OCEANIC METHANE HYDRATE - THE CHARACTER OF THE BLAKE RIDGE HYDRATE STABILITY ZONE, AND THE POTENTIAL FOR METHANE EXTRACTION

Oceanic methane hydrates are mineral deposits formed from a crystallin e ''ice'' of methane and water in sea-floor sediments (buried to less than about I km) in water depths greater than about 500 m; economic hy drate deposits are probably restricted to water depths of between 1.5 km and 4 km. Gas hydrates increase a sediment's strength both by ''fre ezing'' the sediment and by filling the pore spaces in a manner simila r to water-ice in permafrost. Concentrated hydrate deposits may be und erlain by significant volumes of methane gas, and these localities are the most favourable sites for methane gas extraction operations. Seis mic reflection records indicate that trapped gas may blow-out naturall y, causing large-scale seafloor collapse. In this paper, we consider b oth the physical properties and the structural integrity of the hydrat e stability zone and the associated free gas deposits, with special re ference to the Blake Ridge area, SE US offshore, in order to help esta blish a suitable framework for the safe, efficient, and economic recov ery of methane from oceanic gas hydrates. We also consider the potenti al effects of the extraction of methane from hydrate (such as induced sea-floor faulting, gas venting, and gas-pocket collapse). We assess t he ambient pressure effect on the production of methane by hydrate dis sociation, and attempt to predict the likelihood of spontaneous gas fl ow in a production situation.