A method for developing a hydrocarbon reservoir by injecting a gas in the form of foam, comprising a step of determining a foam displacement model, which is a function of an optimal gas mobility reduction factor and of at least one water saturation-dependent interpolation function. The optimal gas mobility reduction factor is determined and constants of the water saturation-related interpolation function are calibrated from a plurality of apparent viscosity measurements for various foam quality values, and by use of a lamella model allowing which relates the lamella density as a function of the water saturation. The invention has an application to petroleum exploration and development.