Comparison of Fracture Surface Properties in Natural and Artificially Generated Fractures in Crystalline Rock

Abstract:

Appropriate descriptions of the mechanical, hydraulic and transport properties of rock fractures are crucial for accurate modelling of flow and transport processes. These properties are intrinsically coupled, related by the geometry of the fracture surface and the physical properties of the rock.
Understanding this coupling is of particular importance, for example, when stimulating naturally low-permeable rocks, e.g. in the development of Enhanced Geothermal Systems or EGS. Natural and stimulated fractures under high pressure are rarely open. Hence, in deep geological systems, the fracture topography becomes a major determinant for the permeability and normal and shear stiffness of the rock. However, due to the variability in natural media; limited accessibility; and the influence of transient mechanical deformation, exact characterization of fractures sampled from deep reservoirs is often difficult, if not impossible.

Instead, laboratory-scale investigations are often used as a proxy to provide insight into characteristic properties, and realistic value ranges for particular fracture properties, from which model parameters can be derived. However, the properties of fractures used laboratory studies depend on many factors: sample size, fracture origin and sampling methods, etc ... which poses the question: What experimental bias is introduced in such artificially generated samples?

In this presentation, we discuss a study comparing the properties of naturally and artificially generated rock fractures. The natural fractures were gathered by overcoring pre-existing fractures in otherwise intact granite samples obtained at depth, while the artificial fractures were generated from the same samples by Brazilian tests performed on intact sub-cores. We characterize and compare the topography of the various fracture types: both natural and artificial, and Mode I and Mode II fractures are considered. Fracture damage during cyclic loading of multiple samples is also presented and the influence of crack propagation on surface topography is investigated. We report results that illustrate the differences in surface roughness with impact on fluid flow, aperture and strength, as well as the scaling of fracture topography with samples of various sizes.