Numerical simulation is used to evaluate the 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. We present the first ever, three-dimensional simulations of CO2 injection-production systems. These show strong effects of gravity on the mass flow and heat extraction due to the large contrast of CO, density between cold injection and hot production conditions. The tendency for preferential flow of cold, dense CO2 along the reservoir bottom can lead to premature thermal breakthrough. The problem can be avoided by producing from only a limited depth interval at the top of the reservoir. Published by Elsevier Ltd.