Theory of lost circulation pressure

Conventional theories predicting borehole breakdown pressure assume breakdown occurs when the tangential stress at the borehole exceeds the tensile strength of the formation. Fracturing tests conducted during the DEA-13 joint industry project, however, showed that when drilling fluid was used as an injection fluid, borehole breakdown did not occur until the well pressure significantly exceeded the pressure which resulted in a tangential stress equal to the rock tensile strength even with a large surface flaw. The test results have shown that all drilling muds have a tendency to seal narrow natural fractures or fractures created by high borehole pressure. The sealing effect of the mud stabilizes fractures and prevents fracture propagation. This effect is one of the primary factors for controlling wellbore stability. In this paper, a theory of fracture initiation and fracture propagation around a borehole whose stability is enhanced by drilling fluid interaction, has been developed and shown to be consistent not only with all the DEA-13 laboratory results, but also with various field evidence. The results show that lost circulation pressure is highly dependent on the Young's modulus of the formation, wellbore size, and type of the drilling fluid, although the conventional theories have ignored these facts.