Experimental study on the feasibility of microwave heating fracturing for enhanced shale gas recovery

Microwave heating fracturing is potentially a green stimulation technology for gas shale recovery. However, the mechanism together with permeability evolution of microwave irradiated reservoir remain unclear. To fill this knowledge gap, the responses of Longmaxi shale from the Sichuan Basin, southwest China, to both continuous and intermittent microwave stimulation along variable microwave heating paths were explored. A complex thermally-induced fracture network can be formed gradually without sudden collapse under intermittent microwave irradiation. Changes in the petrophysical parameters of the shale including wave velocity, weight and volume at different intermittent microwave irradiation steps were measured together with temperature variation. The evolution of permeabilities for the two shale samples with alternately parallel and vertical beddings at different effective stresses was analyzed both before and after microwave irradiation. After the last step of intermittent microwave irradiation in this study, the shale permeability increased by two to four orders of magnitude for the shale sample with flow parallel to bedding and one to two orders of magnitude with flow perpendicular to bedding. Microwave treatment accentuates the anisotropy between bedding-parallel and bedding-normal permeabilities. Evolving pore size was measured by high-pressure mercury porosimetry and thermal-induced fracture characteristics and the changes of mineral composition were characterized by SEM combined with Energy Dispersive Spectroscopy (EDS). Thermally- and chemically-induced swelling stresses are mainly responsible for the development of fractures and micro-porosity in the shale. A permeability model with variable compressibility coefficient was adopted to fit the experimental data for shale permeability across a wide range of effective stresses from 2.5 MPa to 59.5 MPa. Shale fracture compressibility decreases in the later stage of microwave irradiation, suggesting the hardening of thermal-induced fractures.