Produced-Gas (PG) Versus CO2 Foam in Carbonate Porous Media: Residual-Oil Sensitivity

Abstract Foam is commonly evaluated with pure CO2 for gas mobility control, yet field operations may inject a CO2-rich produced-gas stream containing methane and nitrogen. This study compares the transport behavior of foam stabilized by the branched nonionic surfactant G12 in pure CO2 and produced gas (PG) in Indiana limestone under reservoir conditions. Foam apparent viscosity was evaluated as a function of the superficial velocity, foam quality, residual oil, and surfactant concentration. Both CO2 and PG generated strong foam and substantial mobility control. Under oil-free conditions, the PG foam generally exhibited higher apparent viscosity than the CO2 foam over the tested velocity and foam-quality ranges. However, residual oil reduced the PG foam strength more strongly than the CO2 foam strength. The CO2 foam showed similar velocity-dependent behavior with and without residual oil, whereas the PG foam exhibited lower apparent viscosity and a changed shear-thinning response in the oil-bearing core. Power-law fits showed similar velocity dependence for CO2 foam with and without residual oil, whereas the PG system showed a larger difference between oil-free and residual-oil conditions. Foam-quality scans similarly showed that the PG foam was more strongly affected by residual oil, particularly at intermediate foam qualities. These results indicate that pure CO2 does not necessarily provide a complete surrogate for a CO2-rich produced gas stream when designing foam processes for carbonate reservoirs. Gas composition influences both foam oil tolerance and surfactant requirements; therefore, laboratory formulation and process design studies should use field-representative gas compositions whenever possible.