Study on Key Parameters of Directional Long Borehole Layout in High-Gas Working Face

School of Safety Science and Engineering, Xi’an University of Science and Technology, Shaanxi, Xi’an 710054, China Shananxi Yanchang Petroleum(Group) Co., LTD., Xi’an 710075, China Innovation Center of Intelligent Mining Technology in Coal Mine, Ministry of Emergency Management of the People’s Republic of China, Huangling 727307, China School of Energy Engineering, Xi’an University of Science and Technology, Shaanxi, Xi’an 710054, China


Introduction
China is one of the countries with the most serious coal mine gas problem in the world, and gas disaster is the biggest threat to coal mine safety production. It has been proved that gas extraction is the fundamental measure to prevent and control coal mine gas disasters and accidents. With gas extraction, gas accidents in the process of coal mining, such as gas overshoot, gas explosion, and coal and gas outbursts, can be effectively solved. By doing so, the safe production of the coal mine can be guaranteed. However, the existing gas control technology is mainly to solve the gas extraction. To realize three-dimensional comprehensive gas extraction technology with full space coverage, it is necessary to carry out "preproduction gas extraction, in-production gas extraction, and pressure-relief gas extraction" simultaneously. e key problem is to control the borehole trajectory accurately, which is the directional drilling technology of the surrounding rock.
High-gas mines and outburst mines contribute to more than 30% of the coal production capacity in China. In view of this, coordinating the development and use of coal and gas has become an urgent concern for China's coal industry. Wang et al. [1] and Jiang et al. [2] researched the influence of the long borehole when it comes to coal and coal-bed methane extraction based on the example of coal mines in China's mining area. With the increase mining depth of coal mines in China, the quantity of outburst mines and outburst coal seams keeps increasing; Cheng and Yu [3,4] expatiated the necessity of controlling gas in the long drilling hole. Bai et al. [5] and Wang et al. [6] analyzed the influence of different methods on gas adsorption analysis, such as longhole directional drilling technology.
In addition, Liu et al. [7] provided the Jincheng mode based on directional long borehole coal mining. Chae et al. [8] and Zhou and Yan [9] studied the influence of borehole size to the gas flow. Yang et al. [10] established a simplified ROP model for soft coal seams to improve drilling efficiency. e results of Shakib et al. [11] can be used to support the improved directional drilling system design. Jiang and Miao [12] analyzed the necessity and feasibility of the kilometer drill test in the Jinmei Group and its application prospect in gas development and control.
To further improve the application of directional long drilling construction technology in the coal seam surrounding rock, efficient preextraction of coal seam gas and surrounding rock oil-type gas can effectively prevent the problem of gas concentration exceeding the limit in the upper corner of goaf. At the same time, the prepumping drilling can be used for both geological exploration and "one hole for multiple purposes." e layer of the directional long hole in the surrounding rock was selected specifically, and the implementation effect of the directional long hole was tested, so as to provide practical reference for replacing the roadway with holes and provide the development direction for the technical breakthrough of traditional roof highposition hole technology.

Geological Condition of the Test Working Face.
Huangling No. 2 coal mine is located in Huangling County, Shaanxi Province. e geographical coordinates are as follows: east longitude 108°45′06″∼108°58′34″ and north latitude 35°30′30″∼35°48′06″. e mine area is about 352 km 2 , the resource reserve is 970 Mt, and the main mining area is No. 2 coal seam. e mining length of the 205 working face is 3000 m and working face length is 260 m. e overburden rock thickness is 180∼380 m. e main coal seam is No. 2 coal, strip bright coal. e coal seam has simple occurrence conditions and no other geological structures. Only one layer of gangue is distributed in some areas. e thickness of the gangue is 0.1 m, and the lithology of the gangue is mudstone. e thickness of the coal seam is 2.5-5.7 m. e direct bottom plate is dark gray mudstone, which is lumpy and 1.3-1.5 m. e formation tendency is 1-2°. e 205 working face is simple in structure. Within the scope of the working face, the rock layer is nearly horizontal and undulating no more than 4°. e general trend is lower in the middle and higher on both sides. e hydrogeological conditions of the 205 working face are simple, and locally, there will be coal seam fracture water.

