Laboratory Experimental and Numerical Investigations of Heat Extraction From Porous Media by Means of CO2 Application

Application

Geothermal energy systems attempt to extract thermal energy in locations where sub-optimal geological conditions are present. Part of this process involves reservoir stimulation to overcome the lack of porosity/permeability of the rock using either physical or chemical processes with the use of a heat-transfer process fluid. Supercritical CO₂ can be used as the working fluid due to its lower viscosity and greater density difference between the cold and hot fluids used in this process. Researchers at the University of California, Berkeley performed a variety of experimental and numerical investigations to determine the performance of supercritical CO₂ to extract heat from porous rock for geothermal applications. Due to the sparsity of academic literature on how best to construct a lab-scale supercritical CO₂ flow system, the authors took it upon themselves to construct an apparatus to continuously flow temperature-controlled supercritical CO₂ under controlled conditions into a pressure vessel. They measured the temperature of this vessel at 23 locations within the sample using thermocouples. Master Bond EP31 was critical to ensuring that the thermocouples measured the temperature without leaking supercritical CO₂.

Key Parameters and Requirements

The authors inserted 23 thermocouples from the bottom of the vessel through two stainless steel pipes that were sealed using EP31. To optimize the bonding, the authors internally threaded the stainless steel pipes and then cleaned them before assembly. Once the thermocouples were inserted and properly adjusted, a compression fitting cap was attached to one end of the pipes to allow the thermocouples to pass through. The pipes were filled with EP31, and the entire apparatus was vacuumed to remove air bubbles.

Results

The developed apparatus provided data that may be useful for validating CO₂-based porous media flow models for field-scale geothermal applications. When working with supercritical CO₂, effusion of the gas is a major concern, especially at higher temperatures and pressures. The authors investigated a variety of material combinations to obtain seals that prevented CO₂ leaks. After a trial-and-error approach, the authors finally used Master Bond EP31 to seal the thermocouples into their stainless steel housing. Without threading and cleaning, the authors noted that insufficient bonding was obtained, further highlighting the importance of thoroughly cleaning the bonding surfaces before applying and curing EP31. The authors noted that EP31 remained intact and maintained a seal throughout the duration of their experiments, preventing the leakage of supercritical CO₂ during experiments.