A gas injection test, performed on compact bentonite, was carried out at the British Geological Survey. The test is composed by two stages (i.e. hydration followed by gas injection testing). After gas breakthrough and a period of gas flow through the sample, the injection pump was stopped whilst the stresses and porewater pressures were continuously monitored. A Hydro-Mechanical 3D numerical model has been developed to simulate the gas injection test and to achieve similar gas pressure, gas outflow, and stress evolution responses.