Reservoir Fluid Substitution Modelling and 4D Seismic Feasibility Study in an Onshore Niger Delta Field, Processed at NAU

Time-lapse (4D) seismic monitoring has become an essential tool for optimizing production from hydrocarbon reservoirs, yet its feasibility in onshore Niger Delta fields is not well established due to complex near-surface conditions and limited baseline datasets. This study performs rock physics and fluid substitution modelling and assesses the feasibility of 4D seismic monitoring in an onshore Niger Delta field, with all data processing conducted at the NAU Applied Geophysics laboratory. Rock physics models were constructed using wireline log data from three production wells by applying Gassmann fluid substitution to model the seismic response of the primary reservoir interval under varying gas-oil-water saturation scenarios. Acoustic impedance and Vp/Vs ratio changes between modelled saturation end-members were computed for each saturation scenario and compared with estimated seismic noise levels from repeated survey repeatability analysis. The primary reservoir exhibits average porosity of 27 percent and initial gas saturation of 62 percent. Gassmann-predicted P-wave velocity changes between initial and water-flooded scenarios reach 9.4 percent, corresponding to acoustic impedance contrasts of 8.2 percent. Normalized root mean square repeatability values estimated for the field are 14 to 22 percent, placing the expected production-induced seismic signal above the detection threshold in the primary reservoir zone. Synthetic time-lapse seismic sections confirm the theoretical detectability of the water injection front. A minimum monitoring interval of 18 months is recommended based on expected saturation front advance rates. This study provides a quantitative feasibility framework for implementing 4D seismic monitoring in the studied onshore field.

Keywords: 4D seismic, fluid substitution, Gassmann, Niger Delta, time-lapse