Numerical reactive transport modeling of soluble mineral and fluid interactions in the subsurface and application to sedimentary geothermal systems

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Moore, Kayla Renee
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Economics of deep geothermal systems for power production can be improved by targeting warm thermal anomalies. Anomalies can occur near minerals with high thermal conductivity such as halite and dolomite. However, the solubility of these formations may contribute to technical problems associated with geochemistry. In order to evaluate the feasibility and potential benefits of a deep, low-temperature geothermal system targeting thermal anomalies caused by high thermal conductivity minerals, this research investigated the application of thermal, hydraulic and chemical numerical models to problems of high ionic strength mineral and fluid interaction in subsurface flow. Numerical model performance for problems of mineral dissolution flow and transport were studied using laboratory measurements, characterizing sensitivities and accuracy. Field-scale simulations of sinkhole development validated the predictive capability at the field scale. These validations of model performance for fluid-mineral interactions confirmed the validity of numerical models for large-scale geothermal simulations. The geothermal models were binary, doublet systems based in the Williston Basin, Saskatchewan, Canada. A geochemical investigation of produced fluids from a well targeting the halite Prairie Evaporite with 120°C at depth and 60°C at surface, resulted in 0.37 mol L-1 of halite precipitation. Halite precipitation could be inhibited by introducing MgCl2 into the heat exchange fluid and through pressure controls. A second scenario investigated a horizontal production well in the dolomite Dawson Bay formation, including the conversion of an oil and gas well in the Bakken formation for injection. A 6°C temperature anomaly resulted from 371 m of underlying halite and dolomite, increasing power production by 1.5 MW. Produced temperatures ranged from 112°C to 103°C over 30 years. At a flow rate of 0.2 m3s-1 and injection temperature from 60 – 80°C the system produced 6.8 – 10.9 MW. The geochemical analysis indicated the potential for lithium production. Numerical models provided valuable information on produced geochemistry in deep geothermal wells, which can be used to study scale inhibition. Geothermal wells targeting formation above thick high thermal conductivity formations benefit from warm thermal anomalies, reducing drilling costs.
Geothermal, Geochemistry, Reactive transport modeling, Density-driven flow, Subsurface flow
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