Thermoporometry

Gas hydrate growth and dissociation in narrow pore networks: capillary inhibition and hysteresis phenomena

Marine sediments hosting gas hydrates are commonly fine-grained (silts, muds, clays) with very narrow mean pore diameters (∼0.1 µm). This has led to speculation that capillary phenomena could play an important role in controlling hydrate distribution in the seafloor, and may be in part responsible for discrepancies between observed and predicted (from bulk phase equilibria) hydrate stability zone (HSZ) thicknesses. Numerous recent laboratory studies have confirmed a close relationship between hydrate inhibition and pore size, stability being reduced in narrow pores; however, to date the focus has been hydrate dissociation conditions in porous media, with capillary controls on the equally important process of hydrate growth being largely neglected. Here, we present experimental methane hydrate growth and dissociation conditions for synthetic mesoporous silicas over a range of pressure–temperature (PT) conditions (273–293 K, to 20 MPa) and pore size distributions. Results demonstrate that hydrate formation and decomposition in narrow pore networks is characterized by a distinct hysteresis: solid growth occurs at significantly lower temperatures (or higher pressures) than dissociation. Hysteresis takes the form of repeatable, irreversible closed primary growth and dissociation PT loops, within which various characteristic secondary ‘scanning’ curve pathways may be followed. Similar behaviour has recently been observed for ice–water systems in porous media, and is characteristic of liquid–vapour transitions in mesoporous materials. The causes of such hysteresis are still not fully understood; our results suggest pore blocking during hydrate growth as a primary cause.

Dissociation behavior of CO2 hydrate in sediments during isochoric heating

As CO2 is sequestered into sediments in the oceanic environment, CO2 hydrate can form as a byproduct. This study explored the dissociation behavior of CO2 hydrate in sediments in relation to pore fluid pressure evolution and sediment particle size. We synthesized CO2 hydrate in three types of particulate sediments: glass beads, fine sand, and crushed silt. We then dissociated them through isochoric heating. We observed the excess pore fluid pressure build-up and self-preservation behavior, in which the pressure-temperature state evolves along the hydrate phase boundary until either it reaches the second quadruple point or all hydrates dissociate. The pore fluid pressure evolution is limited, however, by the CO2 vapor-liquid phase equilibrium boundary due to the liquefaction of CO2. The presence of CO2 liquid in sediments forces the pressure-temperature evolution to follow the CO2 vapor-liquid phase equilibrium boundary, regardless of hydrate formation and dissociation processes. CO2 hydrate in fine-grained sediments experiences capillary pressure-induced melting point depression, but this effect vanishes when the pores exceed approximately 1 microm, such as in coarse-grained sediments. In particular, any fracture generation in sediments which involves the local release of confinement eliminates the melting point depression induced by the capillary effect.

Homogeneous nucleation of water in mesoporous zeolite cavities

In this paper, we show that water inside mesoporous cavities in zeolites can be supercooled to ca. -40 degrees C at which point homogeneous nucleation of the water takes place. The fundamental phenomena observed here are similar to those reported earlier in for example emulsion droplets or droplets in the vapor phase. However, as these zeolite materials are widely available, they may provide an easily accessible source for studies of supercooled liquids in confinements. Next to this, it is now possible to discriminate with thermoporometry between mesoporous cavities inside the zeolite crystals, in which homogeneous nucleation takes place, and mesopores that are connected to the external surface in which heterogeneous nucleation takes place.