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Coal seam permeability is one of the key factors influencing the gas extraction efficiency, which is of great significance to reduce coal and gas dynamic disasters in gassy coal mines. Hydraulic slotting technique is an effective method to stimulate the coal reservoir, but the selection of slotting key parameters has great impact on gas extraction efficiency. For this reason, the hydraulic slotting model was established by using FLAC^{3D} software to analyze the stress distribution before and after slotting. Then, the influence of borehole diameter, slotting width, and slotting length on coal seam stress relief is also discussed. The results show that the slotting width has a great influence on the stress relief of the coal seam, while the borehole diameter and slotting length have no obvious influence on that. Based on the results of numerical simulation, field tests were carried out in Sangshuping NO.2 coal mine. The results show that the coal seam stress can be fully released, resulting in the improvement of coal seam permeability. The gas extraction efficiency can be highly enhanced by hydraulic slotting. This research achievement provides the guidance basis for high-stress water jet slotting technology with adaptive selection of slotting parameters in different geological conditions.

As the most important energy in China, coal plays an important role in promoting China’s economic development [_{2} fracturing [

Hydraulic slotting technology is being developed rapidly in recent years, which cannot only weaken or eliminate the danger of gas disasters but also change the physical property of the coal reservoir to realize the double effect of stress relief and permeability enhancement in the coal seam. Therefore, this technology has been widely used in coal mines. Yang et al. [

For hydraulic slotting technology, the selection of slotting parameters has a significant effect on the stress relief effect of the coal seam. Wei [

The hydraulic slotting technology first increases the fracture opening generated by the release of the stress of the coal seam. Then, under the effect of stress, the fracture gradually closes, which results in the reduction of permeability of the coal seam [^{3D} numerical simulation software is used to simulate hydraulic slotting. Firstly, the stress relief effect of the coal around the borehole before and after the slotting is discussed. Secondly, the influence of the slotting parameters on the stress relief effect of the coal seam is analyzed. Finally, based on the guidance of numerical simulation results, the hydraulic slotting field test is carried out to verify the improvement of gas extraction by hydraulic slotting. This study provides the theoretical support for the optimization of slotting process and high-efficiency gas extraction.

After the formation of the slotting borehole, stress concentration will happen around the borehole. After slotting, the coal around the borehole damages and deforms, which results in the stress relief. In order to study the influence of slotting on the stress relief of the coal seam around the borehole, this paper chooses the coal seam as the research object and adopts Fast Lagrangian Analysis of Continuous numerical simulation to study the stress distribution around the borehole before and after slotting and then study the impact of different slotting parameters on the stress relief effect by hydraulic slotting. The simulation parameters are based on the test results of physical and mechanical parameters of No. 3 coal in Sangshuping NO.2 coal mine, as shown in Table

Simulation parameters’ form.

Parameter | Minimum-maximum value | Mean value |
---|---|---|

Wave velocity (m/s) | 1974∼2245 | 2105 |

Vision density (kN/m^{3}) | 13.40∼14.13 | 13.36 |

Uniaxial compression strength (MPa) | 12.74∼16.00 | 14.64 |

Uniaxial elastic modulus (GPa) | 3.64∼3.91 | 3.79 |

Uniaxial deformation modulus (GPa) | 2.61∼2.92 | 2.81 |

Poisson’s ratio | 0.37∼0.42 | 0.39 |

Consistent coefficient | 1.3∼1.6 | 1.5 |

Cohesive force (MPa) | — | 10.13 |

Internal friction angle (°) | — | 30.7 |

The simulation adopts the two-dimensional model, with size 8 m × 5 m and a diameter of 100 mm. Symmetrical slotting is conducted along the center of the borehole to both sides, with division into 3538 units in total by 58 × 61, and all units are quadrilateral equal units. The grid around the slots is refined. Taking the far-field effect into consideration, the grid is divided with a combination of equal and unequal spacing. The left edge of the model adopts the symmetric boundary, and the right edge adopts the displacement boundary. The left and right edges only allow the boundary nodes to move along the vertical direction. The lower edge is set as the displacement boundary, of which the horizontal and vertical displacements are both zero. The upper edge adopts the stress boundary, and the applied vertical stress is 0.6 MPa. In order to simulate the effect of hydraulic slotting, a flat slotting with a length of 90 mm and a width of 60 mm is formed along the center of the borehole. The grid model is shown in Figure

Grid model and slotting schematic.

Calculation process of FLAC3D.

The calculation of the model needs to consider the following assumptions: (1) the model is subject to three-dimensional static equilibrium conditions; (2) the rock is a homogeneous isotropic elastic-plastic continuum medium; (3) the rock mass in the plastic zone meets the Mohr–Coulomb strength criterion. The governing equation calculated by the model is as follows [41].

