A New Multi-Factor Model for Calculating Shale Formation Fracturing Pressure

Abstract

A model is set up to calculate shale formation fracturing pressure based on the theory of elastic-plastic fracture mechanics, physicochemical rock fracture mechanics and the maximum tensile stress criterion. The model takes into account such factors as the total stress field and the crustal stress, physicochemical factors including temperature field, shale porosity and the mechanism of seepage of the drilling fluid. The model was verified by field studies of formation fracturing and laboratory investigations of the stress field in triaxial compression of shale core samples with different water contents. The error of the values calculated using the model came to 3.85%, which is close to the measured fracturing pressure. Curves of the dependence of tensile stress and fracturing pressure on water saturation were also obtained. Results show that both the fracturing pressure and the tensile strength decrease with increasing water saturation of the sample, i.e., mechanical characteristics of the rock deteriorate with increasing hydration.