Borowski, W. S., C.K. Paull, W. Ussler III. 1997. Recognition of shallow-subbottom gas hazards in deep-water operation areas: The gas hydrate detection problem. Proceedings Offshore Technology Conference, OTC 8297, 211-216. https://doi.org/10.4043/8297-MS
Recognition of shallow-subbottom gas hazards in deep-water operation areas: The gas hydrate detection problem
We offer a new geochemical method that employs pore-water sulfate gradients as a potential indicator of gas-hydrate presence in deep-water sediments. Gas hydrates are ice-like solids, generally composed of water and methane, which occur naturally in sediments under conditions of high pressure, low temperature, and sufficiently large methane concentrations. As petroleum exploration and development efforts move into deeper water, the possibility of encountering gas hydrate within sediments becomes increasingly likely, and their occurrence may be hazardous to production infrastructure. Operations that significantly alter ambient conditions may cause decomposition of gas hydrate, decrease sediment strength, and potentially induce sediment failure. Thus, recognition of gas hydrate occurrence is necessary for the safe design and emplacement of offshore drilling/production platforms, subsea production equipment, and pipelines. Unfortunately, gas hydrates are not generally sensed by seismic techniques used to identify shallow gas hazards, inviting a geochemical approach to detect gas hydrate in deepwater environments. Methane concentrations will tend to be elevated in areas prone to gas hydrate formation. However, conventional methane concentration measurements in deep-water sediments grossly underestimate the amount of methane due to outgassing. We have determined that under appropriate conditions, pore-water sulfate gradients of marine sediments can be used as a proxy measurement of methane. When methane is present within sediments in sufficient amounts, most of the interstitial sulfate will be consumed by reaction with methane, affecting the shape of sulfate profiles. Fresh sediment samples containing pore waters are collected from a surface ship via piston coring as an independent operation or during geotechnical drilling. Prior identification of the potential for gas hydrate deposits within shallow-subbottom sediments using this geochemical technique can reduce the threat to seafloor structures by safe location of seafloor structures or accommodation of their design.
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