An Improved Model To Forecast Co2 Leakage Rates Along A Wellbore

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Journal Article: An Improved Model To Forecast Co2 Leakage Rates Along A Wellbore

Large-scale geological storage of CO2 is likely to bring CO2 plumes into contact with a large number of existing wellbores. Wellbores that no longer provide proper zonal isolation establish a primary pathway for a buoyant CO2-rich phase to escape from the intended storage formation. The hazard of CO2 leakage along these pathways will depend on the rate of leakage. Thus a useful component of a risk assessment framework is a model of CO2 leakage. Predicting the flux of CO2 along a leaking wellbore requires a model of fluid properties and of transport along the leakage pathway. Leakage large enough to be a concern is most likely to occur along a defect (fracture, microannulus, gas channel) in the steel/cement/earth system, rather than through the cement matrix. This type of discrete leakage pathway has a specific geometry, and its hydraulic conductivity is therefore sensitive to the effective stress (confining earth stresses less pore fluid pressure). Wells that exhibit sustained casing pressure (SCP) are a good analogue to evaluate the likely geometry of leakage pathways. We have implemented a SCP model described in the literature, which yields an estimate of the depth of the leakage source and the effective permeability of the leakage pathway. The latter value can be converted into equivalent geometries of discrete pathways, e.g. the average aperture of a microannulus. We next describe a model for flow of CO2 along a discrete pathway. To obtain worst-case estimates of flux, we assume single-phase flow of CO2 and a continuous pathway of constant aperture. The properties of CO2 vary along the pathway and are computed with the Peng-Robinson equation of state, with an imposed temperature variation (usually geothermal gradient). The new model can assess CO2 leakage provided the information about the depth of leak and effective permeability from the SCP model. Using a range of pathway geometries consistent with observations in SCP wells, we obtain a range of CO2 fluxes for various boundary conditions. For example, through a well whose leakage pathway is 5000 ft long and has effective permeability 50 microdarcies, a CO2 flux of 2 mg/m2/s would leak from the formation, if it were stored at hydrostatic pressure. We estimate the range of CO2 flux based on the range of effective permeability calculated in the SCP model. Generally both the upper bound and lower bound of CO2 flux increases with the increase of effective permeability. We then plot the range of CO2 flux at different leakage depth. It does not show a big change in CO2 flux when the leakage depth increases. We calculate the range of CO2 flux in the case that the wellbore encounters pressure elevation during injection. The CO2 flux increases faster in deep leak than in shallow leak.

Qing Tao, Dean Checkai, Nicolas Huerta and Steven L. Bryant

Published Journal 
Energy Procedia, Date Not Provided




Qing Tao,Dean Checkai,Nicolas Huerta,Steven L. Bryant. . An Improved Model To Forecast Co2 Leakage Rates Along A Wellbore. Energy Procedia. (!) .