Despite attempts to engineer viable deep reservoirs for the recovery of thermal energy at high enthalpy and mass flow rates - dating back to the 1970s - this goal has been surprising elusive. The record is replete with failed attempts, examples on life support and some successes. The key difficulties are in (i) accessing the reservoir inexpensively and reliably at depth, (ii) in penetrating sufficiently far through the reservoir, and (iii) in stimulating the reservoir in a controlled manner to transform permeability from microDarcy to higher than milliDarcy levels with broad and uniform fluid sweep and (iv) to create and retain adequate fluid throughput and heat transfer area throughout the project lifetime. We discuss key controls on permeability evolution in such complex systems where thermo-hydro-mechanical-chemical and potentially biological (THMC-B) effects and feedbacks are particularly strong. At short-timescales of relevance, permeability is driven principally by deformations - in turn resulting from changes in total stresses, fluid pressure or thermal and chemical effects. We explain features of reservoir evolution with respect to both stable and unstable deformation, the potential for injection-induced seismicity and its impact on both reservoir performance and in interrogating the evolving state of the reservoir.
The estimated thermal resource in the upper 5 km of crust below the US is of the order of 107 EJ. This compares favorably both with the hydrothermal resource at a mere 104 EJ and to the annual energy budget for the US, at ∼100 EJ/year. Recovering even a fraction of this baseload resource would contribute significantly to a new low carbon energy economy.
The intrinsic goal of recovering thermal energy from the shallow crust (∼5 km for Engineered Geothermal Systems) requires that high-fluid-throughput and thermally-long-lived geothermal reservoirs may be universally engineered and developed, at will, and at any geographic location. High-fluid-throughput in traditional basement rocks requires that reservoir permeabilities at depth (∼5 km) must be elevated from the microDarcy to the milliDarcy range - this avoids untenable pumping costs and avoids inadvertently fracturing the reservoir by extreme fluid overpressuring of the heat-exchange fluid. Although fracturing would appear desirable in developing conduits with high-fluid-throughput, it typically violates the second tenet of a desired long thermal life, which requires that high heat-transfer area is maintained concurrent with high flow rates. This is only feasible if fluid circulation in the reservoir has a broad and even sweep through media with a short thermal diffusion length (small fracture spacing) thus avoiding short-circuiting and damaging feedbacks of thermal permeability enhancement.