3D observations of the hydraulic fracturing process for a model non-cemented horizontal well under true triaxial conditions using an X-ray CT imaging technique

To investigate the spatial evolution of fractures in non-cemented horizontal wells, cubic cement specimens were fractured in triaxial experiments to reproduce hydraulic fracturing in field applications. Systematic experiments were conducted on specimens with different azimuth angles (0°, 30°, 45°, 60° and 90°), horizontal principal stress anisotropies (10 MPa and 4 MPa) and fracturing fluid viscosities (60 mPa·s and 5 mPa·s). All the tests were performed by injecting fluid at a constant rate of 9 cc/min. High-resolution non-destructive 3D X-ray microtomography was used to record the internal evolution of the induced fractures. The X-ray computed tomography (CT) imaging technique can produce complex 3D images of fracture systems with little artificial noise. The complex fracture geometries observed are a result of the rapidly changing stress conditions at the wellbore wall during injection and of the mechanical interaction among the fractures. The magnitudes of the well azimuth angle, principal stress anisotropy and injection fluid viscosity play significant roles in the evolution of the induced fractures. For a high horizontal differential stress of 10 MPa, the fracture morphologies are primarily determined by the in situ stresses, and the fractures extend in the direction of the maximum principal stress. However, when the horizontal differential stress is 4 MPa, the fracture geometry is influenced by the combined effects of the induced stress from the open-hole wellbore, fracture internal pressure distribution, far-field stresses and azimuth angle. In general, an induced fracture is planar at both a low (0°) and high (90°) azimuth angles, while twisted fractures form at intermediate azimuth angles (30°, 45° and 60°). In addition, the injection fluid with a low viscosity of 5 mPa·s easily infiltrates the formation, inducing a fracture network, in contrast to the smooth fracture plane induced by the injection fluid with a high viscosity of 60 mPa·s. Finally, we propose a new parameter, called the dimensionless net pressure, which can reflect the combined effects of the principal horizontal stress difference, well azimuth angle and injection fluid parameters on the evolution of the hydraulic fractures. The complexity of the fracture geometries is strongly and positively correlated with the dimensionless net pressure.