Study of Hydrocarbon Phase Behavior Under Confinement Effect in Nano-Scale Pores Using Lab-on-a-Chip Technology
Abstract
The characterization of phase behavior hydrocarbons trapped in shale reservoir is an enigma and remains a challenging problem due to the phenomenon known as the “confinement effect”. This effect is mainly caused by the nano-scale pores which are ubiquitous in shale rocks. The confinement effect becomes increasingly significant in nano-scale pores due to domination of surface-fluid interactions over the bulk fluid-fluid interactions as the size of the pores shrinks progressively. This surface-fluid interaction can lead to anomalous and heterogeneous distribution of hydrocarbon molecules that are confined within the nano-scale capillaries, thereby, resulting in a deviation in the phase behavior of hydrocarbons from its original phase behavior in bulk environment (which is termed as the “confinement effect”). Consequently, several simulation studies reported in the literature were conducted to investigate the confinement effect using the equation of state and density functional theory in molecular simulations by incorporating the effect of capillary pressure in these revised models. However, in the contemporary literature, only a limited number of experiments have been conducted on this topic with the goal of performing experimental validation of the numerical predictions obtained from these simulations, especially for channels with nano-scale dimensions below 10 nm depth and also in high-pressure environments.
In this study lab-on-a-chip technology was explored to quantitatively determine the effect of confinement on the phase behavior of hydrocarbons in nano-scale capillary channels. Initially, the fabrication process of experimental device (which is known as the nanofluidic device) consisting of nano-channels with depths as small as 2 nm was developed and refined (with channel depths explored in this study ranging from 50 nm to 2 nm and channel widths of 5 microns) . The experimental apparatus was assembled and experimental protocols were developed for performing quantitative measurement of the confinement effect for test fluids (i.e., hydrocarbons). The experimental results involving large capillaries (where confinement effect is trivial) were compared with the phase behavior data (which corresponds to fluids in bulk quantities) for validating the consistency of the experimental protocols. The experimental results show that the confinement effect is exposed and becomes significant for channel depths less than 10 nm. Hence, the confinement effect is more pronounced for smaller capillary size. Specifically, a 23% deviation of dew-point pressure for n-Butane is observed for experimental results involving channels with a nominal depth of 2 nm. Also, hysteresis effects are observed during the experiments involving repeated condensation and evaporation for fluids under confinement. A significant increase in hysteresis effects were observed for channels with a nominal depth of 2 nm (compared to that of deeper channels explored in this study).
Citation
Yang, Qi (2020). Study of Hydrocarbon Phase Behavior Under Confinement Effect in Nano-Scale Pores Using Lab-on-a-Chip Technology. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /200793.