Oil adsorption in shale nanopores and its effect on recoverable oil-in-place

• We used molecular dynamics (MD) to study oil adsorption in shale nanopores.
• There are always multiple adsorbed layers of liquid hydrocarbons in shale pores.
• We developed a new model to estimate recoverable shale oil-in-place based on MD.
• For Bakken, 13% of the oil-in-place is in adsorbed phase and difficult to produce.
• We showed the potential of thermal processes to improve recoverable oil from shale.

We used molecular dynamics (MD) simulation to study adsorption behavior of oil within nanoscale carbonaceous slits of shale systems. Results show that fluid density is not uniform throughout the pore and that its oscillation from the pore surface to the central plane is attenuated, indicating distinct adsorbed layers and bulk-phase fluid. We studied the effects of slit aperture, temperature, pressure, and oil composition on the volumes and densities of the adsorbed layers. We found that (a) there are always multiple adsorbed layers of liquid hydrocarbons, (b) the number of adsorbed layers depends largely on slit size and oil composition, (c) the adsorption propensity of heavier hydrocarbons is more pronounced, and (d) the fraction of adsorbed volume to total pore space is two times more in circular pores compared to slit-shaped pores. On the basis of our MD analysis on adsorbed and free oil in carbonaceous pores, we developed a new mathematical formulation to estimate recoverable oil-in-place. The application of our model in Bakken shale indicates that about 13% of the oil-in-place is present in adsorbed phase and difficult to produce. We showed the potential of thermal processes to produce adsorbed oil and improve recoverable oil from shale.

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