Wellbore stability analysis for horizontal wells in shale formations

As a serious problem in drilling operation, wellbore instability restricts efficient development of shale gas, especially in formations with weak bedding planes which have strong strength anisotropies. The occurrence of bedding planes has significant influence on wellbore instability. Triaxial compressive tests have been carried out to study the effects of the orientation angle between the major principal stress direction and the weak bedding plane on the strengths of weak bedding shales. According to the laboratory test data, the strength of rock matrix and the strength of weak bedding planes were acquired, and failure criterion of rock matrix and weak bedding planes were established based on Mohr-Coulomb strength criteria. Based on the poroelasticity theory and rock strength theory, this paper proposed a wellbore instability analysis model for horizontal wells to reveal the combined effects of borehole trajectory, occurrence of stratification, strength of weak plane on wellbore instability. The results show that the shape of curve between peak compressive strength and orientation angle is 'U-shaped', and the bedding planes of weakness causing the strong strength anisotropies. When the bedding dip angle is greater than about 45°, the reasonable well azimuth angle would make wellbore stable. In addition, the directions of bedding planes and in-situ stresses pose the significant influences on wellbore instability. This paper provides a method for wellbore stability analysis of weak bedding formations which can help design mud density and well trajectory by elaborating all the effects of various parameters on wellbore stability.