Abstract
The Extended leak-off test, XLOT, is a main tool for obtaining minimum stress data during drilling operations. If performed according to a proper protocol, it is perhaps the most cost-effective and robust method for determining the minimum stress. The low cost of the test means that a lot of data can be collected, allowing for efficient regional analysis. A widely reproduced figure by Gaarenstrom et al. (1993) has become a classic in the industry. It shows the pump-in phase as two distinct linear segments. The point of change in slope is denoted the Leak-off Point or the Leak-off Pressure (LOP). It is sometimes argued that the LOP is a good measure of the in-situ stress, and it is sometimes argued the LOP marks the initiation of a fracture. We analyze this hypothesis and show by basic theoretical considerations and several examples of high quality field data that this is not necessarily the case. The nonlinear behavior during initial pump-in may be due to leak-off to a permeable formation or through a poorly cemented casing shoe.