**7. Constraining the hydraulic fracture propagation in the vertical direction in the presence of weak interfaces and natural fractures**

Interfaces are among the geological features that are known to have an impact on the vertical propagation pattern and the final fracture height in unconventional reservoirs. Interfaces limit adjacent lithologies with similar or contrasting properties. It could be a material or just contact between two adjacent lithologies. Typical material thickness is between 1 and 500 mm for volcanic ash layers, and μm to mm thickness for mineralized veins, highly or partially mineral filled fractures, organic matters layers in the form of bitumen lubricating film or kerogen coating the surface of the interface [21] and bentonite layers [22] than could vary in thickness up to a few centimeters.

Interface mechanical properties can either be strong or weak and can be further weakened by tectonic deformations. They are a source of displacement discontinuities and delamination and are associated with fracture propagation behaviors like kinking, offsets, bifurcation, stepping over and termination. This suggests that displacement continuity hypothesis that are used in many hydraulic fracturing models where contact mechanics between layers with stick conditions (no sliding) is used to model the interface between layers [23, 24], is not valid for modeling real interface effects. In fact, using stick condition within a stratified structure could not account for sliding and de-cohesion between the layers due to hydraulic fracture pressurization. Thus, the need to use a proper interface model that (i) accounts for the displacement discontinuity between layers and (ii) allows the decohesion and interface delamination at the interface.

 Explicitly modeling the interfaces presents some modeling challenges: (i) interfaces could be very thin layers that require the deployment of high-resolution model, (ii) interface mechanical properties are difficult to access given the well logs limitation in detecting them.

Aimene et al. [25] introduced the combined use of Anisotropic Damage Mechanics (ADaM) model and interface models in MPM to model the effects of interfaces in 2D and 3D hydraulic fractures problems. Multiple interface modeling tools were deployed [25] to achieve a better understanding of the impact of the
