Laboratory Devices and Methods for the Study of Processes in Rock Fractures: Fracture Generation, Characterization, and Process Monitoring

ABSTRACT Fractures govern all rock mass properties and the evolution of hydro-thermo-chemo-bio-mechanical processes. The study of fractured rock masses requires impractically large devices; therefore, an alternative hybrid approach combines fracture-scale experimental studies with numerical upscaling to integrate fracture-bound processes into digital rock mass models. A geotechnical laboratory for fracture studies should include devices and protocols for fracture generation, surface characterization, and process monitoring. Fracture generation methods, such as three-dimensional (3D) printing and 3D milling, are versatile and produce repeatable specimens with approximately 30-μm resolution for laboratory-scale specimens. Notably, milling can incorporate the desired rock mineralogy for bio-chemo–coupled processes. Data analyses must account for inherent biases in fracture generation and characterization methods. Material selection for rock surfaces, fluids, and gouge enables a wide range of monitoring strategies to study complex fracture-bound processes. The examples presented in this manuscript demonstrate various monitoring strategies and reveal important fracture-scale processes, including the impact of aperture variability on nucleating instabilities; the interplay between fingered convection, mixing, and diffusion; the strong spatial correlation between preferred flow channels in single-phase fluid flow and the percolation invasion pathways for an immiscible fluid; the emergence of anisotropic aperture fields during shear and their effects on particle migration and entrapment; the potential for fracture sealing through the sequential injection of reactive fluids; and the interplay between advection along fractures and heat diffusion in the matrix.