A new approaching method to estimate fracture gradient by correcting Matthew-Kelly and Eaton's stress ratio

Fracture pressure is one of the most important parameters in wellbore stability. Fracture pressure shows the ability of rock to hold vertical stress before starts to fracture. The application of fracture gradient (FG) affects the well design, such as mud weight profile, casing setting depth, and cementing operation. Fracture gradient could be determined directly by using leak-off test (LOT) and formation integrity test (FIT), and calculation from logging data. There have been a lot of studies in predicting fracture gradient after the fundamental theory of fracture pressure was developed by Hubbert and Willis in 1957. However, most of those studies were performed without considering the geological characteristic, such as precipitation environment, geology structure, and stratigraphy. A new methodology to predict fracture pressure from former calculations, Matthew-Kelly and Eaton, is proposed. The methodology characterized the formation lithology from Poisson's ratio and stress ratio value which is corrected and analyzed by two correcting constants, a and b. As the result, a new value of Poisson's and stress ratio of the formation was generated and the accuracy of fracture gradient was improved. In addition, the flexibility of this methodology indicates that this methodology could be applied in various drilling area.