Plug and abandonment of a well is expected to ensure that the sealing integrity of the cap rock is restored. This also implies that the annulus between casing and rock is sealed off. Some formations may under the right conditions close in on the casing, providing a highly valuable contribution to the sealing. Although acoustic tools may be used to evaluate whether there is casing contact, the hydraulic integrity of such a barrier must be established by field testing for all potential formations.
While the main focus here is on natural shale barriers, the analysis and conclusions apply equally well to testing of cement based and other barriers.
This paper discusses two pressure integrity tests that are used for this purpose, the XLOT-type test and the communications test. The test procedures are outlined, and the expected sensitivity is estimated. In particular, it is shown that temperature effects may complicate test interpretation, possibly giving both false acceptance and false rejections of tests. Hence, it is of vital importance to include temperature measurements in the evaluation of the tests.
It is estimated that the leakage detection limit of both tests is significantly higher than the target value needed to ensure that sealing integrity has been restored. This calls for improvements of the testing procedures.