A New Look at Seafloor Venting: Natural Gas Hydrate Derivatives

Abstract

Natural leaks of multiphase hydrocarbons from the seafloor are widespread. Characterizing the temporal and spatial variability of these emissions provides baselines for comparison to the magnitude of anthropogenic leaks. Determining leakage rates will be important to our understanding the sustainable implementation of subsea infrastructure and the relative contribution of natural sources to global budgets. Multibeam echo soundings from cruise AT21-02 were used to define a region with several~600 to 900 m tall gas plumes in the water column directly above cratered hummocky regions of the sea floor with high backscatter in water depths of~1500 m over the Barbados Accretionary Complex. The relationship of seafloor morphologic features such as faults, craters, and mud volcanoes with vent-gas plumes in the water column is indicative of substantial emissions from this region. Disappearance of the acoustic plumes at~600 m is coincident with the top of CH 4 gas hydrate stability in the water column. In the plumes, natural gas hydrate shells that form at the gas-water interface and armor the gas from dissolution during ascent likely encapsulate bubbles. Ascent-driven pressure change in the bubbles causes shattering of shells forming gas hydrate shards that rise with the ascending plume and add to its acoustic reflection strength. The multiphase fluid consisting of gas, shelled bubbles, shards, and water changes character at the top of gas hydrate stability where the hydrate dissociates and dissolves. Differentiating sources and further characterizing emissions will provide baselines and also contribute to understanding the relative importance of the different emissions sources.

Comprehensive Review on Exploration and Drilling Techniques for Natural Gas Hydrate Reservoirs

“…Although gas hydrate reservoirs are different from conventional gas reservoirs, drilling of a gas hydrate reservoir is very much possible with engineered drilling fluids, cement slurry, and trained manpower. Possible drilling locations are identified on the basis of exploratory studies, which includes seismic surveys, gas seepages, nearby aquifers, gas plumes, gas hydrate mounds in a marine environment, and pressure and temperature profiles. ,, Table categorizes the fundamental differences between a conventional oil and gas reservoir and a gas hydrate reservoir.…”

“…Possible drilling locations are identified on the basis of exploratory studies, which includes seismic surveys, gas seepages, nearby aquifers, gas plumes, gas hydrate mounds in a marine environment, and pressure and temperature profiles. 19,61,134 Table 4 categorizes the fundamental differences between a conventional oil and gas reservoir and a gas hydrate reservoir.…”