Hydrate Stability Research Articles

Variable stoichiometry and a salt-cocrystal intermediate in multicomponent systems of flucytosine: structural elucidation and their impact on stability.

New cocrystals and a salt-cocrystal intermediate system involving the antifungal drug flucytosine (FCY) and various coformers including caffeic acid (CAF), 2-chloro-4-nitrobenzoic acid (CNB), hydroquinone (HQN), resorcinol (RES) and catechol (CAL), are reported. The crystal structures of the prepared multicomponent systems were determined through SC-XRD analysis and characterized by different solid-state techniques. All FCY multicomponent systems crystallize in anhydrous form with different stoichiometric ratios. The cocrystals FCY-HQN, FCY-RES and FCY-CAL crystallize in 2:0.5, 2:0.5 and 3:2 stoichiometric ratios respectively. In contrast, FCY-CAF and FCY-CNB crystallize in a 1:1 stoichiometric ratio. The FCY-CAF cocrystal is formed via an acid-pyrimidine heterosynthon. Due to the partial proton transfer from the acid group of CNB to FCY, a three-point homosynthon is observed between two FCY molecules and the molecules interact via an N-H...O hydrogen bond between FCY and CNB. In FCY phenolic cocrystals, a single-point O-H...O hydrogen bond is observed. The formation of cocrystals and salt-cocrystal intermediate was further confirmed by difference Fourier map analysis and bond angle differences. Except for FCY-CAL, all the multicomponent systems were reproduced in the bulk scale for further characterization. A detailed Crystal Structural Database search was carried out on the multicomponent systems of FCY with acid coformers and we evaluated the formation of cocrystals/salt based on the ΔpKa values, the difference in the bond distances and bond angles. Additionally, the prepared multicomponent systems exhibited hydration stability for one month under accelerated conditions [40 (2)°C and relative humidity 90-95 (5)%].

Methane sealed due to the formation of gas hydrate system in the South China Sea

Although the release of methane into oceans and potentially the atmosphere could accelerate climate change, detailed investigations on the gas source from deep-buried strata and its migration through the gas hydrate stability zone (GHSZ) to the seafloor are limited. These studies are often hindered by the presence of diffracted waves and inaccuracies in seismic velocity models, leading to poor seismic imaging that hampers the understanding of gas sources, migration pathways, and gas hydrate accumulation. In the study, we utilize the technique of common scatter point (CSP) gathers to build an accurate velocity model and obtain high-quality images for a complex gas-hydrate and natural-gas petroleum system. The CSP processing enables the accurate migration of reflected and diffracted waves, resulting in improvements in signal-to-noise ratio and lateral resolution. The improved seismic images offer clearer visualization of various petroleum elements. Specifically, we can identify the top of the hydrate zone and base of the free gas zone within a shallow-buried hydrate system, fault geometries within the free gas zone, a middle-buried natural gas reservoir, gas chimneys as migration pathways, and the deep-buried source rock strata beneath the intrusive volcanic rocks. As a result, we reveal a joint prospect of natural-gas reservoir and gas-hydrate system in the deep-water region of the South China Sea. Our results suggest that methane in the natural gas reservoir has migrated upwardly into the hydrate system, and it is unlikely to leak into the water column.

Gas–Water–Sand Inflow Patterns and Completion Optimization in Hydrate Wells with Different Sand Control Completions

Sand production poses a significant problem for marine natural gas hydrate efficient production. However, the bottom hole gas–water–sand inflow pattern remains unclear, hindering the design of standalone screen and gravel packing sand control completions. Therefore, gas–water–sand inflow patterns were studied in horizontal and vertical wells with the two completions. The experimental results showed that gas–water stratification occurred in horizontal and vertical standalone screen wells. The gas–water interface changed dynamically, leading to an uneven screen plugging, with severe plugging at the bottom and high permeability at the top. The high sand production rate and low well deviation angle exacerbated screen plugging, resulting in a faster rising rate of the gas–water interface. The screen plugging degree initially decreased and then increased as the gas–water ratio increased, resulting in the corresponding variation in the gas–water interface rising rate. Conversely, gas–water stratification did not occur in the gravel packing well because of the pore throat formed between the packing gravels. However, the impact of gas and water led to gravel rearrangement and the formation of erosion holes, causing sand control failure. A higher gas–water ratio and lower packing degree could result in more severe destabilization. Therefore, for the standalone screen completion, sand control accuracy should be designed at different levels according to the uneven plugging degree of the screen. For the gravel packing completion, increase the gravel density without destabilizing the hydrate reservoir, and use the coated gravel with a cementing effect to improve the gravel layer stability. In addition, the screen sand control accuracy inside the gravel packing layer should be designed according to the sand size to keep long-term stable hydrate production.