Selection Principle.
rough the actual monitoring of the 205 working face, it is found that 78% of the gas in goaf is from surrounding rock gas, so the method of drilling and mining is adopted to solve the gas problem. By measuring the development range of the gas migration channel, the proportion of high hole extraction, roof extraction, and floor extraction was determined to be 31%, 41%, and 28%, respectively. In order to achieve multidimensional full coverage in space, it is proposed to carry out three-dimensional comprehensive gas extraction in space by implementing directional long drilling holes in the roof and floor and, at the same time, to achieve "premining (extraction), in-mining, and postmining" gas exploration and extraction mode in terms of time. In order to fundamentally solve the problem of mine gas, it is necessary to improve the efficiency of mine gas extraction and provide strong support for the realization of the intrinsic safety of gas control in mine mining, as shown in Figure 1. And, in the coal, itself, the surrounding rock produced the relief pressure (pressure reduction) and stress concentration phenomenon. ese changes in stress result in changes in the fracture rate of floor rock strata. e area where the stress is less than the original stress is the pressure relief area. At the same time, after coal mining, the rock layer around the production space loses support and gradually moves, bends, and breaks into the goaf.

Key Parameters of Directional
is process gradually expands outward, upward (roof ), and downward (floor) from the stope with the continuous advancement of the mining face. Due to the different position of the roof and floor, the deformation and failure under force are different. e movement and failure of the surrounding rock is a complicated process in time and space. rough long-term observation, it has been proved that the movement and destruction of the overlying strata (roof) have obvious zonation, and its characteristics are related to geology, mining, and other factors. Under the condition of mining the gently inclined medium-thick coal seam with the full strike-wall caving method, as long as the buried depth reaches a certain depth (the ratio of mining depth to mining height is greater than 40), there will be three representative parts of roof overburden failure and movement, which are, respectively, referred to as the caving zone [13], fracture zone [14], and curved subsidence zone [15,16], from the bottom to the top.
According to practical experience, the overburden rock in the caving zone is seriously damaged, and drilling is prone to borehole accidents. e separation fractures are the main part of the bent subsidence zone, which is not suitable for the extraction between gas layers. e influence of rock fracture in the fracture zone on the construction borehole is less than that in the caving zone, and the fracture is more developed than that in the curved subsidence zone. erefore, the favorable stratum for the drainage arrangement of the highgas borehole is selected in the fracture zone. According to the observation and test results of the crack zone in Sipan District of Huangling No. 2 coal mine in 2012, the height range of the crack zone in the fully mechanized coal face is 20∼66 m under the condition of a certain mining height and little lithology change of the roof.

