The Effect of Calcium Ions on Oil-Brine-Surfactant Interfacial Properties and the Relation to Surfactant Enhanced Oil Recovery at Low Salinity
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The present thesis addresses the effect of calcium ions on the interfacial chemistry between oil and solutions of anionic surfactants, and its relevance for enhanced oil recovery (EOR) methods at low salinity. A number of scientific works have recently suggested a combination of low salinity water (LSW) flooding and surfactant flooding as a promising new EOR approach. Surfactant flooding has successfully been established in numerous fields worldwide, and relies on reducing the capillary forces which trap crude oil within the porous reservoir rock. The EORpotential of LSW-flooding, which, however, is still in its trial phase, is supposed to arise from water-mineral interactions, promoting desorption of polar oil components from the rock surface. An additional aim of combining both methods is to reduce retention and precipitation of surfactant within the reservoir, which is of substantial importance for the economic feasibility of the EOR-process. The main focus of our study was the interfacial tension (IFT) between oil and water - one of the basic values in surfactant flooding, and in EOR in general. While the surface activity of ionic surfactants, as generally accepted, depends on the electrolyte concentration of its solution, we chose the molar ratio between calcium and sodium ions as the key parameter. The ionic strength was in most of the experiments kept constant, to evaluate the effect of the calcium ions as precise as possible. IFT measurements between surfactant solutions and oils - either crude oils, or chemically inert hydrocarbons - were performed, both below and above the critical micellar concentration. The respective results are summarized in the journal papers I - IV. In the papers I and II, phenomenological insight in the effect of calcium on IFT is given, while paper III presents a mechanistic approach of explanation; Latter is complemented by molecular dynamics simulations. The interfacial activity of ionic surfactants was found to increase systematically with increasing amounts of calcium in solution, analogous to the case of increasing electrolyte concentrations. This effect seems to correlate directly with the distribution of differently charged ions close to the interfacial adsorbed surfactant layer. In paper IV, the empirical information obtained from the preceding papers is combined with a first oil displacement study from oil saturated sandstone cores, in order to determine the influence of calcium on the EOR-performance of an industrial surfactant formulation. Here, oil recovery efficiency was strongly correlated to IFT changes caused by calcium ions. Paper V and VI are dealing with desorption of polar oil components from silica surfaces upon flushing with surfactant solutions of varying salinity. Here, a novel combination of Quartz Crystal Microbalance and contact angle measurements, for measuring oil desorption and surfactant adsorption quantitatively and to determine consequential wettability alterations of the surfaces, was established.