Institute of Petroleum Engineering

Centre for Gas Hydrate Research Publications

Abstract 154

Capillary Pressure Controlled Methane Hydrate and Ice Growth-Dissociation Patterns in Porous Media: Synthetic Silica versus Natural Sandstone

Ross Anderson, Beau Webber and Bahman Tohidi
6th International Conference on Gas Hydrates, Vancouver, Canada, July 6-10 (2008)

Marine sediments which host gas hydrates are commonly fine-grained (silts, muds, clays) with narrow mean pore diameters (~0.1 μm). This has lead to speculation that capillary phenomena could play a significant role in controlling hydrate distribution in the seafloor, and may be partly responsible for discrepancies between observed and predicted (from bulk phase equilibria) hydrate stability zone (HSZ) thicknesses. Numerous recent laboratory studies have confirmed a close relationship between hydrate inhibition and pore size; clathrate stability being significantly reduced in narrow pores. However the focus of investigations has generally been hydrate dissociation conditions in porous media, with capillary controls on the equally important process of hydrate growth being largely overlooked. Previously, we have demonstrated that hydrate formation and decomposition in synthetic mesoporous silicas is characterised by a distinct hysteresis: solid growth occurring at significantly lower temperatures (or higher pressures) than dissociation. Here, we present experimental methane hydrate and ice growth/melting conditions for a natural medium-grained sandstone (Permian Locharbriggs, Dumfries, Scotland) and compare these with results for synthetic silicas. Although the sandstone pore size distribution extends into the macroporous range (> 50 nm), ice/hydrate growth and melting are still measurably inhibited to lower temperatures at higher saturations in the pore space. Likewise, the distinct growth/melting hysteresis observed for mesoporous silicas also apparently occurs to an extent in the sandstone, although is much more subdued. This hysteresis could be considered analogous to oil/gas reservoir sandstone drainage/imbibition curves; in this case the solid hydrate/ice being the mobile, penetrative (growth) or imbibing (melting) phase, with pore water the continuous wetting phase. The different shape of hysteresis patterns for synthetic silicas and natural sandstone may be attributed to contrasting pore size distribution functions and, particularly, interconnectivity.

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