Mathematical modeling of low salinity waterflooding in sandstone reservoirs: Enhanced Oil Recovery by multicomponent cation exchange

Abstract

Low salinity waterflooding is the injection of water with a smaller salt concentration than the connate water. The control of the pH of injection water, and the amount of dissolved monovalent and divalent cations in the water affect the cation exchange in the reservoir and the mobilization of residual oil. In this work, the solution for the problem of low salinity slug injection driven by seawater considering three dissolved cations and pH effects is described. It was considered that the cations and H+ adsorbed on the rock follow a Langmuir adsorption isotherm type. The adsorption parameters of the cations depend on the water pH. The mathematical problem is split into three parts: a pH equation, a one-phase chromatographic system, and a scalar equation. First, the pH problem is solved, and its solution is used to calculate the one-phase chromatographic problem, and both results are applied in the scalar equation solution. Next, the solution is mapped onto the space-time plane. The solution shows that due to the high adsorption rate, the pH effects take place close to the injection point. Moreover, the high contrast between the adsorption rates of hydrogen and the other cations creates small regions where salinity changes. Cations separation in the porous media, similar to a chromatographic cycle, lead to the generation of several small oil and water banks along the porous media. The oil banks contain residual oil which was mobilized by the cation exchange. The analytical solution was compared with experimental data, showing close agreement. These results, mainly the relation between pH and salinity, and the appearance of water and oil banks along the flooding are new and important to the design of this chemical enhanced oil recovery technique.