A high resolution interferometric method to measure local swelling due to CO2 exposure in coal and shale
Published by: Pluymakers A., Liu J., Kohler F., Renard F., Dysthe D.
Coal swelling, which shows the swelling of epoxy and coal through time. All three experiments are performed on the same sample. a) reflected light image taken with an optical microscope. There is an irregular pattern in the coal sample of light and dark spots, interpreted to be related to the maceral and mineral content. The bright spots could be the inorganic material (ash content 5.2%). b);c) initial map of dolomite and epoxy for experiment dolcoal3, with different vertical scales to highlight the initial epoxy (b) and coal (c) topography. Coal topography is related to the morphology visible in a) (interpreted to be the maceral and mineral distribution); d);e) differential map of dolcoal3 showing δz (swelling), after ~ 66 h of exposure. Color scale for δz in e) is adapted to show the heterogeneous swelling of individual coal macerals. Squares on d) indicate areas from which averages are taken that are shown in Fig. 7. f) Initially high macerals exhibit slightly more swelling, shown by the change in angle of the ‘tail’ compared with the bulk of the data. Contourplots are added to guide the eye.
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 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 × 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.