SHALE AS A SEALING BARRIER AROUND DEEP WELLS

摘要

Well sections that have been completed with an open annulus between the casing and the rock are occasionally found to be sealed by shale that creeps in and closes the annulus. This process usually happens spontaneously, without deliberate actions from the operator, in weak rocks with high shale content. The phenomenon has been studied in a series of laboratory tests. The tests reveal that annulus pressure may play a significant role in the formation of a shale barrier. The study also shows that formation of the barrier implies damage and permanent deformation of the rock in a limited region around the hole. In the field, a shale barrier is hidden behind the casing and is not directly accessible. They are usually detected by sonic logging tools and verified by pressure communication tests. Interpretation of sonic signals from behind the casing is however challenging, and identification of shale barriers in the field is not trivial.