Experimental study on the effect of CO2 storage on the reservoir permeability in a CO2-based enhanced geothermal system

Abstract

A CO₂-based Enhanced Geothermal System (CO₂-EGS) has dual benefits of heat extraction and CO₂ storage. Mineralization storage of CO₂ may reduce reservoir permeability, thereby affecting heat extraction. Solutions require further research to optimize and balance these two benefits. In this study, CO₂ storage and heat extraction were simulated by alternating cyclic injection of water and supercritical CO₂ into fractured granite. By analyzing the changes of ion composition in water samples and the minerals of fracture surface, the mechanisms controlling the fracture permeability with and without proppant were obtained. The results suggest that monticellite and vaterite were formed besides montmorillonite, calcite and illite after increasing the injection cycles. This promotes mineralization storage of CO₂ but reduces reservoir permeability. Without proppant, the permeability decreased in three stages and the reduction rate exhibited a sharp-slow–fast–slow trend. While the use of proppant caused an increase of two orders of magnitude in permeability. Therefore, increasing the non-contact area of the main fracture and the CO₂ flow velocity can avoid a large decrease in permeability, which will increase the heat extraction and mineralization storage of CO₂. The findings provide solutions for the CO₂ emission reduction and the efficient exploitation of hot dry rock.