Adhesion force between cyclopentane hydrates and solid surface materials.

The mechanisms by which hydrates deposit in a petroleum production line are related to pipeline surface properties, fluid composition and properties, and water cut. In this work, adhesion forces between cyclopentane hydrates and solid surfaces were investigated as a function of the solid material, the presence of water and the presence of petroleum acids in the oil phase. The influence of dissolved water on hydrate adhesion forces was also investigated. The results show that the adhesion force between hydrates and solid surfaces was dependent on the surface material; solids with low surface free energy lead to the lowest adhesion forces. The adhesion force was strongly dependent on the presence of water in the system. When a water drop was deposited on the solid surface, the adhesion force between the hydrate and the solid surface was more than 10 times larger than hydrate-hydrate adhesion forces. The presence of a water-saturated oil phase also led to an increase in adhesion force between hydrate particles. Adhesion forces were highest when the solid surfaces are water-wet. Addition of petroleum acids to the oil phase drastically reduced adhesion forces.

NIST Gas Hydrate Research Database and Web Dissemination Channel.

To facilitate advances in application of technologies pertaining to gas hydrates, a freely available data resource containing experimentally derived information about those materials was developed. This work was performed by the Thermodynamic Research Center (TRC) paralleling a highly successful database of thermodynamic and transport properties of molecular pure compounds and their mixtures. Population of the gas-hydrates database required development of guided data capture (GDC) software designed to convert experimental data and metadata into a well organized electronic format, as well as a relational database schema to accommodate all types of numerical and metadata within the scope of the project. To guarantee utility for the broad gas hydrate research community, TRC worked closely with the Committee on Data for Science and Technology (CODATA) task group for Data on Natural Gas Hydrates, an international data sharing effort, in developing a gas hydrate markup language (GHML). The fruits of these efforts are disseminated through the NIST Sandard Reference Data Program [1] as the Clathrate Hydrate Physical Property Database (SRD #156). A web-based interface for this database, as well as scientific results from the Mallik 2002 Gas Hydrate Production Research Well Program [2], is deployed at http://gashydrates.nist.gov.

Thermal expansivity for sI and sII clathrate hydrates.

Knowledge of thermal expansivity can aid in the understanding of both microscopic and macroscopic behavior of clathrate hydrates. Diffraction studies have shown that hydrate volume changes significantly (as much as 1.5% over 50 K) as a function of temperature. It has been demonstrated previously via statistical mechanics that a minor change in hydrate volume (e.g., a 1.5% change in volume or 0.5% change in lattice parameter) can lead to a major change in the predicted hydrate formation pressure (e.g., >15% at >100 MPa for methane). Because of this sensitivity, hydrate thermal expansivity measurements, for both Structures I and II with various guests, are needed help quantify volume distortions in hydrate lattices to ensure accurate hydrate phase equilibria predictions. In addition to macroscopic phase equilibria, the thermal expansion of different hydrates can give information about the interactions between the guest molecules and the host lattice. In this work, the hydrate lattice parameters for four Structure I (C2H6, CO2, 47% C2H6 + 53% CO2, and 85% CH4 + 15% CO2) and seven Structure II (C3H8, 60% CH4 + 40% C3H8, 30% C2H6 + 70% C3H8, 18% CO2 + 82% C3H8, 87.6% CH4 + 12.4% i-C4H10, 95% CH4 + 5% C5H10O, and a natural gas mixture) systems were measured as a function of temperature. The lattice parameter measurements were combined with existing literature values. Both sI and sII hydrates, with a few exceptions, had a common thermal expansivity, independent of hydrate guest. Many guest-dependent correlations for linear thermal expansivity have been proposed. However, we present two guest-independent, structure-dependent correlations for sI and sII lattices, which have been developed to express the normalized hydrate lattice parameters (and therefore volume) as a function of temperature.

Molecular hydrogen storage in binary THF-H2 clathrate hydrates.

The hydrogen storage capacity of binary THF-H(2) clathrate hydrate has been determined as a function of formation pressure, THF composition, and time. The amount of hydrogen stored in the stoichiometric hydrate increases with pressure and exhibits asymptotic (Langmuir) behavior to approximately 1.0 wt % H(2). This hydrogen concentration corresponds to one hydrogen molecule occupying each of the small 5(12) cavities and one THF molecule in each large 5(12)6(4) cavity in the hydrate framework. Contrary to previous reports, hydrogen storage was not increased upon decreasing the THF concentration below the stoichiometric 5.6 mol % solution to 0.5 mol %, at constant pressure, even after one week. This provides strong evidence that THF preferentially occupies the large 5(12)6(4) cavity over hydrogen, for the range of experimental conditions tested. The maximum amount of hydrogen stored in this binary hydrate was about 1.0 wt % at moderate pressure (<60 MPa) and is independent of the initial THF concentration over the range of conditions tested.

Composite thermal conductivity in a large heterogeneous porous methane hydrate sample.

By employing inverse modeling to analyze the laboratory data, we determined the composite thermal conductivity (k(theta), W/m/K) of a porous methane hydrate sample ranged between 0.25 and 0.58 W/m/K as a function of density. The calculated composite thermal diffusivities of porous hydrate sample ranged between 2.59 x 10(-7) m(2)/s and 3.71 x 10(-7) m(2)/s. The laboratory study involved a large heterogeneous sample (composed of hydrate, water, and methane gas). The measurements were conducted isobarically at 4.98 MPa over a temperature range of 277.3-279.1 K. Pressure and temperature were monitored at multiple locations in the sample. X-ray computed tomography (CT) was used to visualize and quantify the density changes that occurred during hydrate formation from granular ice. CT images showed that methane hydrate formed from granular ice was heterogeneous and provided an estimate of the sample density variation in the radial direction. This facilitated quantifying the density effect on composite thermal conductivity. This study showed that the sample heterogeneity should be considered in thermal conductivity measurements of hydrate systems. Mixing models (i.e., arithmetic, harmonic, geometric mean, and square root models) were compared to the estimated composite thermal conductivity determined by inverse modeling. The results of the arithmetic mean model showed the best agreement with the estimated composite thermal conductivity.