Spatial Mapping of Fracture Aperture Changes With Shear Displacement Using X‐ray Computerized Tomography

Abstract

The shearing of fractures can be a significant source of permeability change by altering the distribution of void space within the fracture itself. Common methods to estimate the effects of shearing on properties, such as aperture, roughness, and connectivity, are incapable of providing these observations in situ. Laboratory protocols are needed that enable measurements of the spatial structure of the fracture aperture field in the medium, noninvasively. Here, we investigate changes in rough-walled Brazilian-induced tensile fracture aperture distribution with progressive shear displacement in Westerly granite and Carrara marble using a novel X-ray transparent core holder. The so-called calibration-free missing attenuation method is applied to reconstruct highly resolved (submillimeter) fracture aperture maps as a function of displacement (0 to 5.75 mm) in induced fractures. We observe that shearing increases the core-averaged fracture aperture and significantly broadens the distribution of local values, mostly toward higher apertures. These effects are particularly strong in Westerly granite and may be the result of the higher initial roughness of its fracture surfaces. Also, while the correlation length of the aperture field increases in both parallel and perpendicular directions, significant anisotropy is developed in both samples with the progression of shearing. The results on Westerly granite provide a direct indication that fracture aperture remains largely unaffected until 1 mm of displacement is achieved, which is important when estimating permeability enhancement due to natural and induced shear displacement in faults.

Chemo‐Mechanical Coupling in Fractured Shale With Water and Hydrocarbon Flow

“…The XCT images have an in‐plane spatial resolution, r xy = 230 μ m (Text , Supplementary Methods 3). We note, however, that the CFMA method enables subresolution fracture apertures to be resolved, with an uncertainty of 40–60 μ m, as validated in previous studies using both saw‐cut and rough‐walled fractures (Huo et al., 2016; Q. C. Wenning et al., 2019). A core‐averaged value of the mechanical aperture, , is obtained upon averaging local values estimated using Equation and provide a direct means of comparison against its hydraulic counterpart (Text , Supplementary Methods 4).…”

Hydraulic Transport Through Calcite Bearing Faults With Customized Roughness: Effects of Normal and Shear Loading

“…The usual idea regarding fault reactivation influence on transmissivity is that slip on rough faults generates a permanent increase in hydraulic transport on the fault (Carey et al, 2015; Esaki et al, 1999; Guo et al, 2013; Ishibashi et al, 2012; Lee & Cho, 2002; Olsson & Brown, 1993; Pyrak‐Nolte et al, 1988; Wenning et al, 2019; Yeo et al, 1998; Zambrano et al, 2018). In most of those studies, the shear displacements were in the range of 3 to 20 mm and/or on faults with large roughness (Chen et al, 2000; Wenning et al, 2019). Our results show that when shear slip occurred in the fault (<1 mm), transmissivity remained close to constant with very slight changes, in strong contrast to the usual understanding of shear reactivation described above.…”

“…On the one hand, several studies have shown a decrease of the real contact area with increasing shear stress prior to (and at the onset of) sliding in mortar rock replicas (Grasselli & Egger, 2003;Park & Song, 2013), on hard polymers (Bayart et al, 2016;Ben-David et al, 2010;Svetlizky & Fineberg, 2014), and on soft polymers (Sahli et al, 2018). On the other hand, experiments in hard, coated polymers (Bay & Wanheim, 1976) and in polystyrene-on-glass contacts (Weber et al, 2019) have shown that the real contact area in turn increases with shear stress and initial displacement due to a mechanical degradation of the asperities (plastic deformation). By isolating the effect of normal stress and shear stress separately on transmissivity, our results show that the application of reversible shear load is not the only factor affecting fracture transmissivity but rather the combination of geometry and stress even if its effect is small.…”

“…Complex contact geometries can also lead to flow channeling in fractures (Kang et al, 2016; Watanabe et al, 2009), drastically affecting their hydraulic transport capacity. Most experimental works have been performed either at low stresses (Park & Song, 2013; Patir & Cheng, 1978; Tanikawa et al, 2010; Wenning et al, 2019; Witherspoon et al, 1980) or on faults with constant roughness (Faoro et al, 2009; Rutter & Mecklenburgh, 2017, 2018; Watanabe et al, 2008).…”

Fault sealing and caprock integrity for CO&lt;sub&gt;2&lt;/sub&gt; storage: an in-situ injection experiment

“…Samples from intact rock and fault zone collected before and after the injections will allow characterization of mineralogical and chemical changes due to the rock-fluid interaction; geomechanical tests will reveal eventual changes due to exposure to CO 2 -rich water. This part is not discussed in the present paper and a detailed description of the methodology adopted for lab studies is in Wenning et al, 2019. Numerical modelling assists the design and analysis of different phases of the experiment and a primary aim of the experiment is also to provide parameters to validate and calibrate numerical models that can be used for enhancing process understanding, sensitivity studies and upscaling.…”