Determination of sealing depth of in-seam boreholes for seam gas drainage based on drilling process of a drifter

• The response of drift is used to study the geologic conditions in coal seam.
• Rapid increase of idle pressure is an indication for high stress and gas pressure.
• The sealing depth should be several meters over the stress concentration area.
• A real-time method to determine sealing depth for in-seam boreholes is proposed.

In order to determine a reasonable sealing depth for seam gas drainage boreholes, an innovative method is proposed to study the geological conditions in the coal seam by monitoring the response of the drift while drilling the boreholes. Based on the investigation of the in-situ tests on the drift, we found that the idle pressure of the hydraulic motor, which features a rapid increase at the depth of approximately 14 to 16 m from the roadway wall, appeared to be suitable for characterizing the coal mass condition. Then, a numerical simulation was performed to investigate the reason for the response of the idle pressure. It reveals that high stress and gas pressure are the main factors accounting for the rapid increase of the idle pressure. Accordingly, the rapid increase of idle pressure becomes an indication for high stress and gas pressure.Next, for determining the sealing depth, the gas drainage process with different sealing depths was studied via numerical simulation, and a simplified theoretical analysis was conducted to comprehend the flow of air from the roadway to the borehole. The analysis reveals that the reasonable sealing depth should be several meters larger than the stress concentration area, which is indicated in practice by the idle pressure. Then, a sealing depth of 16 to 18 m is determined for the III1011 mining face in the Haizi coal mine. This research provide a real-time method to determine the sealing depth for in-seam boreholes.