Role of CO₂ in geomechanical alteration of Morrow Sandstone across micro–meso scales

We study the effect of CO₂ on the mechanical properties of Morrow Sandstone in Farnsworth Unit oilfield in Texas, where supercritical CO₂ is being injected for sequestration and enhanced oil recovery (EOR) applications. We conduct mechanical compression tests with acoustic monitoring and microscratch tests on core samples taken from two wells in the Farnsworth field to quantify the role of CO₂ on poroelastic moduli and fracture toughness across micro-to-meso scales. We quantify dynamic poroelastic moduli at different effective stresses and temperatures using argon and CO₂ as saturating pore fluids and analyze the effect of the fluid type on property alteration. Variation in the effective stress is achieved by varying the confining stress from 5 MPa to 20 MPa and varying the pore pressure over 0–20 MPa range. The temperature is varied from 23 degC to 75 degC, which is relevant for sequestration-EOR applications. Microscratch tests are performed to investigate the nonlinear fracture behavior and estimate the change in the scratch toughness resulting from CO₂ exposure. Scanning electron microscopy shows evidence of nonlinear fracture mechanisms. Using the Size Effect Law for fracture analysis, we find that scratch-induced deformation transitions from ductile to brittle regime as the penetration depth increases. We observe a 29%–45% reduction of the scratch toughness in the bedding-parallel direction and a 60%–82% reduction in the bedding-perpendicular direction, both of which are larger in magnitude compared to the 8%–10% increase in the dynamic elastic modulus at mesoscale due to injection-induced cooling. The grain orientation affects the fracture behavior and CO₂-induced geomechanical alterations: a greater decrease in fracture toughness is observed in the bedding-perpendicular orientation.