Mud shale, used in drilling engineering, is prone to hydration and expansion, resulting in creep deformation that leads to wellbore shrinkage and pipe sticking incidents. Studying the creep characteristics of mud shale is significant for designing a reasonable well structure and determining the lower limit of drilling fluid density. The influence of moisture content on rock strength and creep mechanical properties were studied using water absorption, uniaxial compression, and creep tests. Test results show that with an increase in the moisture content, the mud shale was damaged and softened; moreover, the elastic modulus decreased with increase in moisture content. Under the same load level, the instantaneous strain increased with increasing moisture content. Under different loading stresses, the creep of the rock had nonlinear characteristics, which could be divided into three different creep stages: attenuation, second, and accelerated creep. A new improved creep model based on the Nishihara model was established to describe the accelerated creep characteristics of mud shale under different moisture contents. The ageing degradation and water-bearing weakening effects were introduced. The Levenberg–Marquardt nonlinear least-squares method was applied to invert the creep parameters. The results show that the simulated creep curves, generated using the new creep model, conform to the experimental ones. The relationship between the drilling fluid density and wellbore shrinkage ratio can be defined using this model; it provides a reference for reasonably determining the drilling fluid density.

Mud shale is characterized by low strength and strong swelling, with rheological effects and deformation caused by drilling fluid and formation water. Establishing a rheological model to characterize the deformation characteristics is key to solving the problem of wellbore stability. The influence of moisture content on rock strength and creep mechanical properties were studied by means of water absorption, uniaxial compression, and creep tests. The tests showed that with the increase in moisture content, the elastic modulus and strength of hard brittle mud shale decreased. Further, under the same load, the instantaneous strain increased with increasing moisture content. Meanwhile, under various loading stresses, rock creep exhibited non-linear characteristics, which can be divided into three different creep stages: attenuated creep, stable creep, and accelerated creep. Starting with a non-linear viscous dashpot, and then introducing aging degradation and water-bearing weakening effects, based on the water-bearing creep characteristics of hard brittle shale as well as the modeling ideas of the classic component combination model, a new improved creep model based on the Nishihara model was established to describe the characteristics of the accelerated creep stage of hard brittle mud shale with various moisture contents. Subsequently, the Levenberg–Marquardt non-linear, least-squares method was adopted to invert the creep parameters. The results showed that the simulated creep curves achieved by employing the new creep model were consistent with the experimental results, thereby confirming the ability of the new non-linear creep model to provide a theoretical reference for the study of wellbore stability of hard brittle mud shale.