Experimental determination of precipitation and permeability changes in granite fractures with implications for CO2-driven enhanced geothermal systems

Abstract

There is a growing interest in renewable energy and carbon sequestration projects that seek to abate the challenges created by climate change. The exciting prospect of an enhanced geothermal system (EGS) using carbon dioxide as its working fluid offers a combination of renewable energy production and carbon sequestration; however, geo- chemical models are the only way in which the reactions and resulting precipitations in these subsurface systems deep underground are currently understood. Experimentation with a focus on the precipitating reactions and their effect on permeability is lacking. This study aims to use experimentation and image analysis to look into the precipitating reactions and permeability changes that can take place in an enhanced geothermal system with carbon dioxide as its working fluid under different initial conditions. In order to accomplish this task, two experiments were planned and executed: a batch experiment and a column flow experiment through fractured granite cores. The batch experiment confirmed that the reaction between a carbon dioxide acidified brine and a mineral constituent of granite, plagioclase feldspar, could take place over a short time scale. The column flow experiment showed an increase in permeability during the reactionary period, but permeability could not be computed when carbon dioxide was used to displace the brine and “dry-out” the fracture. The subsequent image and electron dispersive x-ray spectroscopy analyses were used to identify the presence of calcite or carbonaceous mineral precipitates along the fracture surface. Additionally, removed surface material originating from the granite core was found in the brines, implying that the reactions or flow removed the material from the surface. This study can be used as a model for future work to delve further into the EGS environment under more realistic subsurface temperature and pressure conditions. Ultimately, these findings contribute to the understanding of EGS reservoir geochemistry by showcasing experimental evidence of precipitations and indicate the importance of addressing these reactions as EGS continues to develop as an energy resource.