Well abandonment is one of the biggest challenges in the oil and gas industry, both in terms of cost and effortas well as the technical hurdles associated with wellbore isolation for an indefinite term. A mechanism thatmay be exploited to simplify well abandonments is using natural shale formations for the creation of annularbarriers. Currently, uncemented annuli often require casing milling and pulling before abandonment plugscan be set, which necessitates the use of a drilling rig. This is an expensive, time- and labor-intensive process,particularly offshore. However, shale creep may naturally form a barrier behind uncemented casing sections.With a qualified annular shale barrier in place, the well may only require the setting of abandonment plugswithin the existing casing string(s), a task that can often be done rigless and with significantly less effort. The work described in this paper presents the results of a rock mechanical investigation into the creepbehavior of North Sea shales and their ability to form effective annular barriers. Field core from theLark-Horda shale was used to conduct dedicated, customized experiments that simulated the behavior ofshale confined under downhole effective stress, pressure and temperature conditions to fill in an annularspace behind a simulated casing string. Full scale tri-axial rock mechanics equipment was used for testingcylindrical shale samples obtained from well-preserved field core in a set-up that mimicked an uncementedcasing section of a well. The deformation behavior of the shale was monitored for days to weeks, and theformation of the annular barrier was characterized using dedicated strain measurements and pressure pulsedecay probing of the annular space.
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