Abstract

Gas hydrates are solid crystalline compounds in which gas molecules are lodged within the lattices of ice crystals. Vast amounts of CH4 are trapped in hydrates, and a significant effort has recently begun to evaluate hydrate deposits as a potential energy source. Class 3 hydrate deposits are characterized by an isolated Hydrate-Bearing Layer (HBL) that is not in contact with any hydrate-free zone of mobile fluids. The base of the HBL in Class 3 deposits may occur within or at the edge of the zone of thermodynamic hydrate stability.

In this numerical study of long-term gas production from typical representatives of unfractured Class 3 deposits, we determine that simple thermal stimulation appears to be a slow and inefficient production method. Electrical heating and warm water injection result in very low production rates (4 and 12 MSCFD, respectively) that are orders of magnitude lower than generally acceptable standards of commercial viability of gas production from oceanic reservoirs. However, production from depressurization-based dissociation based on a constant well pressure appears to be a promising approach even in deposits characterized by high hydrate saturations. This approach allows the production of very large volumes of hydrate-originating gas at high rates (> 15 MMSCFD, with a long-term average of about 8.1 MMSCFD for the reference case) for long times using conventional technology. Gas production from hydrates is accompanied by a significant production of water. However, unlike conventional gas reservoirs, the water production rate declines with time. The low salinity of the produced water may require care in its disposal.

Because of the overwhelming advantage of depressurization- based methods, the sensitivity analysis was not extended to thermal stimulation methods. The simulation results indicate that depressurization-induced gas production from oceanic Class 3 deposits increases (and the corresponding waterto- gas ratio decreases) with increasing hydrate temperature (which defines the hydrate stability), increasing intrinsic permeability of the HBL, and decreasing hydrate saturation- although depletion of the hydrate may complicate the picture in the latter case.

Introduction
Background.

Gas hydrates are ice-like solid crystalline compounds in which gas molecules ("guests") are lodged within the lattices of ice crystals ("hosts"). Under suitable conditions of low temperature T and high pressure P, the hydration reaction of a gas G is described by the general equation: G + NH H2O = G+NH H2O (1) where NH is the hydration number. Hydrate deposits occur in two distinctly different hydrogeologic settings: in the permafrost and in deep ocean sediments.

While there has been no systematic effort to map and evaluate the size of this resource (and current estimates vary widely, ranging between 1015 to 1018 m3), the consensus is that the worldwide quantity of gas hydrates is vast. Gas hydrates are predicted to contain at least twice as much energy, even by the most conservative estimate, as all of the total known fossil fuel reserves recoverable by current methods. Thus, the attractiveness of hydrates, augmented by the environmental desirability of gas (as opposed to solid and liquid) fuels, is undeniable.

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