THIRTY-TWO YEARS OF PROGRESS TOWARDS UNDERSTANDING FLOW AND TRANSPORT IN FRACTURED ROCK

In 1977, fracture flow research began at Sweden’s Stripa Mine. At that time, basement rock was either impervious, or fundamentally unpredictable. The statistical description of fracture geometry was new and untried, simulation was concerned with porous equivalents (Snow tensors), and transport processes were largely untouched. Advanced petroleum concepts used uniform sugar-cube models. Stripa began an era of international underground laboratories and surface-based research sites that have changed our concepts of flow and transport in fractures.

While important problems remain, thirty-two years of work have created a toolbox of field and numerical methods that are becoming standard hydrogeologic practice. The investigations for the block-scale transport experiments at the Kamaishi and Äspö laboratories demonstrate some of the key features of a modern characterization strategy. These volumes were investigated iteratively by flow logging each new hole, isolating the key flow zones with multipoint piezometers, and monitoring subsequent drilling and testing activities. The monitoring results reveal the flow structure of the fracture network including the controlling fractures, connectivity, and compartmentalization. The use of derivative methods in well test analysis, geophysics, and hydrochemistry further define the fracture systems. Tracer tests show the importance of damage-zone structure. The networks are well-represented for simulation using discrete fracture network (DFN) models, which are also an outgrowth of radioactive waste laboratory programs. A DFN approach that represents the controlling features deterministically and background fractures stochastically is efficient and effective.

In addition to defining fracture network geometry, transport investigations at both contaminant-focused and radioactive waste research facilities have highlighted the importance of matrix-fracture interaction. Matrix-fracture interaction has advanced greatly from three lines of research -- the radioactive waste underground laboratories, studies from DNAPL research sites, and evolving petroleum conceptualizations capillarity, imbibition, and oil displacement. These concerns are vital for contaminant transport, oil production, geothermal resource development, and carbon sequestration.