UC Berkeley

The use of CO2 as a heat transfer fluid has been proposed as an alternative to water in enhanced geothermal systems (EGS) and in CO2-plume geothermal systems (CPG). Numerical simulations have shown that under expected EGS operating conditions, CO2 would achieve more efficient heat extraction performance compared to water, especially at sites with low geothermal temperatures and low subsurface heat flow rates. With increased interest in carbon capture and sequestration (CCS), the possibility of combining geothermal energy production with carbon sequestration is actively being explored. Simulations have shown that CO2-based geothermal energy production could substantially offset the cost of CCS. Since numerical models are critical for the planning and operation of geothermal systems that employ CO2 as the working fluid, it is important to validate the results of the current numerical tools against real-world experimental data.

A laboratory apparatus was assembled that is capable of operating at temperatures up to 200◦C, pressures up to 34.5MPa, and flow rates up to 400mL/min. The experimental system was designed such that measurements and controls at the boundaries could be readily modeled. It was found that the dynamic physical behavior and chemical properties of CO2 create problems with sealing, flow control, and safety. The unique challenges of handling, control, and measurement of supercritical CO2 are addressed in this work as well as tools, techniques and materials identified for overcoming them. The described flow system could be applied to the selective extraction of components from organic materials, as well as the extraction of heat from porous media.

Using the assembled apparatus, heat transfer behavior of flowing dry supercritical CO2 through a heated porous medium was investigated and experimental results were compared with a numerical model using TOUGH2 with the ECO2N module. In addition, experiments were performed using (1) CO2 and (2) water as the working fluids under similar operating conditions in order to compare the heat transfer behavior and the overall heat extraction rates. We have made estimates of the density and the effective thermal conductivity of our saturated porous media, and have found that both properties change significantly during the course of experiments. The large changes in CO2 density, due to decreasing system temperatures, can result in fluid accumulation in the system that may have significant impacts on geothermal reservoir management. The large changes in thermal conductivity as a function of pressure and temperature are of concern because the standard TOUGH2 code does not update the thermal conductivity of the system during the course of a simulation.

A detailed TOUGH2 model of the experimental system was created and was calibrated against the experimental data. The calibration results of optional thermal conductivity up- dating code included with the new ECO2N v2.0 module was compared against calibration using the standard constant effective thermal conductivity assumption. It was found that including effective thermal conductivity updating in the model resulted in an simpler calibration process that produced less missfit across all experiments than when a single estimated thermal conductivity value was used.