EOR processes employing gas injection can be very efficient in recovering oil where the gas sweeps. Unfortunately, gas injection has poor sweep efficiency because or reservoir heterogeneity, viscous instability and gravity segregation of injected gas to the top of the formation. Foam can improve the sweep efficiency in gas injection in Enhanced Oil Recovery. The best injection strategy to overcome gravity override in homogeneous reservoirs is a SAG process with a large slug of surfactant followed by a large slug of gas. An additional advantage of SAG injection is increased gas injectivity. Water is displaced from the near-well region where foam weakens, this raises gas mobility and increases injectivity. However, during liquid injection, the injectivity is considered to be poor.
Liquid injectivity is not fully understood during SAG process. In this study several core-flood experiments were conducted to investigate how injectivity is affected by the injection strategy. Through these experiments, it has been found that the gas injection flow rate has no significant effect during the gas injection period, foam collapses after roughly the same number of pore volumes of gas injected regardless of the injection rate.

During the liquid injection period, liquid was injected at different flow rates following a gas injection period. The results suggested a moderately shear-thinning behaviour. Liquid injection rate was increased 10, 40 and 100 times, but the rise in pressure gradient is not proportional to the increment in injection rate.

Furthermore, the effect of the size slug injected was investigated on the subsequent liquid injection. Results show that during prolonged periods of gas injection after foam, a region near the inlet is formed in which the gas mobility is much greater and the liquid mobility is much greater than downstream during the subsequent liquid injection. The bigger the gas slug size, the better the subsequent liquid injectivity in the nearest region to the inlet.

The effect of foam quality was studied; it has been found that foam quality has no big effect during the subsequent gas and liquid injections. The results of foam injection at 0.60 and 0.95 quality followed by gas injection show that foam collapsed after roughly similar number of pore volumes injected regardless of foam quality. During the liquid injection period the trends of the pressure gradient were similar at foam initial quality injection 0.60 and 0.95.

Finally, in order to verify the capability of the radial model developed by Gong et al. \cite{Gong}, the experimental data was fitted to the linear-flow and radial-flow models. The results suggest that the simulation based on Peaceman equation underestimated the gas and liquid injectivity in SAG process, and furthermore, the conventional simulator cannot represent the effect of gas injection on the subsequent liquid injection.