A preserved sample of hydrate-bearing sandstone from the Mount Elbert Test Well was dissociated by depressurization while monitoring the internal temperature of the sample in two locations and the density changes at high spatial resolution using x-ray CT scanning. The sample contained two distinct regions having different porosity and grain size distributions. The hydrate dissociation occurred initially throughout the sample as a result of depressing the pressure below the stability pressure. This initial stage reduced the temperature to the equilibrium point, which was maintained above the ice point. After that, dissociation occurred from the outside in as a result of heat transfer from the controlled temperature bath surrounding the pressure vessel. Numerical modeling of the test using TOUGH+HYDRATE yielded a gas production curve that closely matches the experimentally measured curve.
R ecent studies have concluded that methane hydrate (hereafter hydrate), a naturally occurring clathrate compound consisting of molecular water cages surrounding gas molecules (primarily methane), contains large quantities of methane in shallow sediments throughout the world's coastal margins and polar regions [Boswell et al., 2010; Milkov, 2004; Sloan et al., 1998]. Each cubic meter of hydrate can hold approximately 160 m3 of natural gas at standard temperature and pressure [National Resource Council, 2004]. Even though natural gas from hydrate shows great promise as an energy resource, scientific and engineering questions remain regarding producing gas from hydrates. Three methods are typically considered to produce gas from hydrate: depressurization, where the system pressure is lowered below the hydrate stability pressure; thermal stimulation, where the hydrate-bearing sediments are heated to temperatures in excess of the hydrate stability temperature, and the use of inhibitors such as sodium chloride or alcohols that modify the hydrate stability conditions to destabilize the hydrate [Sloan and Koh, 2008]. Of these techniques, heating the hydrate-bearing sediments and injecting expensive alcohols or corrosive brines may have limited applications, but will often be impractical, particularly considering the size of reservoirs and the lack of control that can be exerted to directly apply the heat or inhibitor to the location needed. Depressurization is expected to be effective in many situations, however the hydrate-bearing sediments must be well enough confined by low permeability strata for the depressurization to be effective.