Exploration and application of directional long boreholes in coal measure strata

: two roadways in adjacent working face of Baode Mine may have the risk of water inrush at the same time, so it is necessary to construct long borehole to cover the roadway excavation. On the basis of the hydrogeological conditions of the mine, the safe water insulation thickness and water inrush coefficient of coal seam No .8 are calculated. The results show that the water inrush coefficient is 0.035-0.037 MPa/m, which is less than the critical value 0.06 MPa/m and the bottom plate has no sudden water hazard. In the construction of No .10 coal seam, the directional long borehole is used to detect whether there is a hidden structure communicating with the floor limestone and to drain water. The test shows that there is no effluent phenomenon in the borehole, which proves that there is no hidden structure in No .10 coal seam.


INTRODUCTION
The directional drilling technology has developed rapidly in recent years because of its controllable drilling track, accurate measurement of drilling track and high target layer drilling rate. At present, it has been widely used in the fields of water exploration, gas prevention and control, floor grouting reconstruction, geological structure exploration and so on [1][2][3][4][5] . The construction of fan-shaped multi-branch directional holes in the target horizon can increase the coverage area of the borehole and better explore the geological conditions of the target horizon. [6][7][8][9][10] The rubber roadway of No .8 coal 81312 face and the auxiliary roadway of 81313 face developed by Baode Coal Mine are restricted by the existence of limestone water inrush in the bottom plate, which affects the rapid recovery of mining area. At present, roadway excavation is needed for mining surface, and it is urgent for construction drilling to explore the water rich condition of roadway floor limestone and cover roadway excavation. Therefore, the multi-branch directional drilling is used to solve the problem in the bottom plate No .10 coal, and the drilling construction of water exploration and discharge hole is carried out to ensure the safety of roadway excavation.

HYDROGEOLOGICAL CONDITIONS IN MINING AREAS
The Baode coal mine is located on the east bank of the Yellow River and on the northwest margin of Shanxi, which belongs to the Loess Plateau. It is characterized by loess gully and hilly landform. The overall terrain is low in the middle and high in the north and south m, with the highest elevation of 1148.1 m, and the lowest in the northern county seat of the mine. The elevation is about 815 m, and the maximum height difference is 335.9 m. The maximum height difference is 335.9The main water source of the deposit is groundwater, including fissure water of sandstone and Ordovician limestone at the top and bottom of coal seam. The type of water filling is mainly karst water filling, coal system fissure water filling, and indirect water filling. The water level of Ordovician limestone karst water in Baode Mine is +839 m,81312 rubber transportation ,8131313 auxiliary transportation channel 0-700 m section heading face floor elevation is 544~565 m, located below Ordovician ash water level 274~295 m, water pressure is 2.74~2.95. It belongs to pressure driving.

RISK ANALYSIS OF ORDOVICIAN WATER INRUSH IN BOTTOM PLATE
The mining of coal seams caused the overlying bedrock to fall and form a new water-conducting fissure zone. However, the basement fractured aquifer of the coalmeasure strata in this area and its overlying third and fourth series loose pore aquifers are recharged by weak water-rich aquifers. The amount is limited, and it forms a complementary relationship with the surface water with the change of topography and valley, which poses little threat to the existing coal seam. With the mining of coal seam, the load reduction effect will be formed on the bottom plate. In case of hidden water conduction structure of coal seam floor, it is an important channel to conduct Ordovician karst water with the characteristics of large recharge, high water head and high water pressure. Water inrush coefficient is a quantitative index to characterize the possibility of mine water inrush. The water inrush coefficient depends on the water pressure on the coal seam floor [11][12] . According to the Code for Geological Exploration of Hydrogeological Engineering in Mining Area (GB12719-1991) and the Rules for Prevention and Control of Water in Coal Mine, the following formula (1) (2) is used to calculate the safe water barrier thickness and water inrush coefficient of coal seam floor in this working face. (1) When the P value is 3.91-4.13 MPa, the water inrush coefficient of the excavated surface is calculated to be 0.035-0.037 MPa/m, less than the critical value 0.06. Therefore, the fan-shaped multi-branch directional drilling in No.10 coal is used to detect whether there are hidden structures in the coal seam, so as to judge the possibility of water diversion of the bottom limestone.

