Water Contact Angle Dependence with Hydroxyl Functional Groups on Silica Surfaces under CO₂ Sequestration Conditions

Functional groups on silica surfaces under CO₂ sequestration conditions are complex due to reactions among supercritical CO₂, brine and silica. Molecular dynamics simulations have been performed to investigate the effects of hydroxyl functional groups on wettability. It has been found that wettability shows a strong dependence on functional groups on silica surfaces: silanol number density, space distribution, and deprotonation/protonation degree. For neutral silica surfaces with crystalline structure (Q³, Q³/Q⁴, Q⁴), as silanol number density decreases, contact angle increases from 33.5° to 146.7° at 10.5 MPa and 318 K. When Q³ surface changes to an amorphous structure, water contact angle increases 20°. Water contact angle decreases about 12° when 9% of silanol groups on Q³ surface are deprotonated. When the deprotonation degree increases to 50%, water contact angle decreases to 0. The dependence of wettability on silica surface functional groups was used to analyze contact angle measurement ambiguity in literature. The composition of silica surfaces is complicated under CO₂ sequestration conditions, the results found in this study may help to better understand wettability of CO₂/brine/silica system

Hydrogen bonds at silica–CO₂ saturated water interface under geologic sequestration conditions

<p>To investigate the effects of sequestration condition on hydrogen bonds between mineral and water, molecular dynamics simulations have been performed. The simulations were conducted at conditions related with geologic sequestration sites: pressure (3.1–32.6 MPa), temperature (318 and 383 K), salinity (0–3 M), salt (NaCl and CaCl₂) and silica surface models Q² (geminal), Q³ (isolated) and amorphous Q³. The hydrogen bonds were classified into four types: silica–silica, silica–dissolved CO₂, silica–water as donors and silica–water as acceptors. The mean numbers of hydrogen bonds for each type were analysed. The results show that: (1) silica surface silanol groups do not form H-bonds with dissolved CO₂ molecules in water (brine); (2) The mean number of hydrogen bonds between silanol groups follows the order: Q² > amorphous Q³ > Q³; (3) The mean number of hydrogen bonds between silanol and water molecules follows the order: Q³ > amorphous Q³ > Q².</p

Pressure and Temperature Dependence of Contact Angles for CO₂/Water/Silica Systems Predicted by Molecular Dynamics Simulations

Wettability controls the capillary behavior of injected CO₂, including capillary entry pressure, relative permeability, and residual fluid saturation, and it is one of the most active topics in geologic carbon sequestration (GCS). However, the large uncertainty of water contact angle (CA) data and its pressure, temperature, and salinity dependence in the literature limit our understanding on wettability. Molecular dynamics (MD) simulations have been performed to investigate the pressure (<i>P</i>) and temperature (<i>T</i>) dependence of water CAs on the silica surface. Three typical molecular surface models for silica, namely, Q², crystalline Q³, and amorphous Q³, were selected, and simulations were conducted at wide pressure (2.8–32.6 MPa) and temperature (318–383 K) conditions. The results show that <i>P</i> and <i>T</i> dependence of water CAs on silica surfaces is controlled by surface functional groups. These findings provide new information to help with the better understanding of wettability alteration under different GCS conditions

Wettability of Supercritical CO₂–Brine–Mineral: The Effects of Ion Type and Salinity

Deep saline aquifers are considered as perfect storage sites to sequestrate CO₂. Interfacial tensions (IFTs) and contact angles (CAs) are key parameters in the heat and mass transfer processes for CO₂/brine/mineral systems in porous media. In the present study, a molecular dynamics simulation method was used to investigate the effects of brine salinity and ion type on wettability of CO₂/brine/mineral systems at 20 MPa and 318.15 K. Four common brines were selected as NaCl, KCl, CaCl₂, and MgCl₂. Interfacial tensions, water contact angles, and hydrogen bond structure and dynamics have been analyzed. The effects of brine salinity and ion type on water contact angles were found to be very complicated. For MgCl₂ and NaCl solutions, the contact angle increases with salinity. For CaCl₂ and KCl solutions, contact angle first increases and then remains constant with salinity. The product of IFT­(CO₂–brine) and the cosine of CA was found to be constant for all brine solutions studied. In the context of large uncertainty of experimentally measured contact angles, this finding is very useful to predict contact angles using interfacial tension data. Due to the fact that IFT­(CO₂–brine) × cos­(CA) is usually related with capillary pressure and residual trapping capacity, this finding is also very helpful to predict these parameters at different brine conditions. More work is required to study the effects of pressure, temperature, and solid surface structure on this relationship

