3D modeling of abnormal pore pressure in shallow offshore Niger delta: An application of seismic inversion

Abstract

Pore pressure is the pressure associated with fluids in the pore spaces of sedimentary rocks. This pressure varies from hydrostatic to severe overpressure. It is classified as abnormal pressure when it deviates from hydrostatic pressure. Incorrect prediction or the absence of pore pressure evaluation can lead to severe damage of the drilling platform, loss of personnel, waste of resources and environmental pollution. This study was carried out in an offshore field in Niger Delta with an objective of predicting the abnormal pressure regime from the seismic volume using seismic inversion technique. First, we employed the Eaton’s and Bower’s model of predicting pore pressure from Well log. The predicted pore pressure from the well was used to calibrate the P-impedance that was used in the inversion process. A pressure-based model for the seismic inversion process was developed. This was constrained by the relationship between the P-impedance and pore pressure in the study area. The predicted pore pressure from the Well logs aligned with the measured pore pressure in the formation. At about 3600 m to end of the Well, depicts a pressure regime that is a deviation from the hydrostatic pressure trend. The abnormal pressure regime is marked by the region where a change is observed in the normal compaction trend line. The cross plot of acoustic impedance against pore pressure reveals a linear relationship; with a direct relationship indicating a normal pressure interval and the indirect relationship at the abnormal pressure regime. The inverted 3D pressure result from the seismic volume validates the normal compaction trend line and also the pressure values estimated from the Well logs. The abnormal pressure regime in the study area is predicted to be found towards the end of Agbada Formation and through the Akata Formation due to the un-compacted shales.