Multiscale Imaging of Gas Adsorption in Shales

Abstract

Abstract Four intact one-inch diameter cores from different shale plays (Barnett, Haynesville, Eagle Ford, and Permian Basin) were analyzed for their gas storage capacity using a novel multiscale imaging methodology spanning from cm- to nm-scale. Gas storage capacity was investigated at the core-scale with carbon dioxide (CO2) and krypton (Kr) using X-ray computed tomography (CT) with voxel dimensions of 190×190×1000 μm. Also, 2D tiled images were acquired using a scanning electron microscope (SEM) and stitched together to form one-inch diameter mosaics with a pixel resolution of 1.5 μm. Multiscale image registration was then carried out to align the CT data with the SEM mosaics. Energy dispersive spectroscopy (EDS) generated elemental spectra maps and subsequent component maps for regions with either substantial or minimal gas storage to assess the interplay of structural features (e.g., fractures) and matrix composition with respect to gas accessibility and adsorption. Registration of CT scans (vacuumed and gas-filled) as well as 190 µm-resolution CT-derived gas storage maps with 1.5 µm-resolution SEM mosaics is straight forward for samples with dense features (such as calcite-filled fractures) that are resolvable by CT imaging. Alignment methods were developed for samples lacking these features including registration marks using silver paint and intermediate resolution microCT scans with cubic voxel dimensions of 27 μm. Once aligned, the relationship of enhanced adsorption zones with open fractures and reduced adsorption regions with secondary mineralization (such as nodules) is apparent for the carbonaceous samples. For the clay-rich Barnett sample, fracture-filling calcite is associated with reduced adsorption similar to the other samples; however, secondary carbonate cementation within the clay matrix aligns with regions with substantial Kr and CO2 gas storage. In contrast, clay-rich matrix regions lacking secondary carbonate cementation exhibit minimal gas storage potential. Causes for this unexpected result include reduced gas accessibility and, possibly, low organic matter content in the clay-rich matrix compared to secondary cemented matrix. These gas adsorption experiments prove the feasibility of dynamic core- to nm-scale CT/SEM/EDS image registration to improve sample characterization. To our knowledge, this is the first investigation of core-scale CO2 gas adsorption employing multiscale imaging. CT and SEM image registration reveal spatial details regarding gas accessibility and storativity at the core-scale. This work also supports the potential of carbon storage in shale formations as well as guides engineers toward optimal CO2 injection zones for enhanced gas recovery.