Speculum Observation and Trajectory Measurement in Gas Extraction Drilling: A Case Study of Changling Coal Mine

Coal will still be China’s basic energy for quite a long time. With the increase of mining depth, gas content and pressure also increase. The problems of gas emission and overrun affect the safety and efficient production of coal resource to a certain extent. In this work, the field test of gas drainage borehole peeping and trajectory measurement in coal seam of Changling coal mine are carried out. These coal seams include C5 b coal seam, upper adjacent C5 a coal seam, C6 a coal seams, C6 c in lower adjacent strata, and C5 b coal seam in high-level borehole. The view of gas drainage borehole peeping and trajectory measurement in the working seam, upper adjacent layer, lower adjacent layer, and high position are obtained. It is found that the hole collapses at the position of about 20m in both adjacent strata and high-level boreholes, and there are a lot of cracks in the high-level boreholes before 12m. The deviation distance of high-level borehole is large, and the actual vertical deviation of upper adjacent layer is small. Finally, the strategies to prevent the deviation of drilling construction are put forward. It includes four aspects: ensuring the reliability of drilling equipment, reasonably controlling the drilling length, standardizing the drilling, and reasonably selecting the drilling process parameters.


Introduction
For quite a long time in the future, coal is still China's main energy and important chemical raw materials, supporting the rapid and stable development of China's national economy [1][2][3][4][5]. More than 90% of China's coal mines are mainly underground mining. The complex coal seam occurrence conditions bring serious threat to coal mine safety production, even inducing personal injury [6][7][8][9][10]. In recent years, relying on scientific and technological progress, coal mine safety production situation continues to improve. Coal mining enterprises have higher requirements for gas drainage technology and pay more attention to the related problems of drainage drilling.
The curvature of drilling trajectory in extraction will lead to the blank area in the coal seam extraction, which greatly affects the gas drainage effect and may lead to coal-gas outburst, gas explosion, and other gas accidents. The main rea-sons are geological factors and technological factors [11][12][13][14]. The main geological factors are the anisotropy of rocks and the interbedding of soft and hard rocks. Coal rock with bedding, schistosity, and other structural characteristics has obvious anisotropy in drillability, which easily leads to borehole bending. In addition, the technical reasons such as uneven drilling foundation, too long drill pipe, incorrect vertical axis installation, and the technological reasons such as too much bit pressure, too high drilling speed, and too much flushing fluid will lead to the bending of the drilling hole. These will lead to the deviation of the final hole position from the target location. Borehole peeper can be used to detect coal seam structure, geological structure, roadway roof separation, borehole quality, and roadway grouting effect. Especially in the study of fracture zone development law, it can be displayed intuitively and clearly through the display, and it can also measure the trajectory of gas drainage borehole [15][16][17].
Scholars in various countries have done a lot of research on the principle and prevention of gas drainage borehole bending. Polak et al. [18,19] established a new theoretical model of drill pipe and hole wall contact. Through the field test, it is found that the model can better estimate and predict the contact force and corresponding deformation of drill pipe, which provides a basis for controlling the drilling direction of drill pipe. Lueke et al. [20,21] analyzed in detail the influence of geological conditions, drill pipe pressure, bit structure, borehole diameter, drilling depth, and other factors on borehole deviation during drilling. Based on the field test, it is proposed that the deviation prevention device can be installed at the appropriate position to reduce the deviation of the borehole. Based on the angular displacement sensor and communication module, Manacorda et al. [22] proposed and designed a bit radar equipment for directional drilling. The device can transmit the actual position of the bit in real time, which provides a reference for effectively controlling the drilling direction. Matheus et al. [23,24] analyzed the shortcomings of existing drilling trajectory control and presented an adjustment method combining drill pipe control and prediction. It focuses on the application value of new rotary steering system (RSS) in drilling, providing a new method to control the movement of drill pipe in real time during drilling. Li [25] used wireless digital compass inclinometer to measure the coordinates of the drilled hole in Chambishi mine, Zambia. He calculated and compared the actual coordinates with the design coordinates to get the final offset. Liang et al. [26] analyzed the causes of borehole deviation from three aspects of geological factors, technological factors, and operation technical factors, summarizing the regularity of borehole deviation and the measures to prevent borehole deviation. Zhu et al. [27] invented a compound drilling device for deviation prevention and straight drilling while drilling, which can effectively improve the drilling speed and the ability of deviation prevention and correction in vertical wells and monitor the deviation change in real time with high stability and low cost. Ding et al. [28] analyzed the problems and error sources in the past drilling construction and gave a method for calculating the trajectory of borehole. Wang et al. [29,30] proposed and designed angle compensation correction method and designed the evaluation index of borehole correction effect in view of serious deviation of pressure relief gas drainage borehole trajectory in 2603 working face of Liyazhuang coal mine. Zhao et al. [31] analyzed the geological, technical, and technological factors the causing borehole bending in drilling engineering construction. They put forward the preventive measures of correctly installing construction equipment and adopting reasonable drilling tool structure. Zhao [32] presents that the main factors of borehole bending can be attributed to three aspects: formation structure, drilling pressure, and equipment installation. He put forward corresponding preventive measures according to different reasons.
In order to investigate the development and trajectory of gas drainage borehole fracture zone in coal mine, the Changling coal mine is taken as a case. By peeping at the highlevel boreholes of C 5 b coal seam, upper adjacent layer C 5 a coal seam, lower adjacent layer C 6 a and C 6 c coal seam, and C 5 b coal seam high-level drilling in Changling coal mine, the development law of borehole fracture zone was investigated, and the trajectory of construction holes of 1250 and 4000 drilling rigs was measured to determine the trajectory of gas drainage holes and explore the deviation law of drilling trajectory.

