Impact of Diffusion and Adsorption on the Performance of CO2, CH4, and Cyclic Gas Huff-n-Puff Injection into Light Oil Reservoirs

Abstract The implementation of cyclic gas injection, commonly known as huff-n-puff, holds significant promise in augmenting hydrocarbon recovery from shale oil reservoirs and addressing condensate blockage in liquid-rich shale formations. The effectiveness of huff-n-puff, however, depends greatly on the composition of both the reservoir fluid and the injected gas. Particularly in ultratight shale reservoirs, where diffusion and sorption play pivotal roles, a precise understanding of their influence on huff-n-puff performance becomes crucial for accurate predictions of oil recovery and solvent retention. To thoroughly assess the huff-n-puff process in shale reservoirs, we conducted extensive large-scale numerical simulations using a dual-porosity naturally fractured compositional model that incorporates molecular diffusion and sorption mechanisms. The Langmuir's adsorption model was employed to account for adsorption effects within the system. Rigorous grid block sensitivity analysis was performed to minimize numerical errors and enhance simulation accuracy. By evaluating the impact of diffusion and sorption on production performance for different fluid and injection gas combinations, we established correlations between the considered characteristics and the huff-n-puff performance. To conduct this evaluation, we selected the Eagle Ford Formation, a highly developed shale with a wide range of pressure-volume-temperature (PVT) windows, from dry gas to black oil. The simulation outcomes revealed that methane (CH4) and cyclic-produced gas exhibited the highest recovery potential, while carbon dioxide (CO2) yielded the lowest production results. The performance of the solvent was notably influenced by the content of light components in the fluid and the gas-oil ratio (GOR). Neglecting molecular diffusion, especially during the soaking period, led to underestimation of recovery factors, whereas disregarding the adsorption effect resulted in overestimation of recovery. Furthermore, we observed that the adsorption of intermediate components on the surface of organic pores in shale gas condensate effectively pushed condensate out of the pores, mitigating condensate blockage around the wellbore. This work aims to provide further insights into the huff-n-puff performance in shale reservoirs by focusing on the reservoir fluid and injection gas compositions. The results of this work will improve our understanding of the relationship between fluid compositions and diffusion and sorption. Furthermore, our findings provide insights into the optimization of the huff-n-puff process in shale reservoirs.