Agro-industrial waste utilization in air-cured alkali-activated pavement composites: Properties, micro-structural insights and life cycle impacts

This study investigates the development and performance of agro-industrial waste-based air-cured alkali-activated composites (AC) for sustainable high-strength pavement applications. The calculated amount of Na2SiO3 and NaOH were employed with water to generate alkaline activator solution. Locally sourced materials, including blast furnace slag, construction and demolition (C&D) waste, and sugarcane bagasse ash (SBA), were utilized to examine mechanical properties, micro-structural behaviour, and life cycle impacts. Optimized AC mixes containing 50% recycled aggregates (RCA) and 15% SBA demonstrated superior compressive, tensile, and flexural strength, while significantly reducing embodied energy and carbon emissions. Microstructural analysis through XRD, SEM, EDAX, and TGA confirmed the formation of stable alumino-silicate hydrate phases, contributing to enhanced superior-strength. The life cycle analysis results indicated considerable environmental benefits compared to traditional OPC pavement concretes. This research presents a sustainable solution for pavement infrastructure, aligning with circular economy principles by promoting the reduction of resource consumption and greenhouse gas emissions.

Hydrate management strategies for CO2 injection into depleted gas reservoirs

Sequestering CO2 in depleted gas reservoirs using carbon capture, utilization, and storage technologies has emerged as a viable strategy for mitigating global carbon emissions. However, CO2 hydrate formation during injection processes is challenging and poses a risk to the operational integrity and efficiency of sequestration projects. This study investigated CO2 hydrate formation dynamics within silica gel environments that closely mimicked the median pore size of natural sandstone reservoirs, particularly targeting depleted gas fields in the East Sea of Korea. Integrating kinetic insights into experimental hydrate formation assessments and molecular dynamics simulations provided a comprehensive understanding of the CO2 hydrate formation mechanisms, as well as the formation dynamics and stability of the CO2 hydrates. For example, the hydrate formation kinetics were significantly reduced in the small pores, suggesting the feasibility of implementing more economical hydrate inhibitor injection strategies during CO2 injection. Moreover, this study evaluated the inhibitory effects of NaCl and methanol on CO2 hydrate stability by accurately measuring the three-phase (H-Lw-V) equilibria and employing silica gels (6 nm pore size). The presence of methanol and amorphous silica significantly impacted hydrate formation, with methanol potentially influencing the local structure around the hydrate phase and silica hindering hydrate growth through dynamic interactions with CO2 molecules. In addition, we elucidated the complex interplay between CO2, water, and methanol (hydrate inhibitor) within the constrained environments of porous media, offering strategic insights for the development of effective CO2 injection and sequestration strategies. Integrating molecular-level observations with real-world geological modeling presents a novel methodology for enhancing the safety, efficiency, and environmental sustainability of carbon capture and storage operations. The findings of this study provide critical insights into hydrate phase behavior, highlighting the importance of precise equilibrium determination under simulated reservoir conditions to effectively anticipate and mitigate hydrate formation challenges.

Kinetics of CO2 hydrate formation in clayey sand sediments: Implications for CO2 sequestration

Hydrate-based CO2 sequestration beneath oceanic sediments is an emerging technique that involves the injection of CO2 into the hydrate stability zone (HSZ) beneath the seabed, forming hydrate cap that structurally traps the injected CO2 and reduces the risk of leaking CO2 from the storage sediment. Gas hydrates are adequately stable in sand sediments saturated with fresh water; however, the salinity of oceanic water impairs hydrate formation kinetics, stability, and CO2 storage capacity. In addition to sandstones, marine sediments are composed of many clay minerals that could affect hydrate formation. Therefore, this study experimentally simulates CO2 injection into sand and clayey sand sediments to assess the potential of CO2 hydrate formation. CO2 hydrates are formed inside a high-pressure reactor, which contains unconsolidated sediment bed/pack (silica sand; mixed sand with bentonite clay: 5 wt% and 10 wt%), saturated with de-ionized water or brine (3.3 wt% NaCl). Hydrate formation experiments were performed at 4 MPa pressure and 274.15 K temperature. Results show that CO2 hydrate formed within the sand sediment, with induction times of 6 and 8.5 h, for the de-ionized and brine systems, respectively. CO2 gas mole uptake in the de-ionized system was 71.54 mmol/mol however, in the brine system the gas uptake was 56.95 mmol/mol. Hence this 20.4% reduction in the gas uptake indicated the inhibition effect of salinity. In contrast, in the brine-saturated 5 wt% clay-sand sediment, the induction time was 6.5 h, indicating the promoting effect of the nano-sized clay particles. However, the gas uptake in this brine-saturated clay-sand sediment was reduced by 45.51% compared to the brine-saturated sand sediment. Increasing the clay content to 10 wt% prevented CO2 hydrate formation due to porosity reduction. Moreover, de-ionized water in clayey sand sediments prevented hydrate formation due to clay swelling. Finally, CO2 hydrate formation at the end of each experiment was visually confirmed.

