Coupled damage-creep mechanisms in shales saturated with sorptive and non-sorptive gases

Time-dependent deformation is a critical factor controlling reservoir integrity and long-term storage security in organic rich shales. Yet the effect of sorbing gases on overall time-dependent and creep behaviour of organic rich shales is not fully understood. This study therefore investigates the influence of sorptive (CO 2 ) and non-sorptive (He) gases on time-dependent deformation of organic rich shale using a combined experimental and modelling approach. Triaxial poro-mechanical creep experiments on Beetaloo shale show that CO 2 exposure significantly enhances creep strain, with deformation increasing systematically as pore pressure and effective stress rise. This enhancement correlates strongly with CO 2 adsorption capacity and cumulative specimen damage, indicating that adsorption-driven swelling initiates adsorption weakening, while progressive damage provides the dominant contribution to creep strain. In contrast, helium exposure results in negligible adsorption and minimal creep, closely resembling dry conditions. Reactor-based adsorption-swelling experiments confirms the relatively minor role of free swelling strain on the overall time-dependent deformation. On the other hand, the shear tests on clay-filled fracture analogs demonstrate that CO 2 reduces the friction coefficient of the clay-filled fracture enabling further creep and damage development. To capture these coupled chemo-mechanical processes, a sorption-damage-creep model grounded in continuum mechanics and non-equilibrium thermodynamics is developed, explicitly linking adsorption kinetics, energy dissipation, and damage evolution to creep strain. Model validation across a range of stresses and pore pressures shows excellent agreement with experimental data, confirming its predictive robustness. These findings reveal fundamental differences between sorptive and non-sorptive gas effects on shale time-dependent deformation and underscore the need to account for adsorption and damage mechanisms in predictive frameworks, with direct implications for unconventional hydrocarbon production, CO 2 sequestration, and the long-term stability of geological storage sites. • Sorptive gas (CO 2 ) significantly enhances shale creep through gas adsorption and damage. • Non-sorptive gas (He) produces negligible creep effects. • A new sorption-damage-creep model integrates gas adsorption, damage, and time-dependent deformation. • Model validation shows strong agreement with experimental results across various stress and pore pressure ranges. • Study provides a mechanistic and predictive framework for unconventional gas and CO 2 storage projects.