Numerical Simulation of the Effect of Nanoconfinement on Hydrocarbon Phase Behavior in Nanometer Scale Pores

Abstract

The proportion of ultra-low permeability oil and gas reservoirs in the low permeability remaining recoverable resources is increasing. Pore sizes of many ultra-low permeability oil and gas reservoirs are in the order of nanometer, which structure is complex and the permeability is very low. In these pores, the phase behavior of hydrocarbon mixture is affected by the capillary pressure and the surface forces and is different from that characterized in PVT cells of conventional reservoirs. In the nanometer scale size multi-component simulation, the capillary pressure is often ignored, which leads to the big difference between the result of prediction and the actual production. In this paper, based on PR EOS, a flash calculation method is used to calculate the multi-component mixed oil and gas phase behavior model under the different capillary pressure and surface forces, aiming at the complex multi component fluid phase behavior in the nanoscale pores of the ultra-low permeability reservoir. The calculation results show that with the decrease of nanometer scale pore radius or the decrease of the contact angle, the influence of the capillary pressure on the deviation of the PVT bubble point pressure curve is more obvious. The degree of deviation of the bubble point pressure curve is obviously affected by the nanometer scale pore radius and the effect of the contact angle. The effect of surface forces on fluid PVT phase in ultra-low permeability reservoirs is not negligible. And, the law of variation in pore radius with the contact angle is given for the judgement of the critical pore radius considering nanoconfinement.