Analysis of Floor Deformation and Failure.
After coal mining, under the dynamic comprehensive action of mining stress, the floor of the coal seam appears to have deformation and failure in different degrees. e changes in stress will lead to changes in the fracture rate of floor rock strata, which will lead to the formation of vertical tensile fractures, stratified fractures, shear fractures, and so on. Because the bottom plate is in the lower layer, the failure deformation of the bottom plate during mining is weaker under the influence of gravity than that of the top plate, so the change of its stress state is analyzed first.
(1) Model Establishment. FLAC (Fast Lagrangian of Continua), a large-scale geotechnical engineering special software designed and developed by the American ITASCA Consulting Group, can be used to solve the stress analysis of deep foundation pits, slopes, foundations, dams, tunnels, underground stopes, and caverns. It can also be used for dynamic analysis.
e "explicit Lagrangian" algorithm and "hybrid-discrete partitioning" technology adopted by FLAC3D can simulate the plastic failure and flow of materials very accurately. Since there is no need to form a stiffness matrix, it can solve large problems based on a small memory space. e mining of coal seam belongs to the damage range of the three-dimensional geological space size. Due to the use of automatic inertia and automatic damping coefficient, it overcomes the limitation of small-time steps and damping problems in explicit formulas. erefore, FLAC3D is one of the most ideal tools for solving 3D geotechnical problems. And, FLAC3D software was used for analysis of floor deformation and failure in this work.
Taking the floor of No. 2 coal seam of the 205 working face as the research object, the working face tends to be 280 m long, and the trend of simulated mining is 120 m. Coal mining is carried out along the model strike. e strike length of the model is 400 m, the trend width is 579 m, the vertical direction is 191 m, and the thickness of No. 2 coal seam is 3.2 m. e average buried depth of No. 2 coal seam is 610 m within the calculation range, and the rock layer of No. 2 coal seam is 105 m above it. e rock mass from the top to the surface of the model applies self-weight to apply vertical load, and the coal seam floor includes a rock layer model of 86 m thick. e thickness of the loose layer and overburden layer is replaced by compensation load. e original stress field is the gravity field.
e Mohr-Coulomb criterion is chosen as the rock failure criterion. To eliminate the boundary effect, a 140 m width boundary is left at both ends and a 150 m boundary is left at both ends of the trend of the model. All constraints are applied to the lower end of the model. e left and right sides constrain the displacement in the X direction, respectively, while the front and rear sides constrain the displacement in the Y direction. e upper end face is the free end. e top surface of the model was subjected to overburden stress, and the vertical stress outside the initial overburden model was calculated as 12.4 MPa according to the calculation formula of rock body weight (P � cH).
e mechanical parameters of the top and bottom slabs of the 2# coal seam in the experimental mine were obtained from laboratory tests, and the model parameters in the numerical simulation were determined according to the mechanical properties of the coal rocks. Obtain the physical parameters of simulated strata, as shown in Table 1.
(2) Simulation Calculation. e simulated mining length is 120 m, and the mining is carried out step by step. Each step  In the goaf, the stress of floor rock strata is in the state of pressure relief (decrease), and the floor coal and rock mass expand. Moreover, with the continuous mining activities, the pressure relief range of floor rock strata keeps increasing. When the pressure relief range increases to a certain extent, the pressure relief range is basically stable. In addition, with the collapse and compaction of roof overburden, the gravity load of floor overburden tends to be stable, and the stress recovery zone appears corresponding to floor abutment pressure. However, the coal that is not affected by the mining action has no change in stress. erefore, in the normal mining stage, the coal and rock on the working face floor appears in the original stress area, stress concentration area (compression area), pressure relief area (expansion area), and stress recovery area (compaction area). is is a description of the failure process of the formation, divided into four stages: advanced stress concentration-concentrated compression-stress release-stress recovery and recompression [17][18][19]. e overlying rock collapse is accompanied by rock fragmentation, and during the destruction of the rock, the surrounding stress is constantly changing, so back to the formation of delaminated fissures and broken fissures. Here, we mainly analyze the failure process of stress release of floor rock strata. According to the data analysis of different vertical depth detection points of the model (Figure 3), the same depth line has obvious zoning on the trend. e middle area is relieved of pressure. According to the different depth detection point data analysis of the model (Figure 4), the same depth line has obvious partition, intermediate regional pressure, and two-side coal region stress concentration, and the remote area of both sides is restored to the original stress, overall with " ear U" type. With the advance of the working face, the pressure is gradually relieved, and the shallow depth line starts to change in a large and drastic range and then gradually decreases and tends to be stable (Figures 4(a)-4(c)). On the direction of the deep depth line, the change amplitude is small at the beginning, then large, and then small. After a certain distance of the working face, compaction and stress recovery begin to occur (Figure 4(d)). It indicates that the shallow part is the first to be damaged, while the deep part is the first to be deformed after the shallow part is damaged. After the failure, the stress changes gradually, and after the working face advances a certain distance, compaction begins to occur along with the collapse of the roof. By comparing the stress variation patterns of different depth lines, it is obvious that the shallow shape curve is sharp, and the deep one is smooth, indicating that the shallow stress variation of the coal floor is drastic. In the deep part, the change is gentle, indicating that the shallow part is violent in failure and deformation, while the deep part is relatively slow.
By analyzing the plastic failure areas of models with different advancing distances on the stoping face, the failure areas of floor rock strata can be intuitively obtained, as shown in Figure 4. With the continuous advance of the stope working face, the floor failure area in the inclined section of the working face is basically distributed in the shape of M, with the two channels with the greatest damage degree of the floor, followed by the middle area. After the stoping surface exceeds 80 m, the failure depth of the floor increases slowly, and the pressure relief range reaches a stable base when the stoping reaches 100 m. e maximum failure depth of the floor is about 40 m.  Contour of SZZ Plane: on Magfac = 0.000e + 000 Gradient calculation -3.9033e + 007 to -3.5000e + 007 -3.5000e + 007 to -3.0000e + 007 -3.0000e + 007 to -2.5000e + 007 -2.5000e + 007 to -2.0000e + 007 -2.0000e + 007 to -1.5000e + 007 -1.5000e + 007 to -1.0000e + 007 -1.0000e + 007 to -5.0000e + 006 -5.0000e + 006 to 0.0000e + 000 0.0000e + 000 to 5.6668e + 005 Interval = 5.0e + 006      presents a downward trend. Qili Town sandstone, whose porosity is less than 5% and permeability is 0.59 × 10 −4 m 2 on average, belongs to the tight reservoir. Select the surface exploration borehole near the 205 working face to make the seam roof connecting well with the section, as shown in Figure 5. It can be seen that, in addition to the Qili Town sandstone, there are two layers of fine-grained sandstone on the top of the coal seam. e first layer is the two-stage two-cycle fine-grained sandstone layer with a thickness of about 6.4 m and a distance of about 30 m from the coal seam. e other layer is a three-cycle fine sandstone layer of the second extension, with a thickness of 6∼14 m and a relatively large spacing of 55∼68 m from the coal seam.  e average spacing between the gas-bearing sandstone beds between No. 2 coal seam and No. 3 coal seam, the two gas-bearing sandstone beds in Fuxian Formation, and the gas-bearing sandstone at the top of the Wayaobao Group and coal seam No. 2 is all less than 40 m. erefore, these reservoirs can be used as the primary consideration for the layout of directional long drilling. Both the roof and the floor contain multiple gas reservoirs within the vertical depth affected by mining. e drilling should be preferentially arranged in the reservoir closer to No. 2 coal. In this way, it is convenient to intercept the gas which is transported from the relief pressure to the stoping space of the gas-bearing layer at the far end of extraction under the influence of working face mining. rough comprehensive analysis, the layout of the comprehensive three-dimensional extraction hole is determined as follows.

