Fracture characterization of the hydraulic fracture test site HFTS2 with X-ray microtomography

Fractures are regular features in subsurface porous media. Fractures are a critical factor in the rock’s strength, stiffness and the creation of preferential flow pathways. Although fractures are an intrinsic feature of the subsurface, current understanding of the geometry and the extent of natural and hydraulic fracture networks in unconventional reservoirs is fairly limited. Measurements such as microseismic surveys, distributed acoustics and strain sensing offer indirect evidence of fracture location and geometry. However, this information is not entirely conclusive. Recent studies have sought direct evidence of hydraulic fracture geometry through coring of the stimulated reservoir. The objective of this project is to obtain three dimensional images of core sections recovered from a hydraulically fractured reservoir located in the Wolfcamp shale in the Delaware Basin obtained through the Hydraulic Fracturing Test Site 2 project (HFTS-2). The preserved cores, three and a half inches in diameter, were moved into the micro-tomograph to ensure minimal disruption. Then, we used micro computerized tomography (micro-CT) to scan different sections of the reservoir in order to create digital models that allowed us to determine parameters such as the fracture shape and orientation. The resulting images have a resolution of 40 microns per pixel/voxel which compared with the resolution of medical-CT logs (200 microns approx.) already represent an improvement of 500%. The high resolution images makes possible to observe microfractures and shale heterogeneity at the micro-scale. We used image analysis methodologies to differentiate the fractures from the rock matrix.