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Gas hydrates are crystalline solid compounds made up of cages of water molecules, within which gas molecules are trapped. Their formation generally requires low temperatures and high pressures. These conditions can be encountered, in particular, during the injection of gas for storage purposes into depleted oil reservoirs, or during the rise of gas bubbles from accidental leaks of stored gas in water-saturated sedimentary layers. The formation of gas hydrates in these contexts can lead to significant changes in sediment properties (decreased permeability, variations in mechanical properties, etc.) and potentially jeopardize the feasibility of storing the gas in the reservoir. X-ray micro-tomography imaging of gas hydrate formation in sediments can provide crucial information (hydrate saturation, shape and distribution of hydrate nodules, formation kinetics, etc.) for understanding the impact of their presence on the properties of these porous media.
To the best of our knowledge, few studies have focused on the visualization of CO₂ hydrates at the pore scale using X-ray tomography [1]. However, several studies on methane hydrates in porous media exist that can serve as a comparison [2-4]. We developed a robust experimental protocol to reproduce the formation of CO2 hydrates within bulk solution and within a model porous medium (VitraPOR® sintered glass) consisting of (1) forming the hydrates in a pressurized carbon reinforced PEEK cell immersed in a cooling bath and (2) scanning the cell at specific moments in time in the TESCAN DynaTOM scanner of the DMEX Centre for X-ray Imaging (Pau, France), while being placed in a customized ice bath for optimal scan quality while maintaining a low temperature. Our study confirmed that the contrast between water and gas hydrate is poor when no contrast agent is added, optimal results were obtained when adding 3 wt% of KI to the water phase. The formation of CO2 hydrates within the porous medium was found to be non-homogeneous, highlighting the crucial role the porous medium plays in the volumetric distribution of the hydrate nodules.
[1] Ta, X. H. , Yun, T. S. , Muhunthan, B. , and Kwon, T.-H. Observations of pore-scale growth patterns of carbon dioxide hydrate using X-ray computed microtomography. Geochemistry, Geophysics, Geosystems 2015, 16, 912–924.
[2] Le, T. X. PhD Thesis: Etude expérimentale des propriétés mécaniques et de la microstructure des sédiments contenant des hydrates de méthane.
[3] Lei, L. , Seol, Y. , Choi, J.-H. , and Kneafsey, T. J. Pore habit of methane hydrate and its evolution in sediment matrix – Laboratory visualization with phase-contrast micro-CT. Marine and Petroleum Geology 2019, 104, 451–467.
[4] Lei, L. , Seol, Y. , and Jarvis, K. Pore‐Scale Visualization of Methane Hydrate‐Bearing Sediments With Micro‐CT. Geophysical Research Letters 2018, 45, 5417–5426.
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