Numerical Assessment and Optimization of Foam-Based EOR Processes in Naturally Fractured Carbonate Oil-Wet Reservoirs

Summary In the recent years, foam process has shown a growing potential in producing naturally fractured reservoirs. While much effort was devoted to fractured reservoir characterization and laboratory design of foaming chemicals, decision-making for field development goes through reservoir simulation. This work assesses foam flooding performance through dual-media reservoir simulations, to optimize injection strategy and foam key features such as stability, mobility reduction, adsorption and interfacial tension reduction. Foam flooding may improve significantly oil recovery in fractured oil-wet reservoirs through a better fluid mobility control and an increase in viscous forces in fractures, improving the sweep efficiency. Additionally, foaming chemicals can yield ultra-low interfacial tension between oil and water, and thus enhance the oil mobilization. This study evaluates the sensitivity of foam flooding performances to foam model parameters and injection strategy at pilot-scale. This sensitivity analysis is based on the variance decomposition (Sobol's index) of the response model (such as oil production), while a surrogate model is built to limit the number of simulations required to evaluate the objective function. Optimization are performed to constrain the process to be more efficient than gas and/or water injection. Compared to other flooding processes such as gas and/or water injection, this work demonstrates how foam flooding can be optimized such that viscous gradients and gravity drainage act optimally to sweep oil in the matrix, but still maintain a sufficient injectivity to yield a maximum recovery. The foam process optimum is a trade-off between oil recovery maximization and costs minimization, which corresponds to a rise in viscous forces that force out the oil from the matrix while respecting the fracture pressure and limiting the amount of injected chemicals. This sensitivity assessment brings new insights for pre-feasibility studies, in particular for the foaming formulation design. Specifically, as foam flooding recovery mechanisms interplay, impacts of chemicals adsorption, injection strategy and optimization parameters will strongly differ whether foaming chemicals achieve or not ultra-low interfacial tension. Hence, for a “low-IFT foam”, chemicals adsorption reduction will be crucial although negligible for a foam process without IFT reduction. Because of different mechanisms involved, the foam process efficiency mainly depends on viscous forces developed in the fracture network whereas the “low-IFT” effect mainly relies on the penetration of chemicals in the matrix, that facilitates oil recovery. The optimization workflow demonstrated its ability to evaluate and to help the selection of the most appropriate process at pilot scale, according to the reservoir specificities.