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Such factors as geologic tectonism and warm-ocean-current circulation may contribute to modern ocean-hydrate disruption.

The concern for seafloor safety is considerably impacted by the fact that BSR indications of hydrates are not totally reliable. For example, on DSDP Leg 164 drilling off the Carolinas, close to the slump (shown in Fig. 2) three holes were drilled—one without a BSR, one with a weak BSR, and one with a very strong BSR. Hydrates were found in all three wells. Such hydrated sediments are fairly dispersed—typically 3.5 vol% in sediments. A more significant concern is the fact that there is not a single clear BSR in the Gulf of Mexico while coring hydrates, one of the most active oil/gas exploration and production regions in the western hemisphere.

Without a clear BSR, but with evidence of near-mudline hydrate deposits, the safety of subsea-equipment foundations is of concern. Companies with subsea equipment typically obtain “drop cores” in the area/route of interest to determine if hydrates are in the vicinity of the foundations. The evidence to date in the Gulf of Mexico suggests that gases have percolated along salt diapirs or geologic faults from deep within the Earth to form hydrates close to the ocean bottom. Gas evolution from the seafloor marks a primary suspected seafloor-hydrate location.

Energy recovery

Because hydrates in ocean sediments are dispersed (typically < 3.5 vol%), substantial ingenuity is required for economic energy recovery. A recent workshop concluded that most critical in-situ issues arise because hydrates are ill-defined in four respects in the geophysical/chemistry domain:

Detection Distribution Sediment properties Hydrate controls

For example, sonic waves are the principal detection tool for ocean hydrate deposits, but sonic quantification and frequently qualitative detection of hydrate is inaccurate, as suggested with BSRs in the Gulf of Mexico. Field tests are required to bound the problem in the field, which will be verified by laboratory experiments.

Pilot drilling, characterization, and production testing of hydrates have begun in permafrost regions that have higher hydrate concentrations (e.g., 30 vol% in the 1998 Mallik 2L-38 well in Canada), with a third Mallik well completed in March 2002. These permafrost-hydrate exploration and production tests will aid the understanding of how to approach the more-dispersed, but far larger, ocean resource in the future. Finite-difference reservoir recovery models indicate that production is only economic at rates greater than 0.5 × 106 SCM/d.

There are three principal energy-recovery methods, as shown schematically in Fig. 3:

Depressurization Thermal stimulation Inhibitor injection

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