Influence of relation between stress field and bedding space on wellbore stability in shale formation

The shale gas reservoir in Jiaoshiba area is well developed with bedding and shows strong anisotropy, which leads to the problems of wellbore instability in the drilling process. At present, for the shale rich in bedding, the conventional calculation model of shale wellbore collapse pressure cannot meet the needs of shale wellbore stability analysis. In this paper, the analytical method considering the influence of bedding is more accurate. Therefore, in this paper direct shear tests and tri-axial mechanic experiments were conducted on the down-hole core rocks of LongMaxi reservoir shale formation, and the obtained experimental results were combined with single discontinuity theory to establish the characterization method of shale strength anisotropy. Then, combining the characterization method of shale strength anisotropy with the sidewall stress distribution model, the calculation method of borehole collapse pressure is established. Finally, the JY1HF well is taken as an example for calculation and comparative analysis. The results show that the LongMaxi reservoir shale strength deteriorating after drilling fluid immersion is not the main control factor of borehole instability; the shear slip along the weak bedding plane is the main mechanical mechanism of borehole instability. The collapse pressure calculated by the conventional model decreases with the increase of the well inclination, and the collapse pressure calculated by the model in this paper shows a trend of increasing first and then decreasing with the increase of the well inclination, and the results calculated by the model in this paper are in good agreement with the actual working conditions. The research results of this paper can provide theoretical basis and reference for drilling design and borehole trajectory optimization of shale gas wells in Jiaoshiba area or other areas.