Hydraulic fracturing is crucial to geothermal and hydrocarbon recovery. This process creates new fractures and reactivates existing natural fractures forming a highly conductive Stimulated Reservoir Volume (SRV) around the borehole. The fracturing process of anisotropic rocks such as shales is examined through this report. We divide the rock anisotropy into two groups:
conventional and
unconventional (shaly) anisotropy.
As the first group, we study two extreme rock types:
Lyons sandstone, a brittle, low porosity and permeability, weakly anisotropic (11%) material and
pyrophyllite, a strongly anisotropic (19%) metamorphic rock similar chemically and mechanically to shale with extremely low porosity and permeability.
As the second group, shale samples (18% anisotropy) from Wolfcamp formation are studied. The calcite filled veins are observed to be mostly subparallel to the fabric direction. Brazilian tests are carried out to observe the fracture initiation and propagation under tension. Strain gauges and Acoustic Emission (AE) sensors record the deformation leading to and during failure. SEM imaging and surface profilometry are employed to study the post-failure fracture system and failed surface topology. Fracture permeability is measured as a function of effective stress. The effect of anisotropy on fracturing is also investigated by rotating the fabric direction of the sample disks relative to the loading axis through increments of 15 degrees.
The rock microstructure, lamination, and brittleness control the activation of the layers. Lyons sandstone shows a wide brittle fracture with larger process zone with twice as much layer activation at lower stress levels. The fracturing process in shale is however a coupled function of rock fabric and calcite veins. The veins easily activate at 15 degrees orientation with respect to the loading axis at 30% of the original failure load. The resulting unpropped fracture has enhanced permeability by orders of magnitude. The findings of this research bring new insights toward an economic and efficient completion design. Fracking through a deviated well reduces the breakdown pressure significantly and activates a large number of veins with enhanced conductivity without the need for proppant injection.
