Pulse generation and propagation in the numerical solution of low salinity water flooding

In the last two decades, it has been widely accepted that injection of reduced salinity water in oil filled porous media results in alteration to the wettability of the invaded zone. As a consequence, this process of Low Salinity Waterflooding (LSWF) will improve the displacement efficiency of hydrocarbons. However, modelling this process is a challenge. Coarse scale numerical models can result in considerable mismatch compared with a fine case scenario. The aim of this study is to analyse pulse-like behaviour that appears in coarse grid models of LSWF. We simulated LSWF using an implicit scheme. We analysed the stability of the numerical models in which pulses were observed to grow in time. The stability analyses were done by adding perturbation to the numerical solution. Then, we investigated the numerical model as well as the physical model to address the causes of the pulses. We show, for the first time, that the numerical solution was stable, despite the apparent growth of the pulses in time. The underlying reason for the pulses is explained in terms of how we model the change to wettability and the impact on relative permeability. Once created, the propagation of the pulses is predicted by analytical theory and fine grid models. Further, the resulting pulses vary in character, including amplitude, width and velocity, from one cell to another due to dispersion combined with the effective salinity range.