When the model causes small plastic strains, the stress-strain relationship is then written as

According to the deformation continuity condition, the total strain tensor is written in terms of the displacement gradient:

Figure

Vertical stress distribution on (a)

Figure

Horizontal stress distribution on (a)

To study the influence of borehole diameter on stress relief of the coal seam after slotting, the diameters of 84 mm, 100 mm, 120 mm, and 150 mm were selected for simulation, respectively. The stress distribution of the coal seam after slotting is shown in Figures

Vertical stress distribution on (a)

Horizontal stress distribution on (a)

From Figures

To study the influence of slotting width on stress relief of the coal seam, the slotting widths of 40 mm, 60 mm, 80 mm, and 100 mm are selected for simulation, respectively. The stress distribution of the coal seam around slots with different slotting widths is shown in Figures

Vertical stress distribution after slotting. (a)

Horizontal stress distribution after slotting. (a)

Stress relief area around slotting.

Slotting width (mm) | Stress relief area of vertical stress | Stress relief area of horizontal stress | ||
---|---|---|---|---|

Slotting up-down side (cm) | Slotting end (cm) | Slotting up-down side (cm) | Slotting end (cm) | |

40 | 2.2 | 0.4 | 0.4 | 0.4 |

60 | 3.2 | 0.6 | 0.6 | 0.6 |

80 | 3.5 | 0.8 | 0.9 | 0.8 |

100 | 3.8 | 1.0 | 1.2 | 1.0 |

It can be seen that, the larger the slotting width, the larger the stress relief area. So, if the slotting width increases to a certain value, the coal seam can be fully stress relieved so as to generate a fully connected fracture network, which provides excellent conditions for high-efficiency gas extraction. Therefore, the increase of slotting width is the key parameter to evaluate the performance of the high-pressure water jet slotting device. However, in the field work, the slotting width is restrained by many conditions, such as the pressure of the pump, slotting nozzle performance, and coal and rock strength. Therefore, the selection of slotting width should depend on the geological conditions and device performance, so as to achieve the best stress relief effect.

To study the influence of slotting length on stress relief of the coal seam, the slotting lengths of 60 cm, 90 cm, and 120 cm are selected for simulation, respectively. The stress distribution of the coal seam around slots with different slotting lengths is shown in Figures

Vertical stress distribution after slotting. (a)

Horizontal stress distribution after slotting. (a)

The hydraulic slotting equipment mainly contains drilling bit, shallow spiral-integral drill rod, high-stress rotary aqua tail, water pump, remote operation floor, stress conversion slotter, high-stress hose, safety protection accessories, etc., and it can be seen in Figure

High-stress water jet slotting set device diagram.

The hydraulic slotting process mainly includes three stages: slotting preparation stage, borehole construction stage, and slotting stage. The operation process of hydraulic slotting can be seen from Figure

Operation process of hydraulic slotting.

The test mine selected is in Sangshuping NO.2 coal mine in Hancheng City, Shanxi Province, which is a coal and gas outburst mine. This extraction seam is 3# coal seam, with average thickness of 5.97 m, gas pressure of 0.4-0.94 MPa, and gas content of 8–14.57 m^{3}/t. The mine mainly adopted gas preextraction by bedding boreholes. However, due to the high gas content and poor permeability of the coal seam, the gas extraction efficiency cannot reach the expected effect. And, there are still some dynamic phenomena occurring in the drilling process, such as spraying hole and clamping drill, which seriously restrict the safe and efficient production of the mine.

The test site is located in the 3306 working face, where the coal seam thickness is 4.35 m∼6.16 m, the coal seam sturdiness coefficient is 0.4∼0.5, and the destruction of the coal type is class II. The maximum original gas content of No.3 coal seam measured in the test area is 12.24 m^{3}/t, which indicates that the test area has a high risk of coal and gas outburst. The coal seam geological conditions of the working face improve the favorable conditions for the hydraulic slotting test [

16 bedding boreholes were drilled in this test. The boreholes are divided into two groups for comparison of the test results, where _{1}–_{8} are slotting boreholes and _{1}–_{8} are ordinary boreholes. Through the comprehensive consideration of the numerical simulation research and the actual conditions on-site, the optimized hydraulic slotting technological parameters were determined, that is, the drilling diameter was 90 mm, the drilling depth was 80 m, the drilling spacing was 12 m, the hole sealing depth was 12 m, the slotting length was 1.2 m, the slotting width was 6 cm, and the slotting spacing was 1 m [

Test site and borehole arrangement.

The better the cutting effect of hydraulic slotting, the greater the permeability of the coal seam, the greater the initial gas flow of the borehole. The natural gas flow is an important index to characterize the initial gas permeability. Therefore, before connecting the drainage pipeline with the borehole, the natural gas flow of the slotted borehole and the ordinary borehole was measured, respectively. The measurement results are shown in Figure

Comparison graph of the natural gas flow in slotting boreholes and ordinary boreholes.

