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

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 a halite-saturated formations can inform decision making regarding well construction, prevention of well clogging, formation dissolution, and thermal short-circuiting. Numerical 1-D and 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, with heating from 60 to 120°C, the solubility of halite decreases resulting in the potential dissolution of 0.57 mol L-1 at the formation. Cooling from 120 to 100°C in the production well results in precipitation of 0.20 mol L-1 halite as well as anhydrite, brucite, carnallite, goergeyite, gypsum, halite, kieserite and polyhalite. Introduction of Mg2+ into the heat exchange brine resulted in a decreased potential for dissolution by 0.35 mol L-1 as the heat exchange fluid entered the formation and within the formation itself, as well as decreased precipitation within the production well, compared to the NaCl-brine. The NaCl-brine solubility was altered by changes in pressure up to 0.18 mol L-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.