Extending the Applicability of the iTOUGH2 Simulator to Supercritical Conditions

Abstract

Supercritical fluids exist near magmatic heat sources in geothermal reservoirs, and the high enthalpy fluid is becoming more desirable for energy production with advancing technology. Furthermore, wells drilled into magma in magmatic geothermal reservoirs have indicated that the heat sources could be located at a shallower depth than assumed in today’s standard modeling practices. In geothermal modeling, the roots of the geothermal systems are normally avoided but in order to accurately predict the thermal behavior when wells are drilled close to magmatic intrusions, it is necessary to incorporate the heat sources into the modeling scheme. Modeling supercritical conditions poses a variety of challenges due to the large gradients in fluid properties near the critical zone. This work focused on using the iTOUGH2 simulator to model the extreme temperature and pressure conditions in magmatic geothermal systems. The study is part of a project on investigating the deep roots of geothermal systems, funded by Geothermal Research Group (GEORG). The IAPWS-95 and IAPWS-IF97 thermodynamic formulations were implemented into iTOUGH2 to provide inverse modeling capabilities of high-temperature magmatic geothermal reservoirs. Thus, the operational range of temperature and pressure in iTOUGH2 was extended from 350°C and 100 MPa to 1,000°C and 1,000 MPa when using the IAPWS-95 formulation, and to 800°C and 100 MPa as well as 2,000°C for pressure within 50 MPa, when using the IAPWS-IF97 formulation. In addition, the possibility of extrapolating the IAPWS-IF97 formulation was investigated because the formulation is significantly faster than the IAPWS-95 formulation, which can be extrapolated to high temperatures and pressures with relatively good accuracy. A five-spot geothermal problem was simulated for a subcritical problem, a supercritical problem with temperatures and pressures close to the critical point, and for very high temperature and pressure conditions likely to occur deep in magmatic reservoirs. Both formulations give equivalent results for temperatures up to 800°C and the difference between the formulations was very low even for extreme temperature and pressure conditions at 1500°C and 150 MPa. Hence, the IAPWS-IF97 formulation is recommended instead of IAPWS-95 due to a significantly faster computational speed.