Summary

Class 1 hydrate deposits are characterized by a hydrate-bearing layer underlain by a two-phase zone involving mobile gas. Two kinds of deposits are investigated. The first involves water and hydrate in the hydrate zone (Class 1W), while the second involves gas and hydrate (Class 1G). We introduce new models to describe the effect of the presence of hydrates on the wettability properties of porous media. We determine that large volumes of gas can be readily produced at high rates for long times from Class 1 gas-hydrate accumulations by means of depressurization-induced dissociation using conventional technology. Dissociation in Class 1W deposits proceeds in distinct stages, while it is continuous in Class 1G deposits. To avoid blockage caused by hydrate formation in the vicinity of the well, wellbore heating is a necessity in production from Class 1 hydrates. Class 1W hydrates are shown to contribute up to 65% of the production rate and up to 45% of the cumulative volume of produced gas; the corresponding numbers for Class 1G hydrates are 75% and 54%. Production from both Class 1W and Class 1G deposits leads to the emergence of a second dissociation front (in addition to the original ascending hydrate interface) that forms at the top of the hydrate interval and advances downward. In both kinds of deposits, capillary pressure effects lead to hydrate lensing (i.e., the emergence of distinct banded structures of alternating high/low hydrate saturation, which form channels and shells and have a significant effect on production).

Introduction
Background.

Gas hydrates are solid crystalline compounds in which gas molecules (referred to as guests) are lodged within the lattices of ice crystals (called hosts).

Gas-hydrate deposits occur in two distinctly different geologic settings where the necessary favorable thermodynamic conditions exist for their formation and stability: in the permafrost and in deep ocean sediments. Because of different formation processes, these two types of accumulations have distinctly different attributes.

Although there has been no systematic effort to map and evaluate this resource, and current estimates vary widely the consensus is that the worldwide quantity of hydrocarbon-gas hydrates is vast (Sloan 1998). Even the most conservative estimate surpasses by a factor of two the energy content of the total fossil-fuel reserves recoverable by conventional methods. The sheer magnitude of this resource commands attention as a potential energy resource, even if only a limited number of hydrate deposits are attractive production targets and/or only a fraction of the trapped gas may be recoverable. As current energy economics make gas production from unconventional resources increasingly appealing (or, at a minimum, less prohibitive), the potential of hydrate accumulations clearly demands technical and economic evaluation. The attractiveness of hydrates is further augmented by the environmental desirability of gas (as opposed to solid and liquid) fuels.

Gas from hydrates is produced by inducing dissociation by one of the following three main methods (Sloan 1998) (or combinations thereof):

  1. depressurization, which involves pressure lowering below the equilibrium hydration pressure at the prevailing temperature;

  2. thermal stimulation, in which the temperature is raised above the equilibrium hydration temperature at the prevailing pressure; and

  3. the use of hydration inhibitors (such as salts and alcohols).

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