A Coupling Mechanical–Chemical–Thermal Wellbore Stability Prediction Model for Shale Formation and Its Application

The intricate interaction of mechanical, chemical, and thermal effects, together with the anisotropy and hydration properties of shale, poses substantial challenges for precisely evaluating wellbore stability. In the presented works, based on a failure criterion derived from the shear slide failure model, a novel wellbore stability prediction model for shale formations is developed, integrating mechanical—chemical—thermal effects. The proposed model predicts that the equivalent drilling fluid density of the collapse pressure for Well W201, which employs oil-based drilling fluid, is 1.33 g/cm3. This guides the actual drilling operation to adopt a ρc of 1.45 g/cm3, thereby successfully ensuring the safe completion of the shale formation drilling work. This result demonstrates the good rationality of the new model. Subsequently, a case study is carried out using the new model to assess the impacts of moisture content, bedding-plane occurrence, drilling fluid temperature, and membrane efficiency on shale wellbore stability. The results are as follows: (1) ρc increases linearly with an increase in moisture content The influence of ω on ρc is minimal at a well azimuth angle (αb) of 60° and most pronounced at αb = 120°. The effect of ω on ρc becomes more intense as the well deviation angle (βb) increases. (2) The range of safe well azimuth angles for deviated and horizontal wells narrows as the bedding-plane trend (TR) increases. Compared with deviated and horizontal wells, the shale wellbore stability of vertical wells is more severely affected by the bedding-plane dip angle (DIP) and TR. (3) ρc increases as the drilling fluid temperature (Tw) rises. The rate of increase in ρc when Tw is greater than the formation temperature is slightly higher than when Tw < T0. An increase in Tw can reduce the range of relatively safe drilling azimuth angles. The influence of Tw on ρc grows with an increase in βb. (4) ρc decreases linearly with an increase in membrane efficiency (Im). The ranges of αb and βb corresponding to a certain high ρc decrease as Im increases.