Roof Directional Long
Hole. It is mainly aimed at the gas-bearing stratum of Qili Town sandstone, controlling the upper corner gas of goaf, and retaining the downward migration gas of the upper gas-bearing stratum under the influence of mining. In addition to minimizing air leakage on the working face, a circulation channel for gas in goaf should also be provided. Negative pressure extraction [20][21][22][23][24][25] is carried out in the construction drainage hole to drain the goaf gas under the effect of the pressure gradient, which provides a circulation channel for the gas in the goaf. When the pumping effect is good, the problem of upper corner gas can be solved well. According to the distribution of gas-bearing strata in Qili Town sandstone and the influence of mining, the boreholes are arranged within the vertical depth of 20∼40 meters, and the sandstone is spread in Shunqili Town including the floor gas-bearing sandstone layer and No. 3 coal seam directional long hole [26][27][28][29][30].

Floor Directional Long Hole.
e directional long drilling hole of the bottom plate selection is arranged in the sandstone gastrointestinal gas in the lower part of the rich county. e gas in the layer can be pumped, and the gas that moves upwards affected by the mining can also be pumped. Which cannot only extract the gas of this layer but also extract upward migration gas of the sandstone gas-bearing layer at the top of Wayaobao formation affected by mining. According to the distribution of sandstone at the lower part of the Fuxian formation in the region, the borehole track is designed to be arranged in the layer 8∼15 m away from the bottom of the No. 2 coal mine, and the sandstone at the lower part of the Shunfu Formation is distributed. e directional long drilling hole of No. 3 coal seam is spread along the direction of the working face, and the gas is mainly extracted from No. 3 coal seam.

e Layout of the Borehole.
e selection of directional long borehole parameters mainly considers the layout of the roadway, the drainage period of the directional long borehole, the coverage area of the borehole, and the cost of construction. Combining with the distribution of the cracks in the mined-out area and the distribution of oil-type gas        reservoirs, the plan layout of the directional long boreholes, the construction trajectory, the construction location, and the spacing are studied. According to on-site construction conditions, it is designed to arrange the roof, floor, and directional drilling of the 3 coal along the layer in the 205 working face with 8 lanes to carry out premining predrainage and in-mining mining drainage and postmining goaf drainage. Drilling to the bearing arrangement is shown in Figure 6. Design parameters are shown in Tables 2-4. In order to understand Figure 6, refer to the data in Tables 2-4.

Directional Drilling of Roof, Floor, and No. 3 Coal
Seams. According to the roof borehole 1-1, methane extraction concentration averaged 28%, methane extraction purity averaged 300 m 3 /d, and gas extraction from surrounding rocks accumulated to nearly 22,000 m 3 , as shown in Figure 7. Corresponding to the drilling concentration and the working surface and the drilling distance, as shown in Figure 8. e placement of directional long boreholes at  different locations backfires because of the different stress changes in the final hole location, thus affecting the efficiency of gas extraction. e drainage concentration of the borehole track is not high in the upper siltstone layer and significantly increases when it enters the Qili Town sandstone member. By tracking the concentration of corner gas on the working surface, a comparison was made between the long drill before and after extraction. As shown in Figure 9, the average statistical concentration was 0.81% in the first 3 months before it entered the roof drilling area and 0.48% in the second month after the extraction of the long drilling hole, which decreased by 41%, indicating a significant decrease.

