Multi-coupling stress field and evaluation of borehole stability in deep brittle shale

In drilling operations of deep brittle shale, borehole stability problems occur frequently, resulting in development difficulties and increased costs. Therefore, studying borehole stability of deep brittle shale is particularly important. The composition of deep brittle shale was analyzed by component content and microscopic observations, and its mechanical strength was experimentally determined by direct shear and triaxial mechanical tests. The multi-coupling stress field was established, and the stability of shale was evaluated using the Jaeger weak-plane strength criterion. Deep brittle shale is mainly composed of quartz (~ 40%), and the hydration effect has a minor influence on its mechanical strength. The rock mass develops natural fractures interlaced with the bedding plane. The cohesive force in the fractures is only 2.02 MPa, and the internal friction angle is 33.39°. There is almost no connection force, which leads to breaking of the rock mass. This study shows that an increase in the number of weak planes, the influence scale of mechanical strength, and collapse pressure increases gradually, and when the number of weak faces increases indefinitely, mechanical strength of the rock mass is infinitely close to a greatly reduced homogeneous state. Further, when the well deviation is 90°, influence from the number of weak planes is most obvious. The weak-plane effect is the main factor influencing stability of the deep brittle shale borehole. Experimental conclusions provide a guiding role in the construction of and research on deep brittle shale boreholes.