Abstract
Gas-injection EOR has good displacement efficiency but poor sweep efficiency. Foam can improve sweep efficiency in these processes, but its direct injection would not be practical due to its poor injectivity. Field results suggest that the well injectivity can be improved if gas and surfactant solution are injected in alternating slugs, leading to foam creation inside porous medium at their (gas and surfactant) contact. This process, referred to as Surfactant Alternating Gas (SAG), aims at reducing gas mobility at the displacement front. Behind the front, because of low water saturation, foam collapses (partially or completely); thus, gas mobility gradually increases to its original mobility at the injection well. This can potentially lead to fingering of the highly-mobile gas into the region with low-mobility gas in the form of foam.
To the best of our knowledge this phenomenon has not been reported in the literature, apparently due to poor grid resolutions employed in the numerical simulations and/or focus on other parameters, from which it is difficult to infer instabilities. In this paper we use a local-equilibrium foam model to simulate a SAG process with high grid resolution simulations. The magnitude of the instabilities decreases with weaker foam, decreasing grid resolution, increasing diffusion, and inclusion of capillary pressure, suggesting that fingering reflects the intrinsic physics of the process and importantly, it is not a numerical artifact. In a successful SAG process, as long as there is enough surfactant ahead of the gas bank to form a region with low mobility, the fluid ahead of it will be efficiently displaced (i.e., stable front exists). In numerical simulations, however, this could cause extremely low discrete mobility values in some of the grid blocks located at the front. In such a process, there can be fingering within the foam bank (unstable displacement), while the foam front appears relatively stable. This work is the first of its kind, i.e., such an unstable front has never been reported in the literature. Further studies are required to verify if the unstable displacements are physics-driven, or due to numerical artifacts at the front with sharp gradients. Our results have practical implications, especially in calculation of the well injectivity and design of optimum slug size and foam strength in a SAG process.
