Electricity and heat generation activities accounted for about 41% of the carbon dioxide (CO2) emissions from burning fossil fuels in 2017. Therefore, developing low-carbon means of power generation and decarbonising the electricity and heat generation sectors are crucial steps in the transition to net zero. Geothermal energy is an adaptable and enormous resource, and when used in combination with carbon capture and storage can prevent CO2 from reaching the atmosphere by sequestering it in saline aquifers located deep underground. CO2 was introduced as an alternative working fluid in geothermal systems, defining the concept of CO2-plume geothermal (CPG). Although scCO2 has a lower heat capacity than water, its lower viscosity results in a lower pressure drop and higher production rates. Moreover, the naturally generated thermosiphon flow (resulting from the density gradient between the injection and the production well) almost removes the pumping requirements at the production well.
However, there are a plethora of geochemical, thermophysical, and subsurface hydrogeological parameters that affect the efficiency of such systems. One of the main concerns about injecting CO2 in saline aquifers is the geochemical reactions (specifically capillary-enhanced salt precipitation) that happen inside the aquifer and can damage the aquifer by reducing its permeability and increasing the pressure build-up near the injection well. Second is the geological uncertainty and heterogeneity that significantly affects the system performance. Last but not least, and arguably the most important, is the naturally-driven thermosiphon flow and power generation from the aquifer.
The present work covers all these concerns in separate but coherent and integrated pieces of research, using a range of porous media and heat and mass transfer modelling studies. First, salt precipitation and the effect of capillary backflow are studied, and an analytical solution is provided to estimate the amount and extent of the precipitation when injecting CO2 in a saline aquifer. Capillary pressure significantly affects the amount of precipitated salt and should not be ignored. Nevertheless, intense salt precipitation mainly occurs in a close area near the injection well. Therefore, its effects on the system’s overall performance and power generation are insignificant. Second, using the developed codes and models, various 2D and 3D heterogeneous braided aquifer realisations are generated, and performance metrics are optimised by studying different affecting parameters. It is observed that heterogeneity significantly reduces the system performance by up to 75%. Finally, a direct CO2 power plant is coupled with the well and aquifer models, and a comprehensive power generation sensitivity analysis is provided. This study has offered a more profound insight into the operation and functionality of CPG power systems and proposes recommendations on their feasibility, performance, and challenges.
Figure 1.2: Geothermal energy technologies: ground source heat pump for relatively shallow depths, and hydrothermal systems, CPG, and engineered geothermal systems (also known as Hot Dry Rock or HDR) for deep geothermal sources (modified from ). Compared to EGS, CPG targets shallower aquifers with higher natural permeability.