The presence of methane hydrates within the shallow seafloor has the potential to significantly impact the physical properties of the sediment. Scientists have observed that sound waves travel noticeably faster in sediments that contain hydrates compared to those that do not. This effect occurs, in part, because the hydrates act as a cement—bonding adjacent grains to one another and thereby increasing sediment rigidity. On the other hand, the dissociation of hydrates and the subsequent release of liquid water and methane will reduce sediment shear strength and may also increase internal pore pressure, both of which promote sediment instability.

The potential connection between the stability of the seafloor and methane hydrate dissociation leads to two important R&D issues: (1) understanding the natural processes of hydrate dissociation and how they may relate to catastrophic seafloor failures along continental margins, and (2) ensuring the safety of drilling operations that may lead inadvertently to hydrate dissociation.

Large-scale Sea floor Instability: There is strong circumstantial evidence that the dissociation of methane hydrates due to ongoing, natural geological processes may play a key role in promoting large-scale seafloor slumps along the continental margins. The best data to support this hypothesis comes from the eastern continental margin of North America, where the initiation points of more than 200 large sediment slumps have been identified and mapped (see the figure above). The location of the slumps is clearly not random, and does not appear to be closely related to the occurrence of steep slopes. Surprisingly, the best correlation appears to be with water depth—most slides start in sediments at depths ranging from 500 to 700 meters. This also marks the approximate landward limit of natural methane hydrate occurrence. Furthermore, the detachment horizon of the slides often correlates with the base of the hydrate stability zone. Clearly, hydrates are not the sole cause of seafloor movement, but they do appear to be a contributing factor.

Several reasons have been cited to explain why seafloor instability might be related to the up-dip termination of the gas hydrate stability zone (GHSZ). First, where the hydrate zone is thicker, the bulk of the hydrate is well within the limits for hydrate stability. Slight changes in temperature or pressure will not cause dissociation. It is only at the base of the hydrate stability zone, where conditions are marginally suitable for hydrate formation, that slight perturbations in the environment can result in dissociation. In deep water where the GHSZ is thick, it could take thousands of years for a rise in sea water temperature to be conducted to affect the GHSZ. However, at and near the up-dip limit, these near-phase boundary conditions lie very near the surface where they can be more easily and quickly impacted by changes in the environment.

Second, due to decreased pressures at shallower water depths, when dissociation does occur near the up-dip edge of the accumulation, it will produce nearly three units of water plus methane gas for each unit of hydrate (Paull et al, 2000). In deeper water and higher pressures, this volume increase is much less. This expansion in the volume of pore fluids increases pore pressure, which negates the weight of the overlying sediment (counteracts the frictional forces that hold the sediment in place on a slope) and thereby increases the likelihood of sediment movement.

Many natural processes can alter the pressure/temperature conditions at the seafloor, and therefore promote hydrate dissociation. The first are changes in sea level. Over geologic time, the volume of water in the oceans periodically increases and decreases, primarily in response to the formation of continental ice sheets that trap water on land during ice ages. These sea-level changes translate directly into changes in ocean-bottom pressures, causing periodic hydrate dissociation. The second most common cause is deposition. As more sediment is piled on the seafloor, layers that contain hydrates are buried to deeper depths and slowly heated, resulting in dissociation at the base of the hydrate zone. Slow and steady erosion will, of course, reverse this effect; however, if the erosion is sudden and massive (for example, a large slump) the pressure drop (removal of overburden) will be felt long before the temperature drop, and hydrate dissociation may occur. Additional natural processes that can lead to hydrate dissociation include changes in oceanic currents that may bring about changes in bottom water temperatures, earthquakes, and tectonic activity that results in subsidence or uplift of the continental shelves.

Understanding the causes of massive continental margin seafloor slumps may not simply be an academic issue. Scientists have determined that geologically recent (within the past 7,500 years) slides in the North Atlantic are likely related to a devastating tsunami that swept over the Norwegian coast. Hydrate dissociation may have contributed to these slides. Clearly, much more needs to be known.

Drilling Safety: One area of immediate concern to both hydrate researchers and industry is preserving the stability of the seafloor in the vicinity of oil and gas drilling and production facilities. As exploration continues to move further offshore into ultra-deep waters, drillers and facility engineers encounter increasingly thick layers of hydrate-bearing, near-surface sediments. Any hydrate dissociation, whether related to natural causes or to unnatural heating (for example, from the transit through wells or pipelines of hot fluids produced from deep horizons) may promote seafloor movement and pose significant safety hazards to personnel and sea bottom installations, pipelines, and production facilities. Consequently, tools and methods to locate favorable areas with reduced risk of seafloor instability are much needed. Similarly, products to effectively insulate hydrate-bearing intervals from sources of heat must also be developed and implemented.