Well permeability estimation and CO2 leakage rates

The risk of leakage along existing wellbores associated with CO2 sequestration depends strongly upon the leakage path permeabilities. Assuming the gas migration path through leaky wellbores in hydrocarbon reservoirs is analogous to the leakage path for CO2 plumes, the distribution of inferred permeabilities enables quantitative estimation of CO2 leakage rates. We estimate the effective permeabilities of about 300 wellbores in six different fields that exhibit sustained casing pressure (SCP) or surface casing vent flow (SCVF). Uncertain parameters that affect the estimated permeability, such as the location of the leak source, are accounted for by a Monte-Carlo simulation approach, yielding an expected value of leakage path permeability for each measurement of SCP or SCVF. The expected permeabilities along most of the leaky wellbores are between 10 μd and 10 md. We further use a data mining approach to seek correlations between the permeabilities and wellbore characteristics. Using the wellbore permeabilities to estimate the aperture of the leakage pathways, we estimate the capillary pressure and hence the minimum CO2 plume heights required for CO2 to enter the leakage paths. We compute worst-case steady CO2 leakage fluxes, finding that over 90% of the fluxes are less than 0.1 ton/m2/yr, which are smaller than large persistent fluxes observed in nature. The majority of the leakage rates along the wellbores are less than 1 kg/yr, and the highest rates are less than 0.1 ton/yr.