Increased Reliability of Supercritical EOS1sc Module in iTOUGH2

Abstract

A reliable numerical simulator capable of modeling supercritical conditions in geothermal reservoirs is crucial for a better understanding and optimized management of magmatic geothermal systems. This paper describes improvements made to the EOS1sc module in iTOUGH2 for increasing the reliability and speed of the supercritical module. In EOS1sc, the IAPWS-IF97 thermodynamic formulation is used to calculate thermodynamic properties across five regions; liquid, vapor, supercritical, two-phase, and high temperature vapor. Backward equations released for the supercritical region of IAPWS-IF97 were implemented into EOS1sc for calculating specific volume as a function of pressure and temperature. The backward formulation is more reliable than the Newton-Raphson method previously used to iteratively calculate density in the supercritical region and it is approximately eight times faster. In EOS1sc, discrepancies across region boundaries in the IAPWS-IF97 formulation can cause convergence issues. Hence, cubic Bézier curves were implemented to provide a smooth continuous function for thermodynamic properties across the supercritical boundaries. Thereby, poor convergence at intersection points between thermodynamic regions were resolved and the reliability of the supercritical EOS1sc module was increased.