An experimental investigation of oil effects on foam flooding under miscible conditions

Miscible gas injection, as an Enhanced Oil Recovery (EOR) method, can improve the recovery factor by swelling the residual oil and phasing out the interfacial tension [1]. However, the sweep efficiency of miscible gas injection is not satisfactory because of conformance problems, such as viscous fingering, channelling and gravity override [2]. Foam can solve the problems associated with gas injection by reducing the mobility of gas and therefore sustaining a stable displacement front [3]. It is known that under immiscible conditions, the presence of oil can damage foam stability through different mechanisms [4]. However, it is not fully understood how interactions between gas and oil under miscible conditions may affect foam flow behaviour. In this study, the effects of oil on foam flow behaviour under miscible conditions are investigated. To this end, several foam flooding experiments have been conducted using CO2 and decane, as a model gas and a model oil, respectively, under miscible conditions. These core-flooding experiments have been conducted with different molar fractions of CO2 and decane. Through these experiments, it has been found that both supercritical CO2 foam and decane emulsion display a low-quality regime and a high-quality regime, whereas, in the presence of oil under miscible conditions, CO2-decane-surfactant floods do not show low and high-quality regimes. Based on CO2 molar fraction in CO2-decane mixture and its effects on foam behaviour, three different regimes have been observed. Regime 1 (CO2 molar fraction in CO2-decane mixture, xCO2 , is larger than 0.8) has the highest apparent viscosity among the three regimes. In regime 2 (xCO2 is below 0.2), the apparent viscosity is still high, although lower than that of regime 1. Finally, within regime 3 (xCO2 ranges from 0.2 to 0.8), the apparent viscosity is the lowest (around 50 cP) among the three regimes and does not experience significant changes. Furthermore, through a set of shear-thinning experiments, it has been shown that both supercritical CO2 foam (xCO2=1) and decane emulsion (xCO2=0), as well as CO2-decane-surfactant floods in regime 3, all exhibit shear-thinning rheology. Moreover, in regime 1 and regime 2, there is a transition at shear rates from 10 s to 100 s, where the apparent viscosity is increased by one order of magnitude. In regime 3, however, the apparent viscosity is at its largest value already (up to 560 cP) when shear rate is even lower than 20 s -1 . Finally, in order to examine the capability of the STARS foam model to capture the oil effects on foam behaviour under miscible conditions, the experimental foam-scan data has been used to estimate the STARS foam model parameters. It has been found that the STARS model is not able to capture the oil effects on foam behaviour under miscible conditions.