General Situation of Mine and
Layout of Boreholes 2.1. General Situation of Mine. Changling coal mine is located in the southeast of Zhaotong City, Yunnan Province. The designed production capacity of the mine is 0.6 million t/a. Mining depth is from +1800 m to +1400 m elevation. The mining area is 11.399 km 2 . The mine is divided into two levels of mining. The first level elevation is +1560 m, and the second level elevation is +1520 m. The mine is divided into five mining areas, and the mining sequence is 101 mining area, 102 mining area, 201 mining area, 202 mining area, and 203 mining area. Plan of excavation engineering in mining area is shown in Figure 1. There are only three minable coal seams in the mine, namely, C 5 b , C 6 a , and C 6 c . The dip angle of coal seam is 2 degrees to 10 degrees, and most of them are below 8 degrees with small coal seam spacing. The coal seam characteristics are shown in Table 1.  Table 2. The layout of coal seam boreholes is shown in Figure 2.
There are six construction boreholes in the C 5 b coal seam. Two short boreholes in the working seam, numbered C 5 b -C 1-1 and C 5 b -C 1-2 , are used for the drilling peeping and trajectory measurement. Two long boreholes in the working seam (constructed by 4000 drilling rig), numbered C 5 b -C 2-1 and C 5 b -C 2-2 , are used for the drilling peeping and trajectory measurement. Two high-level boreholes, numbered C 5 b -A 3-1 and C 5 b -B 2-1 , are used for borehole peeping and trajectory measurement. Two construction boreholes in the C 5 a coal seam, numbered C 5 a -C 2-1 and C 5 a -C 2-2 , are used for the borehole trajectory measurement. Two construction boreholes in the C 6 a coal seam, numbered C 6 a -C 2-1 and C 6 a -C 2-2 , are used for the borehole trajectory measurement. Two construction boreholes in the C 6 c coal seam, numbered C 6 c -C 2-1 and C 6 c -C 2-2 , are used for the borehole trajectory measurement. ZY-1250 full hydraulic drilling rig for coal mine is composed of seven parts: pump station, operating platform, power head, frame, column, and drilling tool. It is mainly suitable for drilling gas drainage holes, drainage holes, coal seam water injection holes, grouting fire prevention holes, geological exploration holes, and various engineering holes in coal mines. It is suitable for all kinds of coal seams and strata with rock firmness coefficient f less than or equal to 8. The cross-2 Geofluids sectional area of roadway for drilling rig is more than or equal to 6.5 m 2 .

LD4000ZYWL00II (SZ2) Crawler Full Hydraulic
Drilling Rig for Coal Mine. LD4000ZYWL00II (SZ2) crawler full hydraulic drilling rig for coal mine is mainly composed of bottom car, support rotation, work surface, frame, operation platform, pump station, gripper, guide sleeve, pipeline system, drilling tool, rear water braid, and other components. It is mainly used for drilling gas drainage hole, grouting fire prevention hole, coal seam water injection hole, outburst prevention and pressure relief hole, geological exploration hole, and other engineering holes. It is suitable for all kinds of coal seams and strata with rock firmness coefficient f less than or equal to 10. The drill can walk independently and turn in place. The cross-sectional area of roadway for drilling rig is more than 9 m 2 . Or the width is more than 3 m, and the height is more than 3 m. The pump station converts the electric energy into hydraulic energy, and the pressure oil from the oil pump drives the motor and the propulsion cylinder to complete various actions of the drilling rig. The hydraulic system adopts the linkage oil circuit. Through the setting of each function selection valve, by operating the rotation and advance/retreat handle, it can realize the positive/negative 3 Geofluids rotation, forward/backward of the power head, the loosening/clamping of the chuck and gripper, and the corresponding linkage functions.

