In-situ Kinetics and X-ray Computed Microtomography Imaging Studies of Methane Hydrates in Host Sediments
Kerkar, Prasad B
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Methane hydrates naturally occur in abundance in permafrost and marine environments. Methane hydrates are ice-like inclusion compounds in which water molecules form a framework through hydrogen bonding and encapsulate methane molecules under conditions of low temperature and high pressure. In natural settings, the sediment-hydrate interaction governs the mechanical strength and other geophysical properties of formations containing methane hydrates. In this study, methane hydrate formation/dissociation kinetics was studied with methane/water (both pure water and seawater) hosted in consolidated Ottawa sand-cores at pressure-temperature (PT) conditions (P: 9.2 MPa; T: 4deg.C) mimicking sub-seafloor settings. The formation study was conducted by charging methane at different pore pressures followed by cooling. The hydrate formation was delayed with increasing pore pressure or consolidation of host sediment. The hydrate dissociation was achieved by incremental step-wise system depressurization during which time, gas output response, sediment cooling due to the reaction endothermicity and post-depressurization PT equilibrium were recorded. The dissociation events due to depressurization were short-lived. During depressurization, thermocouple monitoring showed that the temperature at the center of the core dropped more rapidly than at the middle radius and the boundary. Post-depressurization dissociation was thermally induced where sediments were allowed to warm up to a bath temperature. The post-depressurization PT equilibrium followed theoretical data for methane hydrates on the higher pressure side due to an excess pore pressure generated within confined core. The post-depressurization PT equilibrium was used to calculate the enthalpy of dissociation value as 59.45 kJ/mol. The gas output during depressurization was fit to estimate hydrate dissociation constant. A set of formation/decomposition runs was repeated with seawater. The formation kinetics of hydrates from seawater was found to be delayed with the degree of consolidation. The post-depressurization PT equilibrium values were utilized to calculate the enthalpy of dissociation of methane hydrates. The endothermic effect due to hydrate dissociation was recorded with the highest degree of cooling recorded at the center and the half-radius than that at the core boundary. The cooling responses during depressurization from three thermocouples placed at different lateral and radial positions within core were used as an indicative of presence of hydrates and their preferential dissociation positions. The post-depressurization dissociation was thermally induced, during which the sediments warmed up to the bath temperature. All post-depressurization pressure-temperature (PT) followed theoretical methane-seawater equilibrium on higher pressure side until all hydrates were dissociated. These post-depressurization PT equilibriums were used to estimate the enthalpy of dissociation of methane hydrates from seawater and a consolidated core as 54.774 kJ/mole. The microscopic visualization of time-resolved 3-dimensional (3-D) growth of individual tetrahydrofuran hydrates and methane hydrates formed within a porous media was performed using synchrotron X-ray computed microtomography. Tomographic data were acquired where ~1200 X-ray images were recorded while rotating the sample tube from 0-180deg. at the X2B beamline, National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL). Each tomogram was reconstructed for 2-dimensional cross-sectional images which were compiled to generate 3-D volume. The images of hydrate patches, formed from excess tetrahydrofuran and methane in aqueous solutions, show random nucleation and growth concomitant with grain movement but independent of container-wall effect. Away from grain surfaces, hydrate surface curvature was convex showing that liquid, not hydrate, was the wetting phase, similar to ice growth in porous media. The time-resolved 3-D images show methane hydrate as pore-filling that is well represented by a model reported by Dvorkin et al. (1999). The observed methane hydrate (sI) growth in porous media is similar to that observed for tetrahydrofuran hydrate (sII) reported previously in this study. The contact angle for the methane hydrate system was measured to be 154.25deg. from the CMT data. A combination of patchy and pore-filling microstructure properties could lead to sediment instability, in the event of methane release by hydrate decomposition.