Fracture aperture reconstruction and determination of hydrological properties: a case study at Draix (French Alps)

Abstract

International audienceWe propose two techniques for fracture aperture reconstruction. The first one is a correlation technique that estimates the normal aperture or the tangential shift across a discontinuity whose sides present geometrical similarities. The only required material is a pair of appropriately controlled images of each side. Here, the images are maps of the corresponding side topography, obtained from laser profilometry. Assuming a purely normal opening, it is possible, from two corresponding sides of a given discontinuity in a core log, to infer the precise geometry of the in situ aperture. The second technique allows to retrieve the three-dimensional geometry of a sealed discontinuity from non-independent topography measurements of both sides. Both techniques are applied to discontinuities extracted from a core drilled down to 20 m in a fractured marl formation at Draix (French Alps). The probability density functions of the aperture of the sealed and open discontinuities are shown to be Gaussian. At the sample scale, the sealed fracture aperture is self-affine, while the open one shows a cross-over from a self-affine regime at very small scales to an uncorrelated regime at largest scales. After extrapolating those scaling laws at the scale of the whole formation, we discuss when the aperture roughness affects the hydraulic properties of the Draix fractured bedrock. The overall estimated permeability is significant (10−9 − 10−8 m2), consistently with some previous indirect inferences

Geological discontinuities, main flow path and chemical alteration in a marly hill prone to slope instability: Assessment from petrophysical measurements and borehole image analysis

“…These observations suggest that the possible rapid flow pathways may occur through transmissive discontinuities. This is in agreement with fracture aperture reconstructions in these marls suggesting that the overall permeability is significant (Neuville et al , 2011b). The borehole geophysical data and images allowed evidencing several types of discontinuities in the marls, some of which may be transmissive and may play a role in rapid water flow circulation described above.…”

Hydraulic transmissivity and heat exchange efficiency of open fractures: a model based on lowpass filtered apertures

“…Thus, more realistic models of fractures have been proposed, which often consist in one of the two following models: the void between two identical self‐affine fracture surfaces, opened and shifted with respect to each other, or the void between two statistically independent self‐affine surfaces. Both geometries have been observed, for example, in natural cases, like the joints present at depth in a marl formation analysed in Draix (Neuville et al . 2011a,b).…”

“…The determination of the detailed morphology of the open volume in a fracture or a joint is difficult to perform in situ , and seldom done beyond the characterization of an average aperture. Recently, we have performed such a study on a natural example of rock (black marl) analysed from a log core, and indeed found some example of self‐affine aperture fields at the scale of observation (10 cm) (Neuville et al 2011a,b).…”

“…In practice, some knowledge about the geometry of a fracture can be acquired by the study of core logs (e.g. Neuville et al 2011a,b) at scales between micrometers and centimetres—some scaling hypothesis being required to extrapolate the large scale aperture. Unfortunately, such logs are rare.…”

High-resolution fracture aperture mapping using optical profilometry

“…[6] Different reconstruction techniques have been used to quantify aperture fields in efforts to: (i) relate fracture surface roughness to fracture aperture [Brown, 1995;Lanaro, 2000]; (ii) quantify characteristics of fractures that intercept boreholes [Tatone and Grasselli, 2012;Neuville et al, 2012]; and (iii) understand how surface roughness may influence the hydraulic [Durham and Bonner, 1994] and mechanical [Brown and Scholz, 1986;Sharifzadeh et al, 2008] properties of fractures. In addition, these reconstruction techniques provide the potential to quantify changes in fracture aperture induced by mechanical and chemical processes [Durham et al, 2001;Yasuhara et al, 2006;Elkhoury et al, 2013].…”