Rapid Mineral Precipitation During Shear Fracturing of Carbonate‐Rich Shales

Abstract

Target subsurface reservoirs for emerging low-carbon energy technologies and geologic carbon sequestration typically have low permeability and thus rely heavily on fluid transport through natural and induced fracture networks. Sustainable development of these systems requires deeper understanding of how geochemically mediated deformation impacts fracture microstructure and permeability evolution, particularly with respect to geochemical reactions between pore fluids and the host rock. In this work, a series of triaxial direct shear experiments was designed to evaluate how fractures generated at subsurface conditions respond to penetration of reactive fluids with a focus on the role of mineral precipitation. Calcite-rich shale cores were directly sheared under 3.5 MPa confining pressure using BaCl 2-rich solutions as a working fluid. Experiments were conducted within an X-ray computed tomography (xCT) scanner to capture 4-D evolution of fracture geometry and precipitate growth. Three shear tests evidenced nonuniform precipitation of barium carbonates (BaCO 3) along through-going fractures, where the extent of precipitation increased with increasing calcite content. Precipitates were strongly localized within fracture networks due to mineral, geochemical, and structural heterogeneities and generally concentrated in smaller apertures where rock:water ratios were highest. The combination of elevated fluid saturation and reactive surface area created in freshly activated fractures drove near-immediate mineral precipitation that led to an 80% permeability reduction and significant flow obstruction in the most reactive core. While most previous studies have focused on mixing-induced precipitation, this work demonstrates that fluid-rock interactions can trigger precipitation-induced permeability alterations that can initiate or mitigate risks associated with subsurface energy systems.

From Fluid Flow to Coupled Processes in Fractured Rock: Recent Advances and New Frontiers

“…The precipitation studies of Menefee et al. (2020) illustrate communication between the primary fracture and the damaged matrix (Figure 4). The sorption behavior of tracers has also been used to interrogate fracture surface area (Dean et al., 2015; Roshan et al., 2016).…”

“…They observed substantial reductions of permeability due to dissolution of asperities confirmed by X-ray CT. On the other hand, experiments with HCl on highly reactive limestone showed in one case strong increases in permeability (Noiriel et al, 2013) and, in another case, complex wormhole evolution with strongly enhanced permeability with increasing flow rates (Noiriel & Deng, 2018). In experiments that involve the potential for both dissolution and precipitation, Dávila et al (2016) found that competing calcite dissolution and gypsum precipitation resulted in little change to permeability, while Menefee et al (2020) found extensive precipitation during competing calcite dissolution and witherite (BaCO 3 ) precipitation that concentrated in tight regions of the fracture and within comminuted fracture material (Figure 4). Menefee et al (2020) observed relatively small to negligible decreases in permeability.…”

“… Scanning electron microscopy image of barium carbonate in a fracture network obtained from an induced shear experiment (shear fracture runs vertically through the specimen) involving dissolution of a carbonate‐rich shale and precipitation of BaCO 3 (bright white) in response to coupled injection of BaCl 2 solution (after Menefee et al., 2020). …”

“…Other methods to determine contact areas and enhance CT analysis include the use of injected Wood's metal that resolves apertures down to 2 μm (Pyrak‐Nolte et al., 1997) and pressure‐sensitive film (e.g., Nemoto et al., 2009). CT methods lend themselves to producing time‐sequence images that permit investigation of changes to fracture geometry as a function of displacement, compression, and chemical reactions (Crandall et al., 2017; Frash et al., 2017, 2019; McBeck et al., 2020; Menefee et al., 2020; Renard et al., 2016; Voltolini & Ajo‐Franklin, 2020). A combination of high‐resolution profilometry with aperture‐defining CT provides a nearly complete representation of fracture geometry, lacking only a well‐resolved definition of contact area.…”

“…In experiments that involve the potential for both dissolution and precipitation, Dávila et al (2016) found that competing calcite dissolution and gypsum precipitation resulted in little change to permeability, while Menefee et al (2020) found extensive precipitation during competing calcite dissolution and witherite (BaCO 3 ) precipitation that concentrated in tight regions of the fracture and within comminuted fracture material (Figure 4). Menefee et al (2020) observed relatively small to negligible decreases in permeability.…”