Spatial distribution of reservoir fluids in mature kerogen using molecular simulations

Abstract

This work analyses the distribution of volatile oil and other reservoir fluids in kerogen belonging to the gas condensate/volatile oil window. The kerogen models representing kerogen Type II-C contain pores in the range of micropores (diameter <2 nm) and mesopores (2 nm < diameter < 50 nm). A few of the micropores are isolated pores whereas the mesopores are connected in all spatial directions. Methane is observed to fill the pores while longer n-alkane chains transition from filling the pore to adsorbing onto the pore surface. The heavier components in the reservoir fluids adsorb onto the surface of the pore and can partially or completely clog the pore throats. As a general trend, the longer the alkane chain, the more preferentially the molecule adsorbs onto the pore surface. Additionally, the molecular simulations correctly capture the hydrophobic environment in which water molecules cluster together to form a droplet that resides in or close to a pore throat. Some molecules of carbon dioxide seem to preferentially adsorb onto either the kerogen internal surface or on the molecules of adsorbed heavier components, while the rest of the molecules diffuse into the kerogen body. This observation has not been previously reported to the best of our knowledge. Also the adsorption of the heavier species onto the pore walls significantly decreases the surface area available for adsorption of other species. This work underscores the significance of knowing fluid distributions to model primary recovery as well as secondary or tertiary recovery.