incorporation of water into the growing gas hydrate structure, with salt remaining in solution and reaching concentrations as much as twice that of seawater (Torres et al.,
2011). Massive gas hydrate and chloride brines in nearseafloor sediments along continental margins are not at
all uncommon, and may represent a significant carbon reservoir that is susceptible to perturbations driven by
changes in bottom water temperature.
Gas hydrates and energy
In several review articles, e.g., Boswell and Collett (2010),
four gas hydrate reservoir types are evaluated in terms of
their resource potential: sand-dominated reservoirs, claydominated fractured reservoirs, seafloor massive deposits,
and clay-dominated disseminated reservoirs. Current
exploration, production testing, and modeling target arctic
and marine hydrate-bearing sand reservoirs in the North
Slope of Alaska, Gulf of Mexico, and Japan, with additional research offshore China, Korea, and India. Proposed
methods of gas recovery from gas hydrates were reviewed
by Boswell and Collett (2010). Traditionally, these were
based on: heating the reservoir above the gas hydrate stability, decreasing the reservoir pressure below gas hydrate
equilibrium, and injecting a chemical inhibitor into the
reservoir to decrease the gas hydrate stability. More
recently, laboratory experiments have documented the
feasibility of displacing methane molecules in the hydrate
structure with carbon dioxide molecules, thus releasing
methane and sequestering CO 2 . In 2011 a multiyear project began to field-test the CO 2 for methane exchange concept on the Alaska Ignik Sikumi test well. This field test
showed that CO 2 can be injected into a gas hydrate sandy
reservoir, followed by methane production (Schoderbek
et al., 2012).
Gas hydrates and climate
Methane is a powerful greenhouse gas with a greenhouse
warming potential (GWP) 23 times that of CO 2 on a
per-molecule basis. Sudden release of methane from gas
hydrate therefore has the potential to affect global climate,
and there are interesting although controversial hypotheses that attribute past climate variations to methane release
from marine gas hydrates in response to ocean warming
and/or sea-level change (Dickens, 2011). These hypotheses have yet to be confirmed and more research is needed
to evaluate hydrate response to environmental change: the
fate of methane from marine gas hydrate reservoirs to the
seabed, ocean surface, and the atmosphere.
Gas hydrate observatories
Numerous laboratory and field studies at gas hydratebearing sites, including several drilling expeditions in the
past decades, have provided critical background data on
the conditions of gas hydrate stability and have given an
overall view of the composition and distribution of gas
hydrates in nature. Fundamental questions remain as to
the residence time of gas hydrates near the seafloor and
deeper within the sediment column, the nature and driving
mechanisms for flow and biological interactions in environments where gas hydrates are present and fluid (aqueous
and gas) migration occurs, and the role of the ocean in mitigating gas input to the atmosphere from the seafloor. The
processes involved in modulating carbon transport within
the seabed and in the water column are highly dynamic
and can only be understood through monitoring of complementary parameters over decadal or longer timescales.
Efforts are under way to collect time series data at several
locations, e.g., at the Canadian cabled observatory, NEPTUNE (http://www.neptunecanada.ca/research/researchprojects/project.dot?inode¼30424), at Hydrate Ridge,
structure/ooi-station-map/regional-scale-nodes/hydrateridge/), and in the Gulf of Mexico (http://ieeexplore.ieee.
org/xpls/abs_all.jsp?arnumber¼4625508&tag¼1).
Summary
Methane and other low-molecular-weight gases, such as
ethane and carbon dioxide, can combine with water to
form ice-like substances at high pressure or low temperature in what are known as gas hydrates. The stability of
these compounds is controlled by several factors including
pressure, temperature, salinity, and gas concentrations.
Gas hydrates are commonly imaged by the presence of a
bottom-simulating reflector in the seismic records, which
is the most common tool for identifying hydrate-bearing
sediments. Other geophysical tools and borehole logging
strategies are used to characterize the presence, distribution, and abundance of these deposits. These techniques
are calibrated against geochemical proxies that include
pore fluid freshening by gas hydrate dissociation during
conventional core recovery and controlled depressurization experiments from pressure cores. Interest in gas
hydrate rises from their role in carbon cycling, potential
climate effects, and energy considerations. They constitute a large carbon reservoir, and sand-hosted deposits
are currently being explored for economic uses, with one
of the most innovative approaches being the CO 2 for
methane exchange.
Bibliography
Bohrmann, G., and Torres, M. E., 2006. Gas hydrates in marine sediments. In Schulz, H. D., and Zabel, M. (eds.), Marine Geochemistry. Heidelberg, Germany: Springer, pp. 481–512,
doi:10.1007/3-540-32144-6_14.
Boswell, R., and Collett, T. S., 2010. Current perspectives on gas
hydrate resources. Energy and Environmental Science, 4,
1206–1215.
Claypool, G. E., and Kaplan, I. R., 1974. The origin and distribution
of methane in marine sediments. In Kaplan, I. R. (ed.), Natural
Gases in Marine Sediments. New York: Plenum Press,
pp. 99–139.
Clennell, M. B., Hovland, M., Booth, J. S., Henry, P., and Winters,
W. J., 1999. Formation of natural gas hydrates in marine sediments 1. Conceptual model of gas hydrate growth conditioned
by host sediment properties. Journal of Geophysical Research,
104(B10), 22985–23003.
