Molecular simulation of stripping of crude oil by CO2 in tight reservoirs

Tight oil reservoirs are often oil-wet and contain surface adsorbed layers of hydrocarbon. Improvement of production lies in part in the ability to produce this adsorbed oil for its own sake and to unblock small pores that can improve the relative permeability of the reservoir. In this paper we have used molecular modelling and simulation first to study the formation of adsorbed oil films made from n-alkanes of 5 different molecular weights (nC7, nC12, nC18, nC22, nC25) on an hydroxylated ->-SiO₂ surface, and then to examine the process of stripping oil from these layers using carbon dioxide, nitrogen and water. It was found that all n-alkanes but nC12 formed a monolayer oil film, while nC12 formed a three-layer oil film. Molecular weight, length and flexibility of the n-alkane were all factors in oil film formation. It was found that flooding with CO₂ is able to strip all of the modelled n-alkanes from the α-SiO₂ surface effectively. The time required to strip the n-alkane was longer for n-alkanes with higher molecular weights. The stripping process was divided into three stages: (i) CO₂ diffusion and dissolution, (ii) competitive adsorption, and (iii) oil film push-off. A fourth stage was recognized only for light n-alkanes, and which involved the dissolution of CO₂ in mobilized n-alkane, leading to improvements in its mobility. Comparative simulations using nC12 showed that N₂ and H₂O exhibit no efficacy in stripping n-alkanes from surface adsorbed oil films. The efficacy of CO₂ was attributed to (i) it being a polar molecule that is attracted to the hydroxylated silica surface, (ii) its miscibility in n-alkanes, and (iii) that it is in a supercritical state at reservoir conditions. The failure of N₂ arises because it is a non-polar molecule with no affinity for the surface and exists as an immiscible gas at reservoir conditions. Water was ineffective, because, though polar, it is immiscible in the oil layer and so cannot access the rock surface. Consequently, CO₂-flooding is expected to be particularly effective in improving production from tight oil-wet clastic reservoirs.

Key words: tight reservoir; pore throats; CO₂ flooding; oil film stripping; molecular simulation