PERMEABILITY IN THE UPPER CONTINENTAL CRUST

The permeability (k) of Earth’s upper crust largely controls such processes as advective transport of solutes (k ~ >10^-20 m²) and heat (k ~ >10^-16 m²) and the generation of elevated fluid pressures (k ~ <10^-17 m²). There is a historical dichotomy between the concept of k as a static material property that exerts control on fluid flow and the perspective of economic geologists, crustal petrologists, and others who recognize k as a dynamic parameter that responds to tectonism, devolatilization, and geochemical reactions. “Fracking”, EGS, and GCS are promoting a constructive dialog between these contrasting views of k. Temporal evolution of k can be abrupt or gradual: coseismic streamflow responses imply instantaneous changes in k by factors of up to 20, whereas a 10-fold decrease in the k of a subsiding package of shale may require 10⁷years. Thus in the absence of seismicity, considering k as static parameter is often a reasonable assumption for low-temperature hydrogeologic investigations. However, temporal variation is enhanced by strong chemical and thermal disequilibrium, such that lab experiments involving flow under pressure, temperature, and chemistry gradients often result in 10-fold k decreases over daily to sub-annual time scales. Shear dislocation can increase near-to intermediate-field k by factors of 10² to 10³. Dynamic stresses (shaking) in the intermediate- to far-field corresponding to seismic energy densities >0.01 J/m³ also increase k, albeit at most by a factor of ~20. These k increases are transient, tending to return to pre-seismic k values over months to decades. There is reasonable agreement between the magnitude of near-to intermediate-field k increases directly measured at EGS sites (10²-10³-fold) and those inferred from seismic / metamorphic data, for subduction-zone faults, and in simulations of transient hydrothermal circulation. EGS, GCS, deep injection of waste fluid, and simulations of ore-forming systems all entail similar stimuli, namely fluid-injection rates on the order of 10s of kg/s. In the North American midcontinent, fluid-injection practices have caused a large recent increase in M_w >~3 seismicity. This ongoing injection experiment represents an opportunity to explore and assess dynamic k to midcrustal depths.