A Model for Interfacial Activity of Acidic Crude Oil/Caustic Systems for Alkaline Flooding

Abstract

Abstract The interaction of the alkali in floodwater and the acids in reservoir crude results in the in-situ formation of surfactants, which are responsible for the lowering of interfacial tension (IFT) in caustic flooding. The extent to which IFT is lowered depends on the specific properties of the crude oil and the injection water. Therefore, it is properties of the crude oil and the injection water. Therefore, it is important to establish the relationship between IFT and the essential chemical properties of the acidic oil and the floodwater. This paper presents such a relationship. presents such a relationship. In this discussion, the adsorption and the desorption of the active species at the interface are modeled as ionic processes using the Gouy-Chapman theory of the diffuse double layer. The interfacial potentials calculated using this model show a fair agreement with the potentials calculated using this model show a fair agreement with the experimentally measured trend of electrophoretic mobility. Also, the model rationalizes the experimentally observed effects of alkali concentration, salinity, and the oleic- to aqueous-phase ratio on IFT. We conclude that the acid number of the crude oil may not correlate directly with interfacial activity. Even in cases of low-acid-number crudes, significant interfacial activity could be obtained because of highly hydrophobic active species in the crude. Introduction IFT in acidic crude/alkali systems plays an important role in EOR with alkaline agents. The extent-and probably the mechanism-of oil recovery is highly dependent on the degree to which IFT is lowered in such systems. Because it is possible to estimate the residual oil saturation from capillary number, and hence the IFT through a suitable displacement model, there is a necessity for quantitative evaluation of IFT in an alkaline flooding process. Although it is widely accepted that surfactants are generated by reaction in caustic flooding of acidic crude reservoirs, there have been few attempts to develop a chemistry and relate it to the IFT attained in these systems. A relationship between equilibrium IFT and NaOH concentration (restricted here to caustic) in a batch system permits comparison with experimental data, and hence evaluation of system parameters such as equilibrium constants. Also, from these parameters, an parameters such as equilibrium constants. Also, from these parameters, an equilibrium flow model can be solved for the concentration of chemical agents in place in caustic flooding. The IFT and hence the residual oil saturation can then be calculated in a flow process, on the basis of the batch relationship. This study, by rationalizing the observations of IFT in acidic crude/caustic systems, permits evaluation of the equilibrium constants and the other necessary parameters. The model also reveals the dependence of interfacial potential on variables such as NaOH and NaCl concentrations. Model Description The rationalization of the interfacial activity between the acidic crude oil and caustic solution is based on a simple system chemistry. The proposed system chemistry has been used to solve for the interfacially proposed system chemistry has been used to solve for the interfacially active species in a batch system. Considering adsorption and desorption kinetics at the interface, along with a suitable theoretical description of the electrical double layer, we derived the adsorption isotherm for the active species. The interfacial activity (i.e., the reduction in IFT of the system) was then directly calculated by the Gibbs equation for adsorption at the interface. An alternative method using an equivalent equation of state (EOS) is also presented. To keep mathematics tractable and the physics simple, the formation of micelles is not considered. Only an equilibrium study is conducted here, and it is applicable primarily to an equilibrium flow model. SPEJ p. 602