An experimental study on the damage characteristics of mechanical properties of anthracite induced by supercritical CO_2 injection

摘要

Carbon dioxide (CO2) is a kind of greenhouse gas affects atmospheric temperature. The deep coal seam is a potentially favorable geological body for CO2 storage, as it can be coupled with enhanced coalbed methane production, which can offset partly the cost of storage. CO2 injection to the deep coal seam would be in supercritical state, i.e., ScCO2, which affects the mechanical properties of coal seam more significantly. In this paper, a study was initiated to investigate the effect of ScCO2 adsorption and pore pressure on the mechanical properties of anthracite by the mechanical testing system, and observations were conducted to analyze the micro -damage of anthracite before and after ScCO2 injection by the scanning electron microscope. The results indicated that the strength of anthracite would be reduced with the pore pressure increases. Corresponding to the He injection pressure with 4 MPa and 8 MPa, under the condition of confining pressure 10 MPa and temperature 40 degrees C, the compressive strength reductions were 10.2% and 35.9%, the elastic modulus reductions were 10.9% and 24.7%, and Poisson's ratio increases were 16% and 49%, respectively. The strength of anthracite would also be reduced by ScCO2 adsorption. Corresponding to the ScCO2 injection time for 3 h and 6 h, the compressive strength reductions were up to 42.6% and 61.1%, the elastic modulus reductions were up to 10.4% and 41.3%, and Poisson's ratio increases were up to 8.92% and 35.71%, respectively. The mechanical weak planes were produced by ScCO2 injection, which could reduce the mechanical strength of anthracite. Compared to anthracite samples with the He injection, the elastic modulus of anthracite with ScCO2 injection was reduced by 21.9%, the compressive strength was reduced by 23.9%, and the Poisson's ratio was increased by 5.3%. The results also indicated that the mechanical strength reduction of anthracite was more significantly caused by ScCO2 injection for the superimposed effect of adsorption and pore pressure.