Overpressure prediction by mean total stress estimate using well logs for compressional environments with strike-slip or reverse faulting stress state

Abstract

Predicting correct pore-pressure is important for drilling applications. Wellbore stability problems, kicks, or even blow-outs can be avoided with a good estimate of porepressure. Conventional pore-pressure estimation methods are based on one-dimensional compaction theory and depend on a relationship between porosity and vertical effective stress. Strike-slip or reverse faulting environments especially require a different way to determine pore-pressure, since the overburden is not the maximum stress. This study proposes a method which better accounts for the three-dimensional nature of the stress field and provides improved estimates of pore-pressure. We apply the mean total stress estimate to estimate pore-pressure. Pore pressure is then obtained by modifying Eaton’s pore-pressure equations, which require either resistivity or sonic log data. The method was tested in the Snorre Field in the Norwegian North Sea, where the field changes from strike-slip to reverse stress state. Eaton’s resistivity and sonic equations were used to predict pore-pressure in this region by replacing the vertical stress by the mean total stress estimate. Results suggest that the modified Eaton method with resistivity log data gives better results for the area than the conventional method. The ratio of maximum horizontal stress to minimum horizontal stress throughout each well should be known for best results.