Experimental study on sorption induced strain and permeability evolutions and their implications in the anthracite coalbed methane production

The dynamic changes of strain-induced strains and permeability are two key parameters to determine the production profile of coalbed methane (CBM) wells. Recent field observations from a group of anthracite CBM wells demonstrate that the permeability dramatically changed with depletion. In addition, different drainage strategies will induce different permeability evolution due to different matrix shrinkage behaviors. We carried out a series of experimental measurements on sorption induced strain evolution and its influence on permeability evolution for anthracites using a desorption-seepage testing system. The relationship between sorption pressure and its induced strains was studied. The results show that both axial and radial strains of a coal specimen increase with continuous methane injection pressure. It was found that the strain perpendicular to the bedding plane is higher than that parallel to the bedding plane. Under constant confining and axial stresses, the effective stress decreases with the increase of methane pressure, and permeability initially drops and then rebounds with continuous methane injection. The “check-shape” permeability evolution was observed for all three specimens and the turning points are 4-5 MPa for the tested anthracites. It was also found that sorption swelling strain depends on both petrographical properties and the degree of coalification. The strength of coal increases with the degree of coalification. At a constant confining and axial stresses, both sorption-induced swelling strain and the mechanical-induced strain decline with increase of coal strength. The sorption-induced strain increases with increase of the organic content. During primary CBM production, the permeability of coal initially decreases before reservoir pressure reaches the critical desorption pressure and it starts to increase at low pressure at which the reservoir pressure is below the critical desorption pressure. The results have theoretical and practical significance to guide the reasonable drainage of CBM wells.