Mining and civil engineering industries sometimes use drilling muds for stabilizing a borehole during drilling wells for methane drainage, geothermal energy and radioactive waste disposal. Standard theories predicting borehole breakdown pressure assume breakdown occurs when a small fracture initiates at a location where the largest tangential stress at the borehole reaches the tensile strength of formation. Fracturing tests conducted in this study, how- ever, showed that when drilling fluid was used as an injection fluid, borehole breakdown did not occur even if a fracture initiated at a borehole wall. Bore- hole breakdown occurred when the initiated fracture became unstable after significant growth with O. 76cm (0.3 in.) to 7.62cm [...]
