Coupled multiphase flow and elasto-plastic modelling of in-situ gas injection experiments in saturated claystone (Mont Terri Rock Laboratory)

Understanding gas migration in host rock is one of the most important issues to consider in the final disposal of radioactive waste in deep geological formations. In order to investigate gas transport in water-saturated argillaceous rock, constant gas pressure injection tests on the samples from the Mont Terri Rock Laboratory (Switzerland), as well as pulse injection tests with a stepwise increasing of gas pressure, were carried out. During gas migration, desaturation and hydraulic mechanical interaction can be observed. To simulate gas migration in water-saturated porous media, a coupled multiphase flow and mechanical method within the FEM code family OpenGeoSys (OGS) was developed including the van Genuchten function for the relationship between capillary pressure and water saturation, the approach of Mualem for the relative permeability of both gas and water phases, and an elasto-plastic deformation model. Two approaches were applied, taking into consideration the permeability change by pore gas pressure and by deformation, respectively. Both permeability approaches are based on experimental observations and allow for the significant increase in permeability when gas pressure exceeds the minimal stress. The material parameters for the multiphase model are determined using the measured data from laboratory experiments under well-defined test conditions. With the calibrated model parameters, the in-situ experiment was simulated. Good agreement between the numerical results and the experimental data is achieved.