This paper presents a model for leak-off interpretation that includes evaluation after the borehole is fractured. The Kirsch equation is valid up to the conventional leak-off point, but beyond leak-off the hole is fractured, and an elastoplastic bridge model applies to ultimate failure. Also presented is an evaluation of in-situ stress interpretations from leak-off tests.

Findings of the new model are that the leak-off point using Kirsch is correctly defined by the in-situ stress state and the rock tensile strength. The model also explains the circulation loss problem with oil based drilling fluids where the wellbore strength reduces to the minimum horizontal stress level.

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