A comparative investigation of the effect of gas type on foam strength and flow behavior in tight carbonates

Abstract

Foam generation technique has been practically applied to overcome gas mobility issue during enhanced oil recovery (EOR) processes. Incorporated with the utilized gas, the capability of foaming agent to generate persistent foam determines the flow performance under reservoir conditions. In this study, we first identified the surfactants with excellent phase behavior and foaming properties and, second, evaluated their bulk foam performance with different gases, including N2, CO2, and hydrocarbon gas (CH4) under high-pressure and high-temperature conditions. In bulk, the foam generated with the high-density gas, i.e., CO2, exhibited the greatest foamability but the lowest foam stability. With amidoamine oxide-based surfactant, CH4-foam was found to be more stable than N2-foam, while the opposite was observed for sulfobetaine-based surfactant. The most effective surfactant was later used to perform flow experiments on two tight carbonate rocks, including Minnesota Norther Cream Buff and Edwards limestone, with permeabilities of ∼0.8 and 16 mD, respectively. The core-flooding experiments showed that the optimum quality of CO2-foam was lower than that of N2- and CH4-foams. For a given flow rate and fixed foam quality, the steady-state strength followed the descending order of CH4-, N2-, and CO2-foam. However, in general, the difference in foam strength between N2- and CH4-foams was insignificant when the total flow rate was varied. We also found that the variation in rock permeability had a greater effect on the strength of the CO2-foam compared to those generated with N2 and CH4. In the presence of oil, the shear-thinning behavior of all foams was found more prominent. The increase in oil saturation had a more adverse effect on the strength of N2- and CH4-foams, particularly in the higher permeability rock. The results indicated that the foam strength, which is key in the effectiveness of foam EOR applications, is greatly influenced by several factors such as gas density, gas solubility in water, the presence of oil, and rock permeability.