Hydrocarbon Phase Behaviors and Water-driven Flow  in Kerogen Nanoporous Media
Creators: Zhao, Yinuo
Publisher: name: University of Alberta Library
Year: 2022

<p>Hydrocarbon recovery from shale media has greatly contributed to the global energy supply and has been constantly reshaping the energy sector. Unlike conventional reservoirs, shale has an extensive network of tiny pores in the range of a few nanometers. Besides, an extensive amount of nanopores can be connected to macropores/natural fractures where the confinement effect can be negligible, resulting in the total volume of nanopores comparable to that of connected macropores/natural fractures. The multi-scale structure of shale can significantly affect the phase behaviors of multi-component hydrocarbon mixtures during the production process, which plays a crucial role in the estimation of ultimate oil recovery, well productivity, and reserve estimation as well as ultimately the policy making. <br>
To understand the multi-phase transitions of the multi-component fluid mixtures in multi-scale volumes, Density functional theory (DFT) is used by explicitly considering fluid-surface interaction, inhomogeneous density distribution in nanopores and interplay between nanopores and connected bulk. We find that as system pressure decreases, while lighter components are continuously released from 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. The interplay effect between nanoscale pore and connected fracture/macropore is studied by varying the volume partition of these two regions. We find that due to the competitive adsorption in nanopores, the bulk bubble point pressure increases in line with previous experimental work. vapor-like and liquid-like phases can coexist in nanopores when pressure is between the bubble and dew point pressures of nanoconfined fluids, both of which are much lower than those of the originally injected hydrocarbon mixtures. Furthermore, the heterogeneous PSD effect is studied by including different sizes of nanopores in the multi-scale system. We find phase transitions first occur in the bulk region, then the larger pores followed by the smaller pores. When fluids in one specific pore begin to vaporize, in other pores, the heavier component would be adsorbed, while the lighter component would be released, which suppresses the phase transitions in the smaller pores because of the heavier component accumulation. Thereafter, by comparing the phase behaviors of nanoconfined C1-C3 mixture in a canonical ensemble from Engineering DFT and EOS-based models, we find that adding adsorption layer effect is imperative for vapor phase properties and critical density calculation in the canonical ensemble. These works should provide important insights into the effect of PSD and interplay between nanopores and macropores/fractures in actual shale oil production processes as well as the applicability of PR-EOS in nanoconfinement. <br>
On the other hand, there are numerous ultra-narrow pore throats (sub-2-nm) exist in shale media. The ultra-narrow pore throats can result in excessively high capillary pressure for water-oil two-phase displacement, which is closely related to oil migration and ultimate productivity. we study water-oil two-phase displacement through 2-nm kerogen pore throats from a molecular perspective. MD simulation is used as a benchmark to access the applicability of the Young-Laplace equation. We find that although the Type II-C kerogen is generally oil-wet, water has an excellent displacement efficiency without oil film on the substrate thanks to the hydrogen bonding. Unlike previous works about inorganic pore throats, we find that the capillary pressure from the Y-L equation has an excellent agreement with the breakthrough pressure from the MD simulations for ~2 nm kerogen pore throats. This work should provide important guidance to numerical modeling of the oil recovery process in shale formations as well as optimization of shale/tight oil recovery.</p>