Study and Application of High-Level Directional Extraction Borehole Based on Mining Fracture Evolution Law of Overburden Strata

Abstract

The technical principle of gas drainage using high-level directional extraction boreholes was analyzed. A range of overburden strata was stimulated for pressure relief during mining, the effects of different borehole parameters on gas flow in the goaf and gas concentration in the upper corner were compared, and a field test was conducted to analyze the effect and peculiarities of gas drainage. With the mining of the work plane, overburden mining fissures gradually develop forward and upward, showing a “saddle” shape along the coal seam. The fissures in the middle zone of the goaf are gradually compacted, and a gas accumulation zone is formed around the goaf. High-level directional extraction boreholes arranged in an ellipsoidal belt at the side of the air return can achieve efficient gas extraction in the roof fissure belt. Numerical simulation results showed that the height of the fully depressurized area was 65 m from the roof of the coal seam. In addition, three high-level directional extraction boreholes were drilled in the roof of the coal seam. The gas extraction concentration and gas extraction pure volume of these three boreholes first increased, then decreased, and finally tended to be stable. The sequence of their average values was borehole No.2 > No.3 (twice as much) and > No.1 (2.7 times as much), which are closely related to the evolution law of overburden mining fissures. The research results can provide a reference for the further study of gas extraction technology using high-level directional extraction boreholes in coal and gas outburst seams.

1. Introduction

The co-mining technology mode of coal and gas permits the organic combination of coal mining and gas utilization, which can decrease gas accidents, optimize the energy mix and mitigate the greenhouse effect caused by gas emissions into the air; this is an inevitable trend for the coal industry [1,2,3]. The coal seam thickness, gas content, cleat intensity, lithologic distribution, mechanical parameters of coal, and other factors greatly affect the gas migration and extraction rate [4,5,6,7]. In addition, with the increase in coal mining depth and mining intensity, in the process of coal mining, the gas in adjacent layers or surrounding rock enters the work plane through mining fissures. The gas in the upper corner of the work plane easily causes the gas concentration in the return air flow to exceed the limit, which affects the safe mining of the work plane [8,9]. Former research has shown that a rock drainage roadway and high-level directional extraction borehole can effectively extract gas in the fissure zone and have great operational stability in the upper corner of the goaf and return air flow [10,11,12,13]. However, the rock drainage roadway has the disadvantage of requiring a large amount of site construction, long operation cycles, and high costs [14,15,16]. With improved equipment, the high-level directional extraction borehole can replace the high-level drainage roadway to extract gas in the goaf and fissure zone, which can shorten the construction period, save construction costs, and improve the efficiency of gas control [17,18,19].

To obtain a good gas drainage effect, many scholars and experts have studied the designed position of boreholes based on the mining fracture evolution law of overburden strata [20,21,22]. Coal enterprises have carried out field tests on work planes using high-level directional long borehole technology, which verified the effectiveness of this technology in gas extraction [23,24,25]. The occurring state and geological characteristics of construction sites, including roof and coal seams, play a key role in the design and drilling of high-level directional extraction boreholes [26,27]. Complex distribution, properties and mechanical parameters of roof strata greatly affect the mining fracture evolution law of overburden strata. At present, different multifunctional testing methods and devices have been invented and applied to collect the evolution law of the fissure surface, deformation, displacement, and stress in a timely manner to guide the design and drilling of high-level directional extraction boreholes [28,29,30]. However, a single testing method cannot reflect the detailed changes in the roof strata, and the mining fracture evolution law and deep roof strata cannot be directly observed. At the same time, the methane concentration dynamics in the extracted gas–air mixture from drainage boreholes can indirectly be used to judge the qualitative changes in geomechanical conditions on the longwall panel [31]. Therefore, it is proven that the layer and borehole stability of high-level directional extraction boreholes are very important in obtaining a good gas extraction effect.

