During the directional drilling experiment of drill cutting discharge via Nitrogen in underground soft and outburst-prone coal seam, a depth of 437m inseam borehole has been achieved by CCTEG Xi’an Research Institute. Adopting newly innovated "pneumatic downhole drilling tool plus special lubrication device”, technical bottlenecks such as low-pressure start-up, long-life work, and strong slag discharge to prevent sticking have been broken one by one. The borehole has refreshed directional drilling depth record in soft and outburst-prone coal seam.
Because of its low hardness, crushing coal block and well-developed cracks, inseam borehole drilling in soft and outburst-prone coal seam is facing technical difficulties such as gas outburst, borehole collapse, stuck, etc, which leads to low gas drainage drilling efficiency. Borehole depth is generally less than 150m, which can't meet the requirement of high efficient gas extraction. For conventional drilling, the trajectory is not controllable, and drilling has to stop when the borehole reaches the roof or floor. Thus it is easy to form blank zone of extraction and the blind area of gas control, and create serious security risks.
The 3# main coal seam in Tiandi Wangpo coal Mine is a typical broken soft coal seam. A variety of drilling technique tests have been carried out, but satisfactory results have not been achieved. The difficulty boreholes drilling seriously restricts the safe and efficient mining of Tiandi Wangpo Coal Mine.
CCTEG Xi'an Research Institute faced up to the technical challenges and manage an expert team on drilling technology in April 2021. On the basis of the preliminary test, a new pneumatic directional drilling technology scheme of drill cutting discharge via nitrogen is determined. "High voltage intelligent nitrogen making device with high pressure nitrogen" was used as the source of power to drive pneumatic downhole motor. At the same time, nitrogen is used as drill cutting discharge medium to carry the heat of the drilling process. The new scheme has successfully solved the technical problems such as insufficient air pressure, drilling stuck, dust control and drilling track control. Four directional drilling boreholes were constructed in Wangpo Coal mine, with total footage of 1530m, and the maximum depth reached 437m.
The success test of this project provides a new inseam directional drilling solution for high efficient gas extraction in broken and soft coal seam, which will significant reduce the risk and difficulty of mining in outburst-prone coal field.