Drilling equipment
ZDY6000LD high power deep hole directional drilling rig is used in the drilling construction of the cover roadway, Has 6000 Nꞏm output torque, High gyration, 250 kN, rig feed/drawing capacity Suitable for directional drilling of hole bottom motor, rotary drilling of orifice and compound drilling. Matching BLY390 mud pumps, Hydraulic drive, Can guarantee enough pressure at high displacement, Maximum pump capacity L/min,390 12 MPa, rated pressure It can meet the requirement of directional long drilling. A measurement system using YHD2-1000(A) with drill, It is composed of explosionproof computer, explosion-proof keyboard, explosionproof data memory and probe tube. As the main extension of the drilling is hard limestone formation, the screw drilling tool in the hole uses 4-stage screw motor. Other supporting drilling tools mainly include drill bits, central cable drill pipes and rotary drill pipes. The hole blowout preventer is installed at the orifice, When the drill is out of the hole, the blowout preventer can be held tight, Prevent borehole effluent. The main construction equipment, models and uses are shown in Table 1.

Working face profile
The 0-600 section of 81312 rubber transport and 81313 auxiliary transport channel is located in the southern part of the mine field, with 81312 design face in the east, auxiliary transport roadway in the deep part of the third (lower) plate area in the north, the remaining section of the roadway in the south and the boundary of the mine field, and the unmined solid coal body in the west. The coal seam excavated in the working face is Permian coal seam, the coal and rock strata are generally near the north-south strike, showing a monoclinic structure inclined to the west, and the coal seam inclination angle is 3º~7º, with an average of about 4º. The coal seam thickness tends to be stable basically, the coal seam thickness at the opening is 6.88 m, and the coal thickness is 7.2 m. after entering the ancient river scour zone The structure of coal seam is complex, the gangue is 4~5 layers, the lithology is mudstone and sandy mudstone, the maximum thickness of single layer gangue is 1.4 m.

Design of borehole structure
The formation conditions of borehole test area are good and the hardness of coal seam is large, which is beneficial to drilling hole. In the process of roadway excavation, the stratum stress is released, and the fissure area with a certain area becomes the channel to communicate the limestone water of the floor. Find out whether there is water inrush danger in the process of roadway excavation, cover roadway excavation. According to the design requirements of directional drilling, combined with the geological conditions of the mining area, the directional drilling structure is designed as follows: first, the Φ120mm PDC bit is used to open the hole, then the Φ193mm PDC bit is reamed, and finally the steel casing is Φ146mm 50 m grouting fixed hole. The second open directional section drilling adopts Φ120mm PDC bit +Φ89mm screw motor +Φ89mm without magnetic drill pipe.

Drilling technology construction plan
The drilling water exploration range is 630 m、81312 in front of the heading head and 20 m、 the final hole is located at 10#In coal. The test pressure is 1.5 times the local water pressure and lasts for 30 minutes. The orifice casing is not loose and there is no water leakage around the orifice, indicating that the orifice tube sealing process is qualified. The curvature radius method is used to calculate the trajectory of the borehole in the oblique section of the borehole, The borehole trajectory of the curve oblique section is obtained. After the curve stabilization section successfully enters the target layer, By controlling the tool surface of the drilling tool, the trajectory is approximately extended to the final hole in the target layer by a straight line.

Wireline Drilling Measuring Technology
Coal mine downhole wireline drilling measurement technology is suitable for drilling about 700 m deep hole. It is economical, reliable, convenient and quick to use wireline drilling in this depth. Using YHD2-1000(A) with drilling measurement system to Ф73mm the central cable drill pipe drilling, every 3 m of drilling process drilling inclination and azimuth data collection, combined with the geological conditions of coal seam and hole slag return, Adjust the tool angle of drilling tool, adjust the appropriate drilling pressure and pump quantity, so that the borehole extends according to the design track.

