Numerical simulation study on miscible EOR techniques for improving oil recovery in shale oil reservoirs

Abstract

Shale formations in North America such as Bakken, Niobrara, and Eagle Ford have huge oil in place, 100-900 billion barrels of oil in Bakken only. However, the predicted primary recovery is still below 10%. Therefore, seeking for techniques to enhance oil recovery in these complex plays is inevitable. Although most of the previous studies in this area recommended that CO 2 would be the best EOR technique to improve oil recovery in these formations, pilot tests showed that natural gases performance clearly exceeds CO 2 performance in the field scale. In this paper, two different approaches have been integrated to investigate the feasibility of three different miscible gases which are CO 2 , lean gases, and rich gases. Firstly, numerical simulation methods of compositional models have been incorporated with local grid refinement of hydraulic fractures to mimic the performance of these miscible gases in shale reservoirs conditions. Implementation of a molecular diffusion model in the LS-LR-DK (logarithmically spaced, locally refined, and dual permeability) model has been also conducted. Secondly, different molar-diffusivity rates for miscible gases have been simulated to find the diffusivity level in the field scale by matching the performance for some EOR pilot tests which were conducted in Bakken formation of North Dakota, Montana, and South Saskatchewan. The simulated shale reservoirs scenarios confirmed that diffusion is the dominated flow among all flow regimes in these unconventional formations. Furthermore, the incremental oil recovery due to lean gases, rich gases, and CO 2 gas injection confirms the predicted flow regime. The effect of diffusion implementation has been verified with both of single porosity and dual-permeability model cases. However, some of CO 2 pilot tests showed a good match with the simulated cases which have low molar-diffusivity between the injected CO 2 and the formation oil. Accordingly, the rich and lean gases have shown a better performance to enhance oil recovery in these tight formations. However, rich gases need long soaking periods, and lean gases need large volumes to be injected for more successful results. Furthermore, the number of huff-n-puff cycles has a little effect on the all injected gases performance; however, the soaking period has a significant effect. This research project demonstrated how to select the best type of miscible gases to enhance oil recovery in unconventional reservoirs according to the field-candidate conditions and operating parameters. Finally, the reasons beyond the success of natural gases and failure of CO 2 in the pilot tests have been physically and numerically discussed.

A Literature Review of CO2, Natural Gas, and Water-Based Fluids for Enhanced Oil Recovery in Unconventional Reservoirs

“…Both the contact area and exposure time of CO 2 to the matrix are higher in the laboratory than in the field . For natural gas EOR, higher porosity, natural fracture intensity, and diffusion rates were also beneficial (Table , entry 41). , However, the oil recovery performance of the natural gas was more sensitive to molecular diffusion than CO 2 and increased more dramatically with increases in diffusion rate. These results imply that natural gas is the preferred injection fluid when the permeability of the formation is less than 1 mD because CO 2 has higher molecular weight and needs larger pore throats to invade the matrix .…”

Modeling of the movement of rich gas in a porous medium in immiscible, near miscible and miscible conditions

“…They also investigated their model for fractured gas condensate and showed that diffusion affected the condensate recovery. Alfarge et al 23 investigated the numerical simulation study on miscible EOR techniques for enhancing oil recovery in shale oil reservoirs. Their simulation results showed the significance of molecular diffusion in a gas injection on EOR for the studied reservoirs.…”

Applying the hydrodynamic model to optimize the production for crystalline basement reservoir, X field, Cuu Long Basin, Vietnam

“…However, a field development plan is one of the crucial tasks that can enhance reservoir performance and maximize economic value that has not yet been mentioned in the case of basement reservoir. In recent years, many studies have focused on numerical simulation for improving oil recovery in unconventional reservoirs (Alfarge et al 2018;Izadmehr et al 2018). Alfarge et al (2018) introduced the way to select the best type of miscible gases to enhance oil recovery in shale oil reservoirs.…”

“…In recent years, many studies have focused on numerical simulation for improving oil recovery in unconventional reservoirs (Alfarge et al 2018;Izadmehr et al 2018). Alfarge et al (2018) introduced the way to select the best type of miscible gases to enhance oil recovery in shale oil reservoirs. Furthermore, Izadmehr et al (2018) considered the injector rate, number and installation of submersible pumps to rank the development scenarios for the oil field.…”