Enhanced coalbed gas drainage based on hydraulic flush from floor tunnels in coal mines

Abstract

Gas drainage for the single and low gas permeability coal seam is the key technical problem hampering efficient coal mine gas drainage and without which mining safely cannot be realised in China. To solve this problem, this paper presents an engineering method for enhanced coalbed methane recovery based on high-pressure hydraulic flush from floor tunnels. The first step is to evaluate when the likelihood of coal and gas outburst reaches dangerous levels according to coal seam parameters (including coal seam gas contents, gas pressure, permeability and geological conditions). With these parameters in place, the second step is to determine and optimise borehole parameters, such as the effective influencing radius of hydraulic flush, hydraulic flush space between drills and borehole number to make sure that the coal seam stress is fully released and permeability is dramatically increased. What is also included in this step is the employment of a high-pressure hydraulic flush of coal from boreholes drilled from tunnels developed in the floor of the coal seam. Parameters of water pressure, water flow rate and the volume of coal flushed out are selected based on on-site testing and numerical modelling. Finally, numerical modelling and onsite testing are employed to validate the effects of enhanced coalbed methane recovery, which is whether or not coal and gas outburst danger is eliminated according to the national standards of China. The results show that the technology could improve the permeability of a coal seam and that the gas seepage coefficient was increased by about 10.50 times, the pre-gas drainage ratio was up to 35.5–70.4% and the borehole gas drainage experienced a process of increase-steady-decrease, which delayed 15–20 days of the attenuation time.