Prediction of Overpressure using Effective Stress and Velocity Trend Methods in Unag field Offshore Niger Delta

International Journal of Research and Innovation in Applied Science (IJRIAS) | Volume VI, Issue I, January 2021 | ISSN 2454–6186

Prediction of Overpressure using Effective Stress and Velocity Trend Methods in Unag field Offshore Niger Delta

Unuagba, T.Peter^1*, Ideozu, R.U¹, Eze, Stanley², Osung, E.Wilson³, Abolarin, O. Macpaul³

¹Department of Geology, University of Port Harcourt, Rivers State, Nigeria.
²Department of Marine Geology, Nigeria Maritime University, Okerenkoko, Delta State.
³Department of Petroleum Engineering and Geosciences, Petroleum Training Institute, Effurun, Nigeria.

Abstract: Pore pressure act on subsurface formation fluids where hydrostatic pressures are equivalent to normal pressures, and high formation pressures are greater than normal pressure. Approach used in predicting overpressure are effective stress and velocity methods. The former employs rock stress behaviour as proxy for overpressure prediction while the later uses deviation from normal compaction trend to predict overpressure. This study demonstrates the effectiveness of integrating both methods in overpressure prediction. Well logs comprising of density, sonic and gamma ray logs from three wells Unag-001, 002 and 003 were used. Sonic logs were used to predict overpressure from velocity trend reversals, while density logs were used to generate 2D overburden trend which showed the effective stress of the wells and shale volume logs were generated from Gamma Ray logs. Shales are responsive to overpressure phenomena than sands because they are denser and characterized by low permeability, porosity and less resistive minerals, thus overpressure prediction was centred on shale deformation behaviour. Significant reduction in effective stress and shale density were used to identify overpressure zones while velocity reversal from sonic logs were used to validate this identification. Three overpressure zones A, B and C were identified across the three wells. In well-001, the top of overpressure zones A, B and C were identified at depth of 7600ft, 9200ft and 10500ft, for well-002 at 8100ft, 8700ft, 10300ft, and for well-003 at 8000ft, 10000t, 11800ft respectively. Based on our findings, loading mechanism of under compaction is deduced to be the overpressure mechanism in all the overpressure zones observed except for zone C in well-003, were overpressure is associated with unloading events.

Keywords: Effective stress, Overpressure, Overburden trend, Normal compaction trend, Undercompaction.

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