Carbon Dioxide Foam Rheology in Porous Media: A CT Scan Study

Summary Carbon dioxide (CO2) foam has been widely studied in connection with its application in enhanced oil recovery (EOR). This paper reports an experimental study concerning CO2 foam propagation in asurfactant-saturated Bentheim sandstone core and the subsequent liquid injection with the aid of X-ray computed tomography (CT). The experiments were carried out under various system backpressures. It is found that CO2 foam flows in a characteristic front-like manner in the transient stage and that the water saturation keeps at relatively high level at the outlet of the porous media because of CO2 solubility and capillary end effect. The subsequent surfactant solution injection shows a significant fingering behavior, accompanied by a low flow resistance over the core. It is also found that CO2 foam flow shows higher liquid saturation near the outlet and lower pressure drops under higher system backpressures. This can be attributed to the solubility of CO2 in the liquid phase. The results indicate the advantage of using foam in EOR processes such as water alternating foam (WAF), in which foam flow has higher sweep efficiency and stronger mobility control ability compared, for instance, to water alternating gas (WAG). Nevertheless, care should be taken during the water-injection stage in order not to favor the fingering. Introduction Foam applications in EOR and fluid (acid) diversion have grown considerably over the last three decades. For instance, WAGhas been regularly used in the field as a gasflood mobility control measure. Nevertheless, this technique has not always demonstrated the desired beneficial mobility effects because of the gravity segregation and the unstable preceding of the front between the water and moremobile gas (Holm 1987; Smith 1988). Creating foam by adding surfactant to the aqueous phase has proven to be able to increase the total recovery significantly by increasing the apparent viscosity of the system (Holm and Josendal 1974; Ali et al. 1985; Patzek 1996; Zhdanov et al. 1996; Turta and Signhal 1998). There are many attractive features of EOR using CO2 foaminjection. First, carbon dioxide is a proven solvent for reconnecting, mobilizing, and recovering waterflood residual oil. Many studies (Stalkup 1983) have shown that CO2 can achieve miscible-like displacement efficiency through multiple contacts (partitioning and extraction) with the crude oil. Second, CO2 is available naturally in large quantities and as a byproduct of lignite gasification and many manufacturing processes. Its price is also low, and there are no other large-volume uses competing for CO2. Third, with the push toward sustainable power production and the increasing realization for the need to reduce CO2 emissions, EOR using CO2 is becoming an important alternative for geological CO2 storage.