Bubble Point Pressures of Hydrocarbon Mixtures in Multiscale Volumes from Density Functional Theory

Abstract

Accurate characterization of the bubble point pressure of hydrocarbon mixtures under nanoconfinement is crucial to the prediction of ultimate oil recovery and well productivity of shale/tight oil reservoirs. Unlike conventional reservoirs, shale has an extensive network of tiny pores in the range of a few nanometers. In nanopores, the properties of hydrocarbon fluids deviate from those in bulk because of significant surface adsorption. Many previous theoretical works use a conventional equation of state model coupled with capillary pressure to study the nanoconfinement effect. Without including the inhomogeneous molecular density distributions in nanoconfinement, these previous approaches predict only slightly reduced bubble points. In this work, we use density functional theory to study the effect of nanoconfinement on the hydrocarbon mixture bubble point pressure by explicitly considering fluid−surface interactions and inhomogeneous density distributions in nanopores. We find that as system pressure decreases, while lighter components are continuously released from the nanopores, heavier components accumulate within. The bubble point pressure of nanoconfined hydrocarbon mixtures is thus significantly suppressed from the bulk bubble point to below the bulk dew point, in line with our previous experiments. When bulk fluids are in a two-phase, the confined hydrocarbon fluids are in a single liquid-like phase. As pore size increases, bubble point pressure of confined fluids increases and hydrocarbon average density in nanopores approaches the liquid-phase density in bulk when bulk is in a two-phase region. For a finite volume bulk bath, we find that because of the competitive adsorption in nanopores, the bulk bubble point pressure increases in line with a previous experimental work. Our work demonstrates how mixture dynamics and nanopore−bulk partitioning influence phase behavior in nanoconfinement and enables the accurate estimation of hydrocarbon mixture bubble point pressure in shale nanopores.

Experiments on the Capillary Condensation/Evaporation Hysteresis of Pure Fluids and Binary Mixtures in Cylindrical Nanopores

“…Hysteresis disappears at a lower temperature for smaller nanopores. For the record, a similar hysteretic phenomenon of confined mixture has also been found in several works recently. − As the main purpose of this work, the location of the hysteresis end point for a specific system will be carefully estimated next.…”

Nanoconfinement Effect on Surface Tension: Perspectives from Molecular Potential Theory

“…Previous studies have demonstrated that the pore size and LJ size diameter are the key parameters to determine the shift of critical temperature. ,, Furthermore, the linear relationship in a log–log scale is found to be useful to describe the relationship between the dimensionless shift of critical temperature, Δ T , and the normalized pore dimension, m , which is the ratio of the pore radius to the LJ size parameter. , Meanwhile, MD simulations have shown the difference between Δ T in cylindrical nanopores and that in slit nanopores with the same pore dimension . Therefore, the Δ T as a function of normalized pore dimension, m , in cylindrical nanopores and slit nanopores can be separately expressed as where a 1 and b 1 are the fitting parameters for cylindrical nanopores while a 2 and b 2 are the fitting parameters for slit nanopores.…”

“…Previous studies have demonstrated that the pore size and LJ size diameter are the key parameters to determine the shift of critical temperature. 6,7,29 Furthermore, the linear relationship in a log−log scale is found to be useful to describe the relationship between the dimensionless shift of critical temperature, ΔT, and the normalized pore dimension, m, which is the ratio of the pore radius to the LJ size parameter. 7,29 Meanwhile, MD simulations have shown the difference between ΔT in cylindrical nanopores and that in slit nanopores with the same pore dimension.…”

“…With the prosperity of nanoscience and nanotechnology, the liquid–vapor surface tension (ST) in nanopores attracts great attention because of the envisioned applications in many fields, such as nanomaterial engineering, chemical separation, energy storage, unconventional oil/gas production, and so forth. − The confinement effects introduce significant changes to the thermophysical properties and interfacial conditions of fluids in nanopores. − Therefore, capturing the underlying physics behind the variation of nanoconfined ST and developing practical and rapid determination methods are urgently desired for research studies and industrial applications.…”

Studying key processes related to CO2 underground storage at the pore scale using high pressure micromodels

“…However, further opportunity remains to design micro and nano-models that can capture molecular-scale information. This may include the development of nanofluidics devices [253][254][255] , which have proven to be excellent tools for accessing critical phenomena at the molecular level, including non-conventional phase behaviors. However, challenges still need to be addressed in term of experimentation (clogging, nonvalidity of conventional fluid mechanics equations, etc.).…”