Fluid replacement modelling analysis for C4 reservoir characterisation onshore Niger Delta Basin

Abstract

This study applied Gassman’s fluid substitution theory to model the sensitivity of several saturations of gas and oil in the C4 reservoir using well log data, to determine the applicability of rock physics analysis to evaluate the reservoir in the undrilled area of the field. Results of fluid substitution modeling for gas revealed that P-wave velocity decreased significantly at 5% gas saturation. Upon further increments in the gas saturation, the P-wave velocity remained relatively constant, revealing the difficulty in differentiating between economical and sub- economical gas saturation on seismic. Oil substitution showed that as the saturation of oil introduced into the reservoir increases, the P-wave velocity and density decreases accordingly. The 80% oil saturation model appeared to be greater than the insitu P-wave and density. Hence it can be concluded that the insitu oil saturation may be greater than 80% oil. In both fluid substitution scenarios, the S-wave models remained constant irrespective of the fluid saturation. Cross plot of Vp/Vs ratio against P-impedance and Mu-rho against Lambda-rho technique were done to validate the modeling results. The techniques discriminated both the lithology types and fluid contrast in the reservoir. Hence, it can be applied in the evaluation and characterization of the C4 reservoir in the undrilled area of the field from inversion results.

Keywords: Forward-Modelling, Fluid-Substitution, P-wave, S-wave, Fluid-Saturation, Rock Properties, Petrophysical Evaluation