Drilling Peeping and Trajectory Measurement System.
The ZKXG30 mine drilling imaging track detection device is a high-tech equipment for comprehensive detection of drilling. The product integrates the functions of drilling photography, peeping (video), imaging, and track measurement and completes the workload of the previous four tests in one test. Meanwhile, it can obtain drilling dynamic video, local high-definition photos, full drill hole wall expanded plan, and drilling space track, which is efficient and fast. The instrument includes low-power embedded dual core processor, high-definition high-speed digital camera, and military grade high-precision space angle measuring device, which is supplemented by advanced control algorithm and image processing algorithm and other software systems to realize all functions synchronously. The product design fully considers the actual working environment of the coal mine and strives for simple operation, stable system performance, simple, and reliable. The system structure of the ZKXG30 mining borehole imaging track detection device (track) is shown in Figure 3.

Test Principle and Steps
4.1. Introduction of the Device Structure. The ZKXG30 mine drilling imaging track detection device (track) is mainly composed of host, probe, and sounder. The host is mainly composed of internal system, external interface, and battery pack. The external interface includes probe port, sounder port, operation control knob interface, and charger interface. The internal system mainly includes ARM dual core processor and data storage unit. The internal system is connected with external components through external interfaces. Under       6 Geofluids trajectory detection device has a built-in LED white light emitting diode (with brightness adjustment circuit), and a camera, which is used to capture the hole wall image, and a built-in high-performance three-dimensional electronic compass, which is used to measure the drilling azimuth and inclination of the probe position. The ZKXG30-T(a) mine drilling imaging trajectory detection device has built-in high-performance three-dimensional electronic compass, which is used to measure the drilling azimuth and inclination of the probe position. The video signal, control signal, and compass digital signal in the probe are transmitted to the host through the mine intrinsically safe communication cable. The host receives the probe signal and the depth pulse signal of the sounder, calculates the depth position of the probe, and processes the video signal such as image recording, matching, and splicing. Video recording can be full hole or partial. Video and image matching and splicing are carried out simultaneously. As the probe continues to move into the hole, the whole hole wall is automatically matched and spliced into a complete plane expansion. While processing the image, the host computer can display the real-time monitoring image and the stitched unfolded image and can switch the display of borehole trajectory projection. The saved data can be played back and browsed. After connecting with PC, the instrument can be used as a U disk, which is convenient to copy and paste files.

Drilling Preparation.
The diameter of the drilling probe shall not be less than 30 mm, and the depth shall be within 100 m, so as to ensure that the drilling has a certain elevation angle. Therefore, the water can flow out freely and keep as straight as possible to avoid step holes. After drilling, it needs to be placed for 10 minutes. After the fog in the hole disap-pears, it can be detected to ensure that the probe window will not condense water vapor.

Technical Staffing.
At least 3 field operators are required for drilling exploration in deep roadway, excluding drilling personnel. One probe operator is responsible for using the push rod to push the probe slowly and smoothly into the drilling hole. Another cable operator is responsible for passing the cable through the depth decoder at a constant speed, and the last one host operator is responsible for controlling the host.

Device Connection.
Connecting probe: both ends of the cable are fixed with 7-core waterproof plug. The plug is connected with the probe, and the plug cap is tightly fixed with the probe Detector installation: the tripod is stably placed under the air port, and the probe cable is installed on the pulley Connecting to the host: the other end of the 7-core video cable is connected to the host signal interface, and the 4core cable connects the detector interface with the host deep interface  Through peeping at the coal seam, upper adjacent layer, lower adjacent layer, and high-level gas drainage boreholes, the results show that the integrity of the boreholes in the coal seam is well. There is no obvious macrocracks, but there is water accumulation. Although the water in the borehole is ventilated and drained after the completion of drilling construction, there is still a certain length of water immersion in the depth of the down bedding drilling, which will directly affect the gas drainage effect. Due to the influence of drilling opening at 0-1 m of the hole opening in the lower adjacent layer, the hole collapse is very serious. The integrity of the borehole at 1-6 m is good, but the surrounding fissures are obviously developed. The borehole at 8-10 m begins to collapse obviously, and the borehole at 10 m completely collapses. The overall quality of the borehole is very poor. Compared with the lower adjacent layer, the integrity of the upper adjacent layer is better. However, there are obvious through type axial fractures. The borehole is completely collapsed at about 18 m.

Test Results and Countermeasures
There are a lot of cracks in the high-level borehole before 12 m. When selecting the high-level borehole for gas drainage, attention should be paid to the sealing distance of the borehole greater than 12 m. Otherwise, the gas concentration will decrease, and the gas drainage efficiency will be affected. The hole collapse occurred at the position of about 20 m in both adjacent strata and high-level boreholes, and the highlevel boreholes collapsed seriously, which basically blocked the boreholes. In the follow-up gas drainage process, it will affect the drainage flow in varying degrees, resulting in the decline of drainage efficiency.

