Abstract
Halite formations are attractive geothermal reservoirs due to their high heat conductivity, resulting in higher temperatures than other formations at similar depths. However, halite formations are highly reactive with undersaturated water. An understanding of the geochemical reactions that occur within halite-saturated formation waters can inform decision making regarding well construction, prevention of well clogging, formation dissolution, and thermal short-circuiting. Batch reaction and numerical 3-D flow and equilibrium reactive transport modeling were used to characterize the produced NaCl-brine in a well targeting a halite-saturated formation. The potential for inhibition of precipitation and dissolution using an MgCl2-brine and NaCl+MgCl2-brine were also investigated. Within the injection well for an NaCl-brine, with heating from 70 to 120°C, the solubility of halite decreases resulting in the potential dissolution of 0.479 mol kg-1 halite at the formation. Cooling from 120 to 100°C in the production well results in precipitation of 0.196 mol kg-1 halite as well as anhydrite. Introduction of MgCl2, resulting in a common Cl- ion, into the heat exchange brine resulted in a decreased potential for dissolution by 0.290 mol kg-1 halite within the formation, as well as decreased precipitation within the production well, compared to the NaCl-brine. The halite solubility was altered by changes in pressure up to 0.045 mol kg-1. This indicates that designing and monitoring the composition of heat exchange fluids in highly saline environments is an important component in geothermal project design.
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