Ryo Ohmura

Decomposition of methane hydrates in sand, sandstone, clays, and glass beads

[1]xa0Decomposition conditions of methane hydrates in sediments were measured during formation-decomposition cycles. As test sediments, we used silica sand, sandstone, and clays (kaoline and bentonite), which are typical natural materials known as hydrate bearing sediments, and the range of samples cover a range of water saturating abilities. To better understand the results, we also used uniformly sized glass beads. Pore effects on decomposition of these materials were investigated by analyzing the pore-space distributions of the materials and by varying the initial water content of the samples. The results obtained for sand and sandstone samples indicated that the final decomposition temperatures were shifted lower than those for bulk hydrates at the same pressure. Temperature shifts were more negative for smaller initial water contents with the maximum shift being approximately −0.5 K. The results were consistent with those measured for glass beads with nearly the same particle size. For kaoline clays, the shift was at most −1.5 K. We conclude that the decomposition conditions are mainly affected by the pore sizes. The surface textures and mineral components had less influence on the results. We confirmed that glass beads mimic the effect of sediments for sand, sandstone, and kaoline clays, which have little to no swelling when put in contact with water. On the other hand, for bentonite particles, the results indicated that methane hydrates formed not only between the particles but also in the interlayers. A thermodynamic promoting effect was found for dilute bentonite solutions, although the positive decomposition-temperature shift was at most +0.5 K.

Methane hydrate crystal growth in a porous medium filled with methane-saturated liquid water

The nucleation, growth and ageing of methane hydrate crystals were observed visually in a porous medium filled with liquid water presaturated with methane. The pore space dimensions of the porous medium were 1.0u2009×u2009102u2009µm. The pressure−temperature conditions at which hydrate formation was initiated corresponded to system subcoolings of 3.4u2009K, 6.7u2009K, 12.3u2009K and 14.1u2009K, respectively, where the system subcooling denotes the difference of the system temperature from the triple methane−hydrate−water equilibrium temperature under a given pressure. Faceted (skeletal) hydrate crystals grew and bridged the pore spaces without intervention of a liquid water layer when the subcoolings were equal or smaller than 6.7u2009K. The faceted crystals may form a physical bonding with the walls of the porous medium. At the higher subcoolings, the dispersive formation of dendritic crystals and subsequent morphological change into particulate crystals were observed. The bridging of the dendritic crystals is unlikely in the absence of a large amount of additional methane supply due to the dispersive spatial distribution of the dendritic crystals that have dimensions smaller than those of the pore spaces. As a result of the interpretation of the observed variation in the crystal morphology of the methane hydrate formed in liquid water, the dependence of the crystal morphology on the magnitude of the mass transfer of methane molecules in liquid water observed in the porous medium was consistent with that previously observed in a bulk methane–water system.

Clathrate hydrate crystal growth in liquid water saturated with a hydrate-forming substance: Variations in crystal morphology

This paper reports on our interpretation of our visual observations of the variations in macroscopic morphology of hydrate crystals growing in liquid water saturated with a guest substance prior to the hydrate formation. The observations were made in a high-pressure cell charged with liquid water and gaseous CO2. They revealed distinct variations in the morphology of hydrate crystals depending on the system subcooling ΔT sub, the temperature deficiency inside the cell from the triple CO2–hydrate–water equilibrium temperature under a given pressure. When ΔT subu2009≳u20093u2009K, a hydrate film first grew along the CO2–water interface; then hydrate crystals with dendritic morphology grew in large numbers into the liquid-water phase from that hydrate film. When ΔT subu2009≲u20092u2009K, the dendritic crystals were replaced by skeletal or polyhedral crystals. We present a non-dimensional index for such variations in hydrate crystal morphology. This is based on the idea that this morphology depends on the growth rate of hydrate crystals, and their growth rate is controlled by the mass transfer of the hydrate–guest substance (CO2 in the present experiments), dissolved in the bulk of liquid water, to the hydrate crystal surfaces. The morphology variations observed in the present and previous studies are related to this index.

