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Synchrotron radiation provides the necessary spatial and temporal resolution for non‐invasive operando studies of dynamic processes under complex environmental conditions. Here a new environmental cell for simultaneous in situ dynamic X‐ray imaging and measuring acoustic properties of geological samples is presented. The primary purpose of this cell is to study gas‐hydrate formation in porous geo‐materials and its influence on their acoustic properties. The cell is designed for cylindrical samples of 9 mm in diameter, confining and pore pressures up to 12 MPa, and temperatures from −20°C to room temperature. The cell is portable and can be easily assembled and operated at different X‐ray sources. This cell enables a wide range of experiments studying physical/chemical processes in the Earth subsurface that change the mechanical properties of rocks (geochemical reactions, phase transitions, etc.). [ABSTRACT FROM AUTHOR]

[Display omitted] • Combination of micro-CT and nano-CT used to study methane-hydrate formation in coal. • Hydrates grow differently in coal (thin shells) and sand (shells, pore and water volumes). • Observed water extraction from coal particles along channels during competitive sorption. • Micro-porosity channels in coal particles are confirmed by nano-CT. • Water movement in thin pores at speed 3–4 mm/s observed from synchrotron micro-CT. Fast 3D X-ray imaging has proved to provide crucial insights into multi-phase dynamic processes in various geo-materials. In particular, many in situ imaging experiments have been made to study gas-hydrate formation in porous sandy samples. Such imaging is challenging for the methane gas-hydrate formation in coal samples because of the coal micro-porosity structure and lower X-ray contrast. Here we present results of the first dynamic in situ micro-computed tomography experiment of methane-hydrate formation in coal samples. Synchrotron phase-contrast tomography techniques allowed to achieve necessary contrast levels to separate all the materials of interest (gas, water, coal, and gas hydrate) in reconstructed images with high spatial and temporal resolution. The imaging results are compared to the ones from a similar tomographic experiment with sand samples. Methane-hydrate formation is accompanied by the water movements caused by cryogenic water suction that happens in sequences of short fast movements with longer equilibrium states in between. Only one type of the hydrate formation was observed in coal (growth as shells on grain boundaries) as opposed to three types in sand (shells, growth into gas pockets, and inside water volumes). In particular, this leads to a slower hydrate-formation speed in coal. For the coal sample, we also observed water extraction from grains, and interpreted it as competitive sorption of methane. We visualized the dynamic behavior of this water extraction via micro-channels inside the coal grains, and performed nano-tomography imaging of these channels for a better understanding of this phenomenon. [ABSTRACT FROM AUTHOR]

We present the results of dynamic in-situ 3D X-ray imaging of methane hydrates microstructure during methane hydrate formation and dissociation in sand samples. Short scanning times and high resolution provided by synchrotron X-rays allowed for better understanding of water movement and different types of gas-hydrate formation. Complementing previous observations, we conclude that the process of gas-hydrate formation is accompanied by the water movements caused by cryogenic water suction that happens in sequences of short fast movements with longer equilibrium states in between (when the water is immobile). Based on the 3D microstructure we identified two distinct types of gas-hydrate formation: (i) into the gas pockets and (ii) inside water volumes. For both mechanisms we do not see problems in gas or water supply to support the gas-hydrate formation. The rate of dissociation in the self-preservation mode (pressure drop at negative temperatures) appears to be different for these two types of gas hydrates. This means that the history of the gas-hydrate formation may influence its behaviour at the dissociation stage (e.g. gas-hydrate production). • Different types of hydrate growth mechanisms in porous media: shells, growth into gas pockets and inside water volumes. • Hydrates formed in water volumes show better self-preservation than hydrate grown into gas pockets. • Multiple fast incremental water flows were captured during hydrate formation resembling the Haines jumps. [ABSTRACT FROM AUTHOR]