The method for prediction of formation pore pressure based on mechanical specific energy theory

Abstract

With traditional prediction methods, the accuracy for prediction of formation pressure is low in formations with complex lithology, such as saline aquifers, fractured formations and carbonate rocks. Therefore, in Qinghai Oilfield, in order to ensure the drilling safety, studies have been conducted in order to find the method for predicting the pore pressure of E32 formation, in which the lithology is complex. Mechanical specific energy is the total energy required for breaking and removing rock per unit volume. With the increase of depth, the confining pressure of rock and vertical effective stress increase. Therefore, more energy is required to break and remove rock per unit volume. However, in abnormal high-pressure formation, pore fluid bears some overburden pressure, and the rock effective stress is reduced, leading to less requirement for mechanical specific energy. Therefore, it is feasible to evaluate the formation pressure qualitatively based on the mechanical specific energy theory. In this study, according to the actual drilling conditions, a model for prediction of formation pressure has been constructed and optimized on the basis of mechanical specific energy theory. With this model, the prediction accuracy on formation pressure has been improved obviously. This model has been used in prediction of formation pressure of the E32 formation in several wells in Qinghai Oilfield, which are carbonate rocks with fractures developed. The application results show that this formation pressure prediction model based on the mechanical specific energy works well in prediction of formation pore pressure, with an error less than 12%, which meets the engineering requirements. It provides a new method for predicting the pore pressure in complex lithologic formations such as carbonate rocks.