Yan Li

The exploitation of natural gas hydrates by the pressure reduction method is affected by the decomposition heat absorption effect, and the range of the formation temperature reduction area is expanding. At the same time, the temperature reduction phenomenon is more significant around the production wells under the influence of gas throttling and expansion effects, and hydrate formation will occur under certain temperature and pressure conditions, leading to blockage of effective seepage channels in the reservoir in the region and elevation of seepage resistance, which may affect the output of hydrate decomposition gas. A numerical simulation model is constructed for the purpose of studying the secondary hydrate generation pattern around the well, analyzing the impact of secondary hydrates around wells on the production capacity, and assessing the effectiveness of prevention methods to inform the actual production of hydrates. The results demonstrate that secondary hydrate is typically formed in the near-well area of the upper part of the production well, and the secondary hydrate around the upper part of the production well is the first to be formed, exhibiting the highest saturation peak and the latest decomposition. The formation of the secondary hydrate can be predicted based on the observed change in temperature and pressure, and the rate of secondary hydrate formation is markedly rapid, whereas the decomposition rate, approximately 0.285 mole/d, is relatively slow. Additionally, the impact of secondary hydrates on cumulative gas production is insignificant, and the effect of secondary hydrates on capacity can be ignored. Hot water injection, wellbore heating, and reservoir reconstruction can effectively eliminate secondary hydrates around the well. Reservoir reconstruction represents a superior approach to the elimination of secondary hydrates, which can effectively enhance production capacity while preventing the generation of secondary hydrates.

Carbon dioxide (CO₂) hydrates have garnered significant interest as a promising technology for CO₂ capture and storage due to its high storage capacity and moderate operating conditions. The kinetics of CO₂ hydrate formation is a critical factor in determining the feasibility of hydrate-based CO₂ capture and storage technologies. This study systematically investigates the promotional effects of the amino acid _L-tryptophan (_L-trp) on CO₂ hydrate formation kinetics and morphology under stirred and unstirred conditions. In the stirred system, experiments were conducted in a high-pressure 100 mL reactor with 0.05, 0.10, and 0.30 wt% _L-trp solution. CO₂ gas uptake kinetics and morphological evolution were monitored using a high-resolution digital camera. Results showed that _L-trp promoted CO₂ hydrate formation kinetics without delay, with rapid CO₂ consumption upon nucleation. Morphological evolution revealed rapid hydrate formation, wall-climbing growth, and dendritic morphology filling the bulk solution. Under unstirred conditions, experiments were performed in a larger 1 L reactor with 0.1 wt% and 0.5 wt% _L-trp solutions to assess the influence of additive concentration on hydrate formation thermodynamics and kinetics. Results demonstrated that _L-trp influenced both thermodynamics and kinetics of CO₂ hydrate formation. Thermodynamically, 0.1 wt% _L-trp resulted in the highest hydrate formation, indicating an optimal concentration for thermodynamic promotion. Kinetically, increasing _L-trp concentration from 0.1 wt% to 0.5 wt% reduced formation time, demonstrating a proportional relationship between _L-trp concentration and formation kinetics. These findings provide insights into the role of _L-trp in promoting CO₂ hydrate formation and the interplay between additive concentration, thermodynamics, and kinetics. The results can inform the development of effective hydrate-based technologies for CO₂ sequestration, highlighting the potential of amino acids as promoters in gas hydrate.

Zircon (ZrSiO₄) provides a good pressure-holding environment for ultra-high-pressure metamorphic minerals during crust exhumation due to its high incompressibility and chemical stability. At high pressure, the zircon can transform to reidite. Previous studies show much higher phase-transition pressures at room temperature than those at high temperature (>1000 K) due to kinetic hindrance. To further investigate the kinetics of the zircon–reidite phase transition at relatively low temperatures, the phase boundary at 298–800 K was determined using a diamond anvil cell combined with in situ Raman spectra. The results show that reidite becomes thermodynamically more stable compared with zircon at 8 GPa at room temperature, and the slope of the phase boundary at 298–800 K abruptly differs from that of previous studies at 1100–1900 K. Compared with the equilibrium phase boundary calculated by the density functional theory, it indicates that the kinetic effect of the zircon–reidite phase transition is obvious, and there exists a sufficiently large energy driving force provided by an overpressure to overcome the activation energy barrier below a critical temperature of approximately 880 K. The temperature dependence of overpressure is about 0.023 GPa/K.