Hydro-mechanical properties of a low-clay shale with supercritical CO2 imbibition

Abstract

The mechanical properties and permeability of shales after supercritical carbon dioxide (SC-CO2) imbibition are vital to shale gas production and CO2 sequestration in shale reservoirs. In this study, triaxial compressive strength tests, steady state permeability tests, and CT scanning tests are performed on shale samples with 30 days of SC-CO2 imbibition. The results show that minerals dissolution and precipitation occur during SC-CO2 imbibition. The porosity calculated by the digital core increases by 19.190%. The axial stresses of samples with or without SC-CO2 imbibition increase with increasing confining pressure. When the confining pressure changes between 5 and 20 MPa, the reductions range from 6 to 13% for strength and from 11.26 to 16.47% for Young’s modulus, respectively. The strength of the samples with or without imbibition agrees well with the Mohr–Coulomb criterion. The values of cohesion and internal friction angles are very close which indicates that shale samples with SC-CO2 imbibition can still refer to the failure criterion for samples without CO2 imbibition. The permeability of the sample after SC-CO2 imbibition increases exponentially with the increase in injection pressure and decreases exponentially with an increase of effective pressure. The permeability of the sample before and after imbibition is more sensitive to the change of pore pressure than confining pressure. SC-CO2 imbibition enhances the increase of permeability. The ratio of the shale permeability after imbibition to that before imbibition varies from the minimum of 42 times to the maximum of 827 times. The reduction of strength and increasing of permeability are beneficial for shale gas recovery.