Multiscale Imaging of Gas Storage in Shales

Four intact 2.54-cm-diameter cores from different shale plays (Barnett, Haynesville, Eagle Ford, and Permian Basin) were analyzed for their gas-storage capacity by use of a novel multiscale-imaging methodology spanning from centimeter to nanometer scale. Gas-storage (free and sorbed gas) capacity was investigated at the core scale with carbon dioxide (CO2) and krypton (Kr) by use of X-ray computed tomography (CT) with voxel dimensions of 190 x 190 x 1000 mu m. Also, 2D tiled images were acquired with a scanning electron microscope (SEM) and stitched together to form 2.54-cm-diameter mosaics with a pixel resolution of 1.5 mu m. 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 storage. Registration of CT scans (vacuumed and gas-filled) as well as 190-mu m-resolution CT-derived gas-storage maps with 1.5-mu m-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 by use of silver paint and intermediate-resolution microCT scans with cubic voxel dimensions of 27 mu m. After alignment, the relationship of enhanced storage zones with open fractures and reduced storage 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 storage 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 with secondary cemented matrix. These gas-sorption experiments prove the feasibility of dynamic core- to nanometer-scale CT/SEM/EDS image registration to improve sample characterization. To our knowledge, this is the first investigation of core-scale CO2-gas storage using 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 and guides engineers toward optimal CO2-injection zones for enhanced gas recovery.