ABSTRACT:

Characterizing the heterogeneous structure of shale rocks plays a critical role in understanding their gas storage, fluid transport, and geomechanical properties. In this study, multiple imaging scales are used to better view features involved in the geomechanical properties of shales. Imaging techniques are combined with routine analytical measurements (e.g., X-ray diffraction and mercury injection) to investigate multiscale heterogeneity of a carbonate-rich 2.54-cm core sample from the Eagle Ford shale of southern Texas. A stitched computed tomographic (µCT) image was collected using an X-ray microscope with a voxel dimension of 13.4 µm. Within this image, several regions representing different matrix densities and features such as fractures were selected and imaged using a non-destructive method called “interior tomography” with an improved resolution of 3.6 to 3.7 µm. After the interiors were imaged, 5-mm diameter cores were drilled from the regions corresponding to the interior tomography regions using fiducial marks on the exterior of the core as reference points. High-resolution µCT scans of 5-mm cores have voxel lengths of 4.5 to 5.1 µm whereas interior tomography images have voxel sizes of 1.1 µm. The 5-mm cores were similarly subsampled as 1-mm cores that are imaged as whole cores and an interior tomography. The resulting data collection allows the comparison of µCT data at different scales and core diameters as voxel dimensions alone are not sufficient for this task. For instance, the interior tomography for the 2.54-cm core has smaller voxel size (3.6 – 3.7 µm) compared to that generated by the 5-mm core scan (4.7 – 5.1 µm); however, small features in the 5-mm core scan are better resolved than in the 2.54-cm core interior scan. Image quality and ability to segment properly is affected by the different µCT settings and additional rock volumes that the X-rays penetrate. In addition, volumetric data generated by segmenting 3D reconstructions of µCT data are compared to independent mineral, organic matter, and porosity measurements. The interpretation of multiscale µCT images improves our understanding as to the porosity distribution and rock fabric in this shale sample. These two properties factor into the geomechanical behavior of carbonate-rich shales.

1. INTRODUCTION
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