Mechanistic Insights and Engineering Strategies for Wellbore Integrity Control in Polar Permafrost Reservoirs

Permafrost is typical temperature-sensitive sediment, and geo-mechanical issues such as wellbore collapse and sand production are prone to occur during the oil and gas development. Therefore, it is crucial to elucidate the patterns and mechanisms of wellbore instability throughout the drilling process and to develop/propose effective control strategies. In this study, based on sensitivity analysis of wellbore stability, a method for determining the safe windows of drilling fluid temperature, density, and salinity was proposed, along with corresponding measures for preventing wellbore collapse. The investigation results indicate that the collapse of sediments around the wellbore was primarily distributed within an elliptical region oriented along the direction of the minimum horizontal principal stress. Furthermore, the worsening of wellbore collapse gradually slows as drilling operation progresses, and the final borehole enlargement rate corresponding to the default investigation case reaches 89.84\%. Sensitivity analysis showed that higher drilling fluid temperature, lower density, and elevated salinity all negatively affect wellbore stability. However, wellbore stability is easier to maintain when the permafrost has high initial ice saturation, low stress difference, and shallow burial depth. Ultimately, the safe mud temperature, density, and salinity windows derived from the wellbore stability simulations are beneficial for accurately controlling wellbore stability within the required range.