Molecular Origin of Wettability Alteration of Subsurface Porous Media upon Gas Pressure Variations

Wettability of Chemically Heterogeneous Clay Surfaces: Correlation between Surface Defects and Contact Angles as Revealed by Machine Learning

Quantifying the wettability of clay surfaces and how it changes in the presence of gas mixtures is crucial for designing geo-energy applications such as underground hydrogen storage and carbon capture and sequestration. While computational studies exist for the wettability of atomically perfect mineral substrates, actual minerals possess heterogeneities. This study employs molecular dynamics simulations to examine the impact of surface defects on the wettability of kaolinite surfaces exposed to hydrogen, methane, and carbon dioxide. The results show that siloxane surfaces become more hydrophilic as defect densities increase and that the gases can strongly affect wettability. Carbon dioxide, in particular, shows stronger adsorption on heterogeneous surfaces than hydrogen and methane. As a consequence, carbon dioxide can strongly affect wettability. Additionally, our results show that higher salt concentrations reduce water contact angle, which is important because salt is likely present in the subsurface. A machine learning classification algorithm is applied to interpret the results and develop predictive capabilities. Our findings highlight the importance of surface defects on wettability, which is essential for designing geological repositories for geo-energy applications ranging from enhanced gas recovery to carbon sequestration and intermittent hydrogen storage.

Energetics of water expulsion from intervening space between two particles during aggregation

Solvent expulsion away from an intervening region between two approaching particles plays important roles in particle aggregation yet remains poorly understood. In this work, we use metadynamics molecular simulations to study the free energy landscape of removing water molecules from gibbsite and pyrophyllite slit pores representing the confined spaces between two approaching particles. For gibbsite, removing water from the intervening region is both entropically and enthalpically unfavorable. The closer the particles approach each other, the harder it is to expel water molecules. For pyrophyllite, water expulsion is spontaneous, which is different from the gibbsite system. A smaller pore makes the water removal more favorable. When water is being drained from the intervening region, single chains of water molecules are observed in gibbsite pore, while in pyrophyllite pore water cluster is usually observed. Water-gibbsite hydrogen bonds help stabilize water chains, while water forms clusters in pyrophyllite pore to maximize the number of hydrogen bonds among themselves. This work provides the first assessment into the energetics and structure of water being drained from the intervening region between two approaching particles during oriented attachment and aggregation.

The role of water bridge on gas adsorption and transportation mechanisms in organic shale

This work introduces and discusses the impacts of the water bridge on gas adsorption and diffusion behaviors in a shale gas-bearing formation. The density distribution of the water bridge has been analyzed in micropores and meso-slit by molecular dynamics. Na+ and Cl- have been introduced into the system to mimic a practical encroachment environment and compared with pure water to probe the deviation in water bridge distribution. Additionally, practical subsurface scenarios, including pressure and temperature, are examined to reveal the effects on gas adsorption and diffusion properties, determining the shale gas transportation in realistic shale formation. The outcomes suggest carbon dioxide (CO2) usually has higher adsorption than methane (CH4) with a water bridge. Increasing temperature hinders gas adsorption, density distribution decreases in all directions. Increasing pressure facilitates gas adsorption, particularly as a bulk phase in the meso-slit, whereas it restricts gas diffusion by enhancing the interaction strength between gas and shale. Furthermore, ions make the water bridge distributes more unity and shifts to the slit center, impeding gas adsorption onto shale while encouraging gas diffusion. This study provides updated guidelines for gas adsorption and transportation characteristics and supports the fundamental understanding of industrial shale gas exploration and transportation.

Confinement-induced clustering of H2 and CO2 gas molecules in hydrated nanopores

Gas molecule clustering within nanopores holds significance in the fields of nanofluidics, biology, gas adsorption/desorption, and geological gas storage. However, the intricate roles of nanoconfinement and surface chemistry that govern the formation of gas clusters remain inadequately explored. In this study, through free energy calculation in molecular simulations, we systematically compared the tendencies of H2 and CO2 molecules to aggregate within hydrated hydrophobic pyrophyllite and hydrophilic gibbsite nanopores. The results indicate that nanoconfinement enhances gas dimer formation in the nanopores, irrespective of surface chemistry. However, surface hydrophilicity prohibits the formation of gas clusters larger than dimers, while large gas clusters form easily in hydrophobic nanopores. Despite H2 and CO2 both being non-polar, the larger quadrupole moment of CO2 leads to a stronger preference for dimer/cluster formation compared to H2. Our results also indicate that gases prefer to enter the nanopores as individual molecules, but exit the nanopores as dimers/clusters. This investigation provides a mechanistic understanding of gas cluster formation within nanopores, which is relevant to various applications, including geological gas storage.

