REAL-TIME PORE PRESSURE AND FRACTURE-PRESSURE DETERMINATION IN ALL SEDIMENTARY LITHOLOGIES

Pore pressure and fracture gradient are the two natural limits that ex ert the greatest influence on drilling costs and safety. Traditional e mpirical ''pore pressure'' models are limited to one lithology type (s hale) and rely on incorporating petrophysical or drilling data vs. dep th trend lines. We describe a new method that quantifies the effective -stress law, p = S - sigma(nu). This method uses petrophysical data (g amma ray, resistivity, density) and mineralogic stress/strain relation ships to calculate pore pressure and fracture gradient, on a foot-by-f oot basis, through all sedimentary rock types. Cretaceous marls and li mestones have proven to be an obstacle for traditional pore-pressure e valuation methods. With numbers of high-pressure exploration wells in the North Sea Central graben increasing, there is a need for a better understanding of pressures through the Cretaceous and into the pressur ed formations below. This effective-stress-law method, which is used t o determine pore pressure/fracture gradient, has been tested successfu lly in the complex limestone/shale and sandstone-shale sequences of th e North Sea as an aid to well planning and real-time drilling operatio ns decision making by use of measurement while drilling (MWD) petrophy sical data. This method has recently been used successfully on two Cen tral 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 traditio nal pore-pressure methods, to help set an intermediate casing string a t an optimum depth. We discuss results of these case studies and techn ical content of this pore-pressure method.