Evaluating the integrity of millions of wells worldwide and maintaining them over their lifetime is a daunting challenge cross cutting subsurface resource utilization. This is especially true for geologic carbon storage (GCS) in depleted oil and gas fields, where the integrity of many of the wells is uncertain. United States Department of Energy (DOE) research programs have invested significant resources to understand well integrity. Through these efforts the national lab system has advanced the understanding of well integrity related to GCS. One such effort is the National Risk Assessment Partnership (NRAP), which is a collaborative effort among five DOE labs to develop science-based methods and tools to quantitatively assess and manage risks and related uncertainties in GCS. Well cement undergoes complex reactions when exposed to subsurface fluids e.g., CO2-saturated brine. Alteration of the cement phases advances as moving fronts, liberating calcium, and leaving behind a silica-rich material. The altered material is more porous, permeable, and mechanically weaker than the original cement. At favorable pH, calcium carbonate precipitates to form bands which reduce pore space and retards the rate at which alteration fronts advance. Research has shown that in a properly-cemented annulus, where cement provides zonal isolation and protects the casing, well degradation is unlikely. However, when wells fail to maintain zonal isolation, the corresponding pathways are conduits for CO2 and brine leakage. Through work by NRAP we have observed that while leakage does occur, pathway sealing is possible. Experiments were conducted to quantify the permeability reduction due to precipitation in fractures or due to mechanical deformation of reacted cement. These observations were then used to calibrate a multi-physics model representing interactions in the cement/carbonated brine system. The model, which couples transport, geochemistry, and geomechanics, was used to identify conditions that lead to sealing over an extended parameter space. These observations provided constraints for the development of a reduced-physics model that can rapidly assess a well’s leak-flux evolution. This reduced-physics model can be tuned by new insights to expand the range of application to fit site-specific needs. As the reduced-physics model is computationally efficient it can be incorporated into system-level simulations and be used in stochastic approaches to predict well leakage evolution.

Experiments, simulations, and reduced physics modeling for risk assessment of well integrity at CO2 storage sites

253rd National Meeting (2017)

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