Given the above analysis, the Ji₁₅-22080 work plane in the Pingmei No.8 Coal Mine was taken as the research focus for this paper. The three-zone height calculation formula and numerical simulation software were applied to study the evolution law of overburden mining fissures. High-level directional extraction boreholes were designed for a field test using a directional drilling rig, and the construction layer and gas drainage effect of high-level directional extraction boreholes were analyzed. The research results can provide a reference for the further study of gas extraction technology using high-level directional extraction boreholes in coal and gas outburst seams.

2. Mechanism of the High-Level Directional Extraction Borehole

2.1. Layer Analysis of the High-Level Directional Extraction Borehole

Coal mining increases mining-induced fissures and the permeability of overburden strata. The developed fissures provide good conditions for gas desorption, migration, and enrichment. Therefore, mining-induced fissures are channels and storage places for coalbed methane. The caving zone, fissure zone, and bending subsidence zone are formed vertically, while the coal wall supporting the influence zone, separation zone, and recompaction zone is formed horizontally in the overlying strata of goaf, as shown in Figure 1.

In the mining process of the work plane, due to the movement of overlying strata and the effect of the gas pressure gradient, the pores in the fissure zone increase greatly, and a horizontally penetrating mining-induced fissure zone is formed. Due to buoyancy, high-concentration gas rises along the fracture channel and accumulates in the fissure zone, so the fissure zone is the best area for gas extraction. If the high-level directional extraction borehole is arranged in a caving zone, it will be destroyed by the continuous caving roof, which will affect the gas extraction effect. If it is arranged in the bending subsidence zone, even if the borehole can remain intact for a long time, the fissures in this area are not developed, which also affects the gas extraction effect. That is, the upper part of the caving zone and the lower part of the bending subsidence zone are the best areas for drilling directional extraction boreholes. The orifice is always in a negative pressure state, which can fully use the mining fissure channel to achieve the purpose of gas extraction. Therefore, the vertical height (H_z) of the high-level directional extraction borehole should satisfy H_m < H_z < H_l. H_m is the height of the caving zone, H_z is the vertical height of the high-level directional extraction borehole, and H_l is the height of the fissure zone.

The vertical height of high-level directional extraction boreholes is generally determined by theoretical calculation, similar material simulation, numerical simulation, and field measurement methods. In a field test, the comprehensive histogram of geological exploration boreholes near the work plane or construction parameters of adjacent high-level boreholes and their gas extraction effects can be referenced. The high-level directional extraction boreholes are arranged in layers that are easy to drill and have good borehole stability, so the mudstones that are swollen in water and strata that are easy to collapse should be avoided as much as possible. Based on the lithology and mechanical parameters of roof strata, the height of three zones can be calculated according to the empirical formula in

Table 1. Calculation Formula of Three Zones Height in Goaf.

Dip Angle of Coal Seam (°)	Compressive Strength (MPa)	Rock Type	Height of Caving Zone (m)	Height of Fissure Zone (m)
0~54	40~60	diabase, limestone, siliceous quartzite, conglomerate, glutenite, sandy shale	Hm = (4~5) M	$H_l=\frac{100M}{2.4n+2.1}\pm 11.2$
	20~40	sandy shale, argillaceous sandstone, shale	Hm = (3~4) M	$H_l=\frac{100M}{3.3n+3.8}\pm 5.1$
	<20	weathered rock, shale, argillaceous sandstone, clay rock, Quaternary and Tertiary loose layers, etc.	Hm = (1~2) M	$H_l=\frac{100mh}{4.1n+133}\pm 8.4$
55~85	40~60	diabase, limestone, siliceous quartzite, conglomerate, glutenite, sandy shale	—	$H_l=\frac{100mh}{4.1n+133}\pm 8.4$
	<40	sandy shale, argillaceous sandstone, shale weathered rock, shale, argillaceous sandstone, clay rock, Quaternary and Tertiary loose layer, etc.	Hm = 0.5M	$H_l=\frac{100mh}{7.5n+293}\pm 7.3$

M is cumulative mining height, m; n is coal layer number; m is coal seam thickness, m; and h is small stage vertical height of work plane, m.

.

