealing section. The slots are annular slots surrounding the gas drainage hole, so that each slot is connected to the sealing section of the gas drainage hole. The optimal number of slots is determined according to the following steps: The first step is to prepare the grouting coal rock sample; Sampling is performed at a predetermined position of the coal rock layer, and the taken coal rock sample is divided into four cylinders of the same size. Each cylinder is drilled and grouting is performed to produce four grouting coal rock samples, which are numbered as grouting coal rock sample No. 0, grouting coal rock sample No. 1, grouting coal rock sample No. 2, and grouting coal rock sample No. 3, respectively; a hole is drilled at the center of grouting coal rock samples No. 0 to No. 3, and 0 slits are set around the drill hole on grouting coal rock sample No. 0, 1 slit is set on grouting coal rock sample No. 1, 2 slits are set on grouting coal rock sample No. 2, and 3 slits are set on grouting coal rock sample No. 3; The second step is a post-loading CT scan; First, the triaxial confining pressure of the true triaxial loading device is set according to Formula 1; Formula 1 is: σ=γz; In formula 1, σ is the stress at the sampling location in the coal rock layer, in MPa; γ is the weight of the coal rock per unit volume at the sampling location, in kN / m3; z is the burial depth of the sampling location in the coal rock layer, in meters; the triaxial confining pressure of the true triaxial loading device is σ; Then triaxial compression tests were conducted; A true triaxial loading device was used to apply a predetermined confining pressure to four grouting coal and rock samples for triaxial compression tests. After the triaxial compression tests, the four grouting coal and rock samples were scanned again with industrial CT to obtain scanned slice images of multiple locations of the loaded grouting coal and rock samples. The third step is to process the image to obtain the crack rate; Image analysis software ImageJ was used to analyze scanned slice images at multiple different positions of the grouting coal-rock specimens after loading. The scanned slice images at different positions were binarized to obtain the crack ratios of grouting coal-rock specimens No. 0 to No. 3 after loading. Specifically, for each scanned slice image, the crack ratio of the scanned slice image was obtained by calculating its crack area and the total area of ​​the scanned slice image. The average crack ratio of all scanned slice images of a grouting coal-rock specimen was taken as the crack ratio of the grouting coal-rock specimen. The fourth step is to use Origin software to obtain the relationship formula between permeability and the number of slits (Formula 2), and the relationship formula between fracture rate and the number of slits (Formula 3) through curve fitting; The first sub-step of the fourth step is to input the permeability of grouting coal rock samples No. 0 to No. 3 into the Origin software for curve fitting, obtain the dimensionless fitting coefficients y10, C11 and d11, and establish the relationship between permeability and the number of slits, that is, Formula 2: y1=y10+C11 ×e (-x / d11 ) ; In formula 2, y1 is the permeability, the unit is millidarcy or mD; x is the number of slits, in strips; y10, C11 and d11 are dimensionless fitting coefficients; Formula 2 is used to predict the permeability of grouting coal rock layer when the number of cuts is greater than 3; The second sub-step of the fourth step is to input the crack ratio of grouting coal rock samples No. 0 to No. 3 into the Origin software for curve fitting, obtain the dimensionless fitting coefficients y20, C21 and d21, and establish the relationship between the crack ratio and the number of slits, that is, Formula 3: y2=y20+C21 ×e (-x / d21 ) ; In formula 3, y2 is the crack ratio expressed in percentage; x is the number of slits, in strips; y20, C21 and d21 are dimensionless fitting coefficients; Formula 2 is used to predict the fracture rate of grouting coal rock layer when the number of cuts is greater than 3; The fifth step is to determine the optimal number of slits; According to Formula 2 and Formula 3, with the number of slits as the horizontal coordinate and the permeability and fracture ratio as the vertical coordinates, a curve showing the relationship between fracture ratio and the number of slits, as well as a curve showing the relationship between permeability and the number of slits, are plotted in the same coordinate system. The integer adjacent to the horizontal coordinate corresponding to the intersection of the two curves is the optimal number of slits.

2. The coal seam gas extraction method with integrated perforation drilling and isolation according to claim 1 is characterized in that: The specific position and length of the sealing section are determined by the following method: Before inserting the gas extraction pipe into the gas extraction hole, a camera of a borehole imager is installed at the end of a measuring rod, and the camera is inserted into the gas extraction hole through the measuring rod, starting from the middle and rear part of the gas extraction hole, and the image taken by the camera is manually observed; there is a scale on the measuring rod, and the scale of the measuring rod indicates the depth of the camera inserted into the gas extraction hole; while manually observing the image taken by the camera, the camera is slowly pushed deeper into the gas extraction hole until it reaches the coal seam, that is, the end of the gas extraction hole; in this process, the picture is manually observed to determine the looseness of the coal and rock layers around the gas extraction hole, and the depth of the gas extraction hole corresponding to the position that needs to be reinforced is recorded. The gas extraction hole section between the deepest and shallowest depths in the position that needs to be reinforced is used as the sealing section to ensure that the sealing section covers all positions that need to be reinforced.

3. The coal seam gas extraction method with integrated perforation drilling and isolation according to claim 1 or 2, characterized in that: The slit is an annular slit surrounding the gas extraction hole, which is used to make it easier for the grouting slurry to penetrate and diffuse into the cracks around the gas extraction hole under the action of the grouting pressure. After the grouting liquid solidifies, it forms a barrier to prevent external gas from entering the gas extraction section deep under the action of the extraction negative pressure, thereby improving the sealing effect.

4. The coal seam gas extraction method with integrated perforation drilling and isolation according to claim 3 is characterized in that: Each grouting coal rock sample is a cylinder with a diameter of 50 mm and a length of 100 mm.

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