<i>In Situ</i> Observation of Methane Hydrate Dissociation under Different Backpressures

Depressurization has been considered an economic and practicable method for natural gas hydrate (NGH) exploitation. To obtain the kinetic data of methane hydrate (MH) dissociation under different backpressures, MH dissociation by depressurization in a porous medium was investigated <i>in situ</i> using magnetic resonance imaging (MRI). MH was dissociated under backpressures that were varied from 2.8 to 2.2 MPa, and the hydrate saturation variation during dissociation was analyzed. One experimental case was carried out with constant backpressure, and four cases of variable backpressure depressurization experiments were carried out. The radial dissociation pattern during depressurization was confirmed. During hydrate dissociation, free water was observed to move toward the outlet of the vessel and decreased the water saturation after the hydrate totally dissociated in the field of view (FOV). The MRI data provided excellent information on the spatial distribution of water in the porous media during hydrate dissociation

High Temperature CO₂ Sorption on Li₂ZrO₃ Based Sorbents

In this study, the Li₂ZrO₃ based sorbents with different compositions were synthesized by the solid-state reaction method from the mixtures of Li₂CO₃, K₂CO₃ and ZrO₂. CO₂ sorption properties of Li₂ZrO₃ based sorbents were investigated by analyzing the phases and microstructure changes with the help of thermogravimetric analysis, X-ray diffraction and scanning electron microscopy. The thermodynamic calculations were carried out based on the second law of thermodynamics. Li₂CO₃/K₂CO₃-doped Li₂ZrO₃ sorbent with the composition of 36.23 wt % Li₂CO₃, 55.12 wt % ZrO₂ and 8.65 wt % K₂CO₃ was considered to achieve excellent capability for high temperature CO₂ sorption and presented the maximum sorption rate at 525 °C and 0.15 atm of CO₂ partial pressure. The sorbent kept rather stable for multicycles sorption and regeneration, and maintained its original capacity during 12 cycle processes. There were three distinct phases in the nonisothermal CO₂ sorption process while the main CO₂ sorption occurred during the second phase. An improved iterative Coats–Redfern method was used to evaluate nonisothermal kinetics of the CO₂ sorption process, and the kinetic parameters were derived by the MATLAB model. The Fn <i>n</i>^th-order reaction model predicted accurately the main phases and differences in the activation energies and the frequency factors for different sorbents in the sorption phases corroborated different mechanism integral functions and reaction orders

Measurement of Interfacial Tension of CO₂ and NaCl Aqueous Solution over Wide Temperature, Pressure, and Salinity Ranges

Interfacial tension data and models are of great importance to the storage of CO₂ in deep saline aquifers. In this study, the pendant-drop method combined with axisymmetric drop shape analysis was used in the interpretation of the interfacial behavior of CO₂ and brine. Extensive experimental measurements of the interfacial tension between CO₂ and an NaCl solution were acquired for pressures ranging from 3.0 to 12.0 MPa, temperatures from 300 to 353 K, and NaCl molalities from 0 to 1.8 mol·kg^–1, for a total of 1,254 valid data points. All experiments were conducted in a pressure cell fitted with a capillary tube to create pendant droplets in a CO₂-rich atmosphere. The experimental results indicated that interfacial tension decreased with increasing pressure and increased with temperature and salinity. As pressure increased to a certain point, the interfacial tension reached a plateau. At a given temperature, the CO₂–aqueous system reached a plateau for different salinities under nearly the same pressure. However, the plateau pressure increased with temperature. The plateau interfacial tension value slightly increased with temperature and salinity. We also found a linear relationship between the change in interfacial tension and the molality concentration of brine. An empirical model was also proposed based on the Parachor model for the prediction of interfacial tension. Most results of this model deviated by less than ±5% from our experimental results, indicating that the model was a good fit to our experiments

