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The wellbore stability of soft mudstone formation is strongly affected by thermal and hydraulic loadings due to its low strength and high plasticity under high-pressure and high-temperature conditions. A nonlinear coupled thermal– hydraulic–mechanical (THM) model was proposed to evaluate the influence of temperature and fluid factors on wellbore stability located in deep and complex soft mudstone formations. The evolution of near-wellbore formation strength and permeability was quantitatively described by introducing a plastic damage variable in the THM model. The influence of mud pressure and temperature on the stress and plastic damage distribution near the wellbore zone was analyzed using the THM model. The accuracy of the models was verified by comparing the radial/hoop stress results with those from classical thermopore elastic theory. Results show that the plastic failure of the near-wellbore zone and wellbore stability are mainly controlled by the coupling effects of thermal and hydraulic deformations of the soft mudstone. Specifically, the size of the damage zone is enlarged by 2.7 mm when the mud temperature increases by 40 °C. By contrast, the damage scale is reduced by 1.7 mm as the temperature decrease by 40 °C due to the compaction effects in the nearwellbore region. The mechanical behavior of the soft mudstone wellbore is time dependent, that is, with the increase in time, the damage gradually extends to the periphery of the wellbore after the plastic equilibrium state near the wellbore is reached. This study provides a THM modelling framework for understanding the mechanism of soft mudstone wellbore instability and formulating corresponding preventive measures.

Formation Damage and Wellbore Stability of Soft Mudstone Subjected to Thermal– Hydraulic–Mechanical Loading