Study on deformation and destabilization characteristics and modes of drainage borehole

ABSTRACT

Using the borehole instability model based on theory of microscopic scale elastic damage, the authors conducted numerical analysis of deformation characteristics of drainage borehole at different burial depths and proposed the deformation instability mode for drainage borehole. Results showed that (1) the borehole at shallow depth had less deformation and remained in quasi-circular shape; as the burial depth increased, the deformation of borehole wall became more significant, the top area of borehole showed slight caving, the lower area of borehole displayed coal spalling, and the whole borehole changed into “ellipse-like shape”; the borehole at large burial depth was subjected to severe collapse or even blocking, and the buckling failure mode of the borehole was featured by collapses at upper area (forming a collapse area) and breaks at left and right sides (forming a broken zone). (2) The deeper the burial depth was, the higher the deformation degree was, the larger the pressure released zone around borehole was, and the more significant the gas permeability change of surrounding coal mass caused by borehole deformation was; in general, the instability and deformation of borehole had a less impact on the gas permeability of surrounding borehole. The gas permeability change of surrounding coal mass approximately followed “V shape” rule. (3) According to the creep deformation properties of coal mass and the deformation mechanism of borehole and combining the numerical analysis results and results of previous researches, this study presents the deformation modes of three types of boreholes (complete, collapsed, and blocked boreholes) and corresponding on-site preventive measures. The results of this study are of great guiding significance for the improvement of coal seam gas extraction effect.

KEYWORDS:

• Coal mine safety
• deformation and destabilization characteristics
• drainage borehole
• destabilization modes

Additional information

Funding

This work was supported by the National Natural Science Foundation of China: [Grant Numbers 51734007, 51704099, and 51604101], Program for Innovative Research Team in University of Ministry of Education of China: [Grant Number IRT_16R22], Key Scientific Research Projects of Higher Education Institutions in Henan Province [Grant Number 19A440003], and the State Key Laboratory Cultivation Base for Gas Geology and Gas Control (Henan Polytechnic University) [Grant Number WS2017B14].