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).
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
27 July 2005
- Meeting paper published:
9 October 2005
- Revised manuscript received:
2 March 2007
- Manuscript approved:
11 March 2007
- Version of record:
20 October 2007
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