Experimental investigation of mechanical properties in cryogenically damaged shale

During cryogenic fracturing, the low thermal conductivity of rock results in varying freezing rates within shale formations, leading to significant differences in mechanical behavior. This study employs innovative experimental methods to examine the effects of cryogenic freezing rates on shale's mechanical properties and thermal damage capacity. Results demonstrate that when the freezing rate is below 2.5 °C/s, shale exhibits enhanced mechanical properties under cryogenic conditions. Conversely, freezing rates exceeding 5 °C/s cause mechanical deterioration. A nonlinear relationship was established between shale's compressive strength/elastic modulus and cryogenic freezing rate. The primary damage mechanisms were identified as micro-fracturing induced by cryogenic thermal stress and ice expansion. This study proposes a novel method for quantifying shale's thermal damage capacity using the average peak thermal stress gradient. This finding provides a direct guidance for optimizing cryogenic fracturing operations in shale oil and gas reservoirs, where maximizing the freezing rate can promote more complex fracture networks and enhance stimulation efficiency.