Interfacial properties of the brine + carbon dioxide + oil + silica system

Molecular dynamics simulations of the H2O + CO2 + aromatic hydrocarbon and H2O + CO2 + benzene + silica (hydrophilic) systems are performed to gain insights into CO2-enhanced oil recovery (EOR) processes. For comparison purposes, an overview of the previous simulation studies of the interfacial properties of the brine + CO2 + alkane + silica system is also presented. In general, the water contact angle (CA) of the H2O + CO2 + silica (hydrophilic) system increased with pressure and decreased with temperature. The CAs of the H2O + hydrocarbon + silica (hydrophilic) system are not significantly affected by temperature and pressure. The simulated CAs were in the ranges of about 58°-77° and 81°-93° for the H2O + hexane + silica (hydrophilic) and the H2O + aromatic hydrocarbon + silica (hydrophilic) systems, respectively. In general, these CAs were not significantly influenced by the addition of CO2. The simulated CAs were in the ranges of about 51.4°-95.0°, 69.1°-86.0°, and 72.0°-87.9° for the brine + CO2 + silica (hydrophilic), brine + hexane + silica (hydrophilic), and brine + CO2 + hexane + silica (hydrophilic) systems, respectively. All these CAs increased with increasing NaCl concentration. The adhesion tension of the brine + silica (hydrophilic) system in the presence of CO2 and/or hexane decreased with increasing salt concentration. The simulated CAs were in the range of about 117°-139° for the H2O + alkane + silica (hydrophobic) system. These CAs are increased by the addition of CO2. At high pressures, the distributions of H2O normal to the silica (hydrophobic) surface in the droplet region of the H2O + silica system were found to be strongly affected by the presence of CO2. These insights might be key for optimizing the performance of the miscible CO2 water-alternating-gas injection schemes widely used for EOR.

Wetting Behavior of Kerogen Surfaces: Insights from Molecular Dynamics

In this study, the wettability of a kerogen surface, a key component of shale reservoirs, is investigated by using molecular dynamics simulations. Specifically, we examined the impact of droplet size and morphology as well as surface roughness on the water contact angles. The findings highlighted that the contact angle dependency on the droplet size intensifies with increased rigidity of the surface. Conversely, as the surface becomes more flexible and rougher, it gains hydrophilicity. The higher hydrophilicity stems from the ability of water molecules to penetrate the kerogen corrugations and form more hydrogen bonds with heteroatoms, particularly oxygen. Notably, the contact angle of kerogen hovers between 65 and 75°, thereby crossing the transition from an underoil hydrophilic to an underoil hydrophobic state. Consequently, minor alterations in the kerogen nanostructure can dramatically alter the wetting preference between water and oil. This insight is of paramount significance for refining strategies in managing fluid interactions in shale reservoirs such as geological carbon storage or oil extraction.

Molecular insights into fluid-solid interfacial tensions in water + gas + solid systems at various temperatures and pressures

The fluid-solid interfacial tension is of great importance to many applications including the geological storage of greenhouse gases and enhancing the recovery of geo-resources, but it is rarely studied. Extensive molecular dynamics simulations are conducted to calculate fluid-solid interfacial properties in H2O + gas (H2, N2, CH4, and CO2) + rigid solid three-phase systems at various temperatures (298-403 K), pressures (0-100 MPa), and wettabilities (hydrophilic, neutral, and hydrophobic). Our results on the H2O + solid system show that vapor-solid interfacial tension should not be ignored in cases where the fluid-solid interaction energy is strong or the contact angle is close to 90°. As the temperature rises, the magnitude of H2O's liquid-solid interfacial tension declines because the oscillation of the interfacial density/pressure profile weakens at high temperatures. However, the magnitude of H2O vapor-solid interfacial tension is enhanced with temperature due to the stronger adsorption of H2O. Moreover, the H2O-solid interfacial tension in H2O + gas (H2 or N2) + solid systems is weakly dependent on pressure, while the pressure effects on H2O-solid interfacial tensions in systems with CH4 or CO2 are significant. We show that the assumption of pressure independent H2O-solid interfacial tensions should be cautiously applied to Neumann's method for systems containing non-hydrophilic surfaces with strong gas-solid interaction. Meanwhile, the magnitude of gas-solid interfacial tension increases with pressure and gas-solid interaction. High temperatures generally decrease the magnitude of gas-solid interfacial tensions. Further, we found that the increment of contact angle due to the presence of gases follows this order: H2 < N2 < CH4 < CO2.

