Impact of hydraulic fracturing and borehole spacing on gas drainage along a coal seam

Abstract

Hydraulic fracturing is a technology employed to improve coal seam permeability and gas drainage. This study uses theoretical analysis and field testing to examine the mechanisms of hydraulic fracturing of a coal seam, including the initiation conditions of boreholes and cracks, main crack forms, and expansion and extension processes. RFPA2D-flow software was used to develop a numerical simulation of double-hole hydraulic fracturing, from crack initiation to expansion and extension in the coal body. The numerical simulation revealed characteristics and evolution of the stresses, water pressure and acoustic emission around the boreholes and estimated the permeability improvement range of fracturing holes under conditions of different hole spacing. Results of a field industrial test showed that the daily gas drainage pure flows increased by 5.8-17.6 times, the average gas drainage velocity reached 3.76 m3/min (maximum 10 m3/min), the gas drainage rate of the working face reached 57.32%, and the influence radius of single-hole hydraulic fracturing is approximately 5 m. This study demonstrates that hydraulic fracturing and permeability improvement technology can greatly improve the gas drainage rate of boreholes in a coal seam.