Stress-Dependent Elasticity of Rocks During the Triaxial Loading and Reloading Cycles

Stress-dependent elastic properties of rocks play a significant role in the exploration and in the 4D seismic monitoring. Periodic cycles of the loading and unloading are common for the underground hydrocarbon or CO2 storages. This work presents an experimental and a theoretical study of the stress-dependent elasticity during the multistage triaxial loading and reloading cycles. The ultrasonic measurements confirmed the vertical transverse isotropy of the shale sample. Analysis of the P-wave velocities propagated in the axial and in the radial directions suggested opening of the cracks in the sample, during the first loading cycle. Application of the porosity deformation approach resulted into the description of the nonlinear stress-dependency of the elastic parameters, and moreover, was used for the quantitative and qualitative characterization of the stress-induced alterations in the rock. The created during the first loading cycle pore space was characterized as the compliant pore space with a similar stress-sensitivity as by the already existed pore space. The volumetric estimation of the created pore space is equal to 0.02%.