| Miscible injection of carbon dioxide (CO2) has seen a significant increase in interest for the purpose of enhanced oil recovery in non-fractured reservoirs. However, naturally fractured reservoirs, which are among the largest oil reserves in the world, are considered poor candidates for this process due to presumed low performance efficiency. In naturally fractured reservoirs, gravity drainage is considered to be the main production mechanism. In such environments, matrix permeability is very low compared to fracture permeability, leading to early gas and water breakthrough and subsequent low displacement performance from secondary oil recovery techniques in most of these reservoirs.For huff-and-puff and gravity drainage experiments, effects of matrix permeability, initial water saturation, oil viscosity, and operating pressure were investigated during both miscible and immiscible CO2 injection.The results show that in a fractured system saturated with light synthetic oil (Normal Decane), when CO2 is injected at pressures slightly above the Minimum Miscibility Pressure (MMP), the recovery is increased by a factor of 2 compared to immiscible conditions.The huff-and-puff process was found to be very efficient when CO 2 was injected at near or slightly above the MMP in a core saturated with synthetic oil or light crude oil. Also, it was found that the presence of residual water saturation had no significant impact on the recovery factor when implementing the huff-and-puff technique.In this study, a CO2 huff-and-puff process has been investigated in fractured environments for the first time, both experimentally and through mathematical modeling and simulation. Performance of huff-and-puff was studied under different operating conditions. Additionally, the efficiency of CO 2 flooding and its potential to improve oil recovery from gravity drainage mechanisms has been investigated at different operating pressures covering both immiscible and miscible conditions.In addition, this study indicated that under immiscible conditions, injecting CO2 at pressures below the MMP and above the supercritical condition is not beneficial to improving oil recovery from gravity drainage. This was clearly seen during gravity drainage experiments when operating pressures were set to 10372 kPa and 13829 kPa for a core saturated with light crude oil at residual water saturation.Oil recovery due to immiscible and miscible CO2 injection in conditions similar to the above experiments were also simulated using a fully compositional simulator CMG-GEM(TM) provided by the Computer Modeling Group (CMG). The simulation results obtained for gravity drainage were compared with the experimental results for both normal decane and light crude oil under different operating conditions. Through simulation runs, a set of sensitivity analysis was conducted on the effects of parameters such as the diffusion coefficient, matrix, and fracture permeability on the recovery factor under immiscible and miscible CO2 injection. The results obtained from this study, both experimentally and through simulation, address the knowledge gap related to the best practices for utilizing CO2 for improving oil recovery from fractured reservoir environments. |
