Alterations in coal mechanical properties and permeability influenced by liquid CO2 phase change fracturing

Abstract

Liquid CO2 phase change fracturing (LCPCF) is a promising approach for stimulating coal reservoirs and enhancing gas drainage efficiency of low-permeability coal seams. In this work, low pressure CO2/N2 adsorption, CH4 permeability measurements and UCS tests are combined to quantitatively analyze the changes in coal mechanical properties and permeability before and after LCPCF treatment. The results show that the axial peak strain is increased and ductile failure takes place in treated specimens after 1.26 L of liquid CO2 was used to conduct physical blasting. The UCS and elastic modulus after LCPCF are reduced by 10.82–25.78% and 12.92–28.57%, respectively. Muti-scale pore structures within coal are altered to varying degrees. After LCPCF treatment, mesopore volume and macropore volume are increased by 11.33–41.57% and 33.82–67.5%, respectively, while micropores are slightly affected. Due to the adsorption swelling and Klinkenberg effect, CH4 permeability of all the specimens is reduced with the increase of injection pressure. Comparative analysis of gas permeability has been made between the raw coal and treated specimens. Coal permeability is enhanced by 14.48-29.43% and 10.54-24.82% after LCPCF treatment at the injection pressure of 0.5 MPa and 1.4 MPa, respectively. During the LCPCF process, shock waves and stress waves trigger tensile damage to coal mass, and a large number of new macropores/fractures are generated within coal. This provides more accessible paths for gas flow and coal permeability are greatly promoted. Crack propagation is accelerated by the released high-pressure CO2, and both coal porosity and pore connectivity are further enhanced to facilitate gas seepage in coal seams.