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 100 bar 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 10 mm) were exposed to pressurized CO₂ (20–35 bars) and subsequently deformation over time was monitored with a white light interferometer. This provided a lateral spatial resolution of 979 nm and a vertical spatial resolution of 200 nm, i.e. sufficient resolution so that coal and shale constituents can be tracked individually. Within 72 h epoxy swells homogeneously up to 11 μm, coal swells 4 ± 1 μm 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 swell more. The average or bulk swelling exhibits an approximate t^½ 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 × 20 μm). These patches exhibit finite swelling of on average 250 nm in 4 h (in the Pomeranian shale) and 850 μm in 20 h (in the Green River shale), where total swelling is assumed to be related to the volume of the patches of organic matter.