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A GLOBAL OUTLOOK ON METHANE GAS HYDRATES
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et al.
(2011) and Rutqvist and Moridis (2012) have reviewed
first-order assessment of the first four years of production
and the associated geomechanical response for hypothetical
marine and permafrost gas hydrate deposits. As shown in
Figure 3.6, these simulations suggest substantive changes
in water and gas production over time, as well as significant
surface displacement.
A critical element in the field production testing phase is the
detailed evaluation and monitoring of associated environ-
mental impacts. This evaluation will require study of base-
line conditions within the environment prior to the test, as
well as monitoring of any changes in these conditions during
and after the test (Fujii
et al.
2012; Nagakubo
et al.
2011). Pa-
rameters that will be monitored include the impact of subsid-
ence or other geomechanical instability and possible release
of methane or other substances into the ocean or atmos-
phere. An important environmental issue is the impact of
the release of colder, anoxic, and low-salinity water (originat-
ing from the dissociation of marine hydrates) near the ocean
floor, with potentially significant consequences for chemos-
ynthetic communities there (Moridis and Reagan 2007a, b).
3.4.6
Potential for extending
production beyond sand-dominated
gas hydrate reservoirs
At this time, only gas hydrate deposits in which the hydrate
occurs as a pore-fill within clay-poor sediments of high per-
meability are seen as well suited to sustained production with
currently available technologies employed for production from
conventional oil and gas resources. However, gas hydrates in
such reservoirs are likely to represent only a small fraction of
the global gas hydrate inventory. The bulk of global gas hydrate
occurrences probably consists of dispersed, low-concentration
gas hydrate accumulations (perhaps occupying five per cent
or less of sediment pore space) in fine-grained marine sedi-
ments. These are unlikely to be candidates for commercial
production due to low resource density, limited permeability,
and low sediment strength (Moridis and Sloan 2007).
Recent drilling investigations carried out in offshore India
(Collett
et al.
2008) and Korea (Park 2008), however, have
identified thick sedimentary sections containing a variety of
macroscopic gas hydrate forms, including fracture fillings
and nodules. Gas hydrate concentrations in these marine
settings can be in the range of 20 to 40 per cent of bulk sedi-
ment pore space, making them plausible production candi-
dates if significant geomechanical challenges can be over-
come (Moridis
et al.
2013). In addition, highly concentrated
gas hydrate occurrences associated with cold vent features
have been observed within 100 metres of the seabed in sev-
eral offshore locations, including offshore Korea (Bahk
et
al.
2009), the Cascadia margin (Riedel
et al.
2006a, b), and
the Gulf of Mexico (Sassen
et al.
2001). While such depos-
its may hold potential as future production targets, they are
not suited to conventional oil and gas production methods.
Thus, it is likely that new technologies and approaches will
be required to achieve economic production of gas hydrates
in fine-grained, unconsolidated marine sediments.