A high resolution interferometric method to measure local swelling due to CO2 exposure in coal and shale
Seminar by Anne Pluymakers and co-authors:
Pluymakers, A.1), Liu, J.2,3), Kohler, F.1), Renard, F. 1,4), Dysthe, D. 1)
1) PGP, University of Oslo, postboks 1048, 0316 Oslo, Norway; 2) Department of Mechanical Engineering, Eindhoven University of Technology, 5600 MB, Eindhoven, Netherlands; 3) Department of Earth Sciences, Faculty of Geosciences, Utrecht University, 3584 CD, Utrecht, Netherlands; 4) University Grenoble Alpes & CNRS, ISTerre, Grenoble, France
Surface deformation of a sample Pomeranian shale due to CO2 exposure (~30 bars, ~40 hours), measured using a custom-build pressure cell and a white light interferometer optical microscope. The circles indicates the location of a patch of organic matter (black), interspersed with pyrite (bright), an area which is investigated in greater detail in Fig. e-g); a) backscattered electron image taken before completing sample assembly. The dotted rectangle indicates the approximate location of topography measurements shown in b) where the initial topography is measured with the white light interferometer; c) map with the standard deviation of each pixel through time. High numbers indicate the pixel moved in height during the experiment. In approximately the same location as where the organic matter was present in the SEM image (white circle) there are measurable topography changes; d) histogram of the height data at different times. There is a small change in the histogram shape, close to the measurement error of 200 nm. e-g) Close-up of the area containing organic matter; e) a 20 x 20 µm region exhibits more swelling than the surrounding shale surface. This is shown in further close-up in f). Note the change in scale of the z-axis; g) mean swelling of the area without organic matter and the smaller area with organic matter versus time. The solid line is the same data set smoothed with a median filter with logarithmic binning of time. On average this patch swells about 250 nm in the first hours of the experiment.
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 2-10 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 hours 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 (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 × 20 µm). These patches exhibit finite swelling of on average 250 nm in 4 hours (in the Pomeranian shale) and 850 µm in 20 hours (in the Green River shale), where total swelling is assumed to be related to the volume of the patches of organic matter.