33 Unsaturated Soils – Alonso & Gens (eds) © 2011 Taylor & Francis Group, London, ISBN 978-0-415-60428-4 Energy geotechnology: Implications of mixed fluid conditions J. Carlos Santamarina & Jaewon Jang School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA, USA ABSTRACT: Multi phase fluids are common in energy-related geotechnical problems, including gas- water, gas-oil, ice-water, hydrate-water, and oil-water fluid conditions. The generalization of classical unsaturated soil mechanics concepts to energy geotechnology requires physical understanding of surface tension, contact angle, capillary pressure, solubility and nucleation. Eventually, these pore-level processes affect the granular skeleton. Together, pore and particle-scale interactions upscale through the sediment structure to affect its macroscale response. Possible emergent phenomena include fluid percolation, resid- ual saturation and recovery efficiency; fluid driven fractures, lenses, fingering and pipe formation; bubble migration and bottom blow up. presented in the following section capture the essential characteristics; references are provided for detailed information. 2 ATOMIC-SCALE PHENOMENA Geotechnical implications of mixed-fluid condi- tions arise from interactions at the atomic scale where surface tension and contact angle are defined. Interfaces are in a state of dynamic equilibrium: molecules are continuously jumping from one phase to the other. The average residing time for a 1 INTRODUCTION Energy geotechnology involves geotechnical phe- nomena and processes related to energy, from resource recovery to infrastructure and waste man- agement. Energy resources include fossil fuels (90% of all primary sources—coal, petroleum, and gas), nuclear, hydroelectric, and other renewable sources (wind, geothermal, solar, tidal, biomas). The most critical energy-related waste storages include: CO 2 geological storage (from fossil fuels), fly ash (from coal), nuclear waste, and coal-mining waste. Resource recovery, energy infrastructure and waste management often involve multi-phase fluid conditions (Table 1—classical infrastructure related conditions are not addressed in this manu- script). The most relevant cases are: •฀ L-G: water-air, water-CO 2 and water-methane interfaces (as well as other biogenic and ther- mogenic gases). The liquid L has molecules of the gas in solution, and the gas contains mol- ecules of the liquid. •฀ L 1 -L 2 : water-liquid CO 2 (geological C-storage), and water-oil (petroleum reservoirs). Both liq- uids include molecules of the other liquid in solution. •฀ L-I: water-ice and water-hydrate. Related analy- ses can often be interpreted as the “solid” ice or hydrate phase behaving as a high viscosity fluid. The purpose of this manuscript is to extend fun- damental concepts in unsaturated soil behavior to address mixed-fluid conditions in energy geotech- nology. First, we explore interfacial processes at the atomic scale; then, we identify emergent phenom- ena that affect field-scale applications. Concepts Table 1. Mixed fluid conditions in energy geotechnology. Fossil fuels (oil, coal, gas. Unconventional: coal-bed methane, shale-gas, tight-gas sandstone, CH 4 hydrates) Recovery water(l), oil(l) CO 2 (g), CO 2 (l) CH 4 (g), CH 4 (h) CO 2 storage water(l), oil(l) CO 2 (g), CO 2 (l), CO 2 (h) CH 4 (h) Nuclear Spent fuel storage air/vapor(g), water(l) Renewable: solar, wind, tidal Compressed air storage air/vapor(g), water(l) Renewable: bio, geothermal Production steam(g), water(l) Note: Mixed fluid conditions in infrastructure are not listed. Phases shown in parenthesis (g: gas, l: liquid, h: hydrate).