The permeability of the cement annulus surrounding a casing is measured by locating a tool inside the casing, placing a probe of the tool in contact with the cement annulus, measuring the change of pressure in the probe over time, where the change in pressure over time is a function of among other things, the initial probe pressure, the formation pressure, and the permeability, and using the measured change over time to determine an estimated permeability. The estimated permeability is useful in determining whether carbon dioxide can be effectively sequestered in the formation below or at the depth of measurement without significant leakage through the cement annulus.