Hydraulic fracturing in unconventionals ascribes to "fractures should be perpendicular to the axis of least stress". First published in 1957 by, this theory stands the test of time. They determined this by injecting a plaster slurry into "ordinary gelatine" (homogeneous medium) under different principal stress orientation and observing the final geometry of the set plaster. All tests confirmed their theory, except for one test that "shows a horizontal fracture (that was tested) in stratified gelatin". They discounted this result stating that it was an outlier because of the "weakness due to bubbles between two gelatin layers". However, as all unconventional source reservoirs have weak interfaces associated to the presence of bedding planes, could the rock fabric influence the development of the stimulated rock volume (SRV) more than principal stress?
Utilizing a True Triaxial testing system the authors conducted a series of laboratory tests, conceptually similar to the "stratified gelatin" tests by Hubbert and Willis, on 80 × 80 × 80 mm3 cubes cut from a samples from the Montney Formation, Canada. The laboratory hydraulic fracturing experiments used slickwater injected in the center of the cube to mimic a single stage open hole completion, while under true triaxial conditions in the lab (σHmax 63 MPa, σhmin 43 MPa, σV46 MPa). All tests were monitored with acoustic emissions sensors. All tested cubes were then impregnated with UV epoxy, scanned via x-ray micro-CT (110 um res) and ultimately serial-sectioned at 50 μm intervals. Using deep-learning-assisted stacked-photo reconstruction, a high-resolution high-contrast 3D geometry of the resultant completion stage and the fracture network making the stimulated rock volume (SRV) was obtained. Very complex, but consistent fracture network geometries, grouped into three fracture types, were observed: one pre-existing natural axial fracture, a pseudo bi-wing fracture (⊥ to σ3), and a dominant set associated to bedding plane parting (⊥ to σ2). Indeed, bedding plane parting (⊥ to the σ2) dominated the fracture network and volume percent of the stimulated rock volume (over 60% of the SRV) within the cubes. Very few acoustic emissions were detected in samples where the bedding parallel fractures dominated the SRV.
From our laboratory experiments, in highly laminated reservoirs, the geometry of hydraulic fractures appears to be more strongly influenced by the rock fabric itself and the frequency of layered anisotropy within the completion zone then the local principal stress orientation. Based on these results we can speculate that regional markers/contacts could act as conduits for fluid pathways associated with frac hits, thus contributing to explain phenomena such as parent child interactions or casing deformation, as well as if further propagated, to allow completions fluids to lubricate critically stressed planes/faults, potentially leading to induced seismicity. Understanding the dominant driver of fracture geometry, including the characterization of bedding planes, could allow for the development of an improved risk mitigation workflow and production optimization.
