Pore Pressure Prediction in Onshore West Niger Delta Using Inverted Seismic Velocity and Derived Velocity (Vp) - Vertical Effective Stress (VES) Coefficients

Abstract

In this study, pore pressure has been predicted using seismic data and derived compressional wave velocity (V_p) - Vertical Effective Stress (VES) coefficients. Post Stack Time Migration (PSTM), angle stack gathers, seismic horizons, checkshot, wireline logs, drilling and pressure data from six wells in the Onshore West Niger Delta, Nigeria were analysed and interpreted. Using generated velocity and density crossplots, the active overpressure generating mechanisms for the studied area were deduced. The V_p-VES coefficients were modelled using the direct pressure data and the overburden profile computed from density log. Post stack seismic inversion was performed to improve the seismic resolution as well as derive acoustic impedance using well velocities and stacking velocities from velocity analysis of the 3-D seismic data.  The derived V_p-VES coefficients were used to transform the seismic acoustic impedance velocity into seismic pore pressure volume. Pore pressure profiles were accordingly extracted along well paths so as to test the accuracy of the model. Interpreted density-velocity crossplots revealed a decrease in velocity at constant density of 2.4 g/cc, an indication that unloading mechanisms contribute to overpressure in the field. The Bowers’ V_p-VES coefficients of 7.43 and 0.77 were determined for A and B parameters respectively. Based on the results obtained, the top of overpressure occurred at a depth of 3750 ft and 3800 ft in UMO-001 and UMO-002 wells respectively with a corresponding average pore pressure gradient of 0.47 psi/ft for both wells, indicating that the wells are mildly overpressured. Onsets of unloading were observed in UMO-001 and UMO-002 wells at depths of 6250 ft and 6800 ft with pore pressure gradients of 0.51 psi/ft and 0.60 psi/ft respectively. The Derived Seismic Pore Pressure (DSPP) matched the measured pressure value (kick) of 5300 psi at a depth of 7450 ft and this validated and further increased confidence on the values of the V_p-VES coefficients derived. These results show that the derived seismic acoustic impedance volume, vertical effective stress and overburden model produce high resolution seismic pore pressure cube in both time and space. The derived models when applied especially, with seismic acoustic impedance volume can be used to plan and drill future wells with great successes in the studied area.

Keywords: pore pressure, vertical effective stress, seismic velocity, acoustic impedance

DOI: 10.7176/JEES/9-10-07

Publication date:October 31^st 2019