Controlled Growth of Polycrystalline Ethane Hydrate and Inferences for Gas Recovery from Hydrate Deposits

Abstract

Abstract Profuse formation of hydrate shells on gas bubbles and nucleation of at least seven different morphologies or species of gas hydrate have been achieved in natural seawater. By using newly collected natural seawater, which is a chemically and biologically complex fluid, our experiments in pressurized, refrigerated vessels closely track the growth mechanisms of gas hydrate in oceanic seafloors. Nearly complete pore fill by hydrate can best be accomplished by nucleation of hydrate at point sources within the pore water or on sediment particulate, with growth outward into water space that is refreshed with circulating pore water having high concentrations of hydrate forming gas. Recovery of natural gas from hydrate deposits will depend on ensuring a large enough surface area of hydrate exposed to the circulating pore water. Fracking may be required. Introduction Conditions governing the nucleation and growth of gas hydrates, particularly the abundant natural gas hydrate found in oceanic and permafrost environments (Max 2003), are imperfectly known. Formation of gas hydrate in fresh water or in water that has been "salted" to yield sea water salinity analogues under laboratory conditions has been observed to be much slower and accomplished often with more difficulty than has been observed to take place in natural seawater (Brewer et al. 1998; Sloan 1996), where nucleation and growth appear to be spontaneous (Brewer et al. 1997). Seawater is a chemically complex fluid containing substantial organic, inorganic, and living material. The influence of each of these naturally occurring constituents and their combined effect on nucleation and growth of gas hydrate under different conditions of pressure and temperature is not yet well understood. Where Hydrate Forming Gas (HFG) is bubbled into seawater under essentially isobaric conditions, such as from seafloor seeps, much of the gas (mainly methane) dissolves in the seawater and hydrate shells can form on the gas bubbles where the seawater becomes saturated with HFG. These bubbles naturally rise buoyantly into warmer and shallower water where hydrate is no longer stable. Upon hydrate dissociation, the gas may largely dissolve in seawater rather than reaching the atmosphere, especially where the sea floor seeps are not unusually large. Additionally, when the hydrateshelled bubbles are brought into conditions of reduced ambient pressure the shells may fracture due to expansion of the HFG. Where bubbles fracture where hydrate is stable, the gas will dissolve until local saturation conditions are reached. Large masses of solid hydrate have been observed at a number of locations on the seafloor near natural gas seeps in the northern Gulf of Mexico (Sassen et al. 2001) and on hydrate ridge on the U.S. Cascadia margin (Seuss et al. 2001), amongst other places. These are held to the sea floor by intergrowth with sea floor sediments. The large masses of natural polycrystalline hydrate are associated with profuse gas seeps and conditions of local natural gas saturation of the seawater. It is most likely that the large masses of solid hydrate have been produced by hydrate growing outward into water space surrounding the hydrate where substantial reactant (HFG) is dissolved in the seawater.