Influence of Mechanical Contrast and Confining Pressure on Fracture Behaviour in Layered Rocks

Abstract

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.