Numerical investigation of the gas-induced fracturing behavior of the Callovo-Oxfordian claystone

This paper presents a synthesis of the numerical approaches adopted by three research teams to reproduce gas fracturing initiation in the Callovo-Oxfordian claystone. This collaborative work has been carried out within the framework of the DECOVALEX-2023 project. First, the research teams investigated the impact of gas migration and fluid pressurization within the Callovo-Oxfordian claystone and the fracturing threshold pressure through a series of benchmark exercises under plane strain conditions with increasing complexity. The three numerical approaches accounted for couplings between the mechanical part and hydraulic parameters, such as permeability, through different variables such as damage, fracture aperture, or equivalent plastic strain. Then, the research teams utilized their models to reproduce two injection tests at the field-scale. A challenge faced by the research teams was dealing with a single study point per injection test, complicating the study of responses near the injection interval. This part included interpretative analyses with simplified approaches for a better understanding of gas pressure build-up. Overall, the numerical simulations yielded acceptable results in reproducing the in-tests after a calibration process and provided insights into the hydromechanical response of the Callovo-Oxfordian claystone under two-phase flow conditions. Nonetheless, the benchmark exercises showed that the numerical results using different mechanical constitutive models yielded different outcomes when reaching critical values leading to fracturing, which strongly depend on how the mechanical part influences the hydraulic response through the changes in hydraulic properties. • Synthesis of three research teams participating in the modelling of gas fracturing in claystone. • Approaches: embedded fractures, damage, and a model combining a failure index with dilatancy-dependent permeability. • Six benchmark gas injection tests followed by validation against two in-situ injection tests. • Numerical approaches reproduced in-situ observations, including gas percolation through the borehole damage zone.