Institute of Petroleum Engineering

Centre for Gas Hydrate Research Publications

Abstract 096
Experimental Investigation of methane Hydrate Growth and Dissociation Hysteresis in Narrow Pores
Anderson, R., Llamedo, M., and Tohidi, B.
European Geosciences Union 1st General Assembly, Nice, France, 25 - 30 April (2004).
The phase behaviour of organic and inorganic compounds confined to mesoporous materials is an extensively studied phenomenon. In narrow pores, high capillary pressures generally result 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 in fine-grained sediments has been previously postulated as a possible explanation for disagreements between predicted and actual hydrate stability zones in seafloor sediments. In light of this, numerous recent literature studies have reported experimental data concerning the effect 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 hydrates in porous materials, neglecting potential pore size controls on crystal growth. It is likely that this oversight has in part contributed to questionable assumptions regarding the shape of solid-liquid interfaces in pores, the consequences of which have been detrimental to the interpretation of experimental data and the development of predictive models. In this work, we present the results of an experimental investigation into the growth and dissociation phase behaviour of methane hydrates confined to narrow pores of synthetic mesoporous silica glass. Using a fixed-volume, isochoric, equilibrium step-heating method, we have determined equilibrium pressure/temperature (PT) pathways for hydrate formation and decomposition for various pressure/temperature conditions and pore size distributions. Results show that hydrate formation and dissociation 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. The observed hysteresis pattern is very similar to that reported for gas absorption/desorption in mesoporous materials, and may be attributed to differences in the nature of solid-liquid interfaces during solid phase crystallisation and melting, with pore shape playing a further important role. Results are of importance for interpretation and modelling of gas hydrates in seafloor sediments, and demonstrate that, in addition to depressing the hydrate dissociation point, narrow pores can act as an additional barrier to hydrate stability by restricting hydrate growth conditions to even lower temperatures or higher pressures.