Thermal Injection. With this technique, heat is introduced into the hydrate formation to raise the temperature of the material and promote dissociation. An example of this is the injection of relatively warm seawater into an undersea gas hydrate layer. Once the gas is released within the layer, it can be brought to the surface.

Inhibitor Injection. Certain alcohols, such as methanol or ethylene glycol, act as inhibitors when injected into a gas hydrate layer, and cause the hydrate material to change. They shift the pressure-temperature conditions needed for hydrate stability, allowing the hydrate to dissociate and release its methane.

Depressurization. In some hydrate reserves there are zones in which natural gas is already in its free state. If a well is drilled into such a zone to extract the natural gas, it can also reduce the pressure within the overlying gas hydrate layer. If this pressure reduction is enough to cause dissociation, then gas from the hydrate layer is released and can be extracted at the same time.

Computer simulations for thermal injection using hot water and steam suggest that enough gas would be released to be recoverable. However, the cost of this technique is prohibitive. Similarly, inhibitor injection seems to be feasible but, again, the economic and environmental costs outweigh the production results. At present, the most economically promising technique appears to be depressurization. This technique is limited only to areas with existing reservoirs of natural gas in its free state, and the extraction of gas from the gas hydrate may be hampered by the formation of ice or the reformation of gas hydrate during the dissociation and extraction process.

Other Impacts

Extracting natural gas from hydrate formations using any of the above techniques would have an impact on the formation itself and its surrounding area. In the case of undersea hydrate reserves, the dissociation and extraction would have to be done without contributing to the instability of the seafloor.

A discontinuity in the strength of the sediment column may already exist at the base of an undersea hydrate zone. This could be caused by gas hydrates inhibiting normal sediment consolidation and compaction in the region. Also, any free gas trapped below the hydrate zone could be overly compressed, resulting in an abrupt change in pressure at the zone boundary. Such a discontinuity represents a potentially unstable condition in the seafloor where gas hydrates are situated.

An example of such an instability has been observed in the undersea gas hydrate deposits off the coast of South Carolina, USA. Strong reflections from seismic profiles indicate a discontinuity at the base of the gas hydrate zone. In this area, a slope of about 5° or less indicates a generally stable seafloor. However, many submarine landslide scars are present at depths near the boundaries of the stable hydrate zones. For one particularly large scar, indicating a landslide extending for as much as 66 km (about 41 mi), weaker reflections from seismic profiles within the area of the landslide suggest that the discontinuity may not exist there. This would indicate that hydrates are not now present in that particular area. Scientists theorize that perhaps a change in the pressure from a falling sea level during a glacial period could have caused the hydrate in that area to dissociate. The resulting release of gas could have triggered the long-ago landslide. Future efforts to dissociate and extract natural gas from the hydrate deposits may run the risk of setting off similar undersea disturbances.