Simulation Analysis of Wellbore Instability Considering the Influence of Hydration Effect on the Physical Properties of Brittle Shale

Shale gas, as an important unconventional oil and gas resource, its efficient development can alleviate the current severe energy demand situation. However, the water absorption and expansion characteristics and hydration effects of shale pose a great threat to the drilling safety of shale, mainly wellbore stability. Therefore, based on mechanical property experiments, the influence of hydration expansion on the mechanical property parameters of brittle shale was analyzed, and an evolution model of the mechanical property parameters of shale with hydration expansion was constructed. In addition, a finite element model for numerical simulation of wellbore stability in shale formations was established, and the effects of factors such as the addition of hydration inhibitors in drilling fluid on wellbore collapse were analyzed. Research has shown that the hydration and expansion of shale can reduce its elastic modulus and cohesive force, but the effect of hydration and expansion on Poisson’s ratio and internal friction angle shows the opposite pattern. After being immersed in drilling fluid for 12 hours, the elastic modulus of shale decreased from 5.3 GPa to 3.9 GPa, and the cohesion decreased from 4.6 MPa to 3.0 MPa. In addition, wellbore collapse and instability in shale mainly occur in the early stages of drilling operations, while wellbore collapse will significantly slow down in the later stages. The wellbore enlargement rate increased to 40% within the first three hours of drilling operations. Moreover, the addition of hydration inhibitors in drilling fluids will prevent further collapse of the wellbore by inhibiting the invasion of water. When the hydration inhibitor in the drilling fluid was increased from 0 to 45 g/m3, the wellbore enlargement rate decreased from 66.2% to 27.8%. This study can provide theoretical reference for maintaining wellbore stability and drilling safety during shale drilling.