A pore size is generally very small in tight oil reservoirs. It is 30 nm to 2,000 nm in diameter for tight sandstone reservoirs and 2 nm to 50 nm in diameter for shale reservoirs. In a confined system, the interaction between molecules and pore wall surfaces can alter the hydrocarbon thermal-dynamic properties, resulting in a variation in phase equilibrium. It has been recorded in previous research work. However, there is no clear statement for the reason of the confinement effect on phase equilibrium, and the minimum pore size at which the confinement effect is strong but meaningless is not recorded.
In this paper, multi-component phase equilibrium with the confinement effect is investigated, and the enthalpy and fugacity are recorded. Furthermore, molecular dynamic simulation is performed to study the flow of molecules inside nanopores and the energy is recorded. It can be seen that the effect of confinement contributes to a higher enthalpy in a mixture, which helps to keep a single phase fluid, so the vapor fraction for the fluid can be reduced. Also, the effect of confinement drives a reduction in the fugacity of light components and an increment of heavier components, which makes the vapor and liquid phase equilibrium composition become closer to each other. The critical size for molecules to flow through nanopores is two times the biggest molecular diameter. The confinement effect works until a pore diameter is within 5 nm, and it is extremely difficult for the heaviest hydrocarbon component to move out of pores with a diameter less than 5 nm.
Tight oil resources contribute to a tremendous crude supply in recent years.1 Its primary recovery still remains low even with the advanced technology in hydraulic fracturing and horizontal wells.2 Furthermore, the production characterization of tight oil reservoirs is quite different from the conventional one.3 Recent studies have mainly focused on the fluid phase behavior in tight reservoirs.4–12
