Measuring hydraulic fracture apertures: a comparison of methods

Abstract

Abstract. Hydraulic fracture apertures predominantly control fluid transport in fractured rock masses. Hence, the objective of the current study is to investigate and compare three different laboratory-scale methods to determine hydraulic apertures in fractured (Fontainebleau and Flechtinger) sandstone samples with negligible matrix permeability. Direct measurements were performed by using a flow-through apparatus and a transient-airflow permeameter. In addition, a microscope camera permitted measuring the mechanical fracture apertures from which the corresponding hydraulic apertures were indirectly derived by applying various empirical correlations. Single fractures in the sample cores were generated artificially either by axial splitting or by a saw cut resulting in hydraulic apertures that ranged between 8 and 66 µm. Hydraulic apertures, accurately determined by the flow-through apparatus, are used to compare results obtained by the other methods. The transient-airflow permeameter delivers accurate values, particularly when repeated measurements along the full fracture width are performed. In this case, the derived mean hydraulic fracture apertures are in excellent quantitative agreement. When hydraulic apertures are calculated indirectly from optically determined mechanical apertures using empirical equations, they show larger variations that are difficult to compare with the flow-through-derived results. Variations in hydraulic apertures as observed between methods are almost certainly related to differences in sampled fracture volume. Overall, using direct flow-through measurements as a reference, this study demonstrates the applicability of portable methods to determine hydraulic fracture apertures at both the laboratory and outcrop scales.

Toluene NAPL Oxidation by Ferrous Activated Persulfate in a Fractured Rock Glass Replica

“…The arithmetic mean of the aperture distribution from the ATOS analysis was 339 μm (standard deviation of 133 μm). One would expect the mechanical aperture to be larger than the hydraulic aperture (Cheng et al, 2020;Choi et al, 2019;Liu, 2005), while our results indicate that there was no significant difference between these two apertures. In assembling the two halves of the glass fracture in the frame, any offset in the two halves compared to the surface matching done with the ATOS surface scans could result in a larger aperture than calculated from the ATOS surface scans.…”

Lithology-Controlled Hydrodynamic Behaviour of a Fractured Sandstone–Claystone Body in a Radioactive Waste Repository Site, SW Hungary

“…The aperture can be altered by water-rock interaction processes, which may affect the original aperture differently from part-to-part along the fracture plane. For understanding hydrodynamic processes, on the other hand, the hydraulic aperture should be used, which is defined as the width of a theoretical pipe that produces the same flow rate as the actual fracture does [45,46].…”

“…By definition, an HFU is a volume of a rock body in which the geological and petrophysical properties controlling fluid flow are uniform internally. FZI values are related to porosity, permeability, capillary pressure, lithology distribution, mineralogy, and sedimentary structures and are also influenced by diagenetic processes [46]. The FZI can be calculated as follows [47]:…”

Mechanical and hydraulic properties of the excavation damaged zone (EDZ) in the Opalinus Clay of the Mont Terri rock laboratory, Switzerland

“…Generally, a h is nonlinearly linked to the mechanical fracture aperture a m as a function of fracture surface roughness (Blum et al, 2009;Renshaw, 1995), for example via the Barton-Bandis model using the joint roughness coefficient (JRC) (Barton, 1982;Barton et al, 1985). The mechanical fracture aperture represents the average geometrical distance between the fracture surfaces (e.g., Hakami and Larsson, 1996) and is needed to examine the response of fracture networks due to normal or shear stresses (e.g., Blümling et al, 2007;Cuss et al, 2011;Zhang, 2016) and mechanical self-sealing of artificial fractures (e.g., Marschall et al, 2017;Nagra, 2002).…”