Determination of hydraulic, mechanical and chemical fracture apertures

In various reservoirs such as geothermal reservoirs or host rocks for nuclear waste, fractures and in particular fracture apertures play a crucial role in acting as conduits or even barriers, and therefore control fluid flow and solute transport in such reservoirs or host rocks. Often such reservoirs are simulated by discrete fracture network (DFN) models, whose performance however rely strongly on reliable input parameters such as fracture apertures under different conditions. Hence, in this study we examine various novel field and numerical methods, which are able to determine hydraulic, mechanical and even chemical apertures of natural fractures. First, we compare three different methods, (1) syringe air permeameter, (2) microscope camera and (3) laser scanner for determining hydraulic fracture apertures. Our results prove that the air permeameter allows direct and reliable measurements of hydraulic apertures in the laboratory and also in the field. Additionally, the novel air permeameter could be successfully validated by flow through experiments using various types of fractured core samples. In contrast, microscope camera and laser scanner only provide reliable mechanical apertures. In order to also simulate fracture closure under normal stresses, an innovative contact mechanical approach is introduced and validated using a granodiorite fracture. The simulations indicate the best performance for an elastic–plastic (EP) model, which fits almost perfectly the experimentally derived normal closure data. Finally, a phase-field model (PFM) for hydro­thermally induced quartz growth is used to understand the effect of sealing fractures on the flow behaviour. Our results demonstrate that flow behaviour and hydraulic properties of such chemically altered fractures, i.e. chemical fractures, significantly depend on the evolving crystal geometries. Consequently, a novel equation to estimate hydraulic apertures is derived, which includes a geometry factor α for dissimilar crystal geometries (α = 2.5 for needle quartz and α = 1.0 for compact quartz). Finally, the outcome of our studies clearly demonstrate that nowadays novel experimental and numerical methods exist to precisely determine various fracture apertures improving our understanding of coupled processes on the fluid flow behaviour in fractured media.

Acknowledgements to Florian Amann, Christoph Butscher, Frieder Enzmann, Christoph Naab, Jens Oliver Schwarz and Daniel Vogler