Coupled geomechanical phenomena in the two-phase flow analysis of air-injection tests in clay cores

Abstract

Understanding gas transport processes is one of the key issues in the assessment of radioactive waste repository performance. The actual gas migration mechanisms may entail standard two-phase flow or more complex mechanisms involving coupled two-phase geomechanical and possibly geochemical phenomena. Laboratory tests on OPA cores from a shallow borehole in the Mont Terri Underground Research Laboratory (URL) and from a deep borehole in northern Switzerland were performed by two different laboratories, which include retention behavior and geomechanical tests, as well as specific water and air injection tests to determine single-phase liquid and two-phase properties. For the investigation of the laboratory measurements, numerical models were developed implementing the geometry of the rock core and the boundary conditions on the injection and outflow sides. The specific information from the laboratory experiments in terms of the stress dependence of void ratios and changes in permeability and capillary pressure were implemented in the numerical model, which reproduced well the measured injection pressure and outflow responses of both air injection tests parallel and normal to bedding.