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FROZEN HEAT
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Figure 2.2:
Permeability of gas-hydrate host
sediments. Right: The most promising
gas hydrate occurrences are distinguished
primarily by the nature of the enclosing
sediment (after Boswell
et al.
2011). The
high initial permeability of sand-rich units
enables gas hydrates to accumulate to
high saturations. After gas hydrate is
emplaced measurable permeability is
maintained in the sediments sufficient
to enable existing well-based extraction
technologies. Left: Gas hydrates limited to
thin sand intervals in cores obtained from
the Ulleung basin, Korea, in 2010 (from
Bahk
et al.
2011b with permission).
The criteria that determine the attractiveness of a gas hy-
drate accumulation for production are similar to those ap-
plied in other hydrocarbon-rich basins around the world. An
extensive discussion on the subject can be found in Moridis
et al.
(2009). Initial production will likely target large and
richly concentrated occurrences that can be produced safely
and at rates that make the venture profitable. It is not suffi-
cient that the in-place resource simply be there. There must
also be a way to extract the gas that is economically viable,
safe, and environmentally responsible.
As discussed in Chapter 3, the cumulative results of work in
the field, in the lab, and through application of numerical
models – conducted mainly within the past decade – indicate
that the richest gas hydrate occurrences discovered to date,
as well as those most likely to support economically viable
production of methane, are found in sand-rich sediments
(Fig. 2.2). Production would proceed through specially-tai-
lored applications of well-drilling technologies used in con-
ventional oil and gas production. While it appears that, glob-
ally, most gas hydrates are housed in clays, assessments in
the highest-studied areas (the Alaska North Slope, the Gulf
of Mexico, and offshore southeastern Japan) show large po-
tential resources in sand-rich deposits (Boswell and Collett,
2011). What makes sand reservoirs attractive is the sedi-
ment’s high natural permeability, a measure of the ease with
which fluid can flow. This permeability is key to enabling
gas hydrates to accumulate at high concentrations. Once hy-
drates form, that permeability is dramatically reduced, but
it is still sufficient to allow pressure (and even advection-
based thermal) changes to be delivered into a reservoir from
a drilled well.
2.2
WHAT ARE THE MOST PROMISING
ACCUMULATIONS FOR PRODUCTION?