Study on the application and mechanism of enhanced methane recovery from hydrate

Energy is a key element for human survival and development. The current dominant primary energy sources worldwide are natural gas, oil, and coal, the supply of which is dwindling and causing environmental problems. Highly efficient and clean energy sources are of great importance for sustainable development.

Natural gas hydrates, the main component of which is methane, have received growing attention in the global energy system due to their abundance in nature and CO2 neutrality, if properly extracted with CO2 injection, compared to conventional fossil fuels. Although discovered in 1883, extensive research was only initiated during the last 50 years by scientists from the micro to macro scales.

Major gas hydrates exploitation methods include chemical injection, thermal stimulation, pressure reduction, and CO2 replacement. Each method has advantages and disadvantages, but the depressurization and CO2 replacement methods show relative superiority compared to the others. The combination of depressurization and CO2 replacement shows a higher recovery rate and efficiency.

At the present research stage, the replacement gas employed in the laboratory studies varies from pure CO2 (either in liquid or gaseous form) to simulated flue gas (a CO2 and N2 gas mixture). Limited studies on hydrate production with the injection of air, which is cheap and abundant, have been conducted.

Many factors of the swapping recovery process have been considered, but the hydrate decomposition mechanism behind these factors is complex and challenging to elucidate. To investigate the effects of certain factors on the depressurization production process, three groups of spherical methane hydrate samples with variant diameters of 11mm, 17mm, and 22mm were prepared to simulate hydrate particles macroscopically.

Each sample group has approximately the same overall volume of 8980 mm3. Hydrate decomposition starts with an initial pressure of 6.1 to 6.4 MPa and ends at a final constant pressure between 1.6 MPa and 2.4 MPa. The results show that the effect of depressurization is significant on the methane recovery ratio, while the effect of the surface-area-to-mass ratio is less significant.

During the hydrate decomposition process, the methane production rate increases with increasing operating pressure and surface-area-to-mass ratio. The methane decomposition is jointly governed by two processes: 1) the dissociation process, in which methane molecules leave the hydrate cage, which is controlled by the pressure difference between the equilibrium pressure and the system pressure; and 2) the gas diffusion process, in which methane molecules travel from the hydrate surface through the ice layer.

The experiments show that the gas production process can be divided into three main periods: excess gas release, fast hydrate production, and slow hydrate production. The production rate at low operation pressure is rapid due to the initially prevailing pressure driving force control, whereas gas diffusion with ice coverage on various pellet sizes becomes more dominant at higher operating pressure.

In addition, the experimental results indicate that hydrate decomposition is time-dependent. Initial ice nucleation and conglomeration play an important role in the hydrate decomposition rate. Based on the depressurization investigation, a series of experiments were conducted aiming to obtain a new method for improved recovery by the combination of the depressurization and gas replacement methods, in which air/CO2-enriched air was injected into an artificial multilayer hydrate sediment at pressures ranging from 8.5 to 18.7 MPa.

The recovery efficiency was investigated using a method combining three-stage depressurization assisted with CO2-enriched air injection. The initial production pressure was found to have a positive effect on the recovery of methane via injecting of air, with an opposite influence via injecting CO2-enriched air.

Compared with injecting air, injecting CO2-enriched air promotes the performance of gas hydrate production with up to a 74.4 % recovery ratio. A novel multilayer hydrate cap mechanism is therefore proposed to describe the improved efficiency during the replacement-depressurization process for the first time.

The multilayer hydrate cap and its composition are largely dependent on the initial condition of injected gas, thereby causing limited recovery efficiency. The results obtained from this study are beneficial for the future optimization of operating conditions to maximize efficiency and develop planning for natural gas hydrate resources.

To further explain the experimental results and study the mechanism of methane hydrate decomposition behavior, a molecular dynamics simulation method was performed under one-step and multi-step depressurization processes with NVT ensemble. The influence of temperature was also examined. The effect of hydrate structural properties on the decomposition process was theoretically investigated, including configuration, potential energy, the radial distribution function (RDF), the F4 order parameter, mean square displacement (MSD), and the diffusion coefficient.

MSDs and RDFs showed similar behaviors in line with increasing temperature, which can reduce hydrate stability. A sudden decrease in potential energy was observed for one-step depressurization during simulation times ranging from 1.5 ns to 3 ns. The F4 order parameter confirmed the tendency for the regeneration of hydrates during this period.

The diffusion coefficient can also be improved by an increase in temperature. Multi-step depressurization compensates for energy loss by including the released methane molecules dissolved in the liquid water phase, thus breaking the tendency for hydrate reformation during decomposition. The application of multi-step depressurization in molecular simulation can provide significant insights for on-field hydrate resource exploitation and help to understand the mechanisms behind hydrate production at the molecular scale.