The Geothermal Doublet
The fundamental unit of geothermal heat extraction is the doublet — a pair of wells, one for injection and one for production, connected by permeable rock or engineered fractures. Cold water descends through the injection well, absorbs heat from the surrounding rock as it travels through the reservoir, and returns to the surface hot through the production well. This simulator models the thermodynamics and fluid mechanics of this heat mining process.
Balancing Power and Longevity
Increasing flow rate extracts more thermal power but accelerates the advance of the cold front from injector to producer. The thermal breakthrough time — when cooled fluid reaches the production well — sets the economic lifetime of the system. Reservoir engineers must find the sweet spot: enough flow for commercial power output, but not so much that the reservoir is thermally depleted within a decade.
Permeability and Pressure
The reservoir's permeability dictates the pressure drop required to drive fluid from injector to producer. In high-permeability fractured rock (>100 mD), flow is easy and pump costs are low. In tight granite (<10 mD), massive pressure drops make production uneconomic without hydraulic stimulation. The Darcy equation governs this relationship, and pump power consumption can become a significant fraction of electric output in low-permeability systems.
Optimization Strategies
Advanced strategies include multi-lateral wells that increase reservoir contact area, supercritical CO₂ as a working fluid with better thermosiphon properties, and carefully designed injection schedules that delay thermal breakthrough. Numerical reservoir simulators (TOUGH2, FEHM) model the complex 3D flow and heat transfer to optimize well placement and flow management for each unique reservoir.