Gravitational Fingering Due to Density Increase by Mixing at a Vertical Displacing Front in Porous Media

CO2-improved oil recovery has been applied in petroleum production for many years. CO2 injection has attracted increasing attention because it represents a possible solution for the reduction of the CO2 concentration in the atmosphere to reduce greenhouse gas emissions and improve oil recovery. The density increase from CO2 dissolution in oil creates an unstable high-density layer and leads to gravitational fingers, which may have a significant effect on the mixing and flow path. In this work, we modeled gravitational fingering during injection by a miscible fluid pair with nonlinear density property and investigated the properties of the fingering resulting from the density increase by mixing in porous media using three-dimensional X-ray computer tomography. During the injection process, the mixing near the interface was enhanced, and slight fluctuations appeared at the interface, which grew into high-concentration fingers that interacted and merged with neighboring fingers and extended vertically downward. With increasing Pedet number, the density difference between the downward-moving fingers and upward-moving surroundings increased, resulting in increases of the finger extension velocity and velocity of the upward flow. Furthermore, the mixing near the interface became stronger. Over time, the mixing length and relative volume of the fingers increased. The growth of the mixing length of the fingers was proportional to time with a smaller slope at early times and switched to a larger slope growth at later times. A rapid increase of the relative volume of fingers was observed with increasing Pedet number. In the fingers, the local NaI concentration decreased linearly and the initial interface traveled downward as a result of the enhanced interaction of the fingers during the injection process. The enhanced dispersion with increasing Peclet number affected the broadening of the fingers and reduced the finger number density.