In shale oil reservoirs, Improved Oil Recovery (IOR) methods are relatively considered as new concepts compared with in conventional oil reservoirs. Different IOR techniques have been investigated by using lab experiments, numerical simulation studies, and limited pilot tests. Unconventional IOR methods include injecting CO2, surfactant, natural gas, and water. However, CO2 injection is the most investigated option due to different reasons. CO2 has lower miscibility pressure with shale oils, and has special properties in its supercritical conditions, and CO2 injection also solves greenhouse problems. In this paper, numerical simulation methods of compositional models were incorporated with LS-LR-DK (logarithmically spaced, locally refined, and dual permeability) reservoir models and Local Grids Refinement (LGR) of hydraulic fractures conditions to investigate the feasibility of CO2 injection in shale oil reservoirs. Different mechanisms for CO2 interactions with organic surface, shale brine, and shale oil were implemented in different scenarios of numerical models. Molecular diffusion mechanisms, adsorption effects, and aqueous solubility effects were simulated in this study. In addition, linear elastic models and stress-dependent correlations were used to consider geomechanics coupling effects on production and injection processes of CO2-EOR in shale oil reservoirs. Some of the results for this simulation study were validated by matching the performance of some CO2 fields’ pilots performed in Bakken formation, in North Dakota and Montana portions.

This study extremely found that some of the CO2-EOR pilot tests have a match with the typical simulated diagnostic plots which have CO2 molecular-diffusion rate that is significantly low. Furthermore, this research indicated that CO2 molecular diffusion mechanism has a clearly positive effect on CO2-EOR in huff-n-puff protocol; however, this mechanism has a relatively negative effect on continuous flooding mode of CO2-EOR. Both of dissolution and adsorption mechanisms have a negative effect on CO2 performance in terms of enhancing oil recovery in unconventional formations. Geomechanics coupling has a clear effect on CO2-EOR performance, and different geomechanics models have a different validity in these shale plays. Stress dependent correlations give the best match with CO2-EOR pilots in Bakken formation while linear elastic models would give the best match in Eagle Ford formation. This study explains the effects of different nano and macro mechanisms on the performance of CO2-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 these types of reservoirs.

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