Phase Behavior and Minimum Miscibility Pressure in Nanopores

TLDR

PVT phase behavior in large pores differs from that in nanopores, with confinement altering capillary, electrostatic, van der Waals forces and fluid structure. The study modifies conventional VLE calculations to incorporate capillary pressure and critical shifts in nanopores and applies the modified VLE to investigate phase behavior of reservoir fluids in unconventional reservoirs. Using a multiple‑mixing‑cell algorithm combined with the modified VLE, the authors determine the minimal miscibility pressure of synthetic oil and Bakken oil with CO₂ and CO₂‑methane mixtures. Confinement lowers bubblepoint pressure, gas‑oil interfacial tension, and MMP, while raising the upper dewpoint pressure and reducing the lower dewpoint pressure.

Abstract

Summary Numerous studies indicate that the pressure/volume/temperature (PVT) phase behavior of fluids in large pores (designated “unconfined” space) deviates from phase behavior in nanopores (designated “confined” space). The deviation in confined space has been attributed to the increase in capillary force, electrostatic interactions, van der Waals forces, and fluid structural changes. In this paper, conventional vapor/liquid equilibrium (VLE) calculations are modified to account for the capillary pressure and the critical-pressure and -temperature shifts in nanopores. The modified VLE is used to study the phase behavior of reservoir fluids in unconventional reservoirs. The multiple-mixing-cell (MMC) algorithm and the modified VLE procedure were used to determine the minimal miscibility pressure (MMP) of a synthetic oil and Bakken oil with carbon dioxide (CO2) and mixtures of CO2 and methane gas. We show that the bubblepoint pressure, gas/oil interfacial tension (IFT), and MMP are decreased with confinement (nanopores), whereas the upper dewpoint pressure increases and the lower dewpoint pressure decreases.