Figure _{1}∼_{8} is 0.0203-0.0813 m^{3}/(min·hm); natural gas flow in drill holes _{1}–_{8} is 0.0052∼0.0363 m^{3}/(min·hm). The natural gas flow in drill holes rises 3-4 times averagely after hydraulic slotting. It can be seen that the permeability of the coal seam has been significantly improved in the initial stage after the completion of hydraulic slotting. On the one hand, the hydraulic slotting causes the pressure relief of coal around the borehole, resulting in a large number of new fractures and secondary fractures. On the other hand, slotted stress in the coal body around the borehole is reduced, and the compression of the original coal crack opening increases. Both of them jointly promote the connection of the fracture network around the borehole and provide favorable conditions for gas migration. In addition, the slotting increases the exposed area of coal and accelerates the gas desorption speed, which is also an important reason for increasing the gas flow in the borehole.

The attenuation coefficient of the gas flow in the borehole is an important index to evaluate the difficulty of gas extraction in the coal seam. The larger the attenuation coefficient is, the harder gas exhausting and mining may be. This paper selects _{6} and _{7} and _{7} and _{8} drill holes, respectively, to measure gas emission initial velocity and obtain the attenuation coefficient of drill holes’ gas flow by mathematical fitting. The results are shown in Figure

The attenuation law of the gas flow of slotting and ordinary boreholes.

From Figure _{6} and _{7} are 0.0215 d^{−1} and 0.0237 d^{−1}, respectively. And, the attenuation coefficients of the gas flow of ordinary boreholes _{7} and _{8} are 0.0763 d^{−1} and 0.1231 d^{−1}, respectively. That is, the attenuation coefficient of the gas flow of slotting boreholes is 1/6-1/3 times smaller than that of ordinary boreholes. This is because, under the action of effective stress, the opening degree of slotted fracture gradually decreases or even closes, and the permeability of the coal seam is restrained. However, for ordinary boreholes, the number of cracks produced by coal relief around boreholes is limited, and the closure of cracks under effective stress is also small. It also shows that the effect of hydraulic slotting on coal seam permeability is more significant than that of borehole pressure relief.

The gas extraction purity is an important parameter to indicate the gas extraction effect. The higher the gas extraction purity is, the lower the residual gas content of the coal seam will be, which is of great significance to reduce or even eliminate the coal seam outburst risk. To analyze the improvement effect of gas extraction quantity by hydraulic slotting, the maximum and average values of daily gas extraction quantity of slotting boreholes and ordinary boreholes are counted, as shown in Figure ^{3}/d and 93.0 m^{3}/d, respectively. And, the maximum and average gas extraction quantities of ordinary boreholes are 66.7 m^{3}/d and 40.4 m^{3}/d, respectively. Thus, the gas quantity of slotting boreholes is 2-3 times higher than that of ordinary boreholes.

Daily gas extraction quantity of slotting boreholes and ordinary boreholes.

Before the high-pressure hydraulic slotting test, the maximum original gas content measured in the test area was 12.24 m^{3}/t. According to the requirements of China’s coal industry, the gas content in the test area should be reduced to less than 8 m^{3}/t to meet the drainage standard, which is also the criterion for determining the effective drainage radius. According to the measured data, the steps to calculate the extraction radius are as follows:

Calculate the total amount of gas extraction in the test area, and the formula is as follows:

where _{1} is the length of the extraction hole control area, _{2} is the width of the extraction hole control area, ^{3}, ^{3}/t, and

Calculate the number of boreholes required to reach the standard of extraction under a certain time, and the formula is as follows:

where ^{3}

Calculate the effective radius of drilling drainage, and the formula is as follows:

According to the above calculation process, the results of the extraction radius of the slotted hole and the ordinary hole are shown in Figure

Effective gas extraction radius of slotting boreholes and ordinary boreholes.

The stress distribution around the borehole before and after slotting is analyzed. Before slotting, a small area of stress relief appeared around the boreholes, and a circle caused by stress concentration was formed. After slotting, the stress of the coal seam around the borehole is fully released, and even though the slotting width is only 6 cm, an obvious stress relief area with 6.4 m × 1.2 m would be formed around boreholes.

The influence of slotting parameters on the stress relief of the coal seam was studied. The slotting width has great influence on the stress relief effect, while the borehole diameter and slotting length have little influence on the stress relief effect.

Field results showed that the gas extraction efficiency was highly enhanced by hydraulic slotting. The natural gas flow of slotting boreholes rises 3-4 times averagely. The attenuation coefficient of the gas flow of slotting boreholes is 1/6–1/3 times smaller than that of ordinary boreholes. And, the gas extraction quantity and effective gas extraction radius are improved 2-3 times by hydraulic slotting, respectively.

The data used to support the findings of the study are included within this article.

The authors declare that they have no conflicts of interest.

This work was supported by the National Key R&D Program of China (2018YFC0808305).

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