Title : Experimental investigation and multiscale interpretation of shale hydraulic fracture propagation under non-principal-stress wellbore conditions
Abstract:
Hydraulic fracture propagation in shale reservoirs is controlled by in-situ stress anisotropy, wellbore orientation, and pre-existing bedding fractures. In deep shale gas development, wellbores may deviate from the principal stress direction due to geological constraints, well-pattern deployment, or stimulation design requirements. This non-principal-stress wellbore placement changes the near-wellbore stress concentration and controls fracture initiation pressure, fracture trajectory, and fracture-network complexity. This study presents an experiment-centered workflow that combines true-triaxial hydraulic fracturing tests with CT-based fracture reconstruction, numerical interpretation, and field-scale upscaling.
Cubic shale specimens with visible natural bedding fractures were drilled with small-diameter wellbores to reduce borehole-size effects and to retain the mechanical control of bedding planes and stress anisotropy on fracture growth. The wellbores were arranged with different orientations, including 30° to the principal stress direction. During the experiments, the specimens were loaded under controlled three-dimensional stress conditions. Fracturing fluid was injected at a constant rate, and pressure-time curves were recorded to identify breakdown pressure, multiple breakdowns, and tests constrained by the equipment pressure capacity. After fracturing, surface observations and CT reconstruction were used to identify hydraulic fractures and natural fractures.
Experimental observations show that hydraulic fractures initiated near the wellbore and propagated toward the external surfaces of the shale specimens. Under non-principal-stress wellbore conditions, fracture traces showed asymmetric propagation, indicating that inclined wellbore placement produced uneven near-wellbore stress concentration. Pressure responses differed among specimens: some tests exhibited a single rapid pressure drop after breakdown, whereas others showed multiple breakdowns, reflecting progressive fracture extension and repeated activation of weak planes.
Surface fracture mapping and CT-based reconstructions further indicate that hydraulic fractures frequently intersected natural bedding fractures. At these intersections, fracture deflection, branching, local bedding opening, and bedding-parallel propagation were observed. When bedding fractures were effectively activated, induced hydraulic fractures connected with natural fractures on multiple specimen surfaces and formed a composite fracture system. Representative numerical simulations were used to reproduce the observed fracture geometry and examine the consistency between wellbore orientation, bedding activation, and fracture trajectory.
Field-scale simulation was further introduced as an upscaling step to translate the laboratory-scale mechanism into engineering implications for well placement and fracturing-parameter design. The results indicate that non-principal-stress wellbore placement modifies near-wellbore fracture initiation and promotes direction-dependent fracture propagation in shale. Natural bedding fractures control the final fracture path and determine the complexity of the stimulated fracture network. This study provides physical experimental evidence, supported by multiscale simulation interpretation, for optimizing hydraulic fracturing design in deep shale reservoirs.
