Application of PC-SAFT Equation of State for CO2 Minimum Miscibility Pressure Prediction in Nanopores

Abstract

CO 2 miscible injection is generally one of the most efficient enhanced oil recovery (EOR) methods and widely used in the conventional oil reservoirs. The applicability of CO 2 EOR technology for unlocking the resources from unconventional tight and shale formations and the mechanisms of miscible flooding in these reservoirs still remain unclear. An important parameter used to evaluate the feasibility of CO 2 miscible flooding is the minimum miscibility pressure (MMP). Even though experimental approaches, empirical correlations and theoretical methods have performed well in measuring or predicting MMP between CO 2 and crude oil in conventional reservoirs, they may not be suitable for unconventional formations as phase behavior and MMP can be significantly affected by confinement effect in small pores (e.g., nanopores) in such formations.In this study, a new MMP prediction model based on the modified Parachor Model associated with the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) is developed to determine CO 2 MMP both in the bulk phase and nanopores. The Parachor Model is modified to account for the confinement effect of nanopore walls on the equilibrium interfacial tension (IFT). The Equilibrium IFT reduction in nanopores is related to a temperature-dependent and slit pore width-dependent modification term. The parameters of the new Parachor Model are determined by matching the vapor-liquid surface tension values for CH 4 , C 2 H 6 , C 3 H 8 , n-C 4 H 10 , and n-C 8 H 18 in nanopores, respectively. The prediction ability of the new model is verified by comparing the predicted MMP in the bulk phase with the results of other theoretical approaches and slim-tube experimental data. Finally, the new model is applied to estimate the MMP between Bakken oil and CO 2 stream. The effect of temperature, slit pore width, and impure components in the injected CO 2 on the MMP are also studied. The newly developed model successfully reproduces MMP in bulk phase as compared with both other methods and experimental data. The overall average absolute relative deviation (AARD) for MMP is within 8 %. The calculated equilibrium IFT for liquid-vapor phase has a good agreement with molecular simulation results. For Bakken oil-CO 2 system, if the slit pore width is larger than 10 nm, MMP is independent on pore width; otherwise, it decreases significantly with the decrease of the pore width. If pore width decreases to 3 nm, 67.5 % decrease in the IFT is observed and 23.5% reduction is achieved for MMP between Bakken oil and CO 2 stream, indicating that it is easier to reach miscibility in nanopores, and CO 2 miscible flooding might be a promising enhanced oil recovery (EOR) technology for tight oil and shale oil reservoirs. Furthermore, MMP increases with an increase of temperature in bulk phase, whereas IFT and MMP decrease with an increase of temperature in nanopores.

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

“…They found that using the proposed model, the oil-gas MMP decreases with a decreasing pore radius. Similar results have also been observed in the study of Wang et al [27]. They employed the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) [35] to calculate the vapor-liquid IFT with increasing pressure.…”

“…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].…”

CO2-Fluid-Rock Interactions and the Coupled Geomechanical Response during CCUS Processes in Unconventional Reservoirs

“…However, MMP changes inconsistently with increasing capillary pressure and may decrease, increase, or remain unchanged for different mixtures. When both capillary force and critical shifts are considered, the MMP of the mixture is reduced by nanopores, but when the pore diameter is greater than 10 nm, the change of MMP can be ignored [109,110]. In immiscible displacement, the capillary pressure effect is useful for enhancing oil recovery.…”

A Review of Gas Injection in Shale Reservoirs: Enhanced Oil/Gas Recovery Approaches and Greenhouse Gas Control

“…The MMP would be the same as the critical point of this pseudo-binary mixture, at which interfacial tension (IFT), and subsequently gas-oil capillary pressure would be zero. Wang et al [55] developed a Parachor model to account for the effect of confinement on interfacial tensions (IFTs). They used their model to calculate CO 2 MMP of Bakken oil.…”