A Petrophysical-Mechanlcal Math Model for Real-Time Wellsite Pore Pressure/Fracture Gradient Prediction

Abstract

Abstract Petrophysical-mechanical math modeling is an entirely new approach to Pore Pressure/Fracture Gradient prediction. Traditional petrophysical techniques are used to estimate rock porosity, (ϕ) and relative shale volume (Vshale). In this new methodology, pore pressure is calculated from earth stresses rather than empirical fluid pressure relationships. Overburden stress is an integrated bulk density log derived from porosity, (ϕ) and Vshale. Effective vertical stress is calculated from porosity, (ϕ) for each lithology (Vshale) using the Rubey & Hubbert (1959) relationship. Pore Pressure is the difference between overburden and effective vertical stress, following the Terzaghi (1923) relationship. Fracture propagation pressure is the sum of effective horizontal stress and pore pressure. In relaxed sedimentary basins, effective horizontal stress is calculated from effective vertical stress for each lithology (Vshale). Visco-plastic rather than elastic rock properties are used to calculate effective horizontal stress. This new methodology has been used on 17 wells in the Gulf of Mexico. Well planning is done by batch processing of log data. Drilling operations are supported by real-time processing of MWD log data at the wellsite. Accuracy has been better than 1/2 ppg where lithologic discrimination was good and stress modeling assumptions were met.