Contact Angle on Rough Curved Surfaces and Its Implications in Porous Media

The equilibrium contact angle depends on both the chemistry of the two fluids and solid base and the microstructure on the solid surface. Actual surface of the pore wall in porous media is typically rough and curved, which has not been well-considered in related applications. This work uses a free interfacial energy minimization approach to theoretically derive the equilibrium contact angle on two specific surface structures on flat surfaces and extends the derivation considering the surface curvatures in porous media. Results reveal that the equilibrium contact angle is not dependent on the curvature of spherical surfaces, and we further prove that this conclusion applies to any point along the apparent common line at solid surfaces with any arbitrary curvature. The fundamental physics is the local mechanical balance of a composite contact among three interfacial tensions. Furthermore, the contacting mode can shift from non-wetting to wetting when the pressure difference between two fluids exceeds the entry pressure of the microstructures, which should be considered in relative dynamic scenarios such as rain droplet impact and fluid displacement in porous media. Note that these conclusions are from pure theoretical analysis based on idealistic assumptions, and real circumstances may deviate from these assumptions.

Control of the Structural Charge Distribution and Hydration State upon Intercalation of CO2 into Expansive Clay Interlayers

Numerous experimental investigations indicated that expansive clays such as montmorillonite can intercalate CO₂ preferentially into their interlayers and therefore potentially act as a material for CO₂ separation, capture, and storage. However, an understanding of the energy-structure relationship during the intercalation of CO₂ into clay interlayers remains elusive. Here, we use metadynamics molecular dynamics simulations to elucidate the energy landscape associated with CO₂ intercalation. Our free energy calculations indicate that CO₂ favorably partitions into nanoconfined water in clay interlayers from a gas phase, leading to an increase in the CO₂/H₂O ratio in clay interlayers as compared to that in bulk water. CO₂ molecules prefer to be located at the centers of charge-neutral hydrophobic siloxane rings, whereas interlayer spaces close to structural charges tend to avoid CO₂ intercalation. The structural charge distribution significantly affects the amount of CO₂ intercalated in the interlayers. These results provide a mechanistic understanding of CO₂ intercalation in clays for CO₂ separation, capture, and storage.

Molecular Origin of Wetting Characteristics on Mineral Surfaces

Accurate determination of the wetting characteristics on mineral surfaces is critical for many natural processes and industrial applications where multiphase flow in porous media is involved. The wetting behaviors on mineral surfaces are controlled by water-mineral interactions, giving rise to various wetting characteristics, including contact line advancement, formation of precursor films, etc. However, a fundamental understanding of wetting characteristics on different mineral surfaces is still lacking at the molecular level. Here, utilizing a comprehensive set of molecular dynamics simulations, we investigate the wetting characteristics of water on various mineral surfaces and obtain the corresponding water-mineral interaction properties (including the areal density of water-mineral interaction energy and the work of adhesion of the water-mineral interface), mineral wettability, and structural and diffusion properties of water molecules near the surface. We show that the diffusion properties of water molecules on mineral surfaces play an important role in wetting characteristics. We find that the contact line tends to advance forward in the jumping mode or the rolling mode during the wetting process, which depends on the diffusion capacity of the water molecules on mineral surfaces. The corresponding evolution of the solid-liquid friction coefficient during dynamic spreading is also analyzed. We further demonstrate the strong impact of isomorphic substitution and charge-balancing counterions on wetting characteristics on the surfaces of clay minerals. It is shown that the introduction of charge-balancing counterions can shift the mineral surface from strongly hydrophilic to strongly hydrophobic and lead to completely different wetting characteristics. Our results provide a clearer picture of the molecular underpinnings in mineral wetting phenomena and deepen the understanding of the control of water-mineral interactions on the wetting properties.

Carbon dioxide-enhanced metal release from kerogen

Heavy metals released from kerogen to produced water during oil/gas extraction have caused major enviromental concerns. To curtail water usage and production in an operation and to use the same process for carbon sequestration, supercritical CO₂ (scCO₂) has been suggested as a fracking fluid or an oil/gas recovery agent. It has been shown previously that injection of scCO₂ into a reservoir may cause several chemical and physical changes to the reservoir properties including pore surface wettability, gas sorption capacity, and transport properties. Using molecular dynamics simulations, we here demonstrate that injection of scCO₂ might lead to desorption of physically adsorbed metals from kerogen structures. This process on one hand may impact the quality of produced water. On the other hand, it may enhance metal recovery if this process is used for in-situ extraction of critical metals from shale or other organic carbon-rich formations such as coal.

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.