Abstract: The energy demand has increased dramatically in the last century, and so to have global CO2 emissions. Two critical challenges for the geo-energy sector are to develop different approaches for harvesting energy and to actively decrease atmospheric CO2 emissions. Addressing these challenges requires efficient, sustainable, and affordable technical solutions. Complex fluids are ubiquitous and offer great potential for geo-engineering applications such as enhanced oil recovery, CO2 geological sequestration and soil stabilization. This thesis will present new results on interfacial phenomena in CO2-fluid-mineral systems, including interfacial tension hysteresis, the effects of active components on interfacial tension (surfactants, nanoparticles, organo-bentonites and asphaltenes), and the interfacial pinning of immiscible fluids on substrates. Finally, pore-scale phenomena come together in the study of CO2-assisted gravity enhanced oil recovery. Finally, we provide a better understanding of complex fluids and their interactions within porous media that can lead to efficient and sustainable geo-energy systems.