A constitutive framework for caprocks accounting for viscoplastic cyclic degradation and coupled geo-chemo-mechanical processes

This paper provides an extension of an existing elasto-plastic framework originally proposed by Gens & Nova (1993) for modelling the response of structured soils and soft rocks. The model is enhanced to reproduce not only the mechanical response of caprocks under standard monotonic triaxial loading, but also the effects of the environmental and hydraulic loading induced by modern energy applications, including gas/hydrogen storage and geological carbon storage. The novelty of these applications, compared to the more usual ones developed by the oil and gas industry over decades, lies in the complex pore fluid and stress pressure histories applied and in the strong geochemical interaction of the rock formations with non-native fluids. Cyclic pore pressure histories due to seasonal gas storage may result in a mechanical degradation of the caprock material, while chemical degradation may occur due to pore water acidification resulting from the rock-water-CO2 interaction. To cope with the cyclic mechanical degradation, the framework is first coupled with the extended overstress theory, so to satisfactorily reproduce the time-dependent behaviour of caprocks, which presents inelastic strains even within the yield surface. Such an extension is shown to be essential to reproduce the strong strain-rate dependence and the increase in the number of cycles to failure with the amplitude of cyclic loading observed in experimental data obtained on intact specimens of an Italian stiff carbonatic clay. The elasto-plastic model is then enhanced to account for chemical degradation, using the calcite mass fraction dissolution as a variable controlling damage evolution. Combined with a geochemical reactive transport model, this extension satisfactorily reproduces the progressive degradation of a Chinese shale due to CO2 exposure, showing the ability of the framework to model coupled geo-chemo-mechanical processes.