Deviation Principles of Gas Drainage Drilling in Three-Soft Outburst Coal Seams

The trajectory of the actual gas drainage drilling often deviates from the design in the process of gas control drilling construction, resulting in the deviation of the drilling. The deviation may produce blind areas of gas drainage and induces potential safety hazards. In the present study, we use theoretical analyses and numerical simulations to study the deviation principles of gas drainage borehole in three-soft outburst coal seam. The following conclusions are drawn from our new data: Considering mechanical bending, geometric bending, and other factors that affect the drilling deviation, the deviation mechanism during the drilling process of the drilling rig has been analyzed and we have obtained a mechanical model of the gas drainage drilling rig under the conditions of vertical upward, horizontal and different inclination angles. We have detected the drilling pressure and the weight of the drill pipe as the main influencing parameters of the drilling deviation, providing a theoretical basis for the main influencing parameters of the deviation in the drilling process. In FLAC3d numerical simulation experiment, the displacement around the borehole is limited and the borehole area remains relatively stable and basically unchanged. The overall stress distribution shows a layered structure that largely corresponds to the distribution of the rock sequence. The areas of stress concentration are mainly located in an area of a rock sequence (interlayered mudstone and limestone)) with differing properties, the coal seam area, and the drilling hole location, and the drilling hole is prone to deflection in these areas. The drilling trajectory is mainly influenced by geological factors, equipment factors, and human factors. We propose the combination of a drilling control method and an inclination control method for controlling the influence of machine equipment and human factors in the actual field application. The inclination angle of the borehole deviation is controlled within a certain range to achieve a reduction of the borehole deviation of gas as a suitable reference value in the engineering application.