Laboratory experiments provide a wealth of information related to mechanics of fracture initiation, fracture propagation processes, factors influencing fault strength, and spatio-temporal evolution of fracture properties. Much of the existing literature reports on laboratory studies involving a coupling of thermal, hydraulic, mechanical, and/or chemical processes. As these processes operate within subsurface environments exploited for their energy resource, laboratory results provide insights into factors influencing the mechanical and hydraulic properties of geothermal systems. I report on laboratory observations of strength and fluid transport properties during deformation of simulated faults. The results show systematic trends that vary with stress state, deformation rate, thermal conditions, fluid content, and rock composition. When related to geophysical and geologic measurements obtained from engineered geothermal systems (e.g. microseismicity, wellbore studies, tracer analysis), laboratory results provide a means by which the evolving thermal reservoir can be interpreted in terms of physico-chemical processes. For example, estimates of energy release and microearthquake locations from seismic moment tensor analysis can be related to strength variations observed from friction experiments. Such correlations between laboratory and field data allow for better interpretations about the evolving mechanical and fluid transport properties in the geothermal reservoir ? ultimately leading to improvements in managing the resource.
Successful energy extraction from the vast subsurface thermal resource requires a thorough understanding of the generation and maintenance of permeable pathways. As a majority of geothermal reservoirs are comprised of igneous and metamorphic rocks with low matrix permeability, the pathways for fluid-flow inherently involve a network of interlinked cracks and fractures. Where such a pathway does not exist, or is inadequate for economic energy production, then the permeable fracture network needs to be engineered by creating new fractures or inducing shear on pre-existing fractures. Furthermore, the quality of the permeable network will need to be monitored and adjusted over time in order to properly manage the thermal resource during the several decade lifespan of an exploitable geothermal field.
Results from laboratory experiments are invaluable for helping understand the complexities of the processes that operate within the Earth. Of particular relevance for engineered geothermal systems are concepts stemming from experiments that explore the mechanics of fracture generation [e.g. 1-8], physical factors controlling the strength of existing fractures [e.g. 9-15], the effects of fluidrock interactions on strength properties [e.g. 16-22], and permeability evolution within the fracture network [e.g. 21-26]. The salient details from existing laboratory studies that explore these effects are presented here, with a focus on implications for successful exploitation of fractured geothermal systems.