Multiscale Imaging of Gas Storage in Shales

Abstract

Summary 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 × 190 × 1000 µm. 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 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 storage. 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 by use of silver paint and intermediate-resolution microCT scans with cubic voxel dimensions of 27 µm. 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.

Supercritical methane adsorption and storage in pores in shales and isolated kerogens

“…By quantifying the nature of pores in the various mineral and organic building blocks of shales we can consider the larger scale flow properties of shales and thus their potential either to transmit fluid (useful for a shale gas reservoir) or to retain fluid (useful for a CO 2 or nuclear waste storage site). These findings are consistent with a recent study [85].…”

A Critical Review of the Physicochemical Impacts of Water Chemistry on Shale in Hydraulic Fracturing Systems

“…The nanoscale pores can be interparticle, intraparticle, or even inside the organic matter itself, while the natural fractures can be multiple meters long with a millimeter-scale fracture aperture . The intrinsic shale transport properties are dictated by the smaller densely occurring natural microfractures. , Hydrocarbon gas is stored in these pore spaces as compressed gas and in the kerogen and clay matrix as dissolved gas . Similarly, the polar components of oil facilitate sorption and aid storage in oil-wet nanopores. − When pressure drawdown is applied to such a system, the compressed oil and gas in the pores is recovered first followed by gas desorbed from the pore walls and diffused from the kerogen and clay matrix resulting in the typical production profile shown in Figure .…”

Multi-Scale Microfluidics for Transport in Shale Fabric

“…The µFN includes the addition of 1 µm 'microfractures', at a bearing of 55°, to the MFN's backbone. The geometry of the network and contact angles between intersections is based on an actual fractured shale sample ( Figure 7 of [14] and Figure 10 of [41]). Figure 2 shows the structure of the two micromodels and Table 1 lists the number of fractures of a given aperture for the MFN and the µFN micromodels.…”