Abstract
Natural fractures are common in unconventional reservoirs (e.g., tight oil and gas, shale oil and gas), for which hydraulic fracturing is the main exploitation method. Natural fractures influence the propagation and final transformation effect of hydraulic fractures, which is an important factor for fracture optimization design. In this work, the method of globally embedding zero-thickness cohesive zone elements is applied to establish a dynamic propagation model of hydraulic fractures in naturally fractured reservoirs, and the influence of natural fracture development on the fracture propagation patterns is investigated. The results show that the hydraulic fracture propagation paths in fractured reservoirs are highly complex, and steering, branching, and merging of the propagation paths may occur. When the natural fracture development is small, the fracture propagation pattern is mainly influenced by the minimum horizontal principal stress. In contrast, when the natural fracture development is large, the fracture propagation pattern is influenced by the natural fracture distribution. With increasing natural fracture angle, hydraulic fractures can easily pass through natural fractures and form wide fracturs. Higher numbers of natural fracture groups and fractal dimensions increase the number of fracture propagation directions and communicating natural fractures, and the fractures tend to be narrow and complex.
Highlights
A large portion of global oil and gas resources occur in naturally fractured formations with low permeability
As the distribution direction of natural fractures in a reservoir increases, the hydraulic fracture propagation morphology becomes increasingly complex and forms multiple branching fractures, which are less influenced by the stress field
The hydraulic fracture propagation of fractured reservoirs is influenced by a combination of the natural fracture distribution, which determines the overall fracture propagation path direction
Summary
A large portion of global oil and gas resources occur in naturally fractured formations with low permeability. Hydraulic fracturing technology can improve the reservoir seepage characteristics, connect natural reservoir fractures, and improve the recovery of oil and gas, which are important for achieving economic benefits from fractured reservoirs (Tang et al, 2020; Xia et al, 2020; Jun et al, 2021; Ziyuan et al, 2022). The existence of natural fractures poses serious challenges in hydraulic fracturing design. Mayerhofer et al first introduced the concept of a modified reservoir volume (Rahman and Rahman, 2010), and complex network structures have since become a common assumption in the predictions of the true state of hydraulic fractures in naturally fractured unconventional reservoirs. A quantification of the hydraulic fracture type at the field scale is difficult, but can be achieved by indirect measurements and observations and inter-well interference analysis, which can be used to infer the upper limits of the fracture size
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