Pore pressure prediction using well logs- a case study of a field in the onshore central Niger Delta, Nigeria

Abstract

Well overpressure remains the major cause of blow-outs and oil spills in well/reservoir production.. In this study, we predicted the in-situ pressure of the shale formations using well logs. One well obtained from a field (Field XXX) in the onshore central Niger Delta were analyzed to determine if there is a deviation in the normal compaction behaviour of the shales in this field since mechanical compaction disequilibrium is the major cause of geopressuring in shales. To accomplish this, we investigated the compaction behavior of shales using well data. Three composite parameters- sonic transit time/velocity, resistivity, and porosity- were used. For selected intervals, these composite parameters were each plotted against depth to show how each parameter behaves should the shales compact normally or otherwise. From the results obtained, a similar scenario prevailed in the compaction trend plots of the composite parameters with depth. Overpressure was found to occur in Interval A Section A of the Well (depth range 3691.07m – 3703.54m) and normal pressure in Interval B Section A (depth range 3731.54m – 3873.69m). Interval A Section Aa ( depth range 4299.06m - 4328.32m) and Interval B Section Aa (depth range 4337.16m - 4354.07m) revealed the presence of overpressure. At such depths, the sonic transit time/velocity, resistivity and porosity deviated either to the left (lower values) or to the right (higher values) from the normal compaction trend. These are consistent results since geopressure occurs in zones where normal compaction of shales is mechanically disrupted.

Keywords: Pore Pressure, Geopressure, Compaction, Velocity