FROZEN HEAT | Volume 2

Box 2.2 Gas hydrate evaluation through drilling and coring

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).

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).

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