Extraction of Methane from Oceanic Hydrate System Deposits

Abstract

Abstract No one to date has outlined an economic and safe method for sustained recovery of methane from the newly discovered low-grade oceanic hydrate and associated gas deposits. The potential rewards of unlocking the methane hydrate energy bank are potentially very great. The prize is possibly centuries of energy independence for some industrial states including the United States and Japan, and developing countries such as India, which appear to have considerable deposits of methane hydrate immediately adjacent to their landmasses. A number of nations, including the United States, have established, or are considering establishing, national hydrate research programs. Introduction Oceanic hydrate system deposits, which include both methane hydrate and associated methane gas, are very large, but relatively low grade when compared with conventional hydrocarbon deposits. They differ in character from conventional hydrocarbon deposits in almost every respect except that they encompass significant concentrations of hydrocarbon. It is now not immediately obvious in detail exactly how the methane from oceanic hydrate and related gas deposits will be recovered. It is not even certain whether hydrate or associated gas, or both, will be the preferred initial and eventual best economic target. We examine some of the emerging issues likely to govern hydrate recovery and seafloor stability, and suggest geological models for oceanic hydrate system exploitation. Methane hydrate; its disposition and recognition Methane hydrates(CH4?6.1(±0.1%)H2O] are found in the low temperature-low pressure regimes of permafrost regions and high pressure-moderate temperature (from just below zero degrees C up to about 35 °C max) in ocean sediments. Methane hydrate is stable in seafloors below about 450 meters water depth in open ocean with an average temperate hydrothermal profile that gives hydrate a wide pressure-temperature field of stability1. Methane molecules are compressed closely together in the hydrate lattice. 1 m3 of hydrate yields about 160 m3 CH4 at STP and a residue of 0.87 H2O m32. The hydrate forms in a zone of thermodynamic equilibrium, the Hydrate Stability Zone (HSZ) that extends downward from the seafloor to some depth determined by increasing temperature (Fig. 1). The base of the HSZ is a phase boundary. At constant geothermal gradients the thickness of the hydrate stability zone increases with increasing water depth and increased pressure. Where higher molecular weight thermogenic gases, such as ethane, butane, or propane occur, the hydrate stability field expands considerably. 1% propane in the gas mixture, for instance, can reduce the pressure at which the hydrate forms by nearly 40%1. Figure 1. Position of the HSZ with respect to the hydro-and geothermal gradients and the methane hydrate phase boundary.(Available in full paper)