436
MARINE GAS HYDRATES
2011). Massive gas hydrate and chloride brines in nearseafloor sediments along continental margins are not at
all uncommon, and may represent a significant carbon reservoir that is susceptible to perturbations driven by
changes in bottom water temperature.
Gas hydrates and energy
In several review articles, e.g., Boswell and Collett (2010),
four gas hydrate reservoir types are evaluated in terms of
their resource potential: sand-dominated reservoirs, claydominated fractured reservoirs, seafloor massive deposits,
and clay-dominated disseminated reservoirs. Current
exploration, production testing, and modeling target arctic
and marine hydrate-bearing sand reservoirs in the North
Slope of Alaska, Gulf of Mexico, and Japan, with additional research offshore China, Korea, and India. Proposed
methods of gas recovery from gas hydrates were reviewed
by Boswell and Collett (2010). Traditionally, these were
based on: heating the reservoir above the gas hydrate stability, decreasing the reservoir pressure below gas hydrate
equilibrium, and injecting a chemical inhibitor into the
reservoir to decrease the gas hydrate stability. More
recently, laboratory experiments have documented the
feasibility of displacing methane molecules in the hydrate
structure with carbon dioxide molecules, thus releasing
methane and sequestering CO 2 . In 2011 a multiyear project began to field-test the CO 2 for methane exchange concept on the Alaska Ignik Sikumi test well. This field test
showed that CO 2 can be injected into a gas hydrate sandy
reservoir, followed by methane production (Schoderbek
et al., 2012).
Gas hydrates and climate
Methane is a powerful greenhouse gas with a greenhouse
warming potential (GWP) 23 times that of CO 2 on a
per-molecule basis. Sudden release of methane from gas
hydrate therefore has the potential to affect global climate,
and there are interesting although controversial hypotheses that attribute past climate variations to methane release
from marine gas hydrates in response to ocean warming
and/or sea-level change (Dickens, 2011). These hypotheses have yet to be confirmed and more research is needed
to evaluate hydrate response to environmental change: the
fate of methane from marine gas hydrate reservoirs to the
seabed, ocean surface, and the atmosphere.
Gas hydrate observatories
Numerous laboratory and field studies at gas hydratebearing sites, including several drilling expeditions in the
past decades, have provided critical background data on
the conditions of gas hydrate stability and have given an
overall view of the composition and distribution of gas
hydrates in nature. Fundamental questions remain as to
the residence time of gas hydrates near the seafloor and
deeper within the sediment column, the nature and driving
mechanisms for flow and biological interactions in environments where gas hydrates are present and fluid (aqueous
and gas) migration occurs, and the role of the ocean in mitigating gas input to the atmosphere from the seafloor. The
processes involved in modulating carbon transport within
the seabed and in the water column are highly dynamic
and can only be understood through monitoring of complementary parameters over decadal or longer timescales.
Efforts are under way to collect time series data at several
locations, e.g., at the Canadian cabled observatory, NEPTUNE (http://www.neptunecanada.ca/research/researchprojects/project.dot?inode¼30424), at Hydrate Ridge,
structure/ooi-station-map/regional-scale-nodes/hydrateridge/), and in the Gulf of Mexico (http://ieeexplore.ieee.
org/xpls/abs_all.jsp?arnumber¼4625508&tag¼1).
Summary
Methane and other low-molecular-weight gases, such as
ethane and carbon dioxide, can combine with water to
form ice-like substances at high pressure or low temperature in what are known as gas hydrates. The stability of
these compounds is controlled by several factors including
pressure, temperature, salinity, and gas concentrations.
Gas hydrates are commonly imaged by the presence of a
bottom-simulating reflector in the seismic records, which
is the most common tool for identifying hydrate-bearing
sediments. Other geophysical tools and borehole logging
strategies are used to characterize the presence, distribution, and abundance of these deposits. These techniques
are calibrated against geochemical proxies that include
pore fluid freshening by gas hydrate dissociation during
conventional core recovery and controlled depressurization experiments from pressure cores. Interest in gas
hydrate rises from their role in carbon cycling, potential
climate effects, and energy considerations. They constitute a large carbon reservoir, and sand-hosted deposits
are currently being explored for economic uses, with one
of the most innovative approaches being the CO 2 for
methane exchange.
Bibliography
Bohrmann, G., and Torres, M. E., 2006. Gas hydrates in marine sediments. In Schulz, H. D., and Zabel, M. (eds.), Marine Geochemistry. Heidelberg, Germany: Springer, pp. 481–512,
doi:10.1007/3-540-32144-6_14.
Boswell, R., and Collett, T. S., 2010. Current perspectives on gas
hydrate resources. Energy and Environmental Science, 4,
1206–1215.
Claypool, G. E., and Kaplan, I. R., 1974. The origin and distribution
of methane in marine sediments. In Kaplan, I. R. (ed.), Natural
Gases in Marine Sediments. New York: Plenum Press,
pp. 99–139.
Clennell, M. B., Hovland, M., Booth, J. S., Henry, P., and Winters,
W. J., 1999. Formation of natural gas hydrates in marine sediments 1. Conceptual model of gas hydrate growth conditioned
by host sediment properties. Journal of Geophysical Research,
104(B10), 22985–23003.
436
MARINE GAS HYDRATES