Institute of Petroleum Engineering

Centre for Gas Hydrate Research Publications

Abstract 100
Methane Hydrate Growth and Dissociation Hysteresis in Narrow Liquid Undersaturated Pores
Anderson, R., and Tohidi, B.
32nd International Geological Congress, Florence, Italy, 20-28 Auguest (2004).
The phase behaviour of organic and inorganic compounds confined to mesoporous materials is an extensively studied phenomenon. In narrow pores, high capillary pressures induced by the strong curvature of phase interfaces generally results in a shift of the temperature (or pressure) at which first order phase transitions (e.g. solid-liquid, gas-liquid) occur with respect to bulk (unconfined) conditions. Where pores are of a size sufficient for confined materials retain the physical properties of the bulk phase, the degree of this temperature shift relative to the bulk transition temperature commonly demonstrates a linear relationship with reciprocal pore radius in accordance with the Gibbs-Thomson (or Kelvin) equation. Capillary pressure induced inhibition of gas hydrate has been previously proposed as a potential explanation for disagreements between predicted and actual hydrate stability regions in seafloor sediments. In light of this, numerous recent literature studies have reported experimental data concerning the effects of pore size on hydrate stability, and a number of predictive models have been proposed. However, studies to date have focussed exclusively on the dissociation behaviour of gas hydrates in porous materials, neglecting potential pore size controls on solid phase growth. Furthermore, in all experiments pores have been liquid-saturated, so only the hydrate phase is subject to capillary pressure. In undersaturated systems, gas protrudes into pores, thus there is the potential for high curvature liquid-vapour and vapour-hydrate interfaces to influence phase equilibria. In this work, we present the results of an experimental investigation into the growth and dissociation phase behaviour of methane hydrates in undersaturated (liquid volume < pore volume) synthetic mesoporous silica glass. Using a fixed-volume, isochoric, equilibrium step-heating technique, we have measured equilibrium pressure/temperature (PT) pathways for hydrate growth and decomposition at various pressure/temperature conditions, pore size distributions, and fluid saturations. Results show that fluid saturation plays an important role in the hydrate phase behaviour in porous media. As for saturated systems, hydrate formation and dissociation in undersaturated pores is characterised by a distinct hysteresis between opposing transitions; hydrate growth occurring at lower temperatures (or higher pressures) than dissociation. Hysteresis takes the form of a repeatable closed primary bounding growth/dissociation PT loop, within which various characteristic formation (cooling) and decomposition (heating) specific PT pathways may be followed, depending on initial conditions and fluid saturation. The observed hysteresis pattern may be attributed to differences in the nature of solid-liquid and gas-liquid interfaces during solid phase crystallisation and melting, with pore shape playing a further important role. Results are important for the accurate thermodynamic modelling of clathrate systems, particularly with respect to subsea sedimentary environments, and should prove useful in the simulation of potential methane hydrate exploitation and carbon dioxide sequestration schemes.