By employing a specific particle interaction theory and a high-precision equation of states for the liquid and vapor phases of H2, respectively, a new H2 solubility model in pure water and aqueous NaCl solutions is proposed in this study. The model established by fitting the experimental data of H2 solubility can be used to estimate H2 solubility in pure water at temperatures and pressures of 273.15–423.15 K and 0–1100 bar, respectively, and in salt solutions (NaCl concentration = 0–5 mol/kg) at temperatures and pressures of 273.15–373.15 K and 0–230 bar, respectively. By adopting the theory of liquid electrolyte solutions, the model can also be used to predict H2 solubility in seawater without fitting the experimental data of a seawater system. Within or close to experimental data uncertainty, the mean absolute percentage error between the model-predicted and experimentally obtained H2 solubilities was less than 1.14%.
The formation of gas hydrate reservoir in marine sediments is mainly controlled by methane supply and sedimentary burial. Based on the mass conservation of methane in gas hydrate system, a numerical model of gas hydrate formation was established considering the methane supplied by dissolved methane diffusion, pore water advection, and in situ methanogenesis. A case study of ODP site 1247 at the Hydrate Ridge, offshore Oregon shows that dissolved methane transported by molecular diffusion and pore water advection is the major supply of methane for gas hydrate formation, while in situ methanogenesis contributes little to the gas hydrate reservoir. The gas hydrate reservoir was also evaluated considering the changes of sedimentation rate since 1.67 Ma. Our model results show that the variations of sedimentation rate lead to little change in the size of gas hydrate reservoir at ODP 1247. The calculated hydrate saturation amounts to ∼0%∼3%, which is consistent with the measured values using pressure coring.
Sedimentation and burial rate can vary considerably at continental slopes, due to constantly changing tectonic processes on the geological timescale. This variation may affect the transport of methane in sediment, as manifested in the change of pore water flux, methanogenesis, methane diffusion and hydrate removal from hydrate stability zone (HSZ) and further affect the accumulation of hydrate in submarine sediments. Most of previous models assumed a constant sedimentation rate and thus may result in inaccurate estimates of total amount of hydrate within the HSZ. In this study, we developed a hydrate accumulation model that is capable of handling multiple sedimentation stages. Site 997 of ODP Leg 164, with data indicating its recent history of four sedimentation stages of different rates, is chosen to investigate the significance of sedimentation rate variation experienced at this location. We examined the effect of varying sedimentation rates on hydrate accumulation by taking in consideration sediment compaction, in situ methanogenesis, and composition and component transport processes. Simulation results suggest that the history of hydrate accumulation at Site 997 is characterized by a sequence of increase, decrease, and then increase till the present day. At present, the hydrate deposit has accumulated to 8.30 × 10 4 mol/m 2 , and the average hydrate saturation near the base of the HSZ is 6.3%, which is in general agreement with published estimates in the literature. The accumulation of hydrate at Site 997 was significantly affected by variable sedimentation rates, and nearly one half of the hydrate deposit was accumulated during the last 2.5 Myr. Plain Language Summary Methane hydrate forms in submarine sediments where thermodynamic conditions are satisfied and adequate methane is available. Three main mechanisms have been proposed for adequate methane availability: in situ methanogenesis, advection of methane-bearing fluids, and diffusion of methane. However, in actual environments off continental margins, a variable sedimentation rate may significantly affect the above mechanisms and thus change the amount of methane supply. Therefore, the formation of methane hydrate is influenced eventually. Meanwhile, the dissociation of methane hydrate changes simultaneously with a variable sedimentation rate. Hence, it is unclear how will hydrate reservoirs respond to different sedimentation rate values. In this study, we selected Site 997 of the ODP Leg 164 due to the significant variations in sedimentation rate and developed a hydrate accumulation model reflected several sedimentary stages. We highlighted the importance of consideration of variation of sedimentation rate in the estimation of total amount of hydrate within the HSZ and drawn a conclusion that dissociation of hydrate is more sensitive to the changes in sedimentation rate compared to hydrate formation.
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