Oceanic Methane Hydrate Development: Reservoir Character and Extraction
Authors: Max M.D. , Lowrie A. 
Published: October 2008

Hydrate increases sediment strength by 'freezing' the sediment, similar to water-ice in permafrost. This appears to strongly alter the physical properties of the marine sediments. A number of factors are critical for determining the characteristics of hydrate-trapped gas in order to recover gas from them; most important is the integrity of the hydrate cap/reservoir.

Methane hydrates can be best characterized as shallowly buried strata-bound mineral deposits, from which methane must be derived in-situ and then recovered to the surface as gas. Extraction strategies should focus on adapting mining methodologies for the hydrate-methane conversion process prior to utilizing modified conventional gas recovery procedures. New, more closely monitored and controlled exploration and extraction practices than are used with conventional hydrocarbon deposits must be developed. There will be a constant danger bf blow-out from oceanic gas hydrate reservoirs that are only shallowly buried (less than 1.5 km) in the seabed in water depths over 2 to 3 km deep.

Oceanic Hydrate Character and Energy Potential

Methane hydrate is a methane-water ice-like crystalline material. The bulk of the main natural occurrences of hydrate occupy two distinct pressure-temperature (P-T) regimes, even though there is a continuity of the P-T stability field between them. 1. Polar hydrates are lower pressure and temperature, occurring at shallow depths, while, 2. oceanic hydrates are stable at higher temperatures that are stabilized by the higher pressures of the deep oceans (Fig. 1).

Polar hydrates occur both as part of a compound water-ice and hydrate permafrost on land in continental shelves in which this compound permafrost zone1 formed under subareal conditions during sea level low stands in the last glaciation and were then submerged during inundation of the shelf areas. Other hydrates in Polar continental shelves have formed independent from relic permafrost2. Polar hydrates are much more a really restricted and of likely lesser total volume than oceanic hydrates. They also occur under physical conditions different enough from those of the oceanic hydrates so that methane recovery methods developed in the Siberian and Alaskan Arctic1 cannot simply be used without extensive adaptation for the pressure-temperature conditions and different physical conditions of oceanic hydrates.

Only oceanic hydrates are discussed here. Oceanic hydrates appear to be widespread in the world's ocean seafloors, especially on continental margins, such as the SE U.S. East Coast continental slope, and in ocean embayment's such as the Gulf of Mexico. Methane in them occurs in extremely large concentrations that are becoming recognized as attractive targets for the economic extraction

Hydrocarbon gases (mainly methane) and gas are thermodynamically stabilized in gas hydrates by hydrogen bonding (Van der Walls weak electrical forces) within a crystalline lattice of water molecules3. Hydrates "cement" the sediments they occur within and can impart considerable mechanical strength to the otherwise little compacted sediments. Hydrate fills porosity and restricts permeability similar to water ice freezing sediment in wet permafrost.