In addition, the borehole spacing should be uniform and reasonable. When the borehole spacing is too large, this leads to a blank zone between two adjacent boreholes, and gas is not effectively extracted; too small a borehole spacing will lead to an increase in engineering quantity and construction costs. The horizontal distance from the high-level directional extraction borehole to the air return way can be calculated according to the following formula.

$$L=\left[L_1-\left(L_2+L_1\cot \theta \right)\tan \alpha \right]\sin \alpha +\left(L_2+L_1\cot \theta \right)/\cos \alpha$$

(1)

where L₁ is the vertical distance between the end position of the borehole and the coal seam, m; L₂ is the distance between the borehole and the outer boundary of the “O” ring, m; θ is the angle between the connection of fracture boundary, mining boundary, and coal seam, °; and α is the dip angle of the coal seam, °.

2.2. Technology of High-Level Directional Extraction Boreholes Replacing Roadways

When gas is extracted in an underground coal mine, it is usually necessary to select an appropriate extraction method for the coal quality. A coal seam with hard coal and good porosity can be extracted by a directional borehole in the coal seam. However, in soft coal seams with poor porosity and gas accumulation, gas extraction is usually carried out in high- and low-level roadways in adjacent strata. In the first type of coal seam, a directional extraction borehole is drilled in the coal seam to extract gas. In the second case, large-diameter-roof high-level directional extraction boreholes can be applied to replace the rock extraction roadway and achieve a “borehole instead of roadway”: this technology can greatly reduce the amount of rock roadway construction, improve the efficiency of gas extraction, and has been widely applied to extract gas in the goaf and fissure zone of the work plane in China’s coal mines.

The technology of high-level directional extraction boreholes replacing roadways refers to a certain number of high-level, large-diameter directional boreholes in the roof to replace high-level drainage roadways for gas extraction in the roof (Figure 2). Through the precise control of borehole trajectory, the end positions of boreholes are arranged in the “O” ring or elliptic paraboloid near the side of the air return way, and the borehole trajectory is effectively extended along the fissure zone of the roof to achieve stable and efficient gas extraction in the fissure zone and ensure a high gas concentration, large flow, and long-term gas extraction. The effect reduces the gas migration in the goaf to the upper corner and the return air flow and ensures the safe production of the work plane.

3. Results of Numerical Simulations

3.1. Introduction of the Test Work plane

The elevation of the Ji₁₅-22080 work plane of Pingmei No.8 Coal Mine is from −563 m to −681 m, the ground elevation is from +140 to +180 m, and the burial depth is from 703 to 861 m. The ground surface is mountainous, without other tall buildings and water bodies. The recoverable strike length is 893 m, the inclined length is 200 m, the coal seam thickness generally ranges from 3.4 to 3.85 m, the average coal thickness is 3.3 m, and the dip angle of the coal seam ranges from 15° to 20°, with an average of 17°. The immediate roof of the Ji₁₅ coal seam is siltstone, dark gray, dense, massive, approximately horizontal bedding. The main roof is fine sandstone, dark gray, and mainly composed of quartz with silty mudstone. The immediate floor is mudstone, containing plant root fossils, which can easily expand in water. The main floor is a thin layer of gray sandy mudstone, with fine sandstone in the middle.

3.2. Results and Discussion of Numerical Simulation

3.2.1. Analysis of the Pressure Relief Range of Mining Overburden Strata

To analyze the evolution law of overlying strata stress affected by coal mining, according to the relationship between the coal seam and rock strata in the Ji₁₅ coal seam work plane of the Pingmei No.8 Coal Mine, FLAC^3D simulation software was applied to simplify the height of rock strata and study the failure space of rock strata above the coal seam.

It was assumed that the numerical model (Figure 3) was excavated for 4 m each time; a total of 200 m was excavated; and stress and displacement monitoring points were arranged forward with the trend of the numerical model, key strata, and typical positions. Contour cloud maps of stress, displacement, and plastic zone of the numerical model were made, and changes in the distribution of stress in the plastic zone and overlying strata during excavation were monitored.

