Exploring the effects of four important factors on oil–CO₂ interfacial properties and miscibility 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 properties and minimum miscibility pressures (MMPs) of light oil-CO 2 systems in nanopores. First, the modified PR-EOS is used to calculate the vapour-liquid equilibrium by considering the effects of capillary pressure and shifts of critical temperature and pressure. Second, a thermodynamic formula of the interfacial thickness (IT) between two mutually soluble phases is derived, based on which the novel DIM is developed. The MMP is determined by extrapolating the derivative of the IT with respect to the pressure (vd/vP) T to zero. It is found that at the pore radius of 10 nm, all three quantities, the interfacial tensions (IFTs), ITs, and MMPs, show obvious increases with temperature. The effects of the initial oil composition on the three quantities are measurable but marginal and the MMP is more sensitive to the initial oil composition at a higher temperature. Moreover, the IFTs and ITs are weakly dependent but the determined MMPs are strongly dependent on the injection gas composition. The presence of CH 4 in the injection gas results in a substantial MMP increase in nanopores. At a constant temperature, the effects of the feed ratio of injection gas to oil on the IFTs, ITs, and MMPs are negligible with pure CO 2 injection, especially at low feed gas-oil ratios (less than 0.50 : 0.50 in mole fraction), whereas they become much stronger and cause the MMPs with impure CO 2 (0.65CO 2 + 0.35CH 4 ) 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.

Semi‐analytical nanoscale‐extended surface tension correlation

“…It is found the STs in nanopores, which share similar manner with the bulk results, are decreased with the increasing temperatures but increase with the increasing carbon numbers (i.e., components become heavier). Moreover, the STs of the C 1 , C 2 , C 3 , C 4 , and C 8 at r p = 1.5 nm (i.e., in nanopores) are found to be slightly lower than the bulk data at the same temperatures by comparing and , which can be validated by the literature results . Obviously, the calculated nanoscale STs of different components are in good agreement with the measured data in Figure .…”

A modified cell-to-cell simulation model to determine the minimum miscibility pressure in tight/shale formations

“…The essence of this method is that the oil-gas MMP is determined when the vapor-liquid IFT reaches zero. Later, researchers modified this method and used the Parachor model [24] combined with a cubic equation of states to calculate the vapor-liquid IFT to determine the theoretical oil-gas MMP [26][27][28][29]. The original cell-to-cell simulation method was first proposed by Metcalfe et al [19].…”

“…The proposed model is tested with two crude oil samples and it was found that the oil-gas MMP is reduced in nanopores for both oil samples [28]. Zhang et al [29] applied a similar methodology and observed a linear increase of the confined oil-gas MMP with an increasing temperature.…”

“…4.5 Injection gas impurity effect on the confined MMP calculationsAnother advantage of the proposed MMP algorithm is that it can be used to conveniently investigate the effect of injection gas compositions on the gas-oil MMP. As an example, here we employ the proposed algorithm to examine the effect of CH 4 contamination in CO 2 on the gas-oil MMP.Due to the difficulty of acquiring pure CO 2 for the injection, it is a common practice in the field to reinject the produced CH 4 together with CO 2 back into the reservoir for EOR purposes[29,48]. The previous calculation results for the first and the second fluid systems clearly show that the confined MMP is generally increased when injecting a CH 4 -CO 2 mixture as compared to pure CO 2 injection.…”

Rapid Determination of Interfacial Tensions in Nanopores: Experimental Nanofluidics and Theoretical Models

“…As a quantitative indicator of complex interfacial interactions, the interfacial tension (IFT) and its determination in an accurate and rapid manner are critically important. By definition, the IFT is the amount of work required to bring the molecules from the bulk phases to the contact interface of unit area. , The bulk IFTs, which were widely spread in many areas in the past, have been intensively investigated and their determination methods are various in the experiment and theory. − For example, the bulk IFT can be experimentally measured through the axisymmetric drop shape analysis for the pendant and sessile drop cases , and the spinning drop method. , Alternatively, they can also be calculated by, for instance, the parachor model, , corresponding state model, , and linear gradient theory. , Although some experimental and theoretical methods have been effectively validated and widely accepted to determine the bulk IFTs, they may, at most probability, be restricted to the bulk phase but lose their accuracies when the pore scale reduces to the nanometer scale and is comparable to the molecular size. − With the recently emerging attractions of the IFTs at the micro or even nanometer scale, it is necessary to develop some applicable and accurate experimental and theoretical determination methods.…”