main hole opening branch technology
In order to explore the geological condition of No .10 coal seam more efficiently and accurately, In the construction process, the branch hole should be opened on the basis of the main hole. At present, the main hole branch opening methods commonly used in coal measure strata are, Low speed grinding branch method and repeated grinding branch method [13] . A low-speed grinding branch method sets the tool angle to 180°, Mechanical drilling speed to 6~8 m /h slow drilling until new branch hole [14][15] . Grinding the drill tool angle to 180°, Mechanical drilling speed to 20~30 m /h faster drilling, When all the drilling tools enter the new hole, After repeated grinding 2 times and can normally add drill pipe to continue drilling, Until a new branch hole is opened [16][17] . Average thickness of Baode Coal Mine 10 is only 1.5 m, The target coal seam thin needs to adjust the tool surface angle of the bottom screw drilling tool more accurately through the orifice monitor, Make the borehole track extend in the coal seam.
In order to ensure that the borehole trajectory does not exit the coal seam, The single change of borehole inclination and azimuth is controlled at 0.5°~0.8°. After the main hole construction is completed, the branch hole construction is carried out by using the "backward" branch opening method, Drill the drill bit back to the branch point reserved for drilling trajectory design, Low speed grinding method, Make the drill bend down, slow down, Until the new hole is drilled to the design branch hole depth, Repeat the process of opening the branch hole before drilling the remaining branch hole.

face profile
The working face of 81312 is located in the southern part of the mine field, and the driving roadway of the working face adopts the form of double roadway excavation. The main coal seam is Permian No .8 coal seam, the coal seam is north-south strike, inclined to west, inclined angle is 3º~7º, coal thickness 7.2 m, contains 4~5 layers of sandy mudstone gangue, the maximum thickness of gangue is 1.4 m.

Drilling Structure Design
The hardness of coal seam in drilling exploration area is large, which is beneficial to drilling hole. In the process of roadway excavation, the stratum stress is released, and the fissure area with a certain area becomes the channel to communicate the limestone water of the floor. It is necessary to construct fan-shaped multi-branch directional long borehole to explore the hidden structure of the floor. According to the construction intention of directional drilling, combined with the geological conditions of mining area, the directional drilling structure is designed as follows: first, the Φ120mm PDC bit is used to open the hole, then the Φ193mm PDC bit is reamed, and finally the steel casing is Φ146mm 50 m grouting fixed hole. The second open directional section drilling adopts the Φ120mm PDC bit +Φ89mm the screw motor +Φ89mmΦ89mm without the magnetic drill pipe.

Construction Plan of Drilling Technology
The drilling exploration range is 630 m, 81312 in front of the heading head and 20 m, of the final hole outside the positive side of the 81313 auxiliary transportation channel is located in No . 10  The curvature radius method is used to calculate the trajectory of the borehole in the oblique section of the borehole, The borehole trajectory of the curve oblique section is obtained. After the curve stabilization section successfully enters the target layer, By controlling the tool surface of the drilling tool, the trajectory is approximately extended to the final hole in the target layer by a straight line. The borehole design trajectory plan is shown in figure  1.

ANALYSIS OF TEST CONDITIONS AND EFFECT
As a result of this survey, the expected engineering quantity of the design has been completed, and the total drilling advance is m .3567 In the drilling process, the track is controlled in real time by the wired drilling measurement system. After the construction of the main hole is completed, the construction of the other four branch holes is completed by the "backward" side drilling branch method. Using the fan-shaped multi-branch directional long borehole covering and exploration in the whole area of No .10 coal seam, there is no hole collapse and drilling phenomenon in the construction process, and there is no outlet phenomenon after the final hole of the borehole, which indicates that there is no hidden structure in the bottom plate of No .8 coal seam, and forms a channel to communicate with the high-rise pressure water of the bottom plate. In the later stage, the borehole is used as the gas pre-drainage borehole in No .10 coal seam and the gas interception borehole near No .8 main coal seam. From figure 2, it can be seen that the gas purity increases from unstable to 1.75 m after drilling and pumping3The min fluctuation is 0.77~2.35 m in gas drainage3min, average m 1.833min; gas concentration is up to 82.4%, basically above 60%, high concentration gas can meet the requirements of gas power generation.

CONCLUSIONS
(1) The long-distance water exploration and discharge drilling is used to realize the complete separation between the water exploration and discharge operation and the tunneling operation, which reduces the scope of the cross operation water damage under special circumstances, and improves the working efficiency of the water exploration and discharge operation and the tunneling operation.
(2) A set of fan-shaped multi-branch directional long boreholes were constructed. By adjusting the trajectory of the boreholes in time, full coverage of the detection area was realized, and the precise detection effect was ensured.