Stochastic nature of carbon dioxide hydrate induction times in Na-montmorillonite and marine sediment suspensions

Abstract

We identified stochastic nature of CO2 hydrate induction times in Na-montmorillonite (Na-MMT) and marine sediment suspensions and investigated stochastic induction time patterns at different hydrate forming conditions. Goodness-of-fit tests showed that the distribution of CO2 hydrate induction times can be properly fitted by a lognormal distribution function. As CO2 hydrate formation pressure increased and temperature decreased, both arithmetic and geometric means and their standard deviations of the lognormal distribution significantly decreased due to a super-cooling effect. No CO2 hydrate formation was observed in aqueous solution at 3.0MPa and 0.3°C, while induction times in Na-MMT suspensions were much shorter and lognormally distributed in 40min. Addition of NaCl to Na-MMT suspensions (3.5%) enhanced the hydrate nucleation kinetic by 1.7 times due to the heterogeneity of nucleation sites on Na-MMT surfaces. CO2 hydrate nucleation in Ulleung basin sediment suspension was faster than that in any other suspensions investigated here, showing more consistent lognormal distribution of the induction times with lowest mean and standard deviation. The study manifests the stochastic nature of CO2 hydrate induction times under heterogeneous environmental conditions excluding the ‘memory effect’, which will add to basic knowledge for a successful CO2 hydrate sequestration technology in marine sediment environments.