Rock Mechanical Properties and Wellbore Stability of Fractured Dolomite Formations

Abstract

During the drilling process in the dolomite formation of the Leikoupo formation in western Sichuan, downhole obstruction frequently occurs, which seriously hinders the efficient development of oil and gas resources on site. In view of the problems of borehole stability existing in this type of formation, the geological characteristics, underground complexity, and fracture development of the research block were systematically identified and analyzed, and the physical parameters and mechanical properties of the rock were defined through physical and chemical tests and an understanding of the mechanical properties of the rock. These studies revealed the mechanism of borehole instability and the main factors influencing fractured dolomite formation. Finally, a model of borehole collapse pressure within fractured dolomite formation was established by selecting the appropriate strength criterion as the criterion of borehole instability. The results show that the dolomite fractures of the Leikoupo formation are relatively developed, the formation is dominated by low-angle fractures, and the core integrity is not high. The development of formation fractures has an obvious effect on the mechanical strength of rock, which decreases to different degrees with the decrease in the integrity coefficient of the rock mass, and the anisotropy of the rock’s mechanical strength is obvious. The empirical parameters of the generalized Hoek–Brown (H–B) strength criterion were quantified by introducing rock mass acoustic data, and the accuracy of the improved generalized H–B strength criterion was evaluated using triaxial test data. Taking the M–C criterion, the generalized H–B criterion based on acoustic wave, the D–P criterion, and the weak surface strength criterion as wellbore instability criteria, combined with the distribution of wellbore stress, a prediction model of wellbore stability considering the degree of dolomite formation fragmentation was established. The results demonstrate that the generalized H–B criterion model based on acoustic waves can be used to evaluate the wellbore stability of dolomite fracture formation with more than one group of fractures. These research results have certain guiding significance for efficient and safe drilling in fractured dolomite formations.