Formation, growth and ageing of clathrate hydrate crystals in a porous medium

An experimental study was performed to visually observe the driving force dependence of hydrate growth in a porous medium filled with either liquid water and dissolved CO2 or liquid water and gaseous CO2. The given system subcooling, ΔT sub, i.e. the deficiency of the system temperature from the triple CO2−hydrate−water equilibrium temperature under a given pressure, ranged from 1.7u2009K to 7.3u2009K. The fine dendrites initially formed at ΔT subu2009=u20097.3u2009K changed quickly into particulate crystals. For ΔT subu2009=u20091.7u2009K, faceted hydrate crystals grew and the subsequent morphological change was hardly identified for an eight-day observation period. These results indicate that the physical bonding between hydrate crystals and skeletal materials becomes stronger with decreasing driving force, suggesting that the fluid dynamic and mechanical properties of hydrate-bearing sediments vary depending on the hydrate crystal growth process.

Water Permeability of Porous Media Containing Methane Hydrate as Controlled by the Methane-hydrate Growth Process

This chapter seeks to clarify the relation between fluid permeability and methane-hydrate saturation (Sh). The ultimate purpose is to estimate the theoretical expression by the equation K = K0(1 Sh)N (where K0 is the apparent permeability at Sh = 0 and N is a constant) for input into methane-hydrate numerical simulators. However, the permeability of hydrate-bearing sediment strongly depends on the hydrate saturation, grain-size distribution, porosity, pore-size distribution, hydrate formation method, and so on. To clarify the relation between the permeability and methane-hydrate saturation, we measured the water permeability of methane-hydrate-bearing sediments with different hydrate saturations for three contrasting methane-hydrate formation methods: (1) the connate water reaction method, (2) the gas diffusion method, and the (3) cementing method. The results demonstrate that the rate of decrease in the apparent water permeability (AWP) with increasing methane-hydrate saturation differs for each method of gas-hydrate formation. In addition, the values of K and N in the theoretical expression K = K0(1 Sh)N were estimated for each production method, and a different N value was obtained for each hydrate formation method. It is apparent that the method of gas-hydrate formation leads to a contrasting geometry of methane-hydrate growth at the pore scale and in turn affects the macroscopic AWP saturation relations.

Estimations of Interfacial Tensions Between Liquid CO2 and Water from the Sessile-Drop Observations

Publisher Summary Greenhouse gases, such as CO 2 , have become a serious problem for mankind. To offset the CO 2 emissions into the atmosphere, sequestration of CO 2 in the deep ocean has been proposed. The behavior of CO 2 injected into seawater can be predicted from the CO 2 -H 2 O phase diagram and a depth profile of the seawater temperature. In addition, the interfacial tension between liquid CO 2 and water (or sea water) is an important factor for understanding the behavior of the injected CO 2 droplets into deep water. However, CO 2 hydrate formation on the droplets at depths deeper than 400 m makes the prediction of CO 2 dissolution difficult due to insufficient knowledge of the relevant physical parameters. CO 2 hydrate is an ice-like clathrate compound formed from CO 2 and water under suitable conditions of low temperature T and high pressure P. This crystalline compound will form at the interface between the injected liquid CO 2 and seawater and can reduce the dissolution rate of CO 2 into seawater. The interfacial tension between liquid CO 2 and water or NaCl solution was measured by a simple sessile-drop method at pressures up to 25 MPa and at temperatures of 278 and 288 K. The interfacial tension between liquid CO 2 and pure water was approximately 38 mN m -1 at 288 K and 5 MPa with a small pressure dependence, whereas the values between liquid CO 2 and 3 wt% NaCl solution were more than 10 % larger than those between liquid CO 2 and pure water. At 278 K, CO 2 hydrate is stable and the interfacial tension has larger pressure dependence. This might be related to the supersaturation prior to hydrate formation.