The serial section technique was used to construct a high-resolution and high-quality fracture network image stack of a true triaxial hydraulic fracturing experiment on a shale sample from the Montney formation. The stack was used to create a point cloud and fracture surface meshes that were used for fracture analysis. Fractures were separated by subtracting the fracture intersections from the point cloud then applying a connected components algorithm to separate them. Point clouds were generated from these fractures and were thinned to achieve a 1-voxel thickness. After thinning, they were smoothed to reduce the aliasing effect from the image stack grid structure. Fractures were identified as either a bedding or non-bedding fracture by proxy of their orientation. Then, their surfaces were analyzed using a directional roughness metric. This roughness metric was used along with information about the stress state to evaluate the peak shear strength criterion for each individual fracture. The slipping potential of these fractures under the stress state applied by the true triaxial frame was estimated by the ratio of the actual shear stress on the fracture and the peak shear strength criterion.
Hydraulic fracturing (HF) creates flow channels either by opening pre-existing planes of weakness or by creating new ones within the rock matrix. The geometries of these fractures differ depending on a variety of influencers such as bedding, rock fabric, material strength, the local stress environment, and spatial heterogeneities embedded within the rock mass. The morphologies of these fractures can provide useful information on the expected fracture geometry and production of a reservoir. This can be achieved by fracture geometry quantification with roughness metrics and aperture to gain information for estimating fluid resistance and proppant performance. However, this information is not easy to obtain from the field.
Laboratory HF experiments provide useful insights to the mechanics of hydraulic fracturing performed in the field. Because they are physically accessible, the fractures created by the experiment can be opened and examined. Tan et al. (2017) illustrates an example of an examination of the fracture networks of multiple HF experiments performed under true triaxial stress. Their experiments provided insights on the sensitivity of the fracture network geometry to fluid viscosity and injection rate. However, this required them to take apart the sample to gain access to internal fractures. While they were only interested in the general fracture structure, this action may have potentially lost information on the smaller fractures within the network.