Simulating the Effect of Frozen Soil Thaw on Wellhead Stability during Oil and Gas Drilling Operations in Arctic Waters

Abstract

Wellhead subsidence is one of the most severe engineering disasters during drilling operations in cold regions. Based on the characteristics of thaw-induced settlement of frozen soil, a thermal–fluid–soil coupling numerical model was developed in this study to analyze the probability of settlement at wellbores drilled in the cold sea areas of polar regions. The results show that the thawing of frozen soil due to the heat transfer from hot fluid flowing in the casing to the frozen soil layer is an important reason for the instability of the wellhead. The wellhead subsidence is due not only to an overall formation settlement that results from the thawing of large areas of frozen soil but also to the collapse of the formation surrounding the wellhead due to the accumulation of plastic deformation. The thickness of frozen soil depends on the seafloor temperature, but the thickness of frozen soil and the seafloor temperature exert contrary influences on the stability of the wellhead. A thicker frozen soil layer requires a lower seafloor temperature, leading to higher instability. However, a lower temperature is favorable for the stability of the wellhead. The wellhead stability is a result of the comprehensive interaction of these two factors. Improvement in the heat-insulating property of the casing is an effective method for maintaining the stability of the wellhead. When the thermal conductivity of the casing is lower than 0.1 W · (m°C), the stability time of the wellhead is feasible for the duration of general drilling operations. These research findings provide a theoretical basis for maintaining wellhead stability during drilling operations in cold regions.