Heat Transport in Clathrate Hydrates Controlled by Guest Frequency and Host–Guest Interaction

The underlying mechanism of common limited lattice thermal conductivity (κ) in energy-related host–guest crystalline compounds has been an ongoing topic in recent decades. Here, the guest-triggered intrinsic ultralow κ of the representative xenon clathrate hydrate was investigated using the time domain thermoreflectance technique and theoretical calculations. The localized guest modes were observed to hybridize with acoustic branches and severely limit the acoustic κ contribution. Besides, the strong mode coupling enables the reshaping of the overall lattice dynamics, especially for optical branches. More importantly, we identified that guest fillers prompt great phonon scattering in wide frequencies, which originates from both the guest-frequency-controlled enhancement of phase space and the host–guest-interaction-governed lattice anharmonicity. The extremely low guest frequency and strong host–guest interaction and coupling were thereby underlined to play vital but distinct roles in κ minimization. Our results unveil the dominant factors of guest reduction effects and facilitate the design of efficient thermoelectric or other thermal-related materials

Characterization of Hydrate Formation and Flow Influenced by Hydrophilic–Hydrophobic Components within a Fully Visual Rocking Cell

The intricate interplay of crude oil composition and additives critically affects hydrate formation and flow behavior, which is significant for flow assurance particularly in deepwater. To investigate, we varied the hydrophilic and hydrophobic properties by proportioning two nonionic surfactants (Span 80 and Tween 80) and conducting hydrate formation experiments in a visual rocking cell. The results show that the increasing hydrophilic–lipophilic balance (HLB) value shifted emulsions from water-in-oil to multiple and oil-in-water phases and significantly affected the hydrate formation among different emulsion types, with a marked increase in the hydrate formation rate in the HLB range of 9 to 11. Slurries maintained flowability due to low hydrate conversion, yet higher HLB (>11) led to slight agglomeration and deposition. In addition, the impacts of water conversion were investigated by multiple pressurization, showing that the hydrate formation amount affected slurry flowability but was primarily dependent on the hydrophilicity and hydrophobicity of the surfactants. When the HLB was below 11, the final water conversion was about 80%, but the formed hydrate still exhibited good dispersion and flowability. In contrast, HLB exceeding 13 resulted in extensive adhesion and deposition on the cell walls. When the water conversion reached about 40%, flowability was completely lost and hydrate blockage occurred

Characterization of Hydrate Formation and Flow Influenced by Hydrophilic–Hydrophobic Components within a Fully Visual Rocking Cell

The intricate interplay of crude oil composition and additives critically affects hydrate formation and flow behavior, which is significant for flow assurance particularly in deepwater. To investigate, we varied the hydrophilic and hydrophobic properties by proportioning two nonionic surfactants (Span 80 and Tween 80) and conducting hydrate formation experiments in a visual rocking cell. The results show that the increasing hydrophilic–lipophilic balance (HLB) value shifted emulsions from water-in-oil to multiple and oil-in-water phases and significantly affected the hydrate formation among different emulsion types, with a marked increase in the hydrate formation rate in the HLB range of 9 to 11. Slurries maintained flowability due to low hydrate conversion, yet higher HLB (>11) led to slight agglomeration and deposition. In addition, the impacts of water conversion were investigated by multiple pressurization, showing that the hydrate formation amount affected slurry flowability but was primarily dependent on the hydrophilicity and hydrophobicity of the surfactants. When the HLB was below 11, the final water conversion was about 80%, but the formed hydrate still exhibited good dispersion and flowability. In contrast, HLB exceeding 13 resulted in extensive adhesion and deposition on the cell walls. When the water conversion reached about 40%, flowability was completely lost and hydrate blockage occurred