Effect of methane gas seepage on coal rock strength

In the production stage of coalbed methane (CBM) wells, the pore pressure and effective stress of coal seams change gradually with the continuous methane desorption and seepage, and correspondingly the mechanical strength of coal rock decreases.Consequently, the porosity and permeability of reservoirs and even the wellbore stability are, in a way impacted.In this paper, the coupling relationship between the seepage process of coalbed methane in coal seam and the mechanical property of coal rock was tested indoors.And it provides the theoretical basis for formulating CBM well completion and development schemes.Then, acoustic emission test and uniaxial/triaxial compression test were carried out.It is shown that as the flowing period of CH4 through coal rock cores extends and the elastic modulus of coal rocks decreases gradually, indicating the decrease of its mechanical strength.The uniaxial/ triaxial compression test reveals that the elastic modulus and maximum compression strength of coal rocks decrease with the decreasing of confining pressure.And they decrease slowly (fast) when the confining pressure is high (low).It is demonstrated that downhole coal seams may collapse and drop after coalbed methane is produced for a while, and consequently the production and operation is impacted adversely.