Estimation of formation pressures from electrical surveys - Offshore Louisiana

Abstract

This paper presents a technique for estimating formation pressures from electrical logs. The technique has been found especially applicable in the offshore Louisiana area in detecting abnormally high reservoir pressures. After a brief review of the origin of abnormal pressures and the state of rock stress underground, it is shown that shale porosity is a function of net overburden pressure or effective rock frame stress. Data from electrical surveys are used to calculate formation resistivity factors (F). which are related to porosity. Formation factors are then correlated with net overburden pressure and the formation pressure is calculated, since formation pressure is the difference between total overburden pressure (1.0 psi/ft) and net overburden pressure. Despite certain questionable features, the technique has been found to give reliable pressure estimates, and has been used in over 80 wells. Accuracy is usually within 5 per cent. Several particular applications of the technique are listed. An alternate technique, similar to that discussed in recent papers which utilizes only shale resistivity readings, is also discussed, with a theoretical basis provided. This technique is simpler and quicker, though less reliable, than the primary technique.