Stability analysis and drilling fluid density window optimization considering rock thermal damage evolution in ultra-deep wells

Abstract Ultra-deep well drilling faces challenges of high temperature, high pressure, and narrow density windows, with rock thermal damage significantly affecting wellbore stability. Through high-temperature triaxial compression experiments on granite, sandstone, and shale, we revealed that within 100 °C–300 °C, elastic modulus decreased by 15%–55%, compressive strength decayed by 20%–65%, and cohesion degraded over 50%. A thermal damage constitutive model and time-dependent wellbore stability analysis model were established using thermo-hydro-mechanical coupling theory. Results show thermal stress reaches 30–50 MPa, and thermo-mechanical coupling increases tangential stress concentration by 27%–40%. Under typical conditions, density window width narrows from 0.26 g cm −3 initially to 0.18 g cm −3 after 24 h (31% reduction) and 0.12 g cm −3 after 72 h (54% reduction). A staged dynamic density control optimization method was proposed. Field application in Tarim Basin (7850 m depth, 210 °C) shortened drilling cycle by 15%, reduced non-productive time by 54%, increased safety margin by 62%, and improved wellbore stability score from 65 to 88 points, validating the method’s effectiveness.