Institute of Petroleum Engineering

Centre for Gas Hydrate Research Publications

Abstract 099
Visual Observation of Hydrate Growth from Pure Water and Aqueous Salt Solutions
Yang, J., Anderson, R., and Tohidi, B.
32nd International Geological Congress, Florence, Italy, 20 - 28 August (2004).
Although a large volume of thermodynamic data and predictive models exist for hydrate equilibria in salt-water systems, there is still little information available concerning the influence of salt on hydrate nucleation and growth patterns.We present the results of novel visual experiments investigating hydrate growth in pure water and 3 mass% aqueous NaCl solutions at the micro-scale in synthetic pore micromodels. Experiments were carried on CO2 (s-I), methane (s-I), and a multi-component natural gas (s-II). Hydrates were formed under various degrees of subcooling to investigate thermal influences, and the significance of a previous history of hydrate formation, or 'water history', was examined. Aqueous salt was found to have a significant effect on hydrate growth patterns compared to pure water. Behaviour was similar for all systems, suggesting little dependence on hydrate structure. For both pure water and salt solutions, nucleation generally initiated at gas-water interfaces, with pore centres being the preferential site for hydrate growth, rather than grain surfaces, where a thin film of free water persisted to high hydrate saturations. For pure water, primary fine-grained crystals, which grew rapidly from multiple nuclei, were observed to agglomerate and reform into much larger, secondary, more euhedral crystal masses with lower surface area/energies. Not all gas bubbles converted to hydrate - some were dissolved at the expense of hydrate growing in the liquid phase nearby, while others remained encased in a relatively impermeable coating of solid hydrate. When salt was present, secondary rearrangement and consolidation was severely restricted. Primary fine-grained crystal crusts on bubbles often persisted with little change in morphology, while individual hydrate crystals underwent only minor coalescence, forming highly porous, disordered masses. This contrasting behaviour can be attributed to increased concentration of salt in remnant fluid inclusions and thin films between hydrate crystals.The effect of salts on hydrate morphology was found to be largely independent of subcooling under the conditions studied. In contrast, results suggest a previous history of hydrate formation (water history) played an important role. Water history encouraged nucleation and gas diffusion driven hydrate growth within the aqueous phase, where slower more ordered crystallisation permitted exclusion of salts and development of large, faceted, solid crystal masses.