How to determine hydraulic, mechanical and chemical apertures of natural fractures?
Abstract
In many geothermal, hydrocarbon or groundwater reservoirs or host rocks for nuclear waste disposals, fractures play an important role in acting as conduits or barriers, and can therefore significantly co-determine if reservoirs are productive or not, or if nuclear waste disposals are safe or not. Often such reservoir models are based on discrete fracture network (DFN) models, whose performance however rely strongly on reliable input parameters representing actual fracture properties 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 of a natural sandstone. Our results prove that the air permeameter allows direct and reliable measurements of hydraulic apertures in the laboratory and the field. Additionally, the novel air permeameter could be successfully validated by flow through experiments using various types of fractured core samples. In contrast, the two other methods, microscope camera and laser scanner, only provide mechanical apertures. However, in order to simulate fracture closure under normal stresses, an innovative contact mechanical approach is introduced for a granodiorite fracture. The simulations indicate the best performance for an elastic-plastic 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. These 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 new equation to estimate hydraulic apertures is introduced, which includes a new geometry factor α for different crystal geometries. Our results clearly demonstrate that novel experimental and numerical methods are available to determine various fracture apertures improving our understanding of coupled processes on the fluid flow behaviour in fractured media.
Acknowledgements to Florian Amann, Sally M. Benson, Christoph Butscher, Chaojie Cheng, Frieder Enzmann, Da Huo, David Jaeggi, Harald Milsch, Christoph Naab, Lars Pastewka, Xavier Riess, Jens Oliver Schwarz and Daniel Vogler- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.H32B..02B
- Keywords:
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- 1805 Computational hydrology;
- HYDROLOGY;
- 1822 Geomechanics;
- HYDROLOGY;
- 1829 Groundwater hydrology;
- HYDROLOGY;
- 1832 Groundwater transport;
- HYDROLOGY