Figure 4, Figure 5 and Figure 6 show that the plastic zone basically presents a forward symmetrical shape. In the direction of the trend, the plastic failure area of the overlying strata at the lower end is obviously larger than that at the upper end.

While advancing the work plane, the height of the plastic zone increases with the increase in the advancing length. When the work plane advances to 40 m, the height of the roof fully entering the plastic zone is 13 m, and tensile failure occurs within 10 m from the roof of the coal seam, so it can be considered to be located in the caving zone. When the work plane advances to 80 m, it is found from the trend angle that the rock strata at 30 m from the roof of the coal seam are not destroyed, and plastic failure occurs at 40 m from the roof, which indicates that the rock strata strength is high. When the work plane continues to advance, the layer gradually endures plastic damage. When the work plane advances to 140 m, the height change of the fissure zone decreases significantly and stabilizes gradually, and the highest position of the plastic zone is located at 60 m from the roof of the coal seam.

Figure 7, Figure 8 and Figure 9 show that as the work plane advances forwards, the space of the goaf gradually increases, and overburden strata deform simultaneously. The range and height of the full-pressure-relief zone gradually increase, and the stress gradient gradually increases from the inside to the outside and from the center to the periphery. The fully pressure-relieved zone along the forward direction of the coal seam is basically in the form of an elliptical parabolic shape, which is symmetrically distributed in the middle of the goaf, and the angle of the fully pressure-relieved zone is 53°. Influenced by the dip angle of the coal seam, the fully depressurized zone along the trend direction of the coal seam presents asymmetry. In terms of the mine conditions, the angles of the fully pressure-relieved zone are set at 45° and 61° for the lower- and upper-end positions, respectively.

Using the stress distribution during the stable development of the plastic zone when the work plane advances to 140 m, it can be seen that the maximum height of the fully pressure-relieved zone is 65 m from the roof of the coal seam, which is basically consistent with the height of the plastic zone.

3.2.2. Analysis of the Parameter of High-Level Directional Borehole

To analyze the gas extraction effect of high-level directional boreholes, Fluent numerical software was applied to simulate the variation law of gas concentration distribution in the goaf with distances of 15 m, 20 m, 25 m, and 30 m from the roof of the coal seam. A uniform arrangement of boreholes in the trend direction of the coal seam and a spacing of 5 m between two adjacent boreholes were designed.

Figure 10 shows that the four designed horizontal simulation experiments change the gas flow field in the local area of the goaf so that the gas is intercepted in the upper part of the borehole and the gas concentration in the lower part of the borehole is significantly reduced. The horizontal arrangement of high-level directional extraction boreholes makes the gas appear to have an obvious boundary at this place.

Figure 11 shows the gas concentration in the upper corner of different boreholes’ vertical distances. When the borehole’s vertical distance is 15 m, the borehole is close to the caving zone of the goaf, where fissures develop and are easily affected by the airflow of the work plane; thus, the borehole cannot extract high-concentration gas, and the borehole cannot effectively control the gas in the upper corner. With the increase in the borehole’s vertical distance, the borehole enters the fissure zone, where there is a wealth of separation space, and it is far from the work plane. It can obtain a high gas concentration, and the extraction efficiency improves greatly. The gas concentration in the goaf near the air return way is obviously reduced, and some gas can enter the work plane. When the vertical distance of the borehole continues to rise, the gas concentration in the upper part of the fissure zone becomes higher due to the influence of the gas’s lifting and floating effect, where it is easy to obtain a higher gas extraction concentration in the upper part of fissure zone. However, the borehole can only control nearby gas; it is impossible to control the upper corner area simultaneously. Some gas below the borehole enters the work plane with the air flow and accumulates in the upper corner, which results in gas alarms. Similar results were obtained for converged wells in [32]. The numerical results show that when the borehole is 21 m away from the roof of the coal seam, the gas concentration in the upper corner is the lowest.

