Laboratory Experimental Study of Heat Extraction from Porous Media by Means of Co2

The use of CO2 as a heat transfer fluid has been proposed as an alternative to water in enhanced geothermal systems (EGS). Numerical simulations have shown that under expected EGS operating conditions, CO2 could achieve more efficient heat extraction performance than water. We have conducted laboratory experiments with dry supercritical CO2 in order to test and refine the theoretically derived heat transfer predictions. We have injected cold CO2 into heated porous media held in a laboratory pressure vessel, and have monitored temperature changes at different locations. To date, our measurements cover a pressure range from 77 to 120 bar, temperatures from 20 to 77°C, and a range of CO2 injection rates. Temperature data recorded at different thermocouple locations capture behavior that ranges from broad diffusion-dominated transients at low flow rates to sharp temperature breakthrough curves when conditions are advectively dominated (high-Peclet number). Numerical simulations with TOUGH2/ECO2N using a 2-D axisymmetric model agree reasonably well with the experimental measurements. The laboratory tests have demonstrated the technical challenges involved in working with supercritical CO2, which are due to the sensitivity of the fluid’s physical properties to subtle changes in operating conditions. The results of our experiments and simulations will provide useful guidance as we develop a more capable experimental apparatus.