A Micromechanics-Based Strength Criterion for Porous Rock: Application to Predict Wellbore Failure Behavior

In underground engineering, including oil and gas extraction, mining activities, and CO2 sequestrations, porous rock formations are commonly encountered. The stability assessment of drilling or excavation operations in such formations conventionally relies on strength criteria. Nevertheless, the strength of porous rock shows remarkable nonlinear features and may even experience compressive yield, a phenomenon that is overlooked by existing failure criteria. This paper employs micromechanics and homogenization approaches to derive a novel failure criterion that can depict the compressive yielding of porous rock. Triaxial strength tests were conducted on multiple porous rock types to verify the proposed failure criterion. This validated criterion was then employed to calculate wellbore collapse and fracture pressures for establishing an optimal mud weight window. The research outcomes are as follows. First, the newly developed failure criterion is verified using triaxial strength data from sandstone, limestone, and chalk. A comparative analysis was conducted with the Drucker–Prager cap model and the modified Gurson model. The new model exhibits outstanding predictive accuracy and effectively overcomes the limitations of the aforementioned models. Second, through several field case studies, the rationality of the safe mud weight window predicted by the proposed failure criterion is confirmed. This study offers a more precise and dependable tool for assessing stability and optimizing operations in underground engineering projects that involve porous rock formations.