Investigation of coupled thermo-chemo-mechanical processes for safe carbon geological storage

Jung, Hojung
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Safe and permanent CO₂ storage in geological formations requires reservoir geomechanical stability. Injection of CO₂ into the subsurface changes the local pore pressure and, further, alters the effective stresses due to poro-thermo-chemo-mechanical coupled responses. Changes of pore pressure and effective stress may disrupt the host formation mechanical equilibrium. This alteration may result in geomechanical failure events such as fault reactivation and hydraulic fracturing. Such events can favor fluid migration paths for injected CO₂, induce seismic activity, and cause surface uplift. Examples of field observations during CO₂ injection include: (1) surface uplift at the In Salah project in Algeria, (2) absence if bottom-hole pressure (BHP) increase during injection in Cranfield, Mississippi, and (3) induced seismicity with magnitude M>1 in Decatur, Illinois. In this context, accurate estimations of pore pressure build up and local stress alteration induced by CO₂ injection are critical to avoid geomechanical perturbations. However, current models and predictions often assume relatively homogeneous reservoirs without taking into account compositional behavior. Further, the effects of temperature and chemical reactions have not been rigorously incorporated into the interpretation of local stress alteration and the well response to CO₂ injection. This dissertation shows geomechanical analyses of CO₂ geological sequestrations by three field case studies: Frio CO₂ sequestration pilot test in Texas, Cranfield CO₂ sequestration in Mississippi, and Crystal Geyser in Utah. Both Frio and Cranfield case studies are studied with the help of reservoir simulation and history matching of field data including assimilation of vertical heterogeneity from well-logging analysis and calibration with laboratory experiments. The Frio case study focuses on examination of reservoir capacity of a compartmentalized volume to avert fault reactivation. The Cranfield case study analyzes the influence of thermo-chemo-elastic processes on wellbore fracturing induced by CO₂ injection. The Crystal Geyser case study investigates the long-term chemical effects of CO₂-charged brine on rock mechanical properties through analyses and measurements on rock samples from the field, where a natural CO₂ leakage analog exists. The following conclusions are a result of this dissertation. CO₂ dissolution into brine reduces pore pressure build up significantly in small and compartmentalized reservoirs. Thermo-elastic and chemo-elastic effects alter local stresses and may trigger injector fracturing at bottom-hole pressures lower than expected. Capturing phase behavior, coupled thermo-chemo-mechanical processes, and reservoir heterogeneity are important factors to estimate reservoir capacity and prevent geomechanical perturbations.