Evaluation of NaCl and MgCl2 heat exchange fluids in a deep binary geothermal system in a sedimentary halite formation

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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, heating of an NaCl-brine from 70 to 120 °C caused the solubility of halite to decrease, resulting in the potential dissolution of 0.479 mol kg−1halite at the formation. Conversely, cooling from 120 to 100 °C in the production well resulted in potential precipitation of 0.196 mol kg−1halite. Concurrent precipitation of anhydrite is also expected. Introduction of MgCl2  into the heat exchange brine, which has a common Cl−ion, resulted in a decreased potential for dissolution by 0.290 mol kg−1halite 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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