Mechanical properties of shale following saturation with CO2 and CO2-based fluids: experimental and modeling study

Impacts of saturation with CO₂ and CO₂-based liquids are vital for understanding shale's mechanical properties associated with supercritical-CO₂ optimized shale gas extraction and geological capture and storage of CO₂ in shale reservoirs. A sequence of triaxial compression tests is performed to examine the impact of subcritical CO₂, supercritical CO₂, subcritical CO₂-water, supercritical CO₂-water, subcritical CO₂-NaCl, and supercritical CO₂-NaCl saturation on shale strength. A statistical damage constitutive model of shale after CO₂, CO₂-water, and CO₂-NaCl saturation is established to describe shale's stress-strain relationships under various immersion conditions.

The laboratory findings indicate that the change of the axial stress, Young's modulus, and axial strain of shale after immersion verifies the physical and chemical reactions that occur between shale and the soaking fluids. Mechanical properties of shale show the greatest variations after CO₂-water saturation. The variation in mechanical properties of shale after CO₂-NaCl saturation is smaller than those of shale under CO₂-water saturation owing to the precipitation of NaCl crystals. Pure CO₂ saturation has the smallest influence on shale's mechanical properties among the three types of liquids assessed. CO₂ in a supercritical state shows a stronger impact on shale than the subcritical state for the same sort of fluids. Also, following saturation, all the shales display a mixed tensile-shear failure mode. The cohesion force of shale increments following pure CO₂ saturation, whereas it diminishes following CO₂-water and CO₂-NaCl saturation. Decreases in the internal friction angles are observed for all the soaked shales. The anisotropy of shale leads to a slight difference between the actual failure angle and the failure angle measured by the Mohr-Coulomb criterion.

The stress-strain relationship of shale under different confining pressures is effectively described by the Weibull probability distribution and the principle of strain equivalence. This establishes statistical damage constitutive equations of shale under different soaking conditions. The values of key modeling parameters, including F₀ and m, are highly dependent on the brittleness and strength of shale associated with various soaking conditions.