Nanopore confinement effects on phase behavior and capillary pressure in a Wolfcamp shale reservoir

Abstract

As unconventional oil production is the main focus in petroleum industry, the phase behavior of fluids in nanopore media draws high attention nowadays. The extremely small pore sizes in shale reservoirs affect the fluid phase equilibrium. In this study, the phase equilibrium of Wolfcamp oil in nanopores is studied by coupling the cubic Peng–Robinson equation of state (PR-EOS) with the Young–Laplace capillary pressure equation, fugacity of vapor–liquid calculation, and shifted critical properties. A mercury injection experiment showed that 93.7% of pore diameters in Wolfcamp cores were less than 10 nm. The capillary pressure curve in a real shale reservoir with a multicomponent Wolfcamp oil was generated for the first time. The results show that the bubble point pressure was suppressed by 17.3% when the pore radius (r) is 10 nm, and by 63.8% when r is 1.5 nm. The interfacial tension (IFT) decreases slowly when r is bigger than 50 nm. However, as r becomes smaller, the IFT decreases rapidly, especially when r is less than 10 nm. The nanopore confinement narrows the two-phase region, resulting in a lower capillary pressure and elongating the lower gas–oil ratio production period.