Experimental Study on the Influence Mechanism of Coal–Rock Fracture Differential Deformation on Composite Permeability

Aiming at the gas flow problem in fractured coal–rock assemblage during L-shaped horizontal well hydraulic fracturing and gas extraction in coalbed methane development, permeability–stress test of single fractured coal–rock assemblages was carried out. The influence of gas pressure, effective stress and individual permeability of fractured coal–rock mass on the combination permeability were analyzed. With increase in effective stress, the permeability and its stress sensitivity of the single fractured coal–rock assemblage decreased gradually. Due to the existence of slippage effect, the permeability of coal–rock assemblage decreased gradually with increase in gas pressure within the range of study gas pressures. The gas adsorption of the coal caused the fracture aperture to decrease and led to the uneven deformation of coal–rock assemblage because the adsorption deformation of rock mass can be neglected. Comparison of the permeability variation of single fractured rock and coal sample with the increase of gas pressure further verified that the coal adsorption reduced the permeability of coal–rock assemblage. Finally, the permeability calculation model of single fractured coal–rock combination is presented in this paper. This model can well match the laboratory measured data, and it can clarify the contribution of each permeability part to the overall permeability. The permeability model indicated that the permeability of coal–rock assemblage depended on the smaller part of permeability. The contribution of the high permeability part to the overall permeability can only be increased a little on the basis of the low permeability part.