Minerals Report - International Seabed Authority

6. Harvesting methane hydrates -Some Ideas
The common pool of knowledge acquired to date suggests that
methane hydrates, existing in sediments up to a kilometre or so below the
sea floor, have the potential of becoming a major resource for alternate
energy. Recovering methane from these deposits and transporting it to
shore at a reasonable cost, pose a challenge to technologists and scientists.
Ideas have been conceptualised and research mounted that address these
challenges.
6.1. Production and recovery of methane
The key problem regarding production of methane from the
hydrate layer is 'mobilization' of methane from solid hydrates, i.e.
dissociation of in situ hydrates. Dissociated gas hydrate can serve as one
source of methane; the free methane below the gas hydrate zone is another
possible source. For dissociation of methane hydrates three processes have
been proposed: thermal stimulation, depressurisation, and inhibitor
injection 2,3,60 .
In the thermal stimulation process, thermal energy can be released
into the methane hydrate bearing strata in order to increase the local
temperature enough to cause the gas to dissociate. This process has a
favourable net energy balance as the heat energy required for dissociation
is about 6% of the energy contained in the liberated gas. In simple terms
steam or hot water can be pumped down a drill hole to dissociate the
hydrate and release methane. The released methane could then be
pumped to the surface of the seafloor through another drill hole.
In the depressurisation process, the hydrates are exposed to a lowpressure
environment where they are unstable and decompose to methane
and water. The heat energy for the process comes from the Earth's interior
(geothermal heat flow). The released methane can then be recovered by
conventional technology. The depressurisation method as envisaged
involves horizontal drilling in the free gas zone, which underlies the
hydrate zone. As the free gas is removed, the overlying hydrates become
depressurised and decompose into free gas. Continuous removal of
produced gas is expected to sustain this pressure-induced dissociation of
hydrate zone at its base. This method appears to be most suited to those
deposits where widespread gas occurs in a closure below the hydrate
cap 60 .
INTERNATIONAL SEABED AUTHORITY 540