Experimental study of seismic action influence on gas permeability of coal and hydraulic fractures

Abstract

Mine field development of coal deposits is currently associated with work at depths exceeding 400m. Under these conditions the gas content of promising horizons is one of the main factors constraining the pace of development. An integral part of gas-bearing coals’ extraction is their preliminary degassing. The methane recovery ratio affects the efficiency and safety of longwall work. Degassing is intensified in order to achieve the required values of methane content in coal seams. A well-known method employed to intensify filtration processes in fluid-saturated rocks is the elastic vibrations’ treatment. The laboratory tests and field work show the positive effect of elastic vibrations on the intensification of methane desorption from coal. Modern works in this field are devoted to investigation of connections between the parameters of vibration action and the degree of increase in the methane desorption. This paper provides the results of a series of experimental studies on seismic action’s effect on the permeability of coal and hydraulic fractures. The research has been carried out on a custom designed laboratory bench. The design of the stand provides the effect of elastic vibrations of small amplitude with independent regulation of the average and differential gas pressures, axial and lateral compression of the sample. The experiments have been carried out using solid coal cores, cores with single through longitudinal cracks simulating drainage hydraulic fractures and cores with the single fractures propped with a low-density proppant designed to intensify the degassing of coal seams. The patterns of the seismic impact on the gas permeability of coal under the conditions of all-round compression have been established in accordance with the results of experiments. Also, the experimental results reveal certain patterns of increase of the drainage cracks’ gas permeability observed when the cracks are propped with proppant and are under the low intensity seismic effect under the conditions of all-round compression. The studies show that the effectiveness of seismic action increases with an increase in the accumulated exposure time, followed by stabilization and persistence of the positive effect for at least 3 - 7 days after the cessation of exposure. The obtained results provide the opportunity to assess the possibility of using seismic action to intensify the degassing of non-propped hydraulic fractures in coal mines.