Thermal contraction, mineral dissolution, and secondary mineral precipitation can all affect fracture aperture in enhanced geothermal systems (EGS). For this work, the reactive transport code PFLOTRAN was used to examine the combined effect of both mineral dissolution/precipitation and mechanical thermal contraction of the rock matrix on effective permeability of a fracture zone. PFLOTRAN, which can already consider mineral kinetics and other geochemical processes, was further developed to include the effects of thermal contraction of a rock matrix and the resulting change in fracture aperture, in an equivalent continuous porous medium (ECPM) representation. A single horizontal fracture was represented in PFLOTRAN as a zone of distinct high porosity and permeability in a domain otherwise dominated by low permeability crystalline rock and, from these initial conditions, four 2D simulations (thermal-hydrological, thermal-hydrological-mechanical, thermal-hydrological-chemical, and thermal-hydrological-chemical-mechanical), were run in PFLOTRAN to observe changes in fracture aperture and effective permeability over time. It was found that while mineral precipitation decreased effective permeability of the fracture zone by half an order of magnitude, thermal contraction increased effective permeability by nearly two orders of magnitude, overriding the effects of mineral precipitation, with implications for pumping and thermal drawdown in enhanced geothermal systems. These results will be considered as part of the larger techno-economic model GT-Mod, to speak to the long-term power generating potential and economic practicality of EGS as a carbon neutral energy source.

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