Influence of drilling fluid temperature on borehole shrinkage during drilling operation in cold region

Frozen soil is a porous elastic-plastic-viscous material with temperature-sensitive properties. Borehole shrinkage occurs due to the plasticity and creep of frozen soil that ensues during drilling operations in cold regions. Based on the change of mechanical properties of frozen soil during the heat transfer process that occurs between the drilling fluid and the formation, a heat-fluid-solid coupling model is established in this paper to analyze borehole shrinkage behaviors in frozen soil. The results show that during drilling operations, the plastic and creep deformation of frozen soil will result in borehole shrinkage. When the borehole is drilled, a certain amount of plastic deformation will occur due to stress concentration. As drilling operations continue, both plastic and creep deformation gradually accumulate. The borehole shrinkage is dominated by the initial temperature of the formation, and the higher the formation temperature, the greater the shrinkage. At the same formation temperature, the initial borehole shrinkage decreases with increasing temperature of the drilling fluid, and the final shrinkage increases nonlinearly with increasing temperature. The drilling fluid temperature should be close to 0 °C when the formation temperature is high to reduce the drilling time. When the formation temperature is low, cold drilling fluid should be used to maintain borehole stability. This study provides a theoretical basis for the selection of drilling fluid temperatures during drilling operations in cold regions.