Phase behavior of confined fluids within nanopores have been well proved to deviate significantly from that of bulk fluids. Minimum miscibility pressure (MMP), as a key parameter for the miscible gas injection enhanced oil recovery (EOR) in unconventional reservoirs, has inevitably been affected by the nanoscale confinement. It is of great importance to recognize variations of the MMP within nanopores and its impact on shale hydrocarbon recovery.The objective of this work is to investigate the nanoscale confinement on MMP of CO2/hydrocarbon systems and to compare the accuracy of different theoretical methods in calculating MMP of confined fluid systems. Firstly, a modified PR EOS which is applicable for confined fluid characterization is applied to perform the EOS simulation of the vanishing interfacial tension (VIT) experiments. The MMP of multiple CO2/hydrocarbon systems, such as ternary mixture CO2+n-C4+n-C10, quaternary mixture CO2+C1+n-C4+n-C10, and CO2+Bakken oil at different pore sizes are obtained via the VIT simulations. Secondly, the multiple-mixing-cell (MMC) algorithm coupled with the same modified PR EOS is applied to compute the MMP for the same fluid systems. Comparison of these results to the experimental values is performed to recognize which method has higher accuracy in determining the MMP of fluid systems under nanoscale confinement.Results illustrate that the accuracy of MMP obtained by the VIT simulations depend strongly on the fluid systems. The MMP of some systems agree well with the experimental results, while those of other systems are subject to significant uncertainties. On the other hand, the MMP computed by the MMC method generally have better matches with the experimental results, representing that MMC is a more reliable and accurate method in calculating the MMP of confined fluid systems. Essentially, nanoscale confinement results in the drastic suppression of MMP and the suppression rate increases with decreasing pore size. Calculation results demonstrate that the MMP suppression rate of CO2+Bakken oil within 10 nm pore can be as high as 15%. The drastic suppression of MMP can significantly lower the conditions of achieving miscibility between the injection gas and reservoir oil, which is highly favorable for the miscible gas injection EOR in unconventional reservoirs.The results of this work will not only deepen our understanding of the confined fluid phase behavior but also provide reliable instructions and guidance for the EOR of unconventional reservoirs. In addition, it will shed light on the characterization of the confined fluid systems which may potentially be applied in carbon storage, micro-engineering, and some other nanoscale disciplines.

Minimum Miscibility Pressure of CO2/Hydrocarbon Systems Within Nanopores

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/OMAE2021/volume/85116

OMAE 2021

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