An apparatus with a straddle packer for use within a wellbore. The straddle packer can include a tubular body comprising a first bore and a second bore and a first sealing element configured to seal against an inner surface of a casing installed within the wellbore. A second sealing element configured to seal against the inner surface of the casing, and the second bore can be located between the first sealing element and the second sealing element. An obstruction disposed in association with the tubular body, wherein the obstruction is movable between a first position in which the obstruction opens the second bore and a second position in which the obstruction closes the second bore.