Multiple cluster perforations are typically used for multi-stage hydraulic fracturing of horizontal wells with casing hole completion to create multiple fractures in any single stage. Due to the mechanical interaction of fractures, the local principal stresses field may alter, which affects the fracture complexity. To optimize the network fracturing design, 3D multiple fractures propagation, stress distribution and fracture mechanics need to be well considered.
Firstly, The geometry model for Fuling shale gas formation has been Established, with A depth varying initial in-situ stresses, pore pressure and permeability defined. Then, a linear Drucker-Prager model with hardening is chosen for the rock, while the casing is linear elastic. Based on damage initiation of quadratic nominal stress criterion and damage evolution of B-K critical energy release rate criterion, the tensile and shear mixed mode fracture model consists of the mechanical behavior of the fracture itself and the behavior of the fluid that enters and leaks through the fracture surfaces.
Through the selection of general purpose finite element code ABAQUS, the multiple 3D fractures propagation process and stress interferences between fractures have been modeled. The results illustrate that mechanical interaction exists between any pairs of fractures, which greatly influences the stress distribution and the geometries and widths of the fractures. Induced stress increase is stronger in magnitude and influences larger region for the minimum horizontal stress than for the maximum horizontal stress. Spacing of perforation clusters and net pressures are two important factors for the successful application of network fracturing. It is possible that the cumulative interaction effect of several fractures could neutralize the stress anisotropy in the formation to enhance the fracture complexity. Taking advantage of the altered stress in the rock for a base case with three fractures, an adequate range of perforation clusters spacing and net pressure are determined for the network fracturing of horizontal wells in Fuling shale gas reservoir, and the treatment design is optimized accordingly.
This numerical procedure combined with the post-fracture production rate is also used to evaluate the efficiency of network fracturing for multi-stage fractured horizontal wells in Fuling formation, which can be served as a guidance to optimize the treatment design not only for Fuling but for other shale gas and tight gas reservoirs.