Well Casing Subsidence in Thawing Permafrost: A Case Study

Deep ice lenses or excess ice in permafrost in Arctic oil fields on Alaska's North Slope, the Mackenzie Delta, or Eastern Siberia in Russia create challenging issues for oil well completion design. As thaw subsidence adjacent to oil wells deepens, it induces large drag loads through negative skin friction and leads to strain damage and even failure of the well casing. This paper presents a case study of the surface casing failure mechanism resulting from the subsidence of thawing permafrost by elastoplastic finite-element (FE) modeling. The study site, permafrost thaw evaluation results, and field observations of surface casing damage are summarized. Details of the FE model, material and interface modeling considerations and parameters, and load application steps are described. Modeling results include ground subsidence; stress and strain redistribution in the thawed permafrost near the casing; the shear stress at the surface casing–soil interface; and the vertical stress, strain, and deformation of the casing string. The model limitations are also discussed. Results show that the most likely failure mechanism of the surface casing string is plastic lateral buckling. Surface casing subsidence, possible failure mode, and locations predicted by this model compare favorably with field observation data.