Adaptive utilization of Castagna and Biot-Gassmann equations in fluid replacement mechanism (FRM), with elastic logs analysis and petrophysical cross-plots for fluid discrimination in an offshore basin, Niger Delta, Nigeria

Abstract

Determination of reservoir fluid type is a useful basic procedure in reservoir fluid characterization. Among the ways to achieve this, are through elastic logs and cross plotting in conjunction with fluid replacement mechanism (FRM) which have the capability to discriminate one fluid type from another thereby removing - uncertainties and ambiguity from the reservoir fluid interpretation and isolation. The aim of the study was to accurately identify hydrocarbon and distinguishes it from the brine. This was achieved through elastic log cross-plotting and fluid substitution with clear objectives of diagnosing various reservoir fluid content, removing ambiguities, clearing uncertainties and reