Numerical Simulation Of Low Salinity Water Flooding: Wettability Alteration Considerations

Abstract

Summary The mechanisms of Low Salinity Water Flooding (LSWF), as a cost-effective and environmentally-friendly technique for improved oil recovery, have been extensively investigated during the last years. Although the mechanisms are still subject of research, wettability alteration (or change in relative permeability) of formation rock surfaces from preferential oil wetness to water wetness as a result of multi-component ion exchange (MIE) and geochemical reactions is a feasible and supported pore scale mechanism. Modeling of wettability alteration process is challenging due to the complex interactions among ions in the brine and crude oil on the solid surface. To improve the understanding of the influence of geochemical processes on the LSWF, numerical models were created with parameters identical to those used in the experiments. The low salinity effects were simulated using a numerical reservoir simulator, considering aqueous reactions, ion exchange, and mineral dissolution and precipitation. Characteristic features of the model are explored in order to gain insight into the role of low salinity flooding, and its possible impact on oil recovery. The model was used to predict oil recovery for experiments under a variety of conditions where recovery factor may be increased by about 30 %. The geochemical reactions included in the model control the wetting fractions and contact angles, which subsequently determine the capillary pressure, relative permeabilities, and residual oil saturations. Simulations show that transport of the phases is related to desorption of the divalent ions from the clay surface in such a way that increased desorption gives rise to a change of the relative permeabilities such that more oil is mobilized. Dissolution of calcite tends to reduce desorption of calcium ions from the rock surface and hence the possibility to improve recovery by the MIE mechanism. The release of cations and hence oil recovery depend on several factors like connate water and brine compositions, and clay content. It can be concluded based on this study that the LSWF performance depends on initial wettability conditions, clay content and reservoir minerals, composition of the injected and formation water, and also oil properties.