Simulation of Depressurization for Gas Production From Gas Hydrate Reservoirs

Mathematical Modeling of the Dynamic Temperature Profile in Geothermal-Energy-Heated Natural Gas Hydrate Reservoirs

An analytical model was developed in this study for predicting the dynamic temperature profile in natural gas hydrate (NGH) reservoirs that receive heat energy from a geothermal layer for accelerating gas production. The analytical model was validated by a comparison of its result to the result given by a numerical model. The expression of the analytical model shows that, for a given system, the heat transfer is proportional to the mass flow rate and the temperature drop along the heat dissipator wellbore. Applying the analytical model to the NGH reservoir in the Shenhu area, Northern South China Sea, allowed for predicting the dynamic temperature profile in the NGH reservoir. The model result reveals that the NGH reservoir temperature should increase quickly at any heat-affected point, but it should propagate slowly in the radial direction. It should take more than two years to dissociate NGH within 20 m of the heat dissipator wellbore due to only thermal stimulation. Therefore, the geo-thermal stimulation method should be used as a technique for accelerating gas production with a depressurization scheme. The formation of gas phase due to the NGH dissociation should reduce the thermal conductivity of the NGH reservoir, while the water phase that dropped out from the dissociation should increase the thermal conductivity. The resultant effect should be investigated in the future in laboratories and/or numerical simulation of the dynamic water-gas two-phase flow coupled with a heat–transfer mechanism.

Fluctuation-dissipation analysis of nonequilibrium thermal transport at the hydrate dissociation interface

Herein, a nonequilibrium molecular-dynamics simulation in an NVE ensemble was performed to investigate the spontaneous dissociation of methane-hydrate when it came in contact with liquid water. The nonequilibrium in the interface region is linked to the dissociation process of the hydrate near the interface according to the Onsager's hypothesis. The simulated thickness of the interface was found to be close to the acoustic phonon mean path of methane hydrate and agreed with the reference value. The normalized heat flow autocorrelation function was introduced to study fluctuation-dissipation in terms of the thickness and moving velocity of the interface and the Stefan number. This helped to clearly identify three distinct hydrate-decomposition regimes dominated by sensible heat, latent heat and an intrinsically unstable lattice framework. It was found that the fluctuation-dissipation theory could express the nonequilibrium nature in the front two stages before the threshold was reached, and the dissociation rate increased in the latter stage; this was different from the case of thermal-driven dissociation. The Stefan number decreased rapidly with dissociation in the initial stage and then fluctuated in the intermediate stage; this was analogous to the fluctuation characteristics of the heat flow autocorrelation function. The Stefan number effect shows that thermal dissipation drives the hydrate dissociation and correlates fluctuation to the nonequilibrium nature. It was also found that a small Stefan number was enough to break up the residual hydrate soon after the threshold was achieved. The transient interfacial thermal resistance of the interfacial region was obtained as a typical value in the range of 10^-7-10^-9 m² K W^-1. This justifies that fluctuation-dissipation exists in the nonequilibrium process of hydrate dissociation either in terms of heat flux, as observed in this study, or the diffusion of guest molecules, as reported in other studies.

Research progress on methane production from natural gas hydrates

A review of the research on methane production from gas hydrates, including the research on the characteristics of gas hydrate reservoirs, production methods, numerical simulations and field production tests.