Risk Analysis on Geological and Geomechanical Uncertainties - Learnings from CO2 Sequestration Modelling of a Saline Aquifer in Central Alberta, Canada

Abstract The Government of Alberta has entered into evaluation agreements with 25 hubs to investigate permanent CO2 sequestration options, including deep saline aquifers. There is a rush of optimistic claims from numerous organizations on how much CO2 they are planning to dispose, yet it remains unclear how this will be achieved. In this study, practical models were constructed for the Basal Cambrian Sandstone (BCS) to evaluate its CO2 storage capacity from regulatory, geological and geomechanical perspectives. Field data was collected from the Quest carbon capture and sequestration (CCS) facility, operated since 2015. A geomechanics module, integrated with a dual-permeability fluid model, was utilized to investigate the input variables of the storage formation and associated caprock, such as permeability, Young's modulus, Poisson's ratio, and Biot coefficient, etc. Barton-Brandis model was adopted to investigate CO2 leakage paths through natural fractures in the caprock. An extensive history match was performed and forecast cases were conducted to assess CO2 plume migration, trapping mechanisms, and pressure distribution throughout the life span of the project and over one hundred years after shut-in of CO2 injection. The results have shown that the amount of CO2 disposal could be over four times the original designed capacity if the injection wells can be operated at maximized injection rate while still maintaining injection pressure below the regulated pressure limit. In addition, the CO2 storage potential can be significantly impacted by geological and geomechanical uncertainties, including ambiguities in structural and petrophysical interpretations, characterization of natural fractures, etc. For example, injection pressure that is higher than the minimal effective stress could lead to failure in the caprock. The changes of natural fracture permeabilities are modelled with the effective stress of surrounding rock matrix and the extent of CO2 leakage into the caprock is studied. The results showed that when reactivated, the natural fractures, especially those near the injectors, substantially increased the amount of CO2 leakage into the caprock. When this happens, it could potentially impose a risk to both storage capacity and safe containment of the sequestered CO2. Finally, proxy functions were developed for correlating the calculated CO2 storage capacity and risk factors. This provides a simplified analytical approach for further risk mitigation. This study performed a comprehensive quantification of regulatory, geological, and geomechanical risks in a CCS project with real-world data. It provides an exemplary workflow of risk assessment, both numerically and analytically, to guide exploration, planning, operations, and monitoring of future CCS projects.

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