Calculating Method of Wellbore Collapse Pressure Drilling in Fractured Shale Formation

Abstract

In order to reduce the oil and gas drilling risk and cost, the calculating method of collapse pressure was established to prevent borehole collapse in fractured shale formation. The method was coupled with borehole concentrated stress, induced stress by crack opened and pore pressure change. The influence law of borehole trajectory and crack occurrence was researched. The results show that, collapse pressure increases substantially in small dip fractured shale formation when the well deviation angle exceeds the critical value, and the increase speed is fastest in the direction of maximum horizontal principle stress; collapse pressure is lower when crack is nearly vertical and the tendency parallel to wellbore azimuth for the horizontal well, but when crack dip exceeds 30° and the tendency is not parallel to wellbore azimuth, it is more dangerous for drilling due to the higher collapse pressure. The research results can be used to solve wellbore instability problems and improve drilling efficiency in the similar regions.