Roles of Wettability and Supercooling in the Spreading of Cyclopentane Hydrate over a Substrate

Crystallization Behavior of the Hydrogen Sulfide Hydrate Formed in Microcapillaries

There are no reports on the hydrogen sulfide hydrate growth process and morphology in micropores due to the toxicity of hydrogen sulfide. In this study, the experimental measurements and dissociation enthalpies were provided to assess the effect of the microcapillary silica tube size on hydrogen sulfide hydrate dissociation conditions. To simulate micropore sediments, the H₂S hydrate growth processes and morphologies at different supercooling temperatures were observed in this study. The dissociation temperature depression of the hydrate crystal in the microcapillary was less than 0.001 °C, which shows that the stability of the hydrate is less affected by the microcapillary pore used in this study. The mass transfer from the gas phase to the liquid phase is easily blocked when the hydrogen sulfide hydrate shell covers the gas-water meniscus, causing the growth of the gas hydrate to be inhibited. The hydrate crystal morphology can be divided into fibrous, needle-like crystals and dendritic crystals when ΔT _sub > 12.7; the hydrate crystal morphology can be categorized as dendritic crystals and columnar crystals when ΔT _sub = 7.9-8.9, and the hydrate crystals can form polyhedral crystals when ΔT _sub = 7.9-8.9. Additionally, a new "bridging effect" that a hollow crystal which was filled with the gas phase can connect with two separated gas phases was found at low supercooling temperature.

Carbon dioxide hydrate in a microfluidic device: Phase boundary and crystallization kinetics measurements with micro-Raman spectroscopy

Various emerging carbon capture technologies depend on being able to reliably and consistently grow carbon dioxide hydrate, particularly in packed media. However, there are limited kinetic data for carbon dioxide hydrates at this length scale. In this work, carbon dioxide hydrate propagation rates and conversion were evaluated in a high pressure silicon microfluidic device. The carbon dioxide phase boundary was first measured in the microfluidic device, which showed little deviation from bulk predictions. Additionally, measuring the phase boundary takes on the order of hours compared to weeks or longer for larger scale experimental setups. Next, propagation rates of carbon dioxide hydrate were measured in the channels at low subcoolings (<2 K from phase boundary) and moderate pressures (200-500 psi). Growth was dominated by mass transfer limitations until a critical pressure was reached, and reaction kinetics limited growth upon further increases in pressure. Additionally, hydrate conversion was estimated from Raman spectroscopy in the microfluidics channels. A maximum value of 47% conversion was reached within 1 h of a constant flow experiment, nearly 4% of the time required for similar results in a large scale system. The rapid reaction times and high throughput allowed by high pressure microfluidics provide a new way for carbon dioxide gas hydrate to be characterized.

Contactless probing of polycrystalline methane hydrate at pore scale suggests weaker tensile properties than thought

Methane hydrate is widely distributed in the pores of marine sediments or permafrost soils, contributing to their mechanical properties. Yet the tensile properties of the hydrate at pore scales remain almost completely unknown, notably the influence of grain size on its own cohesion. Here we grow thin films of the hydrate in glass capillaries. Using a novel, contactless thermal method to apply stress, and video microscopy to observe the strain, we estimate the tensile elastic modulus and strength. Ductile and brittle characteristics are both found, dependent on sample thickness and texture, which are controlled by supercooling with respect to the dissociation temperature and by ageing. Relating the data to the literature suggests the cohesive strength of methane hydrate was so far significantly overestimated.

The authors here report tensile properties of polycrystalline methane hydrate at the micron scale by applying a contactless, thermos-induced stress to a tenuous shell of hydrate grown in a thin glass capillary. The results suggest that the cohesive strength of methane hydrate in marine settings may be an order of magnitude less than currently thought.

Gas Hydrate Crystallization in Thin Glass Capillaries: Roles of Supercooling and Wettability

We designed and implemented an experimental methodology to investigate gas hydrate formation and growth around a water-guest meniscus in a thin glass capillary, thus mimicking pore-scale processes in sediments. The glass capillary acts as a high-pressure optical cell in a range of supercooling conditions from 0.1 °C, i.e., very close to hydrate dissociation conditions, to ∼35 °C, very near the metastability limit. Liquid or gaseous CO₂ is the guest phase in most of the experiments reported in this paper, and N₂ in a few of them. The setup affords detailed microscopic observation of the roles of the key parameters on hydrate growth and interaction with the substrate: supercooling and substrate wettability. At low supercooling (less than 0.5 °C), a novel hydrate growth process is discovered, which consists of a hollow crystal originating from the meniscus and advancing on the guest side along the glass, fed by a thick water layer sandwiched between the glass and this crystal.

Morphology Investigation on Cyclopentane Hydrate Formation/Dissociation in a Sub-Millimeter-Sized Capillary

The formation, dissociation, and reformation of cyclopentane (CP) hydrate in a sub-millimeter-sized capillary were conducted in this work, and the morphology of CP hydrate was obtained during above processes, respectively. The influences of the supercooling degree, i.e., the hydrate formation driving force, on CP hydrate crystals’ aspect and growth rate were also investigated. The results demonstrate that CP forms hydrate with the water melting from ice at the interface between the CP and melting water at a temperature slightly above 273.15 K. With the action of hydrate memory effect, the CP hydrate in the capillary starts forming at the CP-water interface or CP–water–capillary three-phase junction and grows around the CP–water interface. The appearance and growth rate of CP hydrate are greatly influenced by the supercooling degree. It indicates that CP hydrate has a high aggregation degree and good regularity at a high supercooling degree (or a low formation temperature). The growth rate of CP hydrate crystals greatly increases with the supercooling degree. Consequently, the temperature has a significant influence on the formation of CP hydrate in the capillary. That means the features of CP hydrate crystals in a quiescent system could be determined and controlled by the temperature setting.

An in situ method on kinetics of gas hydrates

Gas hydrate formation is a high-risk and common flow assurance problem in subsea oil production plants. The modern strategies to mitigate hydrate formation have switched from thermodynamic inhibition to risk management. In this new mitigation strategy, hydrate formation is allowed as long as it does not lead to plugging of pipelines. Thus, understanding the growth kinetics of gas hydrates plays a critical role in risk management strategies. Here, we report a new accurate and in situ approach to probe the kinetics of gas hydrate formation. This approach is based on the hot-wire method, which probes the thermal properties of the medium surrounding the hot-wire. As the thermal properties of gas hydrate and its initial constituents are different, variation in these properties is used to probe kinetics of hydrate growth front. Through this in situ method, we determine kinetics of cyclopentane hydrate formation in both mixing and flow conditions. The findings show that at ambient pressure and a temperature of 1-2 °C, the hydrate formation rate under mixing condition varies between 1.9 × 10^-5 and 3.9 × 10^-5 kg m^-2 s^-1, while in flow condition, this growth rate drops to 4.5 × 10^-6 kg m^-2 s^-1. To our knowledge, this is the first reported growth rate of cyclopentane hydrate. This in situ approach allows us to probe kinetics of hydrate formation where there is no optical access and provides a tool to rationally design risk management strategies for subsea infrastructures.