Borehole breakdown pressure with drilling fluids .1. Empirical results

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, however, 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. Borehole breakdown occurred when the initiated fracture became unstable after significant growth [with 0.76 cm (0.3 in.) to 7.62 cm (3 in.) in length]. The test results showed that all drilling mulls had a tendency to seal narrow natural fractures or fractures induced by high borehole pressure. The sealing effect of the mud stabilized fractures and prevented fracture propagation. This effect is one of the primary factors for controlling wellbore stability. In this work [1], more than 40 large rock samples [76.2 x 76.2 x 76.2 cm (30 x 30 x 30 in.)] were fractured to test the drilling fluid effect on fracture initiation and fracture propagation around a borehole. The results show that borehole breakdown pressure is highly dependent on the Young's modulus of the formation, wellbore size and type of the drilling fluids. Note that the conventional linear wellbore stability theory has ignored all these facts. The results of this experiment are intended to apply to the lost circulation problems from an induced fracture or to the interpretation of the in situ stress measurements with gelled fluids where drilling or fracturing fluids contain significant amount of solid components. Similar phenomena occur for the standard hydraulic fracturing fluids; however, the process zone and the high flow friction at the narrow fracture tip become as important as the gel and solid plugging effect shown in this paper. Hence, the results should be modified before being applied to standard fracturing fluids.