Hydraulic Fracturing In Unconventional Reservoirs: The Impact of Layering and Permeable Frictional Interfaces
Authors: Qian Gao , Ahmad Ghassemi 
Published: September 2019

Abstract
The hydraulic fracture containment and the impact of layering on pumping energy are critical factors in a successful stimulation treatment. Height confinement is needed to ensure effective stimulation of target zones and to maintain the fractures in the target zones. Also, the existence of beds with different ductility can impact the net pressure and pumping requirements. Layered rock properties, in-situ stress, and formations interfaces influence the lateral and height growth of hydraulic fractures. Conventionally, it is considered that the in-situ stress is the dominant factor controlling the fracture height. The influence of mechanical properties on fracture height growth is often ignored or is limited to consideration of different Young's modulus. Also, it is commonly assumed that the interfaces between different layers are perfectly bounded without slippage, and interface permeability is not considered. In-situ experiments have demonstrated that variation of modulus and in-situ stress alone cannot explain the containment of hydraulic fractures observed in field (SPE39950). Enhanced toughness, in-situ stress, interface slip and energy dissipation in the layered rocks should be combined to contribute to the fracture containment. In this study, we consider these factors in a fully coupled 3D hydraulic fracture simulator developed based on finite element method. We use laboratory and numerical simulations to investigate the above factors and how they impact hydraulic fracture propagation, height growth, and injection pressure.
In this work a 3D fully coupled hydro-mechanical model is developed and utilized. The model uses a special zero-thickness interface element and the cohesive zone model (CZM) to model fracture propagation, interface slippage, and fluid flow in fractures. The nonlinear mechanical behavior of frictional sliding along interface surfaces is considered. The hydro-mechanical model has been successfully verified through benchmarked analytical solutions. The influence of layered Young's modulus on fracture height growth in layered formations is analyzed. The formation interfaces between different layers are explicitly simulated through the usage of the hydro-mechanical interface element. The impacts of mechanical and hydraulic properties of the formation interfaces on preventing hydraulic fracture growth are studied.
Hydraulic fractures tend to propagate in the layer with lower Young's modulus so that soft layers could potentially act as barriers to limit the height growth of hydraulic fractures. Depending on the mechanical properties and the conductivity of the interfaces, the shear-slippage and/or opening along the formation interfaces could result in flow along the interface surfaces and terminate the fracture growth. The frictional slippage along the interfaces could be an effective mechanism that contributes to the containment of hydraulic fractures in layered formations. It is suggested that whether a hydraulic fracture would cross a discontinuity depends not only on the mechanical properties but also on the hydraulic properties of the discontinuity; both the frictional slippage and fluid pressure along horizontal formation interfaces contribute to the reinitiation of a hydraulic fracture from a pre-existing flaw along the interfaces, producing an offset from the interception point to the reinitiation point.