Effect of Nanoparticles on Flow Alteration during CO2 Injection

Abstract

Surface-treated nanoparticles have been shown to stabilize CO₂-in-water foam by adhering to the surface of CO₂ bubbles and preventing their coalescence. However, to bring the nanoparticles from the bulk phase to CO₂/water interface requires an input of mechanical energy. Co-injection of CO₂ and an aqueous dispersion of nanoparticles at high rates is known to provide sufficient energy. However, this co-injection is less favorable because of the operational constraint, i.e., injectivity reduction. Here, we show that beneficial effect of nanoparticles, manifested as improved sweep efficiency, occurs even at low shear rates in a drainage displacement. We inject high-pressure liquid CO₂ into sandstone cores initially saturated with brine containing suspended nanoparticles and compare the results with the case with no nanoparticle addition. The water saturation distribution was measured using CT scanning techniques. The results show that the nanoparticles increase sweep efficiency and reduce the gravity override compared to displacements without nanoparticles. The new mechanism described here provides a promising alternative for mobility control in CO₂ floods. Introduction Carbon dioxide injection to recover oil from mature oil reservoirs ("CO₂ flooding??) has been used for over 40 years (Grigg & Schechter, 1997; Jarrell, et al., 2002). CO₂ flooding is now more economically and environmentally viable, because of the high crude oil price and possible large anthropogenic CO₂ sources through carbon capture (Enick & Olsen, 2011). Despite its good local displacement efficiency, CO₂ flooding suffers from poor sweep efficiency due to CO₂'s very low viscosity. Therefore, efficiency of a CO₂ flood can be improved if the mobility of the CO₂ is effectively reduced (Lake, 1989). Previous attempts for controlling CO₂ mobility by using CO₂ direct thickener (Bae & Irani, 1990) or gels (Wagner & Weisrock, 1986), and making foam with surfactants (Rossen, 1996), have made progress, but still have issues to be worked out. For instance, despite over 40 years of research, no CO₂ direct thickener, which could be economically injected, has been found. In addition, surfactant stabilized foam, by its nature, is unstable, and the efficiency of the surfactant under harsh reservoir conditions (high temperature and high salinity) is questionable (Rossen, 1996). Therefore, other more robust methods that can control the mobility of CO₂ effectively and affordably are required. Here, we studied possible use of engineered nanoparticles in controlling the mobility of CO₂ by spontaneous formation of stable CO₂/water foam.