Shale formations in North America such as Bakken, Niobrara, and Eagle Ford have a huge oil volume 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. In shale oil reservoirs, EOR is relatively novel compared with in conventional oil reservoirs. The most investigated technique among EOR methods to be applied in shale oil reservoirs is injecting miscible gases which mainly include CO2, N2 and enriched natural gases. However, these different gases showed different performance in both of lab scale and field pilots conducted in shale reservoirs. In this paper, numerical simulation methods of compositional models have been incorporated with LS-LR-DK (logarithmically spaced, locally refined, and dual permeability) models to mimic the performance of CO2 as well as natural gases (lean gas and rich gas) in different scenarios of unconventional reservoirs. The models of this study are mainly built on the sensitivity analysis for the fluid and rock properties of Bakken formation. Sensitivity analysis methods used in this study were conducted by using two main methods of Design of Experiments (DOE) which are Response Surface Methodology (RSM) and One Parameter At A Time (OPAAT) approach.

This study found that the main parameters affecting CO2-EOR performance are not the same parameters influencing natural-gases (NGs) EOR performance in shale reservoirs. This happens due to the difference in the physical nature among gases, especially the molecular weight difference between CO2 and NGs where the molecular diffusion flow is the dominated flow type in those very tight formations. This study also indicated that NGs-EOR performance exceeds the performance of CO2-EOR in the formations with very small pore throats (Permeability in range of 0.00001-1 mD). However, injecting CO2 is highly recommended over injecting NGs in the reservoirs with the permeability of more than 1 mD. Moreover, it has been observed that NGs are not very strong function of natural fractures intensity as CO2. Furthermore, due to the small molecular weight of NGs, they do not require very large contact areas as CO2 does. This study explains the effects of different nano and macro mechanisms on the performance of CO2-EOR and natural-gases EOR in unconventional reservoirs since these plays are much complex and very different from conventional formations. Also, general guidelines have been provided in this study to enhance success of CO2-EOR in unconventional reservoirs.

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