Prevention Measures of Hole Collapse.
In view of the above experimental results, the prevention of hole collapse can be carried out from the following aspects. In the specific construction, according to the drilling speed, drilling pressure, rock powder, backwater, and lithology changes, the accurate judgment is made. Usually, clean water can be used as the flushing fluid. If the condition is not allowed and only circulating water can be selected, the bottom of cage should be kept in a clean state to avoid contamination by debris and rock powder, which will lead to the drill bit buried and paste, and eventually collapse. The drilling speed and feed pressure of the drilling rig should also be well controlled. If the rock stratum is relatively soft, the speed and pressure should be appropriately reduced according to the actual situation, and the footage height should also be appropriate. In case of hole collapse, the lithology and specific location of the starting section should be judged quickly and accurately, and the effective measures should be taken in time.
The operator should have practical experience and skilled technology, and be able to find the hole collapse in time, and adjust the drilling feed pressure and speed according to the coal and rock conditions. If the coal and rock are relatively soft, the drilling pressure and drilling speed should be appropriately reduced, the lifting frequency of drill pipe should be increased, and the coal and rock powder in the hole should be discharged in time, so as to reduce or avoid the drilling accidents.
According to the hardness of the rock in the hole, the bit type is selected to improve the drilling efficiency. When the rock is soft and the water invasion time is long, the hole collapse is easy to occur. The work should not be stopped during the shift handover, so as to facilitate the rapid construction and reduce the probability of hole collapse.
In the process of drilling, if the rock is soft, the drilling speed should be slowed down at this time, and the hole  Geofluids should be swept repeatedly after passing through the soft rock to avoid sticking. If the rotary resistance is large and the pump is choked, the feed pressure should be stopped to ensure the water circulation. Then, the drilling tool is moved up and down, continuing to work after removing obstacles. When there is abnormal phenomenon, it is forbidden to lift the drilling tool by force. When drilling stops, the drilling tool is lifted to a height of 3 m to 5 m from the bottom of the hole. When there is clear water in the hole, it can be stopped, so as to avoid the phenomenon of buried drilling. When drilling again, the drilling slag in the hole should be washed clean, and the process speed of drilling to the bottom of the hole should be slowed down. It can work normally after reaching the bottom of the hole.  9 Geofluids trajectory measurement is shown in Figure 11, and the horizontal projection, East-West projection, and North-South projection are shown in Figures 12-14.
The trajectory measurement of gas drainage boreholes in C 5 a , C 5 b , C 6 a , and C 6 c coal seams was carried out on site, including 6 boreholes in C 5 b coal seam and 2 boreholes in each adjacent coal seam. The trajectory measurement results and trajectory deviation of gas drainage boreholes in working seam, upper adjacent layer, lower adjacent layer, and high position are shown in Table 3.

Prevention Measures of Deviation in Drilling
Construction. In view of the above experimental results, we think that the prevention of drilling construction deviation can be carried out from the following aspects.
(  11 Geofluids assembly) should be selected according to the geological conditions, and the drilling rig and BHA should be maintained regularly to ensure the stable and reliable state of drilling equipment. Drilling deviation correction technology should be actively applied to reduce the bending degree of drilling as much as possible (2) Reasonable Control of Drilling Length. "Borehole deviation" increases with the increase of borehole depth, and long borehole is easy to form gas drainage goaf at the bottom of borehole. Therefore, from the perspective of controlling borehole deviation, the design and construction length of borehole should be reduced as far as possible on the   14 Geofluids

Conclusions
In this work, the field test of gas drainage borehole peeping and trajectory measurement in coal seam of Changling coal mine are carried out. The conclusions are as follows.
(1) By peeping at the boreholes of C 5 b coal seam, upper adjacent layer C 5 a coal seam, lower adjacent layer C 6 a and C 6 c coal seam, and C 5 b coal seam high-level drilling in Changling coal mine, it can be concluded that the integrity of boreholes in this coal seam is good. There is no obvious macrocracks, but there is water accumulation. The phenomenon of hole collapse occurs at the position of about 20 m in both adjacent strata and high-level boreholes, which will directly affect the gas drainage effect. There are a lot of cracks in the high-level boreholes before 12 m. When high-level boreholes are selected for gas drainage, the sealing distance should be greater than 12 m (3) Through the experiment, we put forward four strategies to prevent the deviation of drilling construction: ensuring the reliability of drilling equipment, reasonably controlling the drilling length, standardizing the drilling, and reasonably selecting the drilling process parameters

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
The authors certify that they have no conflicts of interest.