In-Situ Precipitation of Calcium Carbonate during Carbon Dioxide Injection in Micromodels

Summary Conformance control is a necessary part of enhanced oil recovery (EOR) operations in heterogeneous oil fields. However, many techniques center on near-wellbore conformance or are not well-suited for low matrix permeability formations. We propose the injection of calcium hydroxide (Ca(OH)2) and carbon dioxide (CO2) as a novel conformance control strategy in reservoirs that experience poor sweep efficiency due to heterogeneity. These two chemical agents react to precipitate solid calcium carbonate (CaCO3), potentially blocking already-swept pores and directing injected fluids to unswept regions of the reservoir. Microfluidic devices (micromodels) were used to perform experiments to observe the fundamental, pore-scale behavior of this reaction in porous media. Precipitation grain size, location, and relative quantity were directly observed while changing injection order, CO2 state, and injection rate. It was found that the reaction took place in porous media for both states of CO2 [gaseous and dissolved in deionized (DI) water]. More precipitate formed during experiments in which gaseous CO2 was used compared with when CO2 was dissolved in water. Overall, precipitation occurred where the reactants both contacted each other and mixed, which did not have a preference for high- or low-permeability zones. Precipitation via this method was controlled through CO2 placement in the reservoir and injection rate, as the reaction only occurred where CO2 and Ca(OH)2 were both present and had sufficient convective mixing. Precipitation was also shown to block pore throats and reduce permeability in 2.5-darcy micromodels. This study is a first look at this novel conformance control process and will be used to develop future core-scale experiments.