Laboratory and Numerical Studies of Heat Extraction from Hot Porous Media by Means of Supercritical $\hbox {CO}_{2}$

Abstract

The use of $\hbox {CO}_{2}$ as a heat transfer fluid has been proposed as an alternative to water in enhanced geothermal systems (EGS) and in $\hbox {CO}_{2}$-plume geothermal systems (CPG). Numerical simulations have shown that under expected EGS operating conditions, $\hbox {CO}_{2}$ 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 $\hbox {CO}_{2}$-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 $\hbox {CO}_{2}$ as the working fluid, it is important to validate the results of the current numerical tools against real- world experimental data. In a set of laboratory experiments, we have investigated heat extraction by flowing dry supercritical $\hbox {CO}_{2}$ through a heated porous medium in a laboratory pressure vessel and have compared experimental results with a numerical model using TOUGH2 with the ECO2N module. In addition, experiments were performed using (1) $\hbox {CO}_{2}$ 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. Our laboratory apparatus is capable of operating at temperatures up to 200 $^{\circ }\hbox {C}$, pressures up to 34.5 MPa, and flow rates up to 400 ml/min. The experimental system was designed such that measurements and controls at the boundaries could be readily modeled using TOUGH2. 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 $\hbox {CO}_{2}$ 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 temperature are of concern because the TOUGH2 code does not update the thermal conductivity of the system during the course of a simulation. Our data can be used by geologic reservoir modelers to ensure that their models accurately capture the heat extraction behavior of $\hbox {CO}_{2}$ to aid in the further investigations of EGS, CPG, and CCS.