4. Results of the Field Test and Discussion

4.1. Design of a High-Level Directional Extraction Borehole

The average thickness of the coal seam was 3.3 m, the dip angle of the coal seam was 17°, and the compressive strength of the roof strata ranged from 20 to 40 MPa, so the height of the caving zone and fissure zone can be obtained by substituting the values into Table 1.

After calculation, we found that the height of the caving zone in the Ji₁₅-22080 work plane ranged from 9.9 to 13.2 m, and the height of the fissure zone ranged from 41.4 to 51.6 m.

Based on the principle that the high-level borehole should be arranged in the fissure zone of the work plane, the designed planar graph and cross-section drawn of the high-level directional extraction borehole in drilling site 1# are shown in Figure 12 and Figure 13, respectively.

• Plane projection: The horizontal distances of boreholes 1#, 2#, and 3# in drilling site 1# from the contour line of the lower side in the air return way are 15 m, 33 m, and 45 m, respectively, as shown in Figure 12b.
• Vertical projection: The vertical distances of boreholes 1#, 2#, and 3# in drilling site 1# from the roof of the coal seam are 16 m, 24 m, and 28 m, respectively, as shown in Figure 13b.

4.2. Drilling Parameter Analysis and Drainage Productivity from the Undermined Rock Massif

Based on theoretical analysis and numerical simulation results, the ZYL-15000D crawler full hydraulic directional drilling rig was applied to drill three large-diameter high-level directional boreholes at drilling site 1# of the Ji₁₅-22080 air return way. The lithology of the drilled layer was medium-fine-grain sandstone and sandy mudstone, and the borehole diameter was 120 mm. To ensure gas extraction and borehole stability, a Φ108 mm steel sieve tube was set in the high-level directional extraction borehole. Detailed data on drilling construction are shown in

Table 2. Parameters of High-Level Directional Extraction Borehole.

Hole	Well Design Depth (m)	Wellhead Inclination Angle (°)	Bottom Hole Inclination Angle (°)	Wellhead Azimuth (°)	Bottom Hole Azimuth (°)	Height (m)	Horizontal Distance (m)
3#	444	10	1	305	300	18	15
2#	438	7	3	312	303	26	32
1#	456	14	5	321	302	26	45

, and a planar graph and cross-section drawn of the high-level directional extraction boreholes are shown in Figure 14 and Figure 15, respectively.

Gas extraction data on the high-level directional extraction boreholes of drilling site 1# in the Ji₁₅-22080 work plane are shown in Figure 16 and Figure 17.

In Figure 16 and Figure 17, it can be seen that the gas extraction concentration and pure volume first increase, then decrease, and finally tend to be stable. The analysis shows that as the work plane begins to mine from the open-off cut, the mining-induced fissures in the overburden strata gradually develop forward and upward. As the fissure moves to the borehole, the gas extraction concentration and pure volume begin to increase. When the work plane continues to move forward, the overburden fractures show a “saddle” shape along the trend direction of the coal seam. That is, from the inside to the outside of the work plane, the stress recovery zone, stress reduction zone, stress concentration zone, and original rock stress zone are formed in turn. The fissure zone increases upward, and fractures in the middle of the goaf are gradually compacted. The gas-extraction concentration and gas-extraction pure volume tend to be stable after decreasing in the later stage of extraction. These behavioral features (a sharp decline in methane concentration and net gas consumption) of methane emission dynamics, in our opinion, are due to primary roof breaks. Similar results were obtained for the development conditions of gas-bearing formations in the Donbass when, based on comparisons of methane release dynamics from drainage wells drilled into the undermined massif and the mined-out space of the previously mined panel, the collapse of the main roof occurred 163 m from the installation pass [33].

Table 3. Coal Mine Methane Extraction Data.

