APPLICATION OF GLASS MICROMODEL TO GAS HYDRATE FORMATION AND DISSOCIATION IN POROUS MEDIA

Scenarios for hydrate stability and distribution depend on mechanisms and rates of fluid transport in the pore space, and whether a separate gas phase is present. To quantify the gas hydrates in subsurface layers, we need to understand their distribution on the pore scale to predict the effect on measured geophysical/downhole properties (seismic velocities or electrical conductivity). The models used to invert these petrophysical data for hydrate volume, are highly sensitive to the location of the crystals within the pore space, and particularly whether the hydrate forms within pore bodies, blocks the pore throats or grows on surfaces, cementing grains together.

Glass micromodels provide a unique opportunity for the observation of phase behaviour in reservoir fluid systems, and now we have applied this technique to study hydrates in porous media. Such observations can contribute significantly to our understanding of hydrate distribution within sediments, the mineralogical and pore size distribution controls on crystal growth, and the effect of different hydrate formers and components in the water phase.

Here we present details of a glass micromodel set-up for the visual observation of gas hydrate formation and dissociation in porous media. Several series of tests have been conducted with methane, CO2, tetrahydrofuran, and various mixtures of C1/CO2 to investigate the effect of fluid composition, presence or absence of free gas and the presence of nucleation sites on gas hydrate formation in porous media.

The results showed that gas hydrates could be formed from dissolved gases as well as free gas. Gas hydrates were observed predominately to nucleate and grow in pore bodies rather than pore walls, providing little cementing effect. The cementing improved with an increase in the percent pore filling of gas hydrates and/or a reduction in grain size. A thin layer of water was observed on glass surface of the pore walls. The gas hydrates formed initially under subcooled conditions underwent annealing with adoption of more geometrical crystal shapes.