ABSTRACT
Hydraulic fracturing is a well-established method to improve the exploitation of hydrocarbons or geothermal energy from impermeable reservoir rocks. In homogeneous rock mass hydraulic fractures are expected to be penny-shaped fractures oriented perpendicular to the minimum principle stress. Extent and opening width depend on the mechanical properties of the rock and the hydraulics inside the fracture. However, if the rock mass is layered and/or discontinuous the fracture geometry might deviate from being penny-shaped and aligned with the principle stresses.
We performed laboratory hydraulic fracturing experiments to examine the influence of layering and preexisting discontinuities, such as e.g. coal seams, on fracture propagation. Rock cubes are externally loaded using a true triaxial loading frame. Fractures are induced inside of the cubes by injection of fluid into a borehole. The recordings of acoustic emissions give insights into fracture evolution and geometry.
Our experimental results indicate that—at least on the laboratory scale—low friction discontinuities can have a significant influence on fracture geometry and tortuosity and therefore also on the hydraulic properties of the fracture.
