Research on the negative pressure distribution law and its application for boreholes in coal seam bedding gas extraction

Abstract

Borehole along the seam is a significant borehole distribution process of gas extraction in coal mine, which is a fundamental measure for preventing and controlling gas disasters, but also an important mean to explore coal-bed methane resources. Recently, the technology of borehole along the seam has been widely applied to gas extraction with the development of borehole rigs and kilometer-borehole rig appearing. Presently, the domestic scholars’ researches on the extraction theory mostly focus on pre extraction by borehole through beds and shallow borehole along the seam, few considering the influence of viscous mechanics of gas flow in boreholes. Therefore, the research on the variation of negative pressure inside the borehole along the seam in borehole direction is greatly important to guide the design of borehole along the seam to gas extraction, the determination of key parameters and the application of the technology in theory.

Focusing on research of distribution of negative pressure inside the borehole along the seam in borehole direction and its application, this paper studies two sections: (1)Research on distribution of negative pressure inside the borehole along the seam in borehole direction.  Based on gas occurrence, flow theory, fluid dynamics theory and the definite mechanism of negative pressure inside the borehole along the seam on gas extraction, both gas flow models around boreholes and inside boreholes were established. Distribution of negative pressure inside the borehole along the seam in borehole direction was got by classifying the pressure loss inside boreholes  and coupling the flow conservation equations of two models, so as to its influential factors. Distribution of negative pressure inside the borehole was technically determined by field tests, verifying the feasibility of the proposed models. (2)Application of distribution of negative pressure inside the borehole along the seam in borehole direction. Considering the shortages of traditional extraction processes, two new ones were put forward applying the distribution of negative pressure inside the borehole along the seam in borehole direction, the extraction pipe reaching a certain distance or covering the whole length. Technical processes such as theory analysis, field test and numerical simulation verified the superiority of the new extraction processes.

This study finally got the variation of negative pressure inside the borehole along the seam in borehole direction and the distribution of negative pressure inside the plume borehole along the seam, derived the calculating formula of reasonable borehole length, put forward new extraction processes and verified the superiority of the new extraction processes in some conditions.