Gas Hydrates of the Northern Gulf of Mexico - Geological Conditions and Extraction Methods
Gas hydrate (GH) is a natural-gas source that has not been extracted from the northern Gulf of Mexico (N-GoM). In natural GH, significant amounts of methane are trapped within water ice. A specific combination of P-T conditions is required for GHs to form and remain stable. On a continental shelf like the N-GoM, requisite conditions are temperature just above freezing (~ 2C) and pressure about of 300+ m of seawater. This equates to conditions on the Gulf continental shelf near shelf break. In the N-GoM, GH formation is in two main ways. The first is related to seepage of methane upward through permeable sediments and faults. This methane, which is coming from much older, more deeply buried sediments many 100s to 1000s of feet below the surface, fills open space in upper layers of sediment. In the second way, methane-producing bacteria take natural carbon dioxide in sediment and water and convert it to methane. This process of chemosynthetic gas hydrate production commonly forms mounds that are rich in life forms that thrive on methane. These mounds may mark sites where GHs of the first type are situated at shallow depth. The P-T conditions are favorable, but more than that, there are biotic communities to help precipitate GHs. GHs were initially studied in the N-GoM not for their energy potential, but rather because their presence caused stability problems for offshore drill-rig footings. During the 1990s, USGS, US-MMS, DoE, and NOAA, began collaboration with universities and oil and gas corporations to map GHs in the N-GoM. The result was extensive seismic surveying of the N-GoM area, which produced seismic cross-sections showing evidence of sediment disturbance at various shallow levels that is consistent with sediment displacement during GH formation. Above-mentioned studies confirm that GH deposits in sediment layers at a depth of over several 100 meters from the bottom of the sea floor can be utilized as novel natural-gas resources. In-situ GH decomposition into water and gas is required to produce natural gas economically from these layers because GH is a type of non-mobile solid energy resource. To cause GH decomposition, decompression or temperature increase out of the equilibrium zone is necessary, while dissociation heat should be supplied for continuous gas production. Here, we suggest new economical methods of extraction of GHs given prevailing geological conditions at some sites in the N-GoM.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California