Analytical Modelling of Gas Production From Hydrates in Porous Media

Experimental Investigation into Dissociation Characteristics of Methane Hydrate in Sediments with Different Contents of Montmorillonite Clay

The characteristics of gas production in sediments are crucial to the safe and efficient exploitation of gas hydrate resources. However, research on methane hydrate dissociation in these sediments, particularly in silty-clayey sediments, which are commonly found in nature, remains limited and contains significant gaps. To address this, a series of depressurization experiments were conducted to investigate the dissociation behavior of methane hydrate in silty-clayey sediments with montmorillonite contents ranging from 0 to 20 wt %. The results indicate that montmorillonite significantly inhibits methane hydrate dissociation. When the montmorillonite content increases from 10 to 20 wt %, the average dissociation rate of methane hydrate decreases by approximately 47%-78% compared to sandy sediments. An excess temperature drop of around 0.13 to 0.40 K was observed in the depressurization process as the montmorillonite content increased from 10 to 20 wt %. Methane hydrate dissociates unevenly in montmorillonite clay-bearing sediments due to the nonuniform distribution of the methane hydrate, coupled with the low thermal conductivity and high-water absorption capacity of montmorillonite, which restrict the supply of extra heat. The electrical resistance changes further reveal that the increased bound water content in clayey sediments reduces the impact of water fluctuation on the resistivity changes. Consequently, the resistivity changes in sandy sediments are more pronounced compared to silty-clayey sediments. These findings provide valuable insights for optimizing methane hydrate production technology via depressurization.

Numerical Simulations of Decomposition of Hydrate Particles in Flowing Water Considering the Coupling of Intrinsic Kinetics with Mass and Heat Transfer Rates

During the hydrate exploitation in a shallow marine layer by the mechanical crushing, the hydrate particle decomposition in a wellbore is one of the most concerning problems. In this research, a hydrate dynamic decomposition model coupling intrinsic kinetics with mass and heat transfer rates was established. The model can simulate the hydrate particle decomposition process in flowing water. By comparison, the model calculated results are in good agreement with the measured values. The numerical simulation results show that hydrate decomposition is a non-isothermal process. In the early stage, the hydrate decomposition rate mainly depends on the heat transfer rate. However, it is mainly affected by the hydrate intrinsic kinetics in the late stage. In contrast, the mass transfer rate has little effect on it during the whole decomposition process. By analyzing the influence of sensitivity parameters, it can be found that the activation energy has an important impact on the hydrate decomposition rate, and the hydrate decomposition rate constant decreases significantly at E/R > 9000 K. Increasing the water flowing rate is beneficial to the dissolution of hydrates. System temperature and pressure are two significant factors that directly affect the hydrate decomposition rate, and increasing the temperature or reducing the pressure can effectively increase the hydrate decomposition rate.

Numerical Analysis of Soil Deformation and Collapse Due to Hydrate Decomposition

Natural gas hydrates are an ideal alternative clean energy source. Many countries are currently attempting the trial production of gas hydrates. Japan became the first country to achieve offshore hydrate trial production in 2013, and China conducted 60 days of continuous trial exploitation in 2017. This study analyzes the changes in the internal stress of the hydrate zone and hydrate saturation of the soil throughout the monitoring period and calculates the failure stress of the hydrate deposit layer. The Mohr-Coulomb model is used to simulate Japan's test exploitation conditions to verify the feasibility of the method. Finally, the hydrate decomposition range, the difference in the soil dip angle in the test exploitation area, and the bearing capacity of the hydrate reservoir are numerically simulated to evaluate the stability of the soil. Through the sensitivity analysis of the hydrate decomposition range and the inclination angle of the hydrate sediment layer, it can be found that the hydrate decomposition range has the greatest impact on the deformation, and the soil around the decomposition area may be sheared and collapsed. Within 1 week of decompression and exploitation, the hydrate decomposition radius is approximately 3 m. When the inclination angle increases from 3° to 9°, the sediment deformation increases by 12 times. Therefore, it is necessary to pay attention to the critical value of the decomposition range during the exploitation process.

The influence of porosity and structural parameters on different kinds of gas hydrate dissociation

Methane hydrate dissociation at negative temperatures was studied experimentally for different artificial and natural samples, differing by macro- and micro-structural parameters. Four characteristic dissociation types are discussed in the paper. The internal kinetics of artificial granule gas hydrates and clathrate hydrates in coal is dependent on the porosity, defectiveness and gas filtration rate. The density of pores distribution in the crust of formed ice decreases by the several orders of magnitude and this change significantly the rate of decay. Existing models for describing dissociation at negative temperatures do not take into account the structural parameters of samples. The dissociation is regulated by internal physical processes that must be considered in the simulation. Non-isothermal dissociation with constant external heat flux was simulated numerically. The dissociation is simulated with consideration of heat and mass transfer, kinetics of phase transformation and gas filtering through a porous medium of granules for the negative temperatures. It is shown that the gas hydrate dissociation in the presence of mainly microporous structures is fundamentally different from the disintegration of gas hydrates containing meso and macropores.

A Clathrate Reservoir Hypothesis for Enceladus' South Polar Plume

We hypothesize that active tectonic processes in the south polar terrain of Enceladus, the 500-kilometer-diameter moon of Saturn, are creating fractures that cause degassing of a clathrate reservoir to produce the plume documented by the instruments on the Cassini spacecraft. Advection of gas and ice transports energy, supplied at depth as latent heat of clathrate decomposition, to shallower levels, where it reappears as latent heat of condensation of ice. The plume itself, which has a discharge rate comparable to Old Faithful Geyser in Yellowstone National Park, probably represents small leaks from this massive advective system.