LABORATORY VISUALIZATION AND ACOUSTIC EMISSION MONITORING OF HYDRAULIC FRACTURING IN A COMPLEX, FRACTURED MEDIUM (Invited Presentation)

Fracture propagation in rock can be affected strongly by open and healed (or filled) preexisting fractures, resulting in changes in the propagation paths and complex branching. The resulting network of hydraulically connected complex fractures is considered critical for efficient and economical production of oil and gas, and circulation of geothermal fluids. Evidence for the complex fracturing is mostly from microearthquake (MEQ) observations in the field and numerical simulations while direct observations in the laboratory are still scarce.

In this presentation, we will show recently developed methodologies and the results from laboratory hydraulic fracturing visualization experiments. Optical visualization experiments are conducted in transparent glass cubes containing varying degrees of heterogeneity consisting of fractures and microcracks. These defects are either etched in the rock (glass) matrix using 3D laser engraving based upon specified fracture geometry and patterns, or produced by thermal quenching (and partial rehealing) which result in varying degrees of fracture density, strength, and connectivity (or permeability). Fluorescent dye is used to enhance the images of the thin fractures, assisted by illumination by laser and UV light. In anisotropic (and opaque) shale blocks, the samples are scanned via X-ray CT, assisted by contrast-enhancing fluid (liquid metal) to improve the visibility of thin hydraulic fractures. Using the transparent glass samples for optical visualization, we investigate the impact of fluid viscosity and injection rate on the geometry of the hydraulic fracture and fracture network produced by fluid injection, while the sample cubes are subjected to true-triaxial stresses. In the optical visualization experiments, propagation of the fractures and evolution of fracture networks can be observed through a mirror and a port hole installed in loading platens. Concurrently, we determine the locations of fracturing from acoustic emissions and correlate them to the optical images of the fractures. The experiments indicate that changes in the fluid injection rate (and viscosity) can modify the geometry of the hydraulic fractures, which may be a key to improved control of subsurface permeability.