Simulation of instabilities and fingering in surfactant alternating gas (SAG) foam enhanced oil recovery

Gas-injection enhanced oil recovery (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 as gas and surfactant intermingle in situ. 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 investigated 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 an implicit-texture foam model to simulate a SAG process with fine grid resolution. The magnitude of the instabilities decreases with weaker foam, coarser grid resolution, increasing diffusion, and inclusion of capillary pressure, suggesting that fingering reflects the intrinsic physics of the process: 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 is efficiently displaced (i.e., stable front exists). In numerical simulations, however, there are extremely low discrete mobility values in grid blocks located at the front. In such a process, there can be fingering within the foam bank (unstable displacement), while the foam front itself appears relatively stable. Further studies are required to verify the extent to which simulations of fingering reflect numerical artifacts at the front. These results have practical implications, especially in calculation of the well injectivity and design of optimum slug size and foam strength in a SAG process. (C) 2016 The Authors. Published by Elsevier B.V.