Directional Drilling of is Seam.
Within the range of 0∼400 m from the stop-mining line to the auxiliary lane 205, a total of 3 regional prepumping chambers shall be constructed as the regional prepumping drilling field, and 3 long boreholes shall be arranged in each regional prepumping chamber. From Figures 10-13 , it can be seen that the gas concentration of No. 1-1 borehole drops to 14.0%∼89.0% after 24 months, the gas purity is 0.071∼25,008 m 3 /month, and the cumulative gas extraction is 170,200 m 3 . e gas concentration of No. 1 and No. 2 boreholes was reduced to 7.0%∼66.0%, the gas extraction purity was 0.057∼20,008 m 3 /month, and the cumulative gas extraction volume was 136,500 m 3 . e gas concentration of No. 1-3 drilling holes was reduced to 16.0%∼95.0%, the gas   extraction purity was 0.079∼27,910 m 3 /month, and the cumulative gas extraction was 189,700 m 3 , as shown in Figure 10. Figure 11 shows that the gas concentration of No. 2-1 drilling holes was reduced to 16.8%∼90.0%, 16.8%∼90.0%, the gas extraction purity was 0.011∼28,680 m 3 /month, and the total extraction volume was 194,900 m 3 . e gas concentration of No. 2-2 borehole was reduced to 16.5%∼87.0%, the gas extraction purity was 0.082∼28,920,000 m 3 /month, and the total extraction volume was 196,500 m 3 . e gas concentration of No. 2-3 boreholes was reduced to 14.4%∼ 48.0%, the gas extraction purity was 0.051∼ 8,04 m 3 /month, and the total extraction volume was 122,600 m 3 . e gas concentration of No. 3-1 drilling hole was reduced to 13.4%∼92.4%, the pure gas extraction was 0.079∼27,994 m 3 /month, and the total gas extraction was 188,500 m 3 , as shown in Figure 12. e gas concentration of No. 3-2 borehole was reduced to 15.2%∼80.0%, the gas extraction purity was 0.067∼23.5 million m 3 /month, and the total extraction volume was 159,700 m 3 . e gas concentration of No. 3-3 borehole was reduced to 17.0%∼85.0%, the gas extraction purity was 0.068∼23,87 m 3 /month, and the total extraction volume was 162,200 m 3 . e total gas extraction volume of the 9 prepumping boreholes is 1.520,800 m 3, which shows a good extraction effect. e original gas content in the area 0∼400 m from the auxiliary lane 205 to the stop-production line is 3.01 m 3 /t. en, after 2 years of prepumping, the gas content of the coal seam after regional prepumping is 1.56∼1.73 m 3 /t, and the prepumping rate is 42.52%∼48.17%. e gas pressure was reduced from 0.65 MPa to 0.19∼0.28 MPa, as shown in Figure 13.

Conclusions
(1) Based on the study of the oil-type gas reservoir and the influence range of mining on the working face in the surrounding rock, the paper puts forward the optimization method of the directional long borehole layout of the surrounding rock and the method of borehole layout with comprehensive consideration of the influence of mining on the failure of the roof and floor and the distribution of surrounding rock gas-bearing reservoirs. e roof borehole is arranged in the vertical depth of 20∼40 meters, and the relevant parameters and directional borehole trajectory are determined.
(2) By tracking and monitoring the data of 1164 groups of three-dimensional extraction boreholes in each drilling field, the pure methane extraction from roof boreholes averaged 98∼300 m 3 /d, and the methane extraction concentration averaged 17%∼35%. e average pure methane extraction from the floor borehole is 76∼178 m 3 /d, and the average methane extraction concentration is 3.1∼26%. Methane extraction from No. 3 borehole averaged 56∼190 m 3 /d, and methane extraction concentration averaged 10%∼66%.
(3) e correlation between the extraction data and the exposed layer and the layout of the borehole was studied, and the gas concentration in the corner of the working surface before and after extraction was compared. It can be seen from the concentration and quantity of borehole extraction that the integrated three-dimensional extraction has obvious effects in revealing extraction before relieving pressure during and after extraction. e concentration of gas in the upper corner drops from 0.81% to 0.48%, with a decrease of 41%. It is proved that directional drillingintegrated gas preextraction is a technical method worthy of popularization.
Data Availability e data are available and explained in this article; readers can access the data supporting the conclusions of this study.

Conflicts of Interest
e authors declare no conflicts of interest.

Authors' Contributions
e manuscript was approved by all authors for publication.