Dealing with the Uncertainty in the Prediction of Fracture Gradient

Summary In Geomechanics for drilling engineering, there are two possible methods for predicting the FG: i) method based on the calculation of stress around the wellbore, ii) method using depth correlation established for a specific field from a set of leak-off pressure (LOP) data recorded on offset wells. In Total E&P, the method of calculating stress around the wellbore in order to predict the FG has been used and developed for a decade. It primarily consists of building and calibrating a ID MEM, using proprietary ID poro-mechanical earth model named PoroMEM. The calculation of the FG from the in situ stress model uses two conceptual models of fracturing: shear fracture and tensile fracture. From there, we define two limits for FG: one called FIPmax and the other FIPmin. Such conceptual models help to deal with the uncertainties of rock mechanical behavior because of vertical variability of lithology, help the preparation LOT/FIT during drilling operation, and also aid the interpretation of LOT/FIT. The in situ stress model from PoroMEM also provides the FCP gradient which is useful for well control and is sometimes as the FG. In this paper, we present the conceptual models and an example of FG prediction.