CO2 sorption induced mechanical response of Beetaloo shale

Abstract

Understanding the mechanical response of organic-rich shales to CO₂ adsorption is critical for optimising CO₂ sequestration and enhanced gas recovery operations. However, these responses are often highly complex and demand a thorough understanding of the underlying sorptive poromechanical mechanisms involved. This study examines the sorptive poromechanical response of Beetaloo shale to CO₂ adsorption under varying confining stress and pore pressure conditions, with a focus on quantifying strength reduction mechanisms. Through controlled triaxial compression experiments, the effects of CO₂ exposure on shale strength with varying pore pressures under different confining stresses were analysed. A theoretical model was then developed and experimentally validated to predict the peak strength of shale with CO₂ adsorption integrating triaxial response with Langmuir adsorption mechanics. The experimental results showed a strong linear relationship between confining stress and both peak strength and elastic modulus, within the range of confining stress tested (up to 25 MPa). It was also revealed that while variations in adsorption across different confining stress levels were relatively small for low gas pressures, this difference became more pronounced at higher pore pressures. In addition, it was observed that the shale strength reduction due to gas adsorption under lower confining stresses follows a near-linear trend with increasing gas saturation. However, under higher confinement (e.g., 25 MPa), the relationship becomes nonlinear, with a more pronounced drop in strength occurring at the early stages of gas adsorption. Moreover, the developed model aligned well with experimental data across all tested conditions, offering a practical tool for predicting mechanical alterations in CO₂ saturated formations. The findings of this study enhance our understanding of coupled poromechanical-sorption effects in shale rocks and provide critical insights for optimising CO₂ injection strategies, ensuring formation integrity, and improving long-term sequestration security in shale carbon sequestration.