CO2 Utilization in Tight Oil Formation: Interfacial Properties and Minimum Miscibility Pressures in Nanopores

Abstract

In this paper, a modified Peng-Robinson equation of state (PR-EOS) coupled with the parachor model and a newly-developed diminishing interface method (DIM) are applied to predict the interfacial tensions (IFTs), interfacial thicknesses, and minimum miscibility pressures (MMPs) of Pembina and Bakken oil-CO₂ systems in nanopores at reservoir conditions. First, a modified PR-EOS is used to calculate the vapour-liquid equilibrium in nanopores by considering the effects of capillary pressure and shifts of critical temperature and pressure. Second, the parachor model is coupled with the modified PR-EOS to predict the IFTs. Third, a formula of the interfacial thickness between two mutually soluble phases is derived, based on which the novel DIM is developed. The MMP is determined by using the linear correlation coefficient criterion and extrapolating the derivative of the interfacial thickness with respect to the pressure to zero. The modified PR-EOS coupled with the parachor model and the new DIM are accurate for predicting the phase behaviour, IFTs, interfacial thicknesses, and MMPs in nanopores. It is found that the IFTs and interfacial thicknesses increase with temperature and the MMPs of the Pembina live oil-CO₂ system are linearly increased from 7.0, 8.5, 13.7, 22.5, to 29.7 MPa at T = 15.6, 30.0, 53.0, 80.0, and 116.1°C. The initial oil composition effects on the three quantities are measurable but marginal. The MMP is found to be more sensitive to the initial oil composition at a higher temperature in nanopores. Moreover, the IFTs and interfacial thicknesses are weakly but the MMPs are strongly dependent on the injection gas composition. The presence of CH₄ in the injection gas results in a substantial MMP increase in nanopores. At a constant temperature, the effects of the feed gas-oil ratio on the IFTs, interfacial thicknesses, and MMPs are negligible with pure CO₂ injection, especially at low feed gas-oil ratios (i.e., less than 0.50:0.50 in mole fraction), whereas they become much stronger and cause the MMPs with impure CO₂ (0.65 CO₂+0.35 CH₄) injection to be considerably increased from 26.3 to 40.0 MPa by reducing the feed gas-oil ratio from 0.90:0.10 to 0.10:0.90 in mole fraction. Overall, the effects of four important factors on the interfacial properties and MMPs are specifically studied and a series of optimal strategies are obtained to enhance oil recovery for a miscible CO₂ injection project in the tight formation (e.g., Bakken formation).

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018