Experimental study of the influences of CO₂ injection on stress-strain and permeability of coal reservoir

Based on the coal samples of Chengzhuang coal mine in Qinshui basin, the permeability tests under various effective stresses and CO₂ adsorption pressures of coal were conducted by using the independent-developed experimental simulation device for CO₂ injection and enhanced coalbed methane recovery. The results indicate that the fracture compressibility of coal is affected by CO₂ adsorption, the fracture compressibility coefficients of coal under initial state, subcritical CO₂ adsorption and supercritical CO₂ adsorption are 0.066, 0.086 and 0.089. Two reasons are concluded as:the reaction of CO₂ and minerals in coal improves the internal discontinuity of coal matrix; CO₂ softens the coal matrix and reduces its elastic modulus. The fitting results of measured permeability by adopting the permeability model considering adsorption swelling strain and internal swelling coefficient show that the effective stress is proportional to the internal swelling coefficient. CO₂ adsorption effect blocks some pores and fractures, effective stress not only compress fracture structure but also advances the internal swelling coefficient. The combination of the two effects reduces the injectivity of CO₂ into coal seam, especially for injecting high pressure CO₂ into coal reservoirs with high effective stress.