In many oil and gas wells, long sections of the annulus between casing and rock are left open during completion. These are potential pathways for leakage, and may need to be sealed off in critical places. In numerous field cases in shale formations it has been found that the rock has crept in and closed the annulus by itself. If this happens, it represents huge cost savings for the operator. In order to study this process under controllable conditions, we have designed a specific laboratory test where shale samples are subjected to similar conditions as the rock around a well, and their ability to establish sealing barriers are tested. The test reveals how the sealing efficiency of the barrier and the load on the casing develops with time. Post-test μCT scans reveal information about the permanent structural changes the rock has suffered during the shale barrier forming process. The test results can not be directly transferred to field conditions for a specific rock, however.