The excavation damaged zone (EDZ) of the Callovo-Oxfordian argillites can be regarded as a double-porosity media consisting of blocks of undisturbed argillites separated by cracks generated by shear or traction stresses. However, owing to hardware limitations, most of the hydraulic-gas simulations undertaken on large scales are based on an equivalent-porous-media model of the EDZ, where the fractures are not explicitly represented. The aim of this study is to improve the equivalent-porous-media model’s consistency with the double-porosity flow behaviours shown by experimental data while keeping the same level of simplification. The new model has been developed through a performance assessment (PA)-like approach in two steps: a phenomenological model including explicit fractures has been designed in accordance with the actual geometrical and hydraulic understanding of the EDZ; then, the properties of the simplified model have been calibrated to comply with the hydraulic behaviour of the explicit-fracture model. This methodological approach has been used on an access gallery parallel to the main horizontal stress axis. The new equivalent-porous-media model differs from the previous one on the following three main points: the gallery EDZ has an anisotropic intrinsic permeability that is lower than previously; the same retention law is used as the undisturbed host rock; and there is a higher relative permeability for gas and water than previously.