A Comparative Study of the Effect of CO2 Properties on Thermal Output of Geothermal Wells
Using CO2 as the geothermal power fluid to generate electricity from low-temperature abandoned hydrocarbon wells can help reduce well costs and optimize energy production. In this paper, a previously developed coupled well/reservoir model is extended to study the effects of fluid properties on thermal output.
The problem of a growing carbon footprint calls for the exploitation of cleaner and sustainable energy resources. Geothermal energy is clean, renewable, and in abundant supply under the surface of the earth, which makes it one of the more optimal solutions to this problem. With the depletion of hydrocarbon resources, geothermal energy also helps reliably close the gap between demand and supply of cleaner energy resources, although several problems need to be solved before geothermal energy can be produced globally. This study strives to understand and improve reservoir heat extraction through a geothermal well.
There are approximately 3 million abandoned wells within the US, and this number will only increase. Producing electricity from these abandoned hydrocarbon wells by using them as a source of geothermal energy has intrinsic importance in the context of extending the life of the well and in the generation of options for new wells. Regardless of whether the costs are sunken (existing wells), incremental costs for potential wells can be as minimal as redesigning them to fit future geothermal energy production. Not only is the design of the wells crucial to producing geothermal energy effectively but so is the selection of the right power fluid. Using CO2 as the power fluid to generate electricity from low-temperature abandoned hydrocarbon wells, while simultaneously sequestering it, will help reduce well costs and optimize energy production at lower temperature thresholds.
In this paper, a previously developed coupled well/reservoir model is extended to study the effects of the fluid properties on thermal output. Specifically, the original model considered fluids with constant properties. Several correlations and tables are used in this study for pressure- and temperature-dependent fluid properties (i.e., density and viscosity) to quantify their effect on the thermal balance of the geothermal system. These results are important for understanding the effects of the fluid pressure/volume/temperature (PVT) properties on the physics and economics of the entire geothermal project.
This study is important for the design of closed-loop systems and can be extended to enhanced geothermal systems. For given reservoir intake conditions, it also can be used to perform economic evaluation for abandoned oil and gas wells to assess their feasibility for geothermal energy production while reducing overall CO2 footprint.