Timeline of surfactant solution/gas co-injection (blue) and surfactant solution alternating gas injection (red) scenarios, where W, G and W* denote water, gas and surfactant solution, respectively. Timescale is indicated in injected pore volumes (PV, volume of fluid injected over the pore volume of the porous medium). Both cases have a pre-foam water or gas injection history of 1 PV. In each case the volume of surfactant solution injected of 0.25 PV is identical: in the co-injection case, it is distributed over a total volume of gas and surfactant solution of 1.25 PV according to the imposed quality of f_g = 0.8 that corresponds to optimal foaming conditions. There is no such quality control for the surfactant solution alternating gas injection, where foam is formed by fingering of gas (injected after surfactant solution) through surfactant solution, or at least mixing of the two, due to the different mobility of the fluids.