Scalable En Echelon Shear‐Fracture Aperture‐Roughness Mechanism: Theory, Validation, and Implications

Abstract

Shear fractures can facilitate fluid conductivity through rock. Aperture and roughness are controlling characteristics for a fracture's fluid conductivity. Inspired by en echelon fractures, we develop a shear "fracturelet" model that predicts anisotropic aperture with respect to the direction of shearing, rougher (nonplanar) rather than smoother (planar) fractures, and the bounds of this roughness for a coalesced fracture. This tendency for rougher fracture creation is validated by in situ X-ray images and fluid conductivity measurements from triaxial direct shear experiments on anhydrite and shale. These experiments were conducted at confining stresses from 4 to 30 MPa and shear displacement magnitudes from 0 to 2 mm on initially intact rock specimens. Hydraulic, dilatational, and local fracture apertures were measured in the experiments. Apertures exhibited strong anisotropy with more conductive flow paths forming perpendicular to the direction of shearing. Local and dilatational aperture were found to be positively correlated with increasing shear displacement but hydraulic aperture was found to vary significantly, always having values smaller than the other aperture measures at factors ranging from 0.6 to 0.0. An implication of these results is that shear fractures have a mechanism for simultaneously exhibiting very low fluid conductivity and high fluid storage volume.

Rapid Mineral Precipitation During Shear Fracturing of Carbonate‐Rich Shales

“…Schematic of triaxial direct shear system sample assembly, adapted from Frash et al (2019). The sample assembly is loaded into an aluminum triaxial core holder within a μCT scanner.…”

“…The core holder and all upstream and downstream flow lines are built into a Hytec cabinet equipped with a 150 kV Hamamatsu microfocus X‐ray tube and Varian panel with 127‐μm pixels for live X‐ray imaging and microcomputed tomography. Additional details on the system's design and capabilities are provided by Carey, Lei, et al (2015) and Frash et al (2016, 2019).…”

Hydro‐Mechanical Measurements of Sheared Crystalline Rock Fractures With Applications for EGS Collab Experiments 1 and 2

“…In situ X‐ray imaging was run for PS01‐06 and PS01‐11. In our experiments, the dilation (mechanical aperture) was measured using X‐ray radiography (Frash et al., 2016; Frash, Carey, & Welch, 2019; Frash, Carey, Welch, & EGS Collab Team, 2019) to measure diameter changes (PS01‐06 and PS011‐11) or radial linear variable differential transducers (YA02‐01, YA02‐02, YA02‐03, YA02‐04, and YA02‐05). The error of each method is less than 0.01 mm, which meets our experimental needs.…”

“…Figure 2 shows results from shearing an initially intact, infilled natural‐fracture in Poorman schist (E1) specimen PS01‐06. This test used X‐ray imaging to obtain shear displacement, bulk radial dilation, 3D segmented aperture distribution, specimen rotation, projected 2D fracture void distribution, changes in geometry with time, and location of saturating fluid volumes and the methods were described in Frash, Carey, and Welch (2019). Sequentially, this experiment included: (a) in situ X‐ray microtomography scan for intact specimen, (b) an intact fracture hydroshearing attempt, (c) mechanical shearing at in situ confining stress, (d) a second hydroshearing attempt, (e) mechanical shearing at a different effective confining stress via changing pore pressure, (f) stress‐dependent permeability, (g) in situ X‐ray microtomography scan for the fractured specimen, and (h) a dynamic shear test.…”

“…(2015) and modified by Frash et al. (2016, 2017) and Frash, Carey, and Welch (2019). Here, axial force induces direct‐shear stress on targeted fractures by two L‐shaped steel pistons.…”