HOT & HEAVY: GEOTHERMAL MODELING OF STABILITY FIELDS IN PH-EH SPACE

Fluid-rock interactions in geothermal systems depend upon the speciation of the available chemicals in the system. To address issues like corrosion and scaling in these systems, we need to develop a robust understanding of the plausible geochemical reactions. The dominant chemical species in the system can be modeled by utilizing the pH and Eh of the geothermal fluid. For dynamic, non-steady state systems like geothermal environments, the pH, Eh, P, and T are subject to change with time. As Eh and pH stability fields are determined using the Gibbs free energy of reaction (ΔG_r)_, which itself relies on P and T, the effect of these latter two variables can be visualized. In the H-O-S system, for example, preliminary results indicate that at constant pressure (δΔG_r/δT)_P, there are substantial shifts to stability fields in pH-Eh space. Our modeling results show that temperature is inversely proportional to Eh, pushing the stability field to more reducing regions than would otherwise be stable. Additionally, if solids and liquids are assumed to be incompressible, then our preliminary results indicate that only liquid-gas mixtures (O₂-H₂O and 2H⁺-H₂) are affected by increased pressure (δΔG_r/δP)_T. Continued work will involve repeating the same modeling experiments for the H-O-S system but under high pressures associated with geothermal reservoirs, the utilization of alternate equations of state, and considering the compression of solids and liquids. Alternate equations of state such as the Peng-Robinson, Redlich-Kwong, and van der Waals models are better suited for the high pressures encountered in geothermal reservoirs. Along with the consideration of solid and liquid compressibility, the fluid-rock interaction occurring within geothermal systems will be further elucidated.