# Well-Doublets: A First-Order Assessment of Geothermal SedHeat Systems
## 2 Chemical Condition
### Reservoir Condition  
Mineral dissolution and precipitation, \(\Delta\)T and \(\Delta\)P, and steel goods corrosion are the main issues related to chemical disequilibrium in deep geothermal systems [34]. Geother  
Figure 3: External steel casing corrosion from acidic water attack [32]. Figure 2: Schematic illustration of the process of iron dissolution and sulfide precipitation in a CO2/H2S aqueous system (modified from [29]). mal fluids have diverse chemical components due to the diversity of geological settings (e.g., silica-rich, carbonate-rich, chloride brines, etc.). The recharging water source and associated dissolved gases also define the chemical characteristics of the geothermal fluid, and these may change with time because of the perturbation caused by production and injection. The precipitation of minerals or induced corrosion/scaling leads to small particles carried in suspension into the porous reservoir, altering the system's porosity and permeability. [PERSON] and [PERSON] [35] investigated such changes in a deep geothermal system using coupled Thermal-Hydraulic-Chemical (THC) modeling. The study found that the system's efficiency decreased more rapidly with more mineral dissolution at the start of the project, and it slowed down over a longer time. These results were attributed to the disequilibrium of the system at the start, then changes in the equilibrium condition over time [35]. For production and service life calculations, flow pathway impairment from particles and precipitates is an important disequilibrium effect. Microbially induced (mediated) corrosion, MIC [36], and fouling affect geothermal energy projects (as well as other industrial activities); designing and implementing protection requires multiple knowledge perspectives--material science and metallurgy, electrochemistry, biochemistry, and microbiology [37]. [PERSON] et al. [38] showed MIC's impact on the deterioration of geothermal power plant structures (e.g., vapor ducts and cooling tower supports), noting bacterial activity at temperatures as high as 140 \({}^{\circ}\)C. A MIC problem in the primary surface loop may migrate to the subsurface as corrosion products, and temperature changes impact the reservoir's chemical conditions and change the system's flow properties [39]. Tubular goods may experience general (e.g., steel thinning) or localized corrosion (e.g., pitting) [40]. Uniform corrosion [41], pitting, crevice, and contact (galvanic) corrosion [42; 43; 44], microbially mediated corrosion [45], and oxygen corrosion [38] are all common in geothermal systems. Thermohydrochemical reservoir processes are impacted by stimulation efforts (e.g., hydraulic fracturing with gels, acid injection, etc. ), and injection/production leads to \(\Delta\)T and \(\Delta\)P, triggering mineral precipitation. Even reservoir rock and fluid heat conductivities can be altered by \(\Delta\)T and \(\Delta\)P. [PERSON] et al. [46] showed that significant changes occur in the porosity and permeability of a sandstone reservoir when the effective stress (\(\sigma\)) is increased, as shown in Figure 4.  
Remedial/preventive methods include cleaning, jetting, and waste removal in well workovers, and soft acidizing during stimulation to remove carbonate precipitates, with remediation processes guided by leak-off and injection step-rate testing and data from  
Figure 4: Decrease in measured porosity and permeability of Flechtinger Sandstone geothermal reservoir because of increased effective stress (here called effective pressure) [46]. other chemical and physical monitoring methods [47; 48; 49]. Note that soft acidizing of geothermal systems is taken to mean injecting the same acid volume as used in conventional oil and gas acid treatments but at a much lower rate [48; 50].