Mechanistic simulation study of air injection assisted cyclic steam stimulation through horizontal wells for ultra heavy oil reservoirs

Air injection assisted cyclic steam stimulation (CSS) through horizontal wells is a new technique for the exploitation of ultra heavy oil reservoirs, which has an advantage over other gas or solvent injection processes from the economic point of view. Different from in-situ combustion (ISC) process, the dominating chemical reactions occurring underground in air injection assisted CSS process are low temperature oxidation (LTO) reactions, and owing to the complicated LTO reaction mechanism, this process is still not clearly understood. Therefore, an indepth learning of this process will be of great benefit to its field application and specific project design. In this study, a comprehensive numerical simulation model was established, which accounted for the LTO reactions of different oil components in terms of SARA fractions, as well as permeability reduction induced by coke deposition. A series of simulations were then performed to explore the production performance and elucidate the impacts of various factors. The simulation results demonstrate that air injection assisted CSS using horizontal wells can enhance ultra heavy oil recovery and reduce cSOR in comparison with steam injection alone, which can be attributed to the synergistic effect of steam and air coinjection. Injection of air along with steam can have the same effect as the initial solution gas in reservoir, and the potential of air injection assisted CSS to enhance oil recovery will be more pronounced in oil layers with lean solution gas. In addition, normal air injection can be a viable choice considering the free availability of air, and injection of oxygen-reduced air can become a good option for ultra heavy oils featured with poor LTO reactivity for the sake of safe production.