A Fully Coupled Model for Hydraulic-Fracture Growth During Multiwell-Fracturing Treatments: Enhancing Fracture Complexity

Multiwell-completion techniques, such as sequential fracturing, zipper fracturing, and simultaneous fracturing, have been proposed to improve fracture complexity and connectivity. Critical geomechanics behind the multiwell-fracturing techniques include pore-pressure propagation, cooling stress, tip-induced shear stress, and reversal of stress anisotropy. To optimize multiwell-fracturing treatments, we numerically investigated fracture growth from the perspective of thermo/hydromechanical (THM) coupling. The coupled geomechanics and fluid-heat-flow model is derived from a mixed finite-element (FE) and finite-volume (FV) method, which is capable of simulating multifracture growth in heterogeneous reservoirs. In this study, both hydraulic-fracture (HF) propagation and natural-fracture (NF) reactivation in opening or shearing patterns were taken into account. Particularly, an elastoplastic fracture constitutive model was adopted to predict permanent enhancement of fracture aperture. The effects of perforation-cluster spacing, well spacing, and the fracturing sequence of multiwell completion upon fracture complexity were studied, where the total hydraulically fractured area was treated as the primary indicator of HF effectiveness. By numerical parametric studies, we determined four findings. First, there is an optimal cluster spacing for maximizing the total fracture area for a stage with a given length. Cluster spacing mainly affects stress distribution during HF, which subsequently affects the path of newly created fracture propagation and crossing behaviors (i.e., crossing, arresting, or offsetting). Second, suitable well spacing should be chosen carefully to avoid the hydraulic interconnection between the tip-to-tip stages, as well as to make use of tip-induced shear stresses. Third, the fracturing sequence for multiwell completion is of critical importance. Among three multiwell-completion schemes (i.e., sequential, zipper, and simultaneous fracturing), the zipper-fracturing technique achieves the best fracturing effectiveness for this case study. Fourth, the effect of stress perturbation on NFs can be quite different, depending on the position relative to the created stimulated reservoir volume (SRV). The coupled model significantly improves our understanding of multiwell-fracturing treatments and then provides us with a means to optimize the multiwell completion, enhancing fracture complexity to effectively improve productivity.