Lawrence Berkeley National Laboratory

Numerical simulation is used to evaluate mass flow and heat extraction rates from enhanced geothermal injection-production systems that are operated using either CO2 or water as heat transmission fluid. For a model system patterned after the European hot dry rock experiment at Soultz, we find significantly greater heat extraction rates for CO2 as compared to water. The strong dependence of CO2 mobility (= density/viscosity) upon temperature and pressure may lead to unusual production behavior, where heat extraction rates can actually increase for a time, even as the reservoir is subject to thermal depletion.