Three-dimensional complex fracture propagation simulation: Implications for rapid decline of production capacity

In this study, a simulation model of fracture network geometry in fractured and porous elastic reservoirs is established based on the globally embedded three-dimensional cohesive zone model (3D CZM). The effects of stress interference between natural fractures (NFs), horizontal stress difference (HSD), injection rate, and fluid viscosity on fracture geometry were studied and the phenomenon of rapid failure of unconventional reservoir productivity was explained from a coupling fluid flow/geomechanics perspective. The intersection behavior of hydraulic fracture (HF) and NF is simulated; the simulation results are in good agreement with Blanton's criterion, proving the reliability of the 3D CZM. The research results show that in the process of hydraulic fracturing, in addition to the strong stress interference between HFs, there is also strong stress interference between NFs. Stress interference has an important impact on the traction separation of cohesive elements (CEs) and a continuous dynamic impact on the NF opening. For reservoirs with a small HSD, higher injection rate and fluid viscosity produce smaller stimulated reservoir volume (SRV) length, but larger SRV width. Complex fracture networks tend to develop in the vicinity of horizontal wellbores, which leads to rapid failure of productivity. For this kind of reservoir, minimizing the impact of stress interference with a small injection rate is recommended instead of maximizing the injection rate and fluid viscosity. For reservoirs with a large HSD, lower injection rate and fluid viscosity produce larger SRV length and smaller SRV width. The fracture geometries tend to develop simple straight fractures, which leads to low initial productivity. Thus, adopting a relatively large injection rate and fluid viscosity is recommended to make full use of stress interference effects to increase the development of complex fractures near horizontal wellbores. The results of this study can serve as a guide in evaluating fracture complexity, SRV, proppant migration, drainage reservoir volume (DRV), and well completion design; they can also promote the in-depth understanding of the coupling effect between fluid flow and geomechanics.