Investigating the Subsurface Pressure Regime of Ada-field in Onshore Niger Delta Basin Nigeria

Abstract

Pore Pressure Prediction (PPP) has been proven to be more difficult in basins like the Niger Delta where the subsurface pressure system is complex. Failure of PPP practioners to recognize lateral and vertical variations of formation pressure even within the immediate vicinity of local geology had led to greater uncertainty and safety threat in drilling activities. Hence; this work attempted to investigate subsurface pressure pattern in Ada field, on Onshore Niger Delta depositional belt. Wireling logs, mud weights and reservoir pressures from four wells were analysed for this purpose. The overpressure in field is seen to be mild, i.e. below 0.5 psi/ft but it exceeded 0.6 psi/ft at depth ~12000 ft TVDml with overpressure magnitude of 1682 psi. The study revealed the effect of hydrocarbon production in the field as indicated by drained reservoir pressures. The field is seen to have high net-to-gross ratio as it is common with wells that penetrate the Benin Formation. Result shows that reservoir pressures and that of adjacent shales are not equilibrium hence; caution must be taken when using any offset well as a proxy to pore pressure calculation in the studied area. With is information, it will be easier for PPP experts to estimate pore pressures accurately in these intervals and prevent the risk of drilling dry holes in future exploration activities in the area.