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FROZEN HEAT
36
The pace of scientific expeditions designed to investigate
the occurrence of gas hydrates continues to accelerate. Such
expeditions use specialized drilling vessels staffed by large
teams of scientists, engineers, and technicians. The goal is to
gather data from the subsurface using well-logging tools, as
well as gathering sediment samples and returning them to the
ship deck or to onshore labs for analysis (Figure TB2.2).
While the initial field programs focused largely on coring, amore
recent strategy involves an initial, dedicated, logging-while-
drilling campaign that can test many locations. Gas hydrate
logging operations utilize the same tools used in traditional oil
and gas evaluation to determine sediment lithology, porosity,
and other parameters (Goldberg
et al.
2010).
Based on logging data, the most intriguing sites can be
revisited for more intensive continuous or spot-coring
campaigns, application of specialized tools such as borehole
temperature and seismic devices, and wireline logging
programs (Dallimore and Collett 2004; Collett
et al.
2011).
Standard coring recovers sediment from gas-hydrate-bearing
intervals, although the reduction in pressure and increase in
temperature that occur as the core sample is retrieved often
result in the dissociation of all but the most massive hydrates.
Nonetheless, the dissociation of gas hydrates and release of
nearly pure water into the original saline pore fluids results
in a unique chemical signal called freshening, which can be
exploited to infer the presence and concentration of gas
hydrates (Kastner
et al.
1995; Hesse 2003). The dissociation
of gas hydrates also results in a cooling of the surrounding
sediments due to the endothermic nature of the dissociation
reaction. This phenomenon was first used systematically to
infer gas hydrate presence in sediment cores during ODP Leg
164 at the Blake Ridge (Paull
et al.
1996) and has since served
as the basis for the development of a technology involving the
automated infrared imaging of the recovered core immediately
after it arrives on deck (Long
et al.
2010).
Box 2.2
Gas hydrate evaluation through drilling and coring
The development of pressure coring – recovery of sediment in
devices that maintain pressures near in situ conditions – has
greatly increased our ability to characterize and image gas-
hydrate-bearing formations. Pressure coring is ideally suited to
the problem of gas hydrate sampling, providing the best known
means to determine gas hydrate concentrations and showing
remarkable detail of the morphology of gas hydrate occurrences
(Holland
et al.
2008). Technologies for acquiring samples and
analyzing their physical properties prior to the onset of the
substantial disruption caused by gas hydrate dissociation have
improved steadily (Schultheiss
et al.
2010; Yun
et al.
2006), and
are now an increasingly critical aspect of gas hydrate evaluation.
Wireline logging techniques, typically conducted in the
same hole from which cores were recovered, are identical in
principle to the logging-while-drilling approach. Better vertical
resolution can usually be achieved, but deploying tools on a
wireline is operationally more complex than tool deployment
on the drill pipe. Steady improvement in logging-while-drilling
tools continues to reduce the need for wireline operations. One
data set that is currently best acquired through wireline logging
and that has effective application to gas hydrate studies is
the nuclear-magnetic resonance (NMR) tool (Kleinberg
et al.
2005). At present, NMR provides the best available information
on both sediment permeability and the distribution of various
pore-filling constituents, including mobile liquid water, which is
critical to the most promising production techniques (Volume
2 Chapter 3). Other key data sets, such as shear velocity, are
also best gathered with wireline tools.
Wireline logging allows the deployment of geophones to conduct
a borehole seismic experiment (vertical seismic profile, or VSP)
that can be critical to the calibration of log and conventional
seismic data. The simplest option is to deploy a seismic source
from the drill ship. More advanced techniques, such as 3D-VSP
imaging can be applied, but they require access to a second ship
for the operation of the seismic sources (Pecher
et al.
2010).