Abstract
Spatially and temporally evolving permeability fields are a fundamental feature of many geological environments, both natural and engineered. The dynamic nature of permeability has been studied, documented, and opined upon for many years. Temporally dynamic permeability can influence slope failure in stratovolcanoes, distal volcano-tectonic seismicity, energy recovery from geothermal systems, and resource estimates of geothermal capacity in the United States. It is a fundamental driver in the convection characteristics of high-enthalpy geothermal systems and exhibits curious behavior near large devolatilizing plutons. Our ability to model geothermal systems or, indeed, any system under sustained permeability evolution, remains limited, due to the complexity of physical processes involved. We are building better models to address temporally dynamic permeability at USGS and elsewhere, motivated in part to obtain a better understanding of energy production and resource assessment.
Spatially and temporally evolving permeability fields are a fundamental feature of many geological environments, both natural and engineered. Observed and simulated permeability patterns are of both economic and scientific interest. Shear slip caused by tectonic forcing or fluid injection can increase near- to intermediate-field permeability by several orders of magnitude, mediated by chemical/mechanical mechanisms of permeability degradation and/or enhancement (Fig. 1). Enhanced geothermal systems (EGS), large-scale geologic carbon sequestration, deep and large injections of waste fluid, and devolatilization of large plutons all entail a similar stimulus and a similar, if scaled, response. In all cases, the primary difficulty is one of characterizing temporal evolution or permeability, rather than (or associated with) one-time characterization of permeability, although the latter remains difficult and is a fundamental feature of geothermal resource assessment (Williams et al. 2008).
The dynamic nature of permeability has been recognized for many years. Goodman (1976) provided a series of evaluations related to mechanical deformation, and others many years before in the context of diagenesis. A collection of information on granular behavior can be found in Taron and Elsworth (2010a) and fracture behavior was described in Taron and Elsworth (2010b). More recently, a compendium on crustal permeability was published in Gleeson and Ingebritsen (2016). “There is clear field and laboratory evidence for extreme heterogeneity and dynamic permeability in hydrothermal systems.” (IPGT 2012).