Mechanical deformation destabilizing hydrate within thermodynamic equilibrium region

Natural gas hydrates are a significant potential energy source and key player in global geological climate dynamics, with our all understandings on both sides basing on hydrate stability conditions consisting of pressure (P) and temperature (T). Considering important significance of theoretical researches in two sides, whether there is another factor playing same role with P or T is worth being answered. Using molecular dynamics simulations, changes of non-clathrated water structures and gas bubbles during mechanical deformation processes of methane hydrates were investigated to characterize their destabilizing natures and the deformation is then identified as this new factor. Similar to P-T profile but at a new level of complexity, deformation also characterizes hydrates stability but in three-dimensional space form while combining with T, reflecting deeper influences of specific temperature values, hydrate purities and deformation modes on hydrates stability. Over two tensile andcompressive deformations, the mechanisms responsible for hydrate destabilization vary, with stretching favoring formation of gas bubbles and compressing damaging internal hydrogen-bond structures. Once alternating, such two to-and-fro continuing deformations can act as a piston-like accelerator as well as one-way valve for dissociation of hydrates, destabilizing them rapidly. In summary, P and T act as the prerequisite of hydrate existences, but cannot accurately account for the influence of such dynamic events as earthquakes, ocean tides, and changes of ice shelves on dissociations. This research sheds a new light on the reasonable explanation to dynamic hydrate dissociations, e.g. some abnormal disappearances of reservoirs within appropriate P-T conditions. Stronger, current bottleneck of hydrate exploitation efficiency might also be broken down; likewise, the ultra-rapid dissociation performance during the terminal procedure of hydrate-based natural gas storage/transportation technology might achieve success.

Effect of cement slurry filtrate on methane hydrate stability during deepwater cementing operations

Whether the hydrate layer is stabilized during deepwater cementing operations has a direct impact on the safety of the cementing process and the quality of the cementing. When the cement slurry enters the annular space and waits for solidification, it will be extruded to produce filtrate under the action of formation temperature and pressure. Moreover, the inorganic ions and organic matter contained in the filtrate may penetrate into the formation. However, whether these processes have an effect on the stability of the hydrate layer is not clear at present.In this paper, we initially developed a methane hydrate generation system, termed the water-quartz sand-methane system. This system efficiently produces methane hydrate within a short time and ensures a uniform distribution of the gas-solid phase, which is highly suitable for further research into methane hydrate stability. Subsequently, the primary constituents of the cement slurry filtrate and the principal functional groups of the cement additive were identified. Furthermore, enhancements were made to the existing formula for quantifying methane gas alteration. Additionally, a hydrate testing experimental system was employed to circulate a solution comprising the primary constituents of the cement slurry filtrate over the methane hydrate surface. Ultimately, utilizing the amount of change in the volume of methane gas in the cyclic process as a criterion to evaluate the impact of various components of the cement slurry filtrate on methane hydrate stability, an investigation into the effect of the cement slurry filtrate on methane hydrate stability was conducted. It was confirmed that Ca2+, Na+, Cl− ions within the cement slurry filtrate, along with the pH value, minimally influence methane hydrate stability. Conversely, the cement additive exhibits a discernible disruptive effect on methane hydrate stability, with the retarder demonstrating the most pronounced impact when mass fractions are equated.This study suggests that the adverse effects of amide and carboxyl groups on hydrate stability must be considered in the selection of additives for deepwater cementing slurries. The findings offer guidance for the research and development of additives for deepwater cementing slurries and establish a theoretical foundation for studying cementing slurry systems in hydrate formations.

