Multiscale Laboratory Verification of Depressurization for Production of Sedimentary Methane Hydrates

Summary This study reviews how production of methane from hydrates can be triggered by dissociation of the hydrate structure. Techniques leading to dissociation of hydrates are summarized by pressure depletion, thermal stimulation, and injection of inhibitors. Depressurization is considered to be the most-cost-effective method and is easily implemented in gas reservoirs with overlying hydrate layers. Examples and status of pressure-depletion tests on field scale will be reviewed. In hydrate reservoirs not adjacent to gas zones, the success of pressure depletion is dependent on sufficient permeability to allow for pressure perturbations to reach within the hydrate reservoir and to allow for flow of dissociated gas. This effect has been investigated in this paper by performing controlled pressure depletions on hydrate-filled sandstone cores. Dissociation pressures at given temperatures have been quantified as well as recovery of methane as a function of pressure decrements lower than dissociation pressure. Hydrate dissociation was found to take place over a range of pressure values because of salinity changes in the water phase. A 2D porous silicon-wafer micromodel has been used to gain insight into the mechanisms of hydrate dissociation. Direct visualization of hydrate melting induced by both depressurization and heating is reported from pores replicating authentic sandstone pores. Thermal stimulation led to a more-uniform hydrate melting compared with pressure depletion, and depressurization was most effective when the hydrate was in direct contact with gas bubbles.

Effect of Pd/Mn Substitution on Characteristics of Amorphous Zr-Ni-Ti-V Alloy Hydride Electrodes

ABSTRACTHydrogenation properties of some amorphous Zr-Ni-Ti-V based alloys were investigated. Pressure-composition(P-C) isotherms and hydrogen storage capacities at room temperatures were measured and effects of elemental substitution of the components with Pd or Mn were studied. The alloy electrodes were prepared by using amorphous (Zr-Ni-Ti-V)-(Pd,Mn) alloys prepared by the melt spinning method. The amorphous alloys in the electrode kept their amorphous structures during cycles of charge and discharge. The electrochemical hydrogen storage capacities were strongly affected by the substitution amounts of Pd or Mn. Even a small amount of substitution, changed the equilibrium dissociation pressures of the alloy. In the present study, the rechargeable capacity was optimized up to H/M=0.5 for the alloy electrode with the composition of (Zr45Ni30Ti25)-3at%Pd. The slope in the P-C isotherm suggested that the maximum H/M of the alloy would exceed 1.0 at higher hydrogen pressure than 1.0 MPa, however, the wide distribution of hydrogen site energy in the amorphous hydride resulted in extremely large slope in P-C isotherms, and consequently restricted the rechargeable capacities of the electrodes.

Spatial distribution of air molecules within individual clathrate hydrates in polar ice sheets

We measured the N2/O2 ratios in clathrate hydrate crystals from Vostok Antarctic ice cores using Raman spectroscopy in order to investigate the spatial distribution of air molecules within a crystal. The results showed that the pattern of the spatial distribution of air molecules in clathrate hydrate depends on the crystal. Some clathrate hydrates have inhomogeneous distributions of the N2/O2 ratio within the crystals, while others are practically homogeneous. The spatial distribution of air molecules within an individual clathrate hydrate changes with time due to three processes: (1) the initial selective enclathration caused by the difference between the dissociation pressures of pure N2- and O2–clathrate hydrates, (2) the diffusive mass transfer of air molecules from surrounding air bubbles through the ice matrix, and (3) diffusion of air molecules in the clathrate hydrate crystal. The dissociation pressures and the diffusion rates of air molecules in ice and clathrate hydrate strongly depend on temperature. Therefore, it is concluded that the pattern of the spatial distribution of air molecules in clathrate hydrate is mainly determined by the depth at which they formed and the temperature in the ice sheet.