Properties of CO2 – Methane – Water for CO2 Geothermal Applications
Authors: Arekhov V. , Zhainakov T. , Clemens T. , Wegner J. 
Published: June 2024

Abstract
Using CO2 as a heat extraction medium from the subsurface offers several compelling advantages. These benefits result from CO2's distinct properties, notably its lower viscosity compared to water, as well as its pronounced density variations under varying pressures. Using CO2, it becomes possible to harness higher flow rates and density fluctuations to create a thermosyphon effect. These effects result in significant cost savings when contrasted with conventional water-based geothermal projects. To effectively predict and optimize the utilization of CO2 in geothermal applications, a precise understanding of CO2 properties and CO2-hydrocarbon gas-water mixtures become imperative.
We conducted a series of highly accurate composition, viscosity and density measurements of CO2-methane-water mixtures. To prepare the mixtures, CO2 and CO2-methane gas was humidified in a saturating unit with accurate control of pressure and temperature. The humid gas was then directed through a capillary rheometer and an oscillating U-tube to measure viscosity and density of the mixtures. The outlet was equipped with a multi-stage gravimetric hygrometer to measure the water content and get the composition of the mixture. To ensure reproducibility and high quality of the data, the experiments were conducted repeatedly.
Our research yielded data across a spectrum of CO2-methane-water properties. The measured water solubility in CO2 agreed with existing literature, while significantly extending the available dataset into the realm of high pressure and temperature conditions, pertinent to CO2 geothermal applications. This extension elucidated the non-linear relationship between solubility and pressure.
Brine salinity has a noticeable impact on water solubility in CO2, causing up to a 15% reduction. Our investigation also established the relationship between water solubility and the CO2 content within CO2/Methane mixtures.Furthermore, our study precisely quantified the influence of CO2's water content on phase density and viscosity, revealing that while water's effect on CO2 density is modest, it engenders a substantial difference in CO2 viscosity, with variations of up to 9.1% observed between dry and water-saturated CO2.
This paper addresses critical knowledge gaps pertaining to the modelling of CO2-methane-water fluids for CO2 geothermal applications. Our laboratory findings substantially enhance the dataset for water solubility in CO2 under high-pressure, high-temperature conditions. Furthermore, we shed light on the influence of brine salinity and gas composition, aspects that have not been extensively explored previously. Additionally, we unravel the impact of these properties on the density and viscosity of CO2-rich phases, enabling accurate predictions of volumetric and flow behaviour.
Furthermore, this paper presents a comprehensive account of our experimental methodology and procedures. The results obtained have been rigorously cross verified with existing literature data, affirming the robustness of our chosen approach.