Modelling gas flow in clay materials incorporating material heterogeneity and embedded fractures

A series of gas injection tests on compacted bentonite were carried out at the British Geological Survey. Tests measurements included pressure and rate of gas inflow, gas outflow volume as well as stresses and pore pressure observed at various points of the sample. Tests were performed with two different gas injection systems: injecting the gas from one end of the sample (axial flow) or from one point at the centre of the sample (spherical flow). A coupled hydro-gas-mechanical 3D numerical model has been developed to simulate the tests. Initial permeability is assumed heterogeneous throughout the specimen and embedded fractures are incorporated in the formulation. Gas pressure-induced deformations during the test lead to variations of permeability due to changes in matrix porosity and, especially, fracture aperture. The model is able to reproduce satisfactorily the observed behaviour of the tests including the existence of preferential gas flow paths. A programme of sensitivity analyses involving the variation of different aspects and parameters of the model contributes to a better understanding of the phenomenon and highlights its complexity. The application of the same formulation and parameters calibrated in the axial flow test results in a successful simulation of the spherical flow test.