Real-Time Pore Pressure and Fracture Gradient Evaluation in All Sedimentary Lithologies

Pore pressure and fracture gradient are the two natural limits that exert the greatest influence on drilling costs and safety. Traditional empirical pore pressure models are limited to one lithology type (shale) and rely on incorporating petrophysical or drilling data vs. depth trend lines. The authors describe a new method that quantifies the effective-stress law, p = S {minus} {sigma}{sub v}. This method uses petrophysical data (gamma ray, resistivity, density) and mineralogic stress/strain relationships to calculate pore pressure and fracture gradient, on a foot-by-foot basis, through all sedimentary rock types. This method has recently been used successfully on two Central graben wells. BP drilled a high pressure/high temperature (HPHT) well in the second quarter of 1993 and used on-site pore pressure and fracture gradient (PP/FG) calculations, along with other more traditional pore-pressure methods, to help set an intermediate casing string at an optimum depth. They discuss results of these case studies and technical content of this pore-pressure method.