Borehole wall tensile caving instability in the horizontal well of deep brittle shale

INTRODUCTION: With the increasing drilling depth of shale formation, downhole collapse is a frequent occurrence, which often manifests as borehole wall caving. METHODS: We used the deep shale of the Longmaxi Formation to conduct the mechanical loading and unloading experiments under different downhole working conditions and a theoretical evaluation method of borehole wall caving and instability was proposed. RESULTS AND DISCUSSION: As the confining pressure and axial load increased, the acoustic velocity increased. When a certain value was reached, the acoustic velocity of the rock mass had minimal changes. As the confining pressure continued to unload and decrease, the acoustic velocity decreased. At the moment of core failure, the acoustic velocity suddenly dropped. When the axial force of loading was constant, the unloading speed of confining pressure increased, and shale could easily be destroyed. The pressure at the well bottom changed rapidly, the likelihood of borehole wall failure increased. CONCLUSION: The deep shale has high brittleness. Under the bottom-hole pressure, the borehole wall rock was prone to brittle fracture failure along the parallel bedding surface. Under different working conditions, obvious changes could be observed in the pressure of the effective fluid column at the well bottom. The pressure changed rapidly, which, in turn, caused the rock at the well bottom to break down, thereby resulting in borehole wall caving. After tripping out and turning the pump off, the shale tensile stress in the upper and lower sidewalls of the horizontal well section was responsible for tensile caving.