Molecular Simulation of Enhanced Oil Recovery: A Comparison Between Carbon Dioxide and Nitrogen Injection
Authors: Kazemi Mohammad , Takbiri-Borujeni Ali , Kudrashou Vicheslau 
Published: October 2020

Abstract
In this work, a molecular study of shale EOR is presented to elucidate some controlling parameters, such as soak time, injection pressure, and injection fluid. To create realistic structures for organic nano-porous organic structure, equilibrium molecular dynamics (EMD) is used to create channel-like geometries based on the kerogen type II structure C unit molecules prepared by Ungerer et al. 2014. The atomistic model of the oil residing on the channel is developed using a synthetic oil mixture created based on the experimental study of the phase behavior of petroleum mixtures performed by Turek et al. (1984). N2 and supercritical CO2 (sCO2) molecules are then injected into the channel at different pressures to investigate the effectiveness of this EOR mechanism by computing the oil recovery factors.
In the first step, the density profiles of injected gases and synthetic oil in the pore are investigated at various injection pressures to understand the adsorption of different components. Different oil components are shown to have different adsorption tendencies to the channel surfaces. Heavier oil components, particularly C7+, have the highest adsorption tendency, denoted by an adsorption selectivity of six calculated with respect to methane, i.e., six times higher adsorption tendencies compared to methane.
In the next step, simulations are performed to investigate the importance of injection pressure on the required soaking time. Soaking time is the time required to allow complete mixing of injection gas with oil. The results show that the higher sCO2 injection pressure, the more soaking time is required for the gas to be efficiently mixed with synthetic oil. Furthermore, there is an optimal soaking time after which no significant oil recovery increments can be achieved. Determining the optimal soak time has economic significance and in extreme cases can determine the faith of an EOR project.
Oil recovery factors increase as injection pressures increase but the trend plateaus after reaching an inflection point (called here the minimum diffusivity pressure (MDP), similar to the minimum miscibility pressure (MMP) and minimum miscibility enrichment (MME) defined in conventional reservoirs). Next, a component-wise investigation of the recovery of oil is performed after the EOR fluids are injected. sCO2 molecules tend to be adsorbed to the surfaces of the kerogen pore by replacing (desorbing) the molecules of oil. This phenomenon takes place faster for the lighter oil components compared to the heavier ones.
The computed oil recovery factors range from 24 to 49%, which is consistent with the experimental measurements in Jin et al. (2017). The fraction of the heavier components in the recovered oil is smaller than the lighter components especially when N2 is injected as the EOR fluid (maximum 20 and 34% for N2 and sCO2 injection, respectively). A comparison of the oil recovery factors between sCO2 and N2 shows the superiority of the former as the EOR fluid. This is attributed to sCO2 molecules possessing higher diffusivity in oil and also higher adsorption tendencies to the kerogen atoms on the channel walls compared to N2 molecules.
This work is one of the few studies on the shale EOR at the molecular level. This work along with the extensions of this work to larger scales can shed light on the highly unknown and complex nature of shale recovery.