CO2 Adsorption onto Water-Bearing Shale: Insights from Molecular Dynamics and Implications on CO2 Prestorage Fracturing

Summary Hydraulic fracturing coupled with CO2 injection or CO2 prestorage fracturing is a pivotal technique for enhancing shale oil recovery. Besides, geological CO2 storage offers a feasible solution for mitigating global warming. However, after hydraulic fracturing, the shale matrix is in a water-bearing environment. The complex mechanisms associated with the impact of the injected CO2 on shale oil recovery in the water-bearing kerogen matrix remain unclear. In this work, we explored the adsorption mechanism of five representative components of shale oil in water-bearing kerogen through molecular dynamics (MD) simulation, which may provide useful microscopic insights for industrial CO2 prestorage fracturing. Our research revealed that CO2 could decrease the adsorption capacity of n-octane (OCT; saturated alkanes), thiophene (THIOP), and naphthalene rings (NAPs; aromatic hydrocarbons) onto the kerogen, which consequently improved the recovery of these components. Conversely, the adsorption capacity of pyridine (PYR) and n-octadecanoic acid (STE) was boosted upon the CO2 introduction. This could be attributed to the fact that after CO2 injection, both the quantity and the lifetime of hydrogen bonds between these two components and kerogen were increased. The interaction energy between these two components and the water-bearing kerogen also increased, which was in-line with the changes in molecular van der Waals (vdW) surface electrostatic potential (ESP) and the spatial distribution function (SDF). In addition, to reveal the deeper mechanism, the interactions between the specific sites or functional groups on the kerogen and the different components are analyzed to predict the intermolecular charge transfer. It is believed this work may offer useful insights into the design and implementation of CO2 prestorage fracturing for improved shale oil recovery and CO2 geological storage.