*Article* **Molecular-Scale Considerations of Enhanced Oil Recovery in Shale**

#### **Mohamed Mehana \*, Qinjun Kang and Hari Viswanathan**

Los Alamos National Lab, Computational Earth Science Group, Earth and Environmental Science Division, Los Alamos, NM 87545, USA; qkang@lanl.gov (Q.K.); Viswana@lanl.gov (H.V.)

**\*** Correspondence: mzm@lanl.gov

Received: 1 November 2020; Accepted: 11 December 2020; Published: 15 December 2020

**Abstract:** With only less than 10% recovery, the primary production of hydrocarbon from shale reservoirs has redefined the energy equation in the world. Similar to conventional reservoirs, Enhanced Oil Recovery (EOR) techniques could be devised to enhance the current recovery factors. However, shale reservoirs possess unique characteristics that significantly affect the fluid properties. Therefore, we are adopting a molecular simulation approach that is well-suited to account for these effects to evaluate the performance of three different gases, methane, carbon dioxide and nitrogen, to recover the hydrocarbons from rough pore surfaces. Our hydrocarbon systems consists of either a single component (decane) or more than one component (decane and pentane). We simulated cases where concurrent and countercurrent displacement is studied. For concurrent displacement (injected fluids displace hydrocarbons towards the production region), we found that nitrogen and methane yielded similar recovery; however nitrogen exhibited a faster breakthrough. On the other hand, carbon dioxide was more effective in extracting the hydrocarbons when sufficient pressure was maintained. For countercurrent displacement (gases are injected and hydrocarbons are produced from the same direction), methane was found to be more effective, followed by carbon dioxide and nitrogen. In all cases, confinement reduced the recovery factor of all gases. This work provides insights to devise strategies to improve the current recovery factors observed in shale reservoirs.

**Keywords:** molecular simulation; enhanced oil recovery; methane; shale
