Impact of Surfactant Structure and Oil Saturation on the Behavior of Dense CO2 Foams in Porous Media

Abstract Miscibility with oil lies among the main advantages of dense CO2 injection for pore scale oil displacement during tertiary recovery. At reservoir scale, injecting dense CO2 in the form of foam can also improve its sweep efficiency. However, although the use of such miscible dense CO2 foams has been considered in over twenty pilots since the 1980's, only few lab studies have considered foams formed with CO2 in this particular thermodynamical state. Indeed, dense CO2 has solvation properties and a viscosity higher than that of a gas. This impacts several attributes of its foams in porous media, such as Mobility Reduction Factors (MRF) and behavior in presence of oil. We present new results demonstrating that classical foamers are not effective in improving mobility control of dense CO2, but that relatively high MRF are achieved using carefully formulated surfactants. Based on these findings, we study the impact of foam on miscible flooding efficiency in corefloods. Reversely, we also evaluate how miscibility of CO2 with oil impacts foam MRF. Our approach is based on multiple corefloods experiments, with different formulations, at various oil saturations. Additionally, physical-chemistry measurements such as interfacial tension estimations and foam stability monitoring are performed in reservoir conditions (pressure and temperature). This set of experiments shows that a balance must be found between maximizing MRF and minimizing the risk of emulsion formation in porous media. This paper brings new insights on the interpretation of CO2 foams coreflood results, based on the thermodynamical properties of the CO2 phase. It provides the reader with a clearer view of gas properties that must be considered when analyzing results of dense CO2 foams corefloods. This can help reconcile seemingly contradictory results appearing in the literature, particularly regarding the values of MRF as a function of pressure and in the presence of oil.