Prediction of over pressured zones from well logs in the Niger delta

Abstract

The changes of some geophysical properties such as sonic compressional velocity with depth is an indication of the pressure system of over-pressured zones. Once these anomalous pressures are not forecasted correctly before drilling, calamitous occurrences, for instance kicks, and blowouts may ensue. This paper investigated the distribution and origin of overpressures in the Niger delta sedimentary basin using well log data. A Normal Compaction Trend line was generated by means of sonic velocity data and the overpressure zones were detected by observing the reversals of the sonic velocity data. The outcome of the formation pressure gradient prediction infers that the commencement of overpressure in the study area occurs at 7500ft with a resultant pressure gradient of 0.51psi/ft. Meanwhile, the findings of the assessment of sonic velocity and density cross-plots revealed disequilibrium compaction as the primary cause of overpressure. The prominent zones of overpressure were detected to appear at depths between 11100ft to 11300ft with an associated pressure gradient of 0.52psi/ft. to 0.53 psi/ft. This work will help in precisely predicting zones of overpressure in the research area prior to drilling.