A New Method for Predicting Fracture Propagation Pressure From MWD or Wireline Log Data

Abstract

ABSTRACT A new approach has been developed for predicting fracture initiation and propagation pressure from MWD or wireline log data. The technique combines well established principles of rock mechanics and petrophysics to calibrate earth stresses and pressure to rock properties. Real-time pressure predictions are based upon MWD data derived while drilling the well rather than depth from a regional average or planning well. Fracture propagation pressure (Fp) is the sum of pore pressure (P) and the effective horizontal stress (σh) during fracturing (Fp = p + σh). Pore pressure is first determined as the difference between overburden(S) and effective vertical stresses (σy) from the relationship (P=S-σY). This rock mechanics approach is fundamentally different from the "normal" (Pn) and "excess" (Pe) additive fluid pressure approaches (P=Pn+Pe) for predicting pore pressure (P). The overburden calculation accounts for the water column, average rock densities above the measured interval, and calculated rock densities within the measured interval Effective vertical stress (σV) is calculated for all lithologies in the sand-shale continuum using empirically derived Power Law expressions of lithology (v-shale) and porosity (Φ). The new fracture pressure prediction approach is an extension of the effective stress concept using Drucker Prager failure criteria. Leakoff test data has been used to define effective stress-rock properties limits for fracture initiation and propagation pressures. The new approach is shown to be about four times more accurate than traditional approaches that determine effective horizontal stress as a function of depth. The calibration data and three log examples are shown to demonstrate the accuracy of the technique. The use of Fracture Pressure logs before or during drilling could have prevented lost circulation in two cases and the setting of unnecessary casino in the third.