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FROZEN HEAT
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Commercial production of gas from gas hydrates has not
yet occurred. Several production research and develop-
ment studies have, however, been carried out, most notably
at the Mallik site in Canada (see summary in Dallimore
et
al.
2012) and in the Nankai Trough (Yamamoto
et al.
, 2014).
While this research has clearly identified depressurization as
3.5
TIME FRAME FOR
GAS HYDRATE DEVELOPMENT
Hydrocarbon resources are commonly described as either
“conventional” or “unconventional”. Conventional resources
are those that exist in the subsurface as liquids or gases
under high pressure and within permeable reservoirs such
that commercially-viable production (extraction) rates can be
achieved simply by drilling into the reservoir. In fact, a primary
concern with conventional reservoirs is in controlling and limiting
the production rate, particularly in the early phases. Failure
to maintain this well control can result in well blow-outs and
uncontrolled hydrocarbon release to the environment. In contrast,
reservoir quality in unconventional reservoirs is typically very low,
and as a result, additional engineering means are required to
improve reservoir quality around well bores to achieve desired
flow rates. Gas hydrates, which require some combination of
reservoir depressurization, heating, and/or chemical injection to
be productive, are therefore unconventional reservoirs.
While the vast majority of hydrocarbons produced for energy
continue to come from conventional reservoirs, production
from unconventional resources, most notably shale gas in
the United States, is growing rapidly. It cannot be assumed,
however, that all unconventional resources will be associated
with the same environmental risks. The following discusses
general types of environmental risks with respect to the issue
of gas hydrate production.
Loss of well control/spills: This risk, which is significant
in conventional resource development, can also occur in
Box 3.5
Environmental Impacts of Gas Hydrate Production: Comparison to
Existing Conventional and Unconventional Gas Development
unconventional development, particularly where resources are
deeply buried and under high pressure. Gas hydrates, which are by
definition shallow (and thus relatively low-pressure) resources, are
therefore very unlikely to support uncontrollable flow rates. In fact, a
primary challenge in gas hydrate production is not only establishing
flow, but sustaining it. Because gas hydrate reservoirs only produce
recoverable methane when artificially (and temporarily) removed
from their natural pressure condition (a condition that is imposed
by the simple presence of the overlying sediment for onshore gas
hydrates, and by the water column for offshore gas hydrates), any
cessation in the energy input used to achieve pressure reduction will
immediately re-establish gas hydrate stability conditions and halt the
methane release (see Nagakubo
et al.
, 2011; Moridis
et al.
, 2014).
Lastly, liquid hydrocarbons are not known to pool at the shallow
sediment depths at which gas hydrates occur, so the risk of inducing
oil spills while recovering methane from gas hydrate is minimal.
Water Consumption: Unconventional production, such as shale
gas, shale oil, oil shale, and tar sands are characterized by large
water demands during extraction. Gas hydrate drilling and
production, as now envisioned, would require minimal water
useage as the primary stimulation method will be the imposition
of reduced pressure through simple partial evacuation of the
wellbore, as opposed to water-intensive thermal stimulation or
permeability-creation through artificial fracturing.
Water Quality Impacts: All hydrocarbon production results
in the co-production of reservoir brines along with the oil and