A wellbore stability analysis model with chemical-mechanical coupling for shale gas reservoirs

Chemical and mechanical coupling are the most important factors affecting wellbore instability. The main objective of this research is to propose a wellbore stability analysis model for shale gas reservoirs. A mathematical model was proposed to analyze wellbore stability based on a quantitative solution for stress induced by mechanical, hydraulic and chemical effects, and the effective stress tensor around the borehole in a cylindrical coordinate system was also obtained. Anisotropic mechanical properties and changes in the strength of shale rocks were collected from tri-axial compression experiments, direct shear tests and the literature. To examine for shear failure along the weak plane and across the weak plane, the effective stress tensor in a cylindrical coordinate system was transformed into the weak plane's local coordinate system and integrated into the strength criteria of the weak plane. In addition, the failure regions around a horizontal well were simulated at different drilling times and for different drilling directions, and the real causes of wellbore instability for well X201-H1 in the Sichuan basin were analyzed. The results indicate that the nonlinear evolution equation for the strength parameters obeyed the logistic model; the strength parameters decreased drastically as the soaking time increased over the first five days, after which the strength parameters decreased slowly. In addition, pore pressure increased and solute concentration decreased under the condition Cm < C0, while pore pressure decreased and solute concentration increased under the condition Cm > C0. The decrease in strength and the increase in pore pressure have significant impacts on the stability of wellbores within shale gas reservoirs. Pore pressure propagation changes the effective normal stress on the weak plane of the wellbore and results in the stress concentration exceeding the strength envelope. In traditional models, failure regions occur only on the surface of a borehole; however, in the new model, failure regions can also occur in the interior of a formation, and they can occur within four zones around a wellbore's circumference. The decrease of shale strength and the increase of pore pressure under the condition of water-based mud (WBM) has a greater impact than in oil-based mud (OBM), which help define the critical equivalent mud weight (CEMW) requirements at which the rate of collapse increases rapidly. To maintain borehole stability, a series of approaches must be adopted, including reasonable mud weight (MW), mud system, well path, physical plugging, etc. This model can be used to analyze the failure regions around boreholes and calculate the CEMW needed to maintain wellbore stability at different times. This model is different from, and more practical than, the traditional model.