Varying free-gas zone (FGZ) and bottom-simulating reflection (BSR) response across a deep graben off Trujillo, central Peru

The complexity of marine gas hydrate systems at the Peruvian convergent margin has been linked to the post-Miocene history of vertical tectonics and subduction erosion that affected the forearc. Here, multichannel seismic data and published findings reveal that such a complexity has been further extended by the occurrence of the pre-Miocene deep Morsa Norte Graben (MNG) off Trujillo (8°–9° S) in the Central Peru margin. At water depths of 650–750 m in the upper slope, directly above the MNG depocentre (which has a 4–6 km overburden thickness), continuous bottom-simulating reflections (BSRs) and concentrated sub-BSR high-amplitude reflections are confined beneath a layered basin with a low–moderate near-seafloor heat flow (7–33 mW m −2 ). A deeper BSR modelled with a thermogenic gas composition is associated with the enhanced reflections. At a water depth of 900 m, sub-BSR reflections become less frequent in an area with a layered sediment cover defined by a moderate near-seafloor heat flow (15–33 mW m −2 ). At a water depth of 1200 m, where the MNG is relatively thick (3–4 km overburden thickness), faults connect patchy BSRs with a moderate–high near-seafloor heat flow (52–110 mW m −2 ). There, sub-BSR enhanced reflections are scarce. Immediately above the top of the gas hydrate stability zone (GHSZ), the near-seafloor heat flow reaches 81 mW m −2 . Modelling suggests a water depth-dependent transport of heat towards the seafloor with respect to the top of the GHSZ, implying that the closer the seafloor is to the top of the GHSZ, the lower the advection of heat, and vice versa. Recent seafloor-related depositional and structural features amplify such relations in agreement with near-seafloor heat-flow variability. However, towards the area outside the extent of the MNG (<3 km overburden thickness), continuous BSRs are not linked either to a deeper BSR or to sub-BSR enhanced reflections. The continuity of one of these BSRs is deflected upwards beneath a slump, suggesting an incomplete thermal re-equilibration of the GHSZ. Therefore, we conclude that the BSR responds to: (1) the confinement and thickening of the free-gas zone (FGZ) above the MNG depocentre due to the sealing effect of recent sedimentation close to the top of the GHSZ; (2) the seepage of gas-rich fluids from a thinned FGZ above the relatively thick MNG, due to the enabling effect of faults cutting the GHSZ far from the top of the GHSZ; (3) the undisturbed state of the FGZ outside the extent of the MNG; and (4) the disequilibrium state of the base of the GHSZ due to the unloading of sediments in an unstable slope environment prone to failure.

Hydrate–based SF6 capture and sequestration: Insights from thermodynamics, kinetics, in–situ Raman spectroscopy, and molecular dynamic simulation

Sulfur hexafluoride (SF6) is currently the most potent greenhouse gas to date due to its remarkably long atmospheric lifespan and chemical inertness. Hydrate–based technology provides an innovative solution to capture SF6 under lower pressure conditions and enables the long–term storage of SF6 gas. Herein, we conduct a fundamental study to explore the potential of capturing and sequestering SF6 gas within clathrate hydrates. The three–phase coexistence conditions were measured at the pressure ranging from 0.33 to 1.06 MPa and at ambient temperatures. A thermodynamic model was employed to predict the equilibrium conditions, using an iterative method to calculate the phase equilibrium pressure. Molecular dynamic simulation (MD) was used to observe the growth of SF6 hydrate at the nanosecond scale. In-situ Raman spectroscopy was utilized for analyzing real-time vibrational bands of S–F and O–H in SF6 hydrate, offering crucial insights into hydrate stability and growth. Additionally, microscopic kinetic experiments were also carried out to clarify the impacts of different pressures (0.8, 1.0, and 1.2 MPa) on the nucleation time, gas consumption, and growth rate during hydrate formation in both pure water and seawater. An efficient approach is proposed for the separation of solid–liquid mixtures, fabricating hydrate pellets. The findings of this study establish a foundation for the development of hydrate–based SF6 capture and storage technology.

