Time-Lapse Seismic Monitoring (4D) of Enhanced Oil Recovery in a Mature Niger Delta Field: Rock Physics Modelling and Sensitivity Analysis

Enhanced oil recovery through waterflooding in mature Niger Delta fields is hampered by an incomplete understanding of sweep efficiency and bypass zones that could be identified through time-lapse seismic monitoring, but the theoretical sensitivity of 4D seismic signals to fluid and saturation changes in the high-porosity, low-cementation sands of the Niger Delta has not been rigorously characterised through forward rock physics modelling. This research investigates the theoretical and practical sensitivity of 4D seismic monitoring to reservoir fluid changes in a mature oil field in OML 26, Delta State, using the Gassmann fluid substitution framework extended to account for the specific rock physics character of unconsolidated to weakly consolidated Niger Delta Agbada Formation sands. A comprehensive rock physics database assembled from core measurements, log data, and published experimental data for Niger Delta sandstones is used to calibrate the Hashin-Shtrikman and Hertz-Mindlin contact cement models, determining their applicability limits as a function of effective stress and cementation state. Sensitivity analysis demonstrates that 4D seismic amplitude differences detectable above the 4D noise level require oil saturation changes exceeding 15 percent in cemented sands but only 8 percent in uncemented sands. Application of the calibrated rock physics model to interpret a repeat seismic survey acquired 5 years after the baseline identifies four areas of effective waterflood sweep not evident in earlier interpretations and two bypassed oil zones. Dynamic reservoir simulation constrained by the 4D interpretation improves production history match at seven monitored wells. Keywords: time-lapse seismic, 4D, enhanced oil recovery, rock physics, Niger Delta