Abstract
Upward migration of gas-dissolved pore fluid is an important mechanism for many naturally occurring hydrate reservoirs. However, there is limited understanding in this scenario of hydrate formation in sediments. In this preliminary work, hydrate formation and accumulation from dissolved gas in sandy sediments along the migration direction of brine was investigated using a visual hydrate simulator. Visual observation was employed to capture the morphology of hydrates in pores through three sapphire tubes. Meanwhile, the resistivity evolution of sediments was detected to characterize hydrate distribution in sediments. It was observed that hydrates initially formed as a thin film or dispersed crystals and then became a turbid colloidal solution. With hydrate growth, the colloidal solution converted to massive solid hydrates. Electrical resistivity experienced a three-stage evolution process corresponding to the three observed hydrate morphologies. The results of resistivity analysis also indicated that the bottom–up direction of hydrate growth was consistent with the flow direction of brine, and two hydrate accumulation centers successively appeared in the sediments. Hydrates preferentially formed and accumulated in certain depths of the sediments, resulting in heterogeneous hydrate distribution. Even under low saturation, the occurrence of heterogeneous hydrates led to the sharp reduction of sediment permeability.
Highlights
Natural gas hydrates are icelike crystalline compounds comprising small guest molecules, such as methane or other light hydrocarbons, which are trapped in the cages of a hydrogen-bonded water framework under relatively low temperature and high pressure [1]
At 15 h, inlet pressure P1 abruptly began to rise. This was thought to be an indication of hydrate formation that resulted in a decrease of sediment permeability
Electrical resistivity at all 12 depths experienced a three-stage evolution process, which indicated three morphologies of hydrates presented in the sediments
Summary
Natural gas hydrates are icelike crystalline compounds comprising small guest molecules, such as methane or other light hydrocarbons, which are trapped in the cages of a hydrogen-bonded water framework under relatively low temperature and high pressure [1]. In order to safely and effectively recover methane from hydrate reservoirs, great efforts have been devoted to study hydrate formation and dissociation behaviors in laboratories and field tests, as previously reviewed [3,4,5] These works have greatly improved the understanding of natural-gas hydrates and given hope of unlocking “fire in ice” for humans’ daily lives. In order to understand the behaviors of hydrate formation and accumulation along the upward fluid-migration direction, a one-dimensional visual hydrate simulator (ODVHS) with height/diameter ratios of about 40 was applied In this preliminary experimental work, a flow test was carried out to form hydrates from upward-migrating dissolved gas in sediments. The evolution of sediment permeability during hydrate growth and accumulation was analyzed
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