Shallow subsurface fluid dynamics in the Malvinas Basin (SW Atlantic): A geoacoustic analysis

Using a database consisting of sub-bottom profiles, 2D and 3D seismic data, a series of seep structures and acoustic anomalies associated with a plumbing system in the subsurface of the southern part of the Malvinas Basin off the Argentine coast are presented. The seep morphologies consist of mounds, pockmarks, and a carbonate mound, derived from hydrocarbons migrating from the Early Cretaceous Springhill and Lower Inoceramus formations through extensional faults and overthrusts of the Malvinas Fold-Thrust Belt and accumulating in the gas hydrate stability zone. This configuration is widely observed in the anticlines of the Malvinas Fold-Thrust Belt, which are characterized by a vertical structure highlighting a Bottom Simulating Reflector (BSR) overlain by a blanked-out zone at the seabed, interpreted as a gas hydrate stability zone. The plumbing system is influenced by active transtensional tectonics, as shown by two sets of extensional faults concentrated over some anticlines. These faults lead to displacements that, in many cases, reach the seabed. One nuance of the influence of tectonic activity on fluid escape is most evident in the Malvinas anticline, where all the pockmarks and the carbonate mound are concentrated. In the western part of the Malvinas anticline, five pockmarks are observed in an area characterized by a shallow BSR. In contrast, in the eastern part of the Malvinas Basin, one pockmark and one carbonate mound are derived from a series of extensional faults. The presented data, as well as a comparison between the western and eastern parts of the Malvinas anticline, indicate that the anticlines are the main cause of seepage in this area, as they allow the accumulation of migrating fluids. Likewise, the seepage morphologies outside the anticlines are much less pronounced, as can be observed in the mounds that overlie the bottom vents.

Stability Characteristics of Natural Gas Hydrate Wellbores Based on Thermo-Hydro-Mech Modeling

During drilling operations, drilling fluids undergo heat exchange with hydrate-bearing formations. The intrusion of drilling fluids affects hydrate stability, leading to variations in stress fields around a wellbore and complex scenarios such as borehole collapse, significantly hindering the efficient development of natural gas hydrate resources. This study establishes a thermo-hydro-mech model for hydrate-bearing inclined wells based on linear thermoelastic porous media theory and an appropriately high inhibitive drilling fluid temperature. This research reveals that drilling should follow directions of minimum horizontal stress or perpendicular to maximum horizontal stress during drilling operations to control wellbore stability. Hydrate decomposition due to factors like drilling fluid pressure and temperature can rapidly reduce the strength of low-saturation formations, significantly increasing the risk of wellbore instability. Therefore, selecting appropriate highly inhibitive and low-temperature drilling fluids during drilling operations helps control hydrate decomposition and reduce fluid intrusion, thereby mitigating risks associated with wellbore instability.

Phase Stability of CH4 and CO2 Hydrates under Confinement Predicted by Machine Learning.

Understanding the phase stability of gas hydrates under confinement is fundamental to the geological stability evolutions of gas hydrate systems on Earth. Herein, the phase stability of CH4 and CO2 hydrates under confinement is predicted by machine learning. Three machine learning models, including support vector machine, random forest, and gradient boosting decision tree, are constructed to predict the phase stability of CH4 and CO2 hydrates under confinement. Our machine learning results show that the prediction accuracy of the support vector machine model is highest, yet the prediction accuracy of the random forest model is lowest among those machine learning models in determining the phase stability of confined gas hydrates. Based on their performance in predicting the phase stability of confined gas hydrates, the support vector machine model with a training set fraction of 0.7 is finally chosen to deal with the unknown phase stability of confined gas hydrates. Importantly, the average accuracy of the support vector machine model can reach more than 90% in predicting the unknown phase stability of both CH4 and CO2 hydrates. The trained machine learning models can help us to quickly and accurately determine the phase stability of CH4 and CO2 hydrates under confinement in future applications.

Tuneable mesoporous silica material for hydrogen storage application via nano-confined clathrate hydrate construction

Safe storage and utilisation of hydrogen is an ongoing area of research, showing potential to enable hydrogen becoming an effective fuel, substituting current carbon-based sources. Hydrogen storage is associated with a high energy cost due to its low density and boiling point, which drives a high price. Clathrates (gas hydrates) are water-based (ice-like) structures incorporating small non-polar compounds such as H2 in cages formed by hydrogen bonded water molecules. Since only water is required to construct the cages, clathrates have been identified as a potential solution for safe storage of hydrogen. In bulk, pure hydrogen clathrate (H2O-H2) only forms in harsh conditions, but confined in nanospaces the properties of water are altered and hydrogen storage at mild pressure and temperature could become possible. Here, specifically a hydrophobic mesoporous silica is proposed as a host material, providing a suitable nano-confinement for ice-like clathrate hydrate. The hybrid silica material shows an important decrease of the pressure required for clathrate formation (approx. 20%) compared to the pure H2O-H2 system. In-situ inelastic neutron scattering (INS) and neutron diffraction (ND) provided unique insights into the interaction of hydrogen with the complex surface of the hybrid material and demonstrated the stability of nano-confined hydrogen clathrate hydrate.

