Water Bridges in Clay Nanopores: Mechanisms of Formation and Impact on Hydrocarbon Transport

Hydrocarbon Transportation in Heterogeneous Shale Pores by Molecular Dynamic Simulation

Shale oil in China is widely distributed and has enormous resource potential. The pores of shale are at the nanoscale, and traditional research methods encounter difficulty in accurately describing the fluid flow mechanism, which has become a bottleneck restricting the industrial development of shale oil in China. To clarify the distribution and migration laws of fluid microstructure in shale nanopores, we constructed a heterogeneous inorganic composite shale model and explored the fluid behavior in different regions of heterogeneous surfaces. The results revealed the adsorption capacity for alkanes in the quartz region was stronger than that in the illite region. When the aperture was small, solid-liquid interactions dominated; as the aperture increased, the bulk fluid achieved a more uniform and higher flow rate. Under conditions of small aperture/low temperature/low pressure gradient, the quartz region maintained a negative slip boundary. Illite was more hydrophilic than quartz; when the water content was low, water molecules formed a "liquid film" on the illite surface, and the oil flux percentages in the illite and quartz regions were 87% and 99%, respectively. At 50% water content, the adsorbed water in the illite region reached saturation, the quartz region remained unsaturated, and the difference in the oil flux percentage of the two regions decreased. At 70% water content, the adsorbed water in the two regions reached a fully saturated state, and a layered structure of "water-two-phase region-water" was formed in the heterogeneous nanopore. This study is of great significance for understanding the occurrence characteristics and flow mechanism of shale oil within inorganic nanopores.

Pore-Scale Study on Shale Oil-CO2-Water Miscibility, Competitive Adsorption, and Multiphase Flow Behaviors

Due to the fracturing fluid imbibition and primary water, oil-water two-phase fluids generally exist in shale nanoporous media. The effects of water phase on shale oil recovery and geological carbon sequestration via CO2 huff-n-puff is non-negligible. Meanwhile, oil-CO2 miscibility after CO2 huff-n-puff also has an important effect on oil-water two-phase flow behaviors. In this work, by considering the oil-CO2 competitive adsorption behaviors and the effects of oil-CO2 miscibility on water wettability, an improved multicomponent and multiphase lattice Boltzmann method is proposed to study the effects of water phase on CO2 huff-n-puff. Additionally, the effects of oil-CO2 miscibility on oil-water flow behaviors and relative permeability are also discussed. The results show that due to Jamin's effect of water droplets in oil-wetting pores and the capillary resistance of bridge-like water phase in water-wetting pores, CO2 can hardly diffuse into the oil phase, causing a large amount of remaining oil. As water saturation increases, Jamin's effect and the capillary resistance become more pronounced, and the CO2 storage mass gradually decreases. Then, based on the results from molecular dynamics simulations, the influences of oil-CO2 miscibility on oil-water relative permeability in calcite nanoporous media are studied, and as the oil mass percentage in the oil-CO2 miscible system decreases, the oil/water relative permeability decreases/increases. The improved lattice Boltzmann model can be readily extended to quantitatively calculate geological CO2 storage mass considering water saturation and calculate the accurate oil-water relative permeability based on the real 3D digital core.

Effects of water chemistry on microfabric and micromechanical properties evolution of coastal sediment: A centrifugal model study

Water chemistry alteration induced strength weakening of natural sediment, which leads to the differential settlement of infrastructures in coastal areas, has caused numerous disasters and engineering failures. To thoroughly understand the underlying mechanisms of how water chemistry influences the microfabric and mechanical properties evolution of coastal sediments, herein, the authors adopted centrifuge test, X-ray diffraction (XRD), and atomic force microscope (AFM) to quantitatively study the structure anisotropy index (i.e., orientation index (OI)), micromorphological property (i.e., root mean square height (S_q)), and micromechanics (i.e., microscale apparent modulus) of clay sediments in different water chemistry conditions and gravity gradients. The results show that the change rule of OI is: OI_saline > OI_alkaline > OI_water > OI_acid, along the vertical sedimentary depth. Randomly distributed clay flocs and loose flocculated soil skeleton (mainly consisted by edge-to-face (EF) and edge-to-edge (EE) contact of the kaolinite platelets) are associated with the acidic water chemical conditions. The action of supergravity and face-to-face (FF) repulsive contact mode lead to high degree of anisotropy of kaolinite sediments in alkaline environment. Clay platelets are compacted closely under the synergetic effect of centrifugal pressure and prevailing van der Waals attraction (reduction of electric double layer repulsion) in saline environment. The change of 1/S_q is highly consistent with the change of OI at different depths in different water chemical environments. Along the sedimentary depth (i.e., transition from the normal gravity (1 g) to supergravity (8000 g)), the microscale apparent modulus of kaolinite sediment was found to be the highest in alkaline environment. As the water chemistry changes from alkaline to acidic, however, the microscale apparent modulus of kaolinite aggregate decreased, and it showed the smallest in the saline environment.

Molecular Simulation Study and Analytical Model for Oil–Water Two-Phase Fluid Transport in Shale Inorganic Nanopores

Shale reservoirs contain omnipresent nanopores. The fluid transport phenomena on the nanoscale are significantly different from that on the macroscale. The understandings of fluid transport behavior, especially multiphase flow, are still ambiguous on the nanoscale and the traditional hydrodynamic models are insufficient to describe the fluid flow in shale. In this work, we firstly use a molecular dynamics simulation to study the oil–water two-phase flow in shale inorganic quartz nanopores and investigated the unique interfacial phenomena and their influences on fluid transport in a confined nanospace. The results of the molecular simulation revealed that the water-oil-water layered structure was formed in quartz nanopores. There is no-slip boundary condition between water and quartz surface. The density dip and the extremely low apparent viscosity of the oil–water interface region were observed. The liquid–liquid slip effect happened at the oil–water interface. Based on the nano-effects obtained by the molecular simulation, two mathematical models were proposed to describe the nanoscale oil–water two-phase flow, considering both the solid–liquid and liquid–liquid interfacial phenomena, and the performances of two mathematical models were validated. This study shed light on the flow behaviors of oil and water on the nanoscale, and provides the theoretical basis for scale-upgrading, from the nanoscale to the macroscale.

Transferable Gaussian Attractive Potentials for Organic/Oxide Interfaces

Organic/oxide interfaces play an important role in many areas of chemistry and in particular for lubrication and corrosion. Molecular dynamics simulations are the method of choice for providing complementary insight to experiments. However, the force fields used to simulate the interaction between molecules and oxide surfaces tend to capture only weak physisorption interactions, discarding the stabilizing Lewis acid/base interactions. We here propose a simple complement to the straightforward molecular mechanics description based on "out-of-the-box" Lennard-Jones potentials and electrostatic interactions: the addition of an attractive Gaussian potential between reactive sites of the surface and heteroatoms of adsorbed organic molecules, leading to the Gaussian Lennard-Jones (GLJ) potential. The interactions of four oxygenated and four amine molecules with the typical and widespread hematite and γ-alumina surfaces are investigated. The root mean square deviation (RMSD) for all probed molecules is only 5.7 kcal/mol, which corresponds to an error of 23% over hematite. On γ-alumina, the RMSD is 11.2 kcal/mol using a single parameter for all five chemically inequivalent surface aluminum atoms. Applying GLJ to the simulation of organic films on oxide surfaces demonstrates that the mobility of the surfactants is overestimated by the simplistic LJ potential, while GLJ and other qualitatively correct potentials show a strong structuration and slow dynamics of the surface films, as could be expected from the first-principles adsorption energies for model head groups.