Gas hydrates in nature

In natural settings, such as the ocean bottom, when buried organic matter decomposes to methane and dissolves in water, clathrates form at temperatures greater than 277 K (4°C or 39°F). Biogenically produced methane in dissolved water forms hydrates very slowly, because of mass-transfer limitations. Over geologic time, the total enclathrated methane in the oceans has been estimated at 2.1 × 10¹⁶ standard cubic meters (SCM)—twice the energy total of all other fossil fuels on Earth. The amount of hydrated methane in the northern latitude permafrost is relatively small (7.4 × 10¹⁴ SCM), within the error margin of ocean hydrate estimates.

Fig. 1 shows world hydrate deposits in the deep ocean and permafrost, most of which were determined by indirect evidence such as seismic reflections called bottom simulating reflectors (BSRs). These seismic signals are caused by velocity inversions because of gas beneath some higher-velocity barrier, such as a hydrate deposit. The hydrates contribute only in a minor way to the signal. Substantial efforts are currently underway to perform multidimensional seismic studies to determine the geographic extent.

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  Fig. 1—Hydrate deposit locations in the deep ocean and permafrost(after Kvenvolden)

Seafloor stability

Significant ocean-hydrated-sediment slumps can jeopardize the foundation of subsea structures, such as platforms, manifolds, and pipelines. The single incident off the Carolina coast, shown in Fig. 2, occurred about 15,000 years ago and increased the Earth’s extant atmospheric methane by as much as 4%. Recent subsea experiments have shown that natural methane-hydrate particles, as large as 2 in., have survived the 2.5-mile trip from the ocean bottom to the surface before decomposing. The effect of subsidence on subsea structures and foundations represents the initial meeting point for the two energy communities: the first concerned with hydrates in the Earth and the second with hydrates in man-made production systems. The investigation of seafloor-hydrate safety concerns will positively impact the longer range development of energy-recovery technologies for seafloor hydrates.

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  Fig. 2—Seafloor sediment slump off the Carolina coast associated with hydrate decomposition(after Dillon et al.

Because the atmosphere warmed by 4°F with shallow oceans in the Late Paleocene (55 million years ago), there is evidence for the hypothesis of Dickens et al. that ocean methanehydrate dissociation caused a large greenhouse-gas warming of 14°F, significantly impacting evolutionary processes. Atmospheric-induced changes in the ocean-floor temperature are not likely to occur in current times because deeper oceans effectively constrain temperature changes. 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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  Fig. 3—Three principal energy-recovery methods from hydrates: depressurization, themal simulation, and inhibitor injection.

The most producible permafrost hydrate deposits are those lying in direct contact with a gas reservoir, such that free-gas production causes hydrate dissociation by decreasing reservoir pressures below the hydrate stability pressure. Heat from the Earth allows hydrate decomposition to slowly replenish the gas reservoir. Makogon indicated that the Messoyakha, a Siberian permafrost reservoir, was produced for almost a decade in this manner during the 1970s. With the Siberian exception, no commercial production of hydrates has occurred.

The second and third hydrate production methods are thermal stimulation and inhibitor injection, both of which have also been tried in the former Soviet Union. However, both methods are very expensive, relative to depressurization. Economic estimates indicate that depressurization alone is not viable. Gas production from hydrates requires both depressurization and thermal/inhibitor stimulation.

Production from stand-alone hydrates in the permafrost or in the ocean is much more costly but technically feasible. Bil suggested that the best course of action is for the industry not to invest funds in research of hydrated-energy recovery, until more research is done to provide technical breakthroughs.

Well-documented gas production from hydrates close to conventional permafrost reservoirs will probably begin in the Western hemisphere during the next decade at incremental costs over normal gas production. A new Mallik 3L-38 production test well was drilled in the MacKenzie delta of Canada in the first quarter of 2002, with depressurization and thermal stimulation to recover the hydrated gas. The objective of the well is to extend those findings to recover gas from leaner hydrate deposits off the shore of Japan, and, as such, the work is heavily funded by Japanese and other national hydrate programs. Many national projects (e.g., Japan, India, Korea, and the United States) are currently seeking to find viable methods to recover gas from hydrates.

Noteworthy papers in OnePetro

Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read

External links

Paull, C.K. and Dillon, W.P. ed. 2001. Natural Gas Hydrates: Occurrence, Distribution, and Detection, 124, 315. Washington, DC: Monograph Series, AGU. http://www.agu.org/books/gm/v124/

See also

Hydrates

Transporting stranded gas as hydrates

PEH:Hydrate_Emerging_Technologies