Environmentally friendly production of petroleum systems with high CO2 content

Natural gas hydrates represents a huge source of energy. At the same time substantial leakages of natural gas from hydrates contributes significantly to climate changes. One of the most important reasons for these natural gas fluxes is leakage of seawater in to the hydrates from seafloor, through fracture systems. Hydrate dissociates if surrounding seawater is less than hydrate stability limit. Another interesting aspect of natural gas hydrates is the potential for safe CO2 storage. These different aspects of hydrates in natural sediments put demands on thermodynamic models. In addition to accurate description of pressure temperature hydrate stability there also a need to describe hydrate dissociation in concentration gradients towards surrounding water or surrounding gas as two examples. In this work we present new experimental data and an extensive thermodynamic model for hydrate. In contrast to conventional thermodynamic models for hydrate the model is consistent since all thermodynamic properties are derived from the Gibbs free energy. In this work we examine mixtures of CH4, C2H6, N2, CO2 from the China Sea and some synthetic mixtures, using this model. Maximum CO2 content in these mixtures are 60 mol% and the rest is dominated by CH4. Agreement between experimental data and model calculations are generally good and average deviations are below 5.5% for all the systems and conditions examined. Another aspect of the model is the ability for incorporation of effects of mineral surfaces. Specifically it is illustrated that adsorption of water on rust dominates liquid water drop out from gas as compared to water dew-point. Production of natural gas with such high CO2 content requires a strategy for CO2 separation and storage. It is proposed that the CH4 is separated from the C2H6, CO2 and N2 and cracked to H2 and CO2 using steam. Thermodynamic analysis indicates a significant potential for safe CO2 storage in natural gas hydrate and H2 as the only export product.

Shoaling of the gas hydrate stability zone inferred from 3D seismic data of the Cauvery basin

Shoaling of the gas hydrate stability zone inferred from 3D seismic data of the Cauvery basin

Pore Characteristics of Hydrate-Bearing Sediments from Krishna-Godavari Basin, Offshore India

Pore-filling hydrates are the main occurrence forms of marine gas hydrates. Pore characteristics are a vital factor affecting the thermodynamic properties of hydrates and their distribution in sediments. Currently, the characterization of the pore system for hydrate-bearing reservoirs are little reported. Therefore, this paper focuses on the Krishna-Godavari Basin, via various methods to characterize the hydrate-bearing sediments in the region. The results showed that X-ray diffraction (XRD) combined with scanning electron microscopy (SEM) and cast thin section (CTS) can better characterize the mineral composition in the reservoir, high-pressure mercury injection (HPMI) focused on the contribution of pore size to permeability, constant-rate mercury injection (CRMI) had the advantage of distinguishing between the pore space and pore throat, and nuclear magnetic resonance cryoporometry (NMRC) technique can not only obtain the pore size distribution of nanopores with a characterization range greater than nitrogen gas adsorption (N2GA), but also quantitatively describe the trend of fluids in the pore system with temperature. In terms of the pore system, the KG Basin hydrate reservoir develops nanopores, with a relatively dispersed mineral distribution and high content of pyrite. Rich pyrite debris and foraminifera-rich paleontological shells are observed, which leads to the development of intergranular pores and provides more nanopores. The pore throat concentration and connectivity of the reservoir are high, and the permeability of sediments in the same layer varies greatly. The reason for this phenomenon is the significant difference in average pore radius and pore size contribution to pore permeability. This article provides a reference and guidance for exploring the thermodynamic stability of hydrates in sediments and the exploration and development of hydrates by characterizing the pores of hydrate reservoirs.

