Prediction Model of Casing Wear Shape and Residual Strength under Compound Modes

Abstract

Summary Casing wear is one important reason for wellbore integrity failure in deep and ultradeep wells. Field measurement data show that there are multiple wear areas on the casings in some well sections, and the wear shape is more complicated than a single crescent, which exceeds the scope of conventional casing wear theory. To solve this problem, prediction models of casing wear shape and residual strength under compound modes are built. First, the compound wear mode is divided into four categories according to field measurement results, and the prediction model of wear shape is built. Second, the failure mechanism of casing with the compound wear shape is revealed through experiment and numerical simulation. Next, the influences of various factors on casing strength are revealed, and an intelligent prediction model of casing strength is built. Last, a software platform of casing wear and strength prediction is developed and applied to an ultradeep well. The results show that casing wear shape is not only a single crescent but also has the modes of overlapping, separating, and crossing crescents. Different wear modes are caused by different reasons: overlapping is caused by the change of wear diameter, separating by a large deviation of wear position, and crossing by slight deviation of wear position. The failure mechanisms of worn casing under internal and external pressures are different: Internal pressure failure is one kind of strength failure while external pressure failure is one kind of stability failure. The critical internal pressure of the casing is mainly determined by the main wear area but irrelevant to the secondary wear area, yet the critical external pressure is closely related to both the main and secondary wear areas. Casing strength will be overestimated if casing wear is neglected, so casing wear should be sufficiently considered in ultradeep well design.