3 years ago

A high resolution interferometric method to measure local swelling due to CO2 exposure in coal and shale

We present an experimental method to study time-dependent, CO2-induced, local topography changes in mm-sized composite samples, plus results showing heterogeneous swelling of coal and shale on the nano- to micrometer scale. These results were obtained using high resolution interferometry measurements of sample topography, combined with a new type of experimental microfluidic device. This device is a custom-built pressure vessel, which can contain any impermeable sample type and can be placed under any microscope. The pressure vessel itself has been tested to handle pressures up to 100bar at room temperature conditions. For the experiments reported here we used three sample types: i) epoxy and dolomite, ii) coal, epoxy and dolomite and iii) shale. These model systems (thicknesses between 2 and 10mm) were exposed to pressurized CO2 (20–35bars) and subsequently deformation over time was monitored with a white light interferometer. This provided a lateral spatial resolution of 979nm and a vertical spatial resolution of 200nm, i.e. sufficient resolution so that coal and shale constituents can be tracked individually. Within 72h epoxy swells homogeneously up to 11μm, coal swells 4±1μm and dolomite is unreactive with the dry CO2 injected here, and as such is used as a reference surface. The differential swelling of coal can be correlated in space with the macerals, where macerals with an initial higher topography (interpreted to be related to hardness) swell more. The average or bulk swelling exhibits an approximate t½ relation, indicative of diffusion-controlled adsorption of CO2 on the organic matter. Measurements of the differential swelling of both shale samples enabled tracking of individual patches of organic matter within the shale (max. 20×20μm). These patches exhibit finite swelling of on average 250nm in 4h (in the Pomeranian shale) and 850μm in 20h (in the Green River shale), where total swelling is assumed to be related to the volume of the patches of organic matter.

Publisher URL: www.sciencedirect.com/science

DOI: S0166516217308753

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