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 xii 𝑄 ̇regional heat flow (W m -2 ) Rretardation (-) Re -Reynolds number (-) Ssize of fracture (m) 𝑆 0specific storage (m -1 ) 𝑆 𝑐advection/dispersion sources and sinks (kg m -3 s -1 ) 𝑆 𝑄volumetric flow rate per unit volume representing sources and sinks (s -1 ) ttime (s) 𝑇temperature at depth (K) 𝑇 0reference or surface temperature (K) Udimensionless time (-) 𝑣 ⃑fluid velocity (m s -1 ) 𝑣 𝑥fluid velocity in the x direction (m s -1 ) 𝑣 𝐿fluid velocity in the longitudinal direction (m s -1 ) 𝑊 𝑎𝑞mass of the solvent in water in an aqueous solution (kg) 𝑊 ̇maximum potential power output of a geothermal well (kW) xx-direction (m) yy-direction (m) zelevation head (m) Greek 𝛽 -dimensionless constant relating density and concentration (kg m -3 ) 𝛣thermal expansivity of fluid (°C -1 ) 𝛾 𝑖activity coefficient of species i (-) 𝜀porosity (-) 𝜀frequivalent fracture porosity for a porous medium (-) 𝜃relative liquid phase saturation (m 3 m -3 ) 𝝀heat (thermal) conductivity (W m -1 K -1 ) 𝚲thermal hydrodynamic tensor (kg m s -3 K -1 ) 𝜇fluid dynamic viscosity (kg m -1 s -1 ) 𝜌fluid density (kg m -3 ) 𝜌 0mass density of the reference water (kg m -3 ) 𝜌 𝑠mass density of the saltwater (kg m -3 ) 𝜌 𝑚mass density of the solid matrix (kg m -3 ) 𝜔coefficient related to tortuosity (-) 𝜒buoyancy coefficient (-) 𝛤second-order thermal expansivity of fluid and (°C -2 ) 𝜅fluid compressibility (MPa -1 ) 𝛺pump efficiency in a geothermal system (-) 𝜂 𝑝𝑟𝑜𝑑 and 𝜂 𝑖𝑛𝑗enthalpy of fluid at each the production and injection wells (kJ kg -1 ) 𝜑 1 and 𝜑 0specific entropy of fluid at the production and injection wells (kJ kg -1 °C-1 )
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−1 halite at the formation. Conversely, cooling from 120 to 100 °C in the production well resulted in potential precipitation of 0.196 mol kg−1 halite. 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−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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