A rock fracture is a mechanical break or discontinuity that separates a rock body into two or more parts. The continuity or cohesion of the rock body is lost across a fracture. Fractures are formed in response to stress on a rock. A rock breaks and forms a fracture when the applied stress reaches the rock strength. Vertical fractures improve connectivity between multiple layers, and can aid the production of geothermal and petroleum reservoirs.
The heterogeneity of layered reservoirs leads to significant variation in mechanical properties, which in turn influence fracture nucleation, fracture growth and fracture geometry. This variation in rock mechanical properties, combined with layer thickness, is called mechanical stratigraphy. Natural fractures are subject to controls imposed by mechanical stratigraphy. Focusing on the mechanisms that control natural fracture development can improve fracture characterization. As rock strength is an important part of mechanical stratigraphy, the term mechanical contrast is introduced to examine the effect of contrasts in rock strength of adjacent layers.

This study examines the effect of the mechanical contrast and confining pressure on fracture behaviour in layered rocks in the laboratory. The focus of this study is threefold, it examines the effect of mechanical contrast and confining pressure on fracture propagation, fracture orientation and fracture aperture in layered rocks.
Unconfined and confined compressive strength tests have been performed on layered samples with varying mechanical contrasts at different confining pressures. A total of 169 tests have been performed which include confined and unconfined compressive strength tests on layered and monophase samples and brazilian tensile strength tests and velocity measurements on monophase samples.

The results show that fractures initiate in the weakest layer and propagate through the layer interface or are contained within the weakest layer. Unconfined compressive strength tests showed that differences in rock strength do not always act as a containment barrier.
The combination of mechanical contrast and confining pressure does control the containment of fractures within a layer. Lower horizontal compressive stresses are required to contain fractures when the mechanical contrast increases.
Mechanical contrast does not seem to influence fracture aperture. Confining pressure however greatly influences fracture aperture as it limits the ability of fractures to dilate.
Results show that fracture orientation is controlled by mechanical contrast. Fractures refract at layer interfaces when the mechanical contrast is sufficiently high. Confining pressure does not seem to affect the refraction of fractures.

The experimental results can improve the understanding of fracture containment, fracture aperture and fracture orientation in layered rocks at subsurface conditions. The mechanical contrast of the layered rocks, combined with the stress conditions need to be considered when characterizing subsurface fractures.
Vertical connectivity between layers is of importance when predicting fluid flow through reservoirs. As frac tures often serve as preferential fluid flow paths, correctly interpreting fracture characteristics is important for successful development of layered reservoirs.

Hydrocarbon or geothermal reservoir often consists of several rock layers from different lithology. The various lithologies have their own number of mechanical properties and the layering effect introduced the term of mechanical contrast, which represents the ratio of rock strength between adjacent layers. Mechanical contrast and confining pressure highly influence the fracture behavior in the layered rocks.
In this study, fractures in layered rocks are investigated, starting with its geometry and also the stress field contributed to the fracture generation and development. The fracture geometry such as fracture length, average aperture, aperture distribution and orientation are quantified in a two dimension slice image. The study focused on comparing the fracture behavior when (a) the layered rock compositions are the same between samples with increasing confining pressure or (b) the different compositions of layered rocks (different mechanical contrast) between samples in the same confining pressure.
The results show that fracture tends to propagate through layer interface when the mechanical contrast between adjacent layers and the confining pressure are low. The fracture in the weak layer developed at a gentler dip (shear fracture) with higher fracture aperture compared to the ones in the strong layer which almost vertical (tensile fracture). In addition, the shear fracture in the weak layer usually accompanied by the zone of cataclastic flow while the tensile fracture has a more clear pathway for fluid flow.
However, mode I opening/tensile fractures are less likely to affect fluid flow in the reservoir because their aperture is insignificant at depth. While in mode II sliding/shear fractures, only several parts along the fracture that can provide the open space, which depend on the presence of jogs and irregularities on the fracture surfaces.
The results from fracture measurements show that in the weak layer, average aperture and aperture distribution will reduce with the increasing of confining pressure, but increased with the increasing of mechanical contrast. Average fracture aperture and distribution have a significant role in capillary pressure. The higher average aperture will reduce the amount of pressure needed to flow the fluid, while a higher number of aperture standard deviation (aperture distribution) has a contrasting effect. The average aperture has a bigger impact on capillary pressure compare to aperture distribution. Thus, by increasing the confining pressure or decreasing the mechanical contrast, the required pressure for fluid to flow is increasing.
Furthermore, the numerical modeling is performed by imitating the rock mechanical properties and the fracturing conditions from the laboratory experiment. The results show that under compressive stresses, the layered rocks still generate tensile stresses around the interface within the strong layer. The tensile stresses occur because of the stress transfer between adjacent stiff and soft layer with a bonded interface. The presence of tensile stress and the crack-tip stress are responsible for the generation of the tensile fracture in the strong layer for all samples.
The effect of varying the number of confining pressure, Poisson’s ratio and Young’s modulus on the tensile stresses distribution are also performed. The sensitivity study shows that Poisson’s ratio has a more significant impact compared to Young’s modulus on both maximum tensile stress and thickness of tensile region. Higher Poisson’s ratio resulting in higher tensile stresses, while on Young’s modulus it depends on the contrast between adjacent layers rather than the magnitudes.
Understanding the fracture behavior in layered rocks is beneficial for reservoir characterization, as fractures can enhance the permeability and providing vertical connectivity between isolated reservoirs. Accurately interpret 3D natural fracture distribution can help the estimation of the resource and recoverable potential early in field life. It will also contribute to optimizing the well placement and completion design for efficient production planning.