Abstract
Methane hydrate formation and dissociation kinetics were investigated in seawater-saturated consolidated Ottawa sand-pack under sub-seafloor conditions to study the influence of effective pressure on formation and dissociation kinetics. To simulate a sub-seafloor environment, the pore-pressure was varied relative to confining pressure in successive experiments. Hydrate formation was achieved by methane charging followed by sediment cooling. The formation of hydrates was delayed with increasing degree of consolidation. Hydrate dissociation by step-wise depressurization was instantaneous, emanating preferentially from the interior of the sand-pack. Pressure drops during dissociation and in situ temperature controlled the degree of endothermic cooling within sediments. In a closed system, the post-depressurization dissociation was succeeded by thermally induced dissociation and pressure-temperature conditions followed theoretical methane-seawater equilibrium conditions and exhibited excess pore pressure governed by the pore diameter. These post-depressurization equilibrium values for the methane hydrates in seawater saturated consolidated sand-pack were used to estimate the enthalpy of dissociation of 55.83 ± 1.41 kJ/mol. These values were found to be lower than those reported in earlier literature for bulk hydrates from seawater (58.84 kJ/mol) and pure water (62.61 kJ/mol) due to excess pore pressure generated within confined sediment system under investigation. However, these observations could be significant in the case of hydrate dissociation in a subseafloor environment where dissociation due to depressurization could result in an instantaneous methane release followed by slow thermally induced dissociation. The excess pore pressure generated during hydrate dissociation could be higher within fine-grained sediments with faults and barriers present in subseafloor settings which could cause shifting in geological layers.
Highlights
Methane hydrates remain a fascinating area of research both as a scientific curiosity as well as a potential source of methane, the principle component of natural gas hydrates
To give an improved basis for predicting actual behavior of hydrates in an oceanic environment, this study investigated the stability of methane hydrates during formation, followed by thermally induced dissociation under sub-seafloor conditions mimicked in a laboratory with natural sand as the host, confining pressure, seawater, and representative P-T conditions
The methane charging pressure was varied to study hydrate formation kinetics at different effective pressures that are defined as the difference between the confining pressure and pore pressure
Summary
Methane hydrates remain a fascinating area of research both as a scientific curiosity as well as a potential source of methane, the principle component of natural gas hydrates. Handa and Stupin [1] reported pressure-temperature (P-T) profiles for the hydrate-ice-gas and hydrate-liquid-water-gas for methane and propane hydrates in 70 Å radius silica gel pores. To give an improved basis for predicting actual behavior of hydrates in an oceanic environment, this study investigated the stability of methane hydrates during formation, followed by thermally induced dissociation under sub-seafloor conditions mimicked in a laboratory with natural sand as the host, confining pressure, seawater, and representative P-T conditions. Hydrate dissociation was carried out at different temperatures to study the preferential formation/dissociation locations of methane hydrates within the sand-pack, degree of cooling due to their dissociation and equilibrium conditions during dissociation These equilibrium P-T data were used to compute the enthalpy of dissociation of methane hydrates from consolidated and seawater saturated sand
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