How creeping shale may form a sealing barrier around a well

摘要

For wells to be permanently plugged and abandoned, rigid requirements for sealing barriers apply. Cement filling the annulus outside the casing may provide proper sealing, however in shale sections where the annulus is not filled with cement, the shale may creep towards the casing and eventually form an equally efficient sealing barrier. This paper discusses some geomechanical aspects of the shale barrier forming process, and describes an effort to study the process by a numerical simulation. Creep is implemented as time dependent plastic deformation. The annulus gap is rapidly reduced in the beginning, but the closure rate soon drops significantly, probably due to arching effects. For some shales, this effect may possibly terminate the process before the gap is closed. When contact between formation and casing is established, the closure rate is further reduced. Ovaling or collapse of the casing are possible consequences, due to high and uneven load on the casing. The simulations indicated that elevated temperature may speed up the gap closure process, however no substantial long term effect of temperature was seen.