Thermodynamic and computational approaches to the stability and structural transformation of Xe + large-molecule guest substance (LMGS) hydrates

In this study, the influence of different types of large-molecule liquid substances (LMGSs) and formation conditions on the stability and structural transformation of Xe + LMGS hydrates was investigated using phase equilibria, powder X-ray diffraction (PXRD), 129Xe NMR spectroscopy, and molecular dynamics (MD) simulations. The phase equilibria of Xe hydrates in the presence of three different LMGS types (neohexane [NH], methylcyclopentane [MCP], and methylcyclohexane [MCH]) were measured in the pressure range of 0.1–1.0 MPa. The equilibrium curves of Xe + MCH and Xe + NH hydrates shifted away from that of pure Xe hydrate under low pressure, whereas they coincided with that of pure Xe hydrate under high pressure. Interestingly, the equilibrium curve of Xe + MCP hydrate aligned with that of pure Xe hydrate throughout the pressure range. PXRD and 129Xe NMR analyses demonstrated that the equilibrium curve shift under low pressure was caused by the formation of sH (Xe + LMGS) hydrates, which was attributed to the inclusion of NH or MCH. However, MCP did not participate in sH hydrate formation with Xe. MD simulations revealed that MCP in the large cages did not contribute to the thermodynamic or dynamic stability of sH hydrates. The findings of this study provide valuable insights into the development of hydrate-based noble gas capture and storage.

Gas hydrate inhibition by urea and its blends with vinyl lactam polymer: Paving the way to green hybrid inhibitors

Urea is an environmentally benign substance considered a promising gas hydrate inhibitor in flow assurance. Nevertheless, its effect on gas hydrate formation kinetics remains poorly understood. This study investigates the impact of urea and hybrid urea/polymer KHI samples on the nucleation and growth kinetics of sII natural gas hydrates. Hydrate formation was observed at a lower onset temperature with increasing urea and polymer concentration. The nucleation of sII hydrate in aqueous urea solutions occurred at a higher subcooling (7.50 ± 0.58 K at 30 mass%) than in pure water (5.17 ± 0.69 K). These findings indicate that urea acts as a nucleation inhibitor of sII hydrates, although its capacity is not as potent as that of polymer KHI.The analysis of the hydrate nucleation probability distributions revealed that the sII hydrate nucleation rate exhibited a significant decline, approximately one order of magnitude, at subcooling of 5.3–6 K for 20 mass% urea compared to water. A similar behavior was noticed in an aqueous solution of polymer KHI. Urea has succeeded in providing a total benefit of up to 10 K in hydrate onset temperature due to a shift in the hydrate stability zone and nucleation retardation.Consequently, urea is a green dual-acting anti-hydrate chemical that inhibits the formation of sII hydrates by thermodynamic and kinetic mechanisms. Besides, adding 10 mass% urea increases the cloud point temperature of vinyl lactam polymer Luvicap 55W by 14 K. Urea is an additive that considerably augments the stability of the polymer in aqueous solutions at elevated temperatures. These findings make urea an effective and environmentally friendly top-up inhibitor that significantly enhances the anti-hydrate properties of polymer KHI.

Effect of particle size, water saturation, inorganic salt and methane on the phase equilibrium of CO2 hydrates in sediments

Understanding the thermal stability of gas hydrate in complex marine geological environment is of importance to hydrate-based carbon sequestration. In this work, the factors affecting the equilibrium of CO2 hydrate in ocean sediments, including quartz sands, inorganic salts and gas impurities were quantitatively measured in a temperature range from 273 to 283 K and a phase equilibrium model of hydrate was established. To reveal the distribution in pore structure, the micro-morphologies of hydrate-bearing sediments were measured by cryo-SEM. Results showed that reduction of initial water saturation, addition of NaCl and CH4 were found to have inhibitory effect on CO2 hydrate equilibrium. Initial water saturation reduced the equilibrium temperature by the capillary pressure, but only 0.3–0.7 K temperature depression was observed as the water saturation reduced to 5 %. About 5.7 K in the average temperature depression was found by the addition of 10 wt% NaCl and 24 mol% CH4. NaCl and CH4 influenced the hydrate equilibrium by changing the water activity and chemical potential of hydrate water lattice. SEM images showed that the hydrate formed in pores of quartz sand had porous surface and coated the sand particles like a layer of cells which are 5–20 μm in diameter, suggesting the hydrate layer exists between the liquid and gas phase. Based on the van der Waals-Platteeuw model, a hydrate equilibrium model was developed. The model provided a good prediction of the hydrate equilibrium in the presence of quartz sand, NaCl and CH4 with an averaged deviation of ±4.2 %, which had the potential to be applicated in more complexed ocean sedimentary environment.