A Model for Predicting Borehole Wall Stability in High Stress Fractured Formations

To deal with the wellbore collapse and instability problems encountered in drilling the broken marine carbonate formations in Shunbei and western Sichuan, a visual true triaxial wellbore instability physical simulation experimental platform was prepared and used to study the microstructural characteristics, physiochemical characteristics and mechanical properties of the broken formations, and it was understood that concentrated geostress, formation fragmentation and the mechanical-chemical coupling effect between the drilling fluid and the rocks are the main controlling factors for wellbore instability. By introducing a formation integrity coefficient, a parameter relationship between the “formation integrity coefficient + drilling fluid soaking” and formation mechanics was established. Based on finite element simulation, the distribution of the geostress of the broken formations was revealed. Based on equivalent rock mechanics parameters and taking into account the chemical interaction between drilling fluid and rock, a collapse pressure prediction model based on M-C criterion was constructed. The prediction accuracy of collapse pressure in typical wells, such as the well SHB9X, PZ5-3D and PZ6-5D, is as high as 86.0%-93.9%.