Geomechanical method for wellbore stability prediction: A case study of the Longmaxi formation in the Yongchuan shale gas field, southwestern China

Shale formations display heterogeneous characteristics, and the current methods for assessing the stability of adjacent wells using drilling data have limitations. This paper presents a regional wellbore stability prediction method for shale formations using geomechanics, with a case study on the Longmaxi formation in the Yongchuan area. A 3D structural model was constructed using seismic data and incorporated into finite element software to replicate the heterogeneous mechanical parameters. Numerical simulations were employed to determine the regional in situ stress and predict fracture development based on rock failure criteria. The findings indicate that the maximum and minimum principal stresses in the Longmaxi formation within the Yongchuan area range from 75 to 117 MPa and 54-102 MPa, respectively. The in situ stress is elevated in the dip and fault areas, with fractures primarily occurring in the fault and anticline regions. Both the in situ stress and fracture distribution display significant heterogeneity. Collapse pressure varies from 1.07 to 1.61 g/cm3, with higher values in the fractured zone and maxima in the fault area. Experimental validation supports the simulation results. In addition, this paper examines the influence of elastic modulus, Poisson's ratio, cohesion, and fracture angle on fracture development and collapse pressure. The results indicate that elastic modulus, cohesion, and fracture angle have a substantial impact, whereas Poisson's ratio has a minor effect. These findings provide new insights into wellbore stability assessment in shale formations.