Influence of Drilling Fluid Temperature, Density, and Salinity on Borehole Stability in Permafrost Strata

In the drilling process in permafrost strata, the mass and heat transfer effects may thaw the strata around the boreholes and decrease the content of pore ice, thus causing the mechanical properties of the strata to deteriorate greatly, thus influencing the stability of the borehole walls. In this work, a multiphysics coupling mathematical model was built for the stability of borehole walls in permafrost strata. Based on this model, the leading factors for the influences of the mass and heat transfer effects of drilling fluids on the stability of borehole walls were analyzed, and the influences of different drilling conditions on the stability of borehole walls were studied. The results demonstrate that the heat conduction of drilling fluids to the strata is the most important factor that influences the stability of borehole walls, and the diffusion of salt components affects the freezing temperature of pore water and the pore ice content in the frozen area. As the duration of the drilling increases, the collapsed zones of the borehole walls develop toward the radial and circumferential directions. Decreasing the temperature of the drilling fluids can improve the temperature distribution in the strata around the boreholes and is beneficial to reducing the degree of collapse. The increment in the concentration of salt components in the drilling fluids can decrease the overall temperature distribution in the strata, while the increase in the ionic concentration substantially decreases the pore ice content in permafrost and increases the borehole expansion rate. Enlarging the fluid column pressure of the drilling fluids does not intensify the mass and heat transfer effect of drilling fluids on the strata, while it greatly affects the stress distribution in the strata, shrinks the borehole collapse range, and improves the stability of the borehole walls.