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Summary Large-scale geological storage of carbon dioxide (CO2) is likely to bring CO2 plumes into contact with a large number of existing wellbores. The flux of CO2 along a leaking wellbore requires a model of fluid properties and of transport along the leakage pathway. Knowing the range of effective permeability of faulty cement is essential for estimating the risk of CO2 leakage. The central premise of this paper is that the leakage pathway in wells that exhibit sustained casing pressure (SCP) is analogous to the rate-limiting part of the leakage pathway in any wellbore that CO2 might encounter. Thus, field observations of SCP can be used to estimate transport properties of a CO2-leakage pathway. Uncertainty in the estimate can be reduced by accounting for constraints from well-construction geometry and from physical considerations. We then describe a simple CO2-leakage model. The model accounts for variation in CO2 properties along the leakage path and allows the path to terminate in an unconfined (constant-pressure) exit. The latter assumption provides a worst-case leakage flux. By use of pathway permeabilities consistent with observations in SCP wells, we obtain a range of CO2 fluxes for the cases of buoyancy-driven (post-injection) and pressure-driven (during injection) leakage. Assuming the frequency distribution is representative of SCP wells, we observe that in leakage pathways corresponding to the slow but nonnegligible buildup of casing pressure (several psi/D), the effective permeability of the leakage path is in the range of microdarcies to hundreds of microdarcies, and the corresponding CO2 fluxes are comparable with naturally occurring background fluxes observed at the ground surface. In pathways corresponding to intermediate and fast buildup rate of casing pressure (tens to hundreds of psi/D), the effective permeability is in the range of tenths to tens of millidarcies, and the CO2 fluxes are comparable with surface flux measurements at the Illinois basin and at the natural seep at Crystal Geyser (Utah). In pathways corresponding to very fast buildup rate (thousands of psi/D), the effective permeability is from tens to hundreds of millidarcies and the CO2 fluxes are up to three orders of magnitude higher than those measured at Crystal Geyser.

Large-scale geological storage of CO2 is likely to bring CO2 plumes into contact with a large number of existing wellbores. The flux of CO2 along a leaking wellbore requires a model of fluid properties and of transport along the leakage pathway. The leakage pathway in wells that exhibit sustained casing pressure (SCP) is analogous to the rate-limiting part of the pathway in existing wellbores along which CO2 may leak. Thus field observations of SCP can be used to estimate transport properties of a CO2 leakage pathway. We develop a more robust optimization algorithm to get the best data fit in the SCP model. Constraints from well construction geometry and from physical considerations reduce the range of estimated permeability. We then describe a simple CO2 leakage model. The model accounts for variation in CO2 properties along the leakage path and allows the path to terminate in an unconfined (constant pressure) exit. The latter assumption provides a worst-case leakage flux. Using pathway permeabilities consistent with observations in SCP wells, we obtain a range of CO2 fluxes for various boundary conditions. In leakage pathways corresponding to the slow but nonnegligible buildup of casing pressure, the CO2 fluxes are comparable to naturally occurring background fluxes observed at ground surface. In pathways corresponding to rapid buildup of casing pressure, the fluxes are comparable to measurements at Crystal Geyser (Utah), a natural CO2 seep. Uncertainty in pathway permeability has a first-order effect on uncertainty of CO2 flux. Uncertainty in the length of the pathway has a comparatively minor effect. Increasing the CO2 at the base of the pathway does not dramatically increase the CO2 flux above the purely buoyancy-driven value.

Large-scale geological storage of CO2 is likely to bring CO2 plumes into contact with a large number of existing wellbores. Estimating the flux of CO2 along a leaking wellbore requires a model of fluid properties and of transport along the leakage pathway. Wells that exhibit sustained casing pressure (SCP) in an intermediate annulus have a leakage path along a cement/steel interface, or within the cement in the annulus. The former path is analogous to a leakage path along a cement/earth interface outside the casing. The latter path can occur in cement outside the casing. Thus the likely magnitude of the permeability of leakage paths outside the well can be estimated from the permeability of these analog paths. A sustained casing pressure (SCP) model yields information about effective permeability of the pathway. By choosing reasonable ranges for other well construction parameters, we apply the SCP model to obtain a range of effective permeabilities for a well based on a measured casing pressure build up history. We illustrate the approach with several field examples. For a relatively slow pressure build up (several psi/day), the permeability of the leakage path is in the range of microdarcy to hundreds of microdarcy. Fast pressure build up (thousands psi/day) indicates permeabilities in the range of tens of millidarcy to hundreds of millidarcy. To account for the uncertainty in wellbore construction parameters, we calculate the distribution of effective permeability of a leaky well using Monte-Carlo simulation. The resulting permeability shows an approximately log-normal distribution skewed toward the maximum possible value. The expected value and a confidence interval are obtained for each well, which represents the most probable permeability of the well for a given pressure build up. For the wells studied here the expected values range from 10 microdarcy to 100 millidarcy. The expected leakage path permeability correlates reasonably well with pressure build up rate. This is to be expected from Darcy’s law, and thus a strong correlation between leakage path permeability and other characteristics of the wells in this sample does not exist. Obtaining the statistics of the expected leakage path permeability will thus require more observations of SCP wells. The effective permeability of a leaky well is essential in calculating the CO2 leakage flux. Under the assumption that a leaky well encountered by a CO2 plume has a leakage pathway with the similar properties to an SCP well, we calculate the CO2 flux for the best, worst and most probable scenarios for the example wells. In the most probable scenario of CO2 flux, we calculate the expected CO2 leakage rate. Slow leakage yields a 0.1 kg/y CO2 rate while fast leakage could have a rate of 1000 kg/y.

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