Methane Hydrate Formation and Dissociation on Suspended Gas Bubbles in Water

Abstract

Understanding gas hydrate formation on gas bubbles evolved from an oil/gas blowout and the stability conditions of the hydrates formed are key to controlling hydrates during a blowout and its containment. In this work, methane hydrate formation and dissociation conditions on suspended gas bubbles in water were studied. For the formation process, methane gas was gradually injected into a counter flowing water column until a full hydrate shell on suspended gas bubbles was observed. The hydrate shells were then dissociated by either depressurization or heating. The minimum methane concentration to form hydrate shells on suspended gas bubbles in water was determined for the pressure range of (7.0 to 17.3) MPa at 277 K. The dissociation pressures of hydrates are also reported for temperatures from (276 to 286) K. It is observed that the hydrate shells on gas bubbles formed and remained stable only when a minimum dissolved concentration (∼0.0013 mole fraction) of gas in water was reached. During dissociation, hydrate shells showed a unique morphology, ranging from solid shells, broken shells, plates, and crystals on the surface of the gas bubbles.

Split-beam echo sounder observations of natural methane seep variability in the northern Gulf of Mexico

“…For 32 plume observations, gas transfer out of the bubbles was sufficiently slow to enable survival of bubbles detectable at 18 kHz to depths shallower than 600 m. In one case, a plume reached as shallow as 360 m, though no plume observations extended shallower than this depth. Only one plume observation reaching a depth shallower than 600 m was limited by the echo sounder FOV (at approximately 550 m), suggesting that bubbles which had been consistently acoustically observable at 18 kHz during the 800 m ascent from the seafloor to a depth of 600 m dissipated rapidly over the subsequent 250 m. These observations suggest enhanced survival of bubbles for a small number of plumes, followed by rapid reduction of detectable bubbles at depths shallower than 600 m. Formation of methane hydrate shells on the bubbles has been suggested as a mechanism which may inhibit gas transfer and increase the duration of bubble survival for methane bubbles originating at the depth of the survey area and ascending through the depth range over which methane hydrates are stable [ Rehder et al ., ; Leifer and MacDonald , ; Judd , ; Greinert et al ., ; McGinnis et al ., ; Chen et al ., ]. The shallow (minimum) depth limit of the hydrate stability zone, above which hydrates will dissociate at shallower depths, typically falls between 500 and 600 m in the Gulf of Mexico [ Milkov et al ., ; Tishchenko et al ., ; Weber et al ., ] and is calculated according to Tishchenko et al [, equation (24)] at 610 m for a typical temperature‐depth profile in the study region.…”

The interaction of climate change and methane hydrates

“…During this bubble-stripping process, CH 4 is replaced by oxygen and nitrogen Vielstädte et al, 2015]. Bubbles emitted deeper than the shallowest extent of gas hydrate stability in the water column may develop an armor of gas hydrate [Chen et al, 2014[Chen et al, , 2016Graves et al, 2015;Rehder et al, 2002Rehder et al, , 2009Sauter et al, 2006;Topham, 1984;Wang et al, 2016;Warzinski et al, 2014;Zhang, 2003], but such armoring may or may not reduce the rate at which CH 4 leaves the rising bubbles Wang et al, 2016]. Overall, most CH 4 emitted from the seafloor either above or below the top of the GHSZ and whether originating with gas hydrate dissociation or other processes will be dissolved relatively deep in the water column.…”

High-pressure visual experimental studies of oil-in-water dispersion droplet size

“…The contact between the dispersing hydrocarbon and aqueous bulk phase results in the dispersion of liquid and gaseous hydrocarbon in water. Gas bubbles are unlikely to remain trapped in the water column for a prolonged period, due to both the substantial density difference between gas and water, and the rapid dissolution of light hydrocarbons in seawater (Chen, Sloan et al 2013). Conversely, numerical studies by Paris et al (Paris, Le H«naff et al 2012) suggest that small oil droplets may naturally stratify at depths beyond 1000 m, enabling extensive subsea lateral transport.…”