Number of Hole	Concentration/%		Pure Volume /(m3/min)
	Range	Average Value	Range	Average Value
1#	9.5~14.8	8.7	0.2~1.9	0.6
2#	6.3~9.1	6.5	0.1~0.6	0.2
3#	10~52.4	17.3	0.5~5.7	1.3

shows that the average gas extraction concentration of boreholes 1#, 2#, and 3# are 6.5%, 17.3%, and 8.7%, respectively, and the average gas extraction pure volume of boreholes 1#, 2#, and 3# are 0.2 m³/min, 1.3 m³/min, and 0.6 m³/min, respectively. The order of average gas extraction concentration and gas extraction pure volume is borehole 2# > borehole 3# > borehole 1#. The reasons for the different gas extraction effects of these three high-level directional extraction boreholes are as follows. (1) The height of borehole 1# from the roof of the coal seam is 18 m, which is only 4.8 m higher than the theoretical maximum caving zone (9.9~13.2 m). That is, the borehole is located in the lower part of the fissure zone, so the average value of gas-extraction concentration and gas-extraction pure volume is the smallest. The analysis shows that borehole 1# mainly extracts air in the goaf and fails to extract high-concentration gas. The borehole can solve the problem of gas alarms in the upper corner. (2) The average value of gas-extraction concentration and gas-extraction pure volume of borehole 3# is better than that of borehole 1#. The analysis shows that the height of borehole 3# from the roof of the coal seam (26 m) is greater than that of borehole 1# (18 m). Gas accumulates above the fissure zone due to buoyancy, so the gas extraction effect of borehole 3# is better. (3) The distance between borehole 2# and borehole 3# to the roof of the coal seam is the same, but the horizontal distance from borehole 2# (32 m) is smaller than that of borehole 3# (45 m). The bed separation fissures in the middle of the goaf tend to be compacted with the mining of the work plane, and a connected mining fissure development zone is formed around the goaf. The migration law of high gas concentration also follows the variation law of bed separation fissures. That is, gas accumulates around the goaf during the mining process (as shown in Figure 2), which explains why the extraction effect of borehole 2# is the best.

Therefore, to avoid gas alarms in the upper corner of the goaf and return air flow, the roof lithology and mechanical parameters, mining height, mining advancing speed [34], periodic weighting step, and other factors should be comprehensively considered based on the evolution law of mining-induced fissures in overlying strata during the construction of high-level directional extraction boreholes in the Ji₁₅-22080 work plane of the Pingmei No.8 Coal Mine.

The gas-extraction concentration and pure volume indicate that in the process of work plane mining, high-level directional extraction boreholes can extract gas from goaf and fissure zones, realize safe mining of the work plane, and avoid gas overrun in the return air flow. However, there are many factors affecting the borehole layout and parameters, and it is necessary to combine various factors to optimize the design. The evolution law of overburden rock is one of the key factors to consider.

5. Conclusions

• To resolve the problem of gas alarms in the goaf and upper corner, high-level directional extraction boreholes can replace high-level drainage roadways to extract gas. The high-level directional extraction borehole should be arranged in an ellipsoidal belt at the side of the air return way, which can realize stable and efficient gas extraction in the fissure zone of goaf.
• According to an empirical formula, the height of the caving zone in the Ji₁₅-22080 work plane ranges from 9.9 to 13.2 m, and the height of the fissure zone ranges from 41.4 to 51.6 m. The FLAC^3D simulation results show that the tensile failure occurs within 10 m from the roof of the coal seam, which is located in the caving zone, and the highest position of the plastic zone is located at 60 m in the roof of the coal seam. The numerical simulation results are in good agreement with the empirical formulas.
• The high-level directional extraction borehole can change the gas flow field in the local area of goaf, the gas concentration in the lower part of the borehole is significantly reduced, and when the borehole is arranged 21 m from the roof of the coal seam, the gas concentration in the upper corner is the lowest.
• As the work plane starts to be mined from the open-off cut, mining-induced fractures of overlying strata gradually develop forward and upward, and fractures of overlying rock show a “saddle” shape along the trend direction of the coal seam. The gas extraction concentration and pure volume of the high-level directional extraction borehole first increase, then decrease, and finally tend to be stable with the change in the extraction time. The average value sequence of gas extraction concentration and pure volume is borehole No.2 > No.3 (twice as much) and > No.1 (2.7 times as much).