SAG Foam EOR

Abstract

Surfactant-Alternating-Gas (SAG) is a popular enhanced oil recovery method that utilizes foam to decrease gas mobility and subsequently improve reservoir sweep efficiency. SAG is known to have many benefits, such as mitigating corrosion effects and increasing gas injectivity. Despite this, liquid injectivity is typically very poor during the SAG process. Currently, there is not a consistent model for liquid injectivity during SAG. However, Gong et al. recently conducted core-flood experiments to gain clarity on how gas and liquid injectivity are affected during SAG in near-wellbore conditions. Gong et al. determined that gas and liquid injectivity are represented by the propagation of multiple banks. For gas injectivity, there are two banks: the collapsed-foam bank and the foam bank. For liquid injectivity, there are three banks: the collapsed-foam bank, the gas dissolution bank and the liquid-fingering bank.
Conventional foam simulators, such as CMG’s STARS, use the Peaceman equation to obtain an estimate for well pressure as well as liquid and gas injectivity. Gong et al. came to the conclusion that conventional foam simulators do not take the propagation of the collapsed-foam bank into account. Thus, Gong et al.’s results indicate that the Peaceman equation greatly underestimates gas and liquid injectivity during SAG in a 100 by 100 meter grid block. Subsequently, this paper aims to determine how refining the 100 by 100 meter grid block will alter well pressure and liquid and gas injectivity, especially in the near-wellbore region.
We used CMG’s STARS, a conventional foam simulator, to test the impact of grid refinement on injectivity and well block pressure. Our grid set-ups include a base grid of 5x5 equal blocks and two refined grid cases, in which the center grid blocks are partitioned into 9 and 25 equal block pieces. Our results indicate that as we refine the grid, the dimensionless pressure drops occur quicker and the magnitude of pressure decreases. When compared to the base grid, the pressure drops of the refined grids were discovered to be approximately 9 and 25 times faster for the 3x3 center grid case and the 5x5 center grid case respectively. Additionally, we observed additional dimensionless pressure peaks in the refined cases, which can be explained by a drop in relative mobility when the foam reaches a new grid block.
Although our foam parameters were based on Gong et al.’s foam scan, our foam parameters are not exactly the same as the parameters listed in Gong et al.’s paper because we did not include foam shear-thinning properties. In order to build on this paper’s research, future research should look into comparing our STAR’s results to the fractional-flow theory. Additionally, future models and research should look into reducing the simulation’s time step in order to increase the number of iterations calculated by the simulator, especially around the region in which the pressure drop occurs. Lastly, it would also be important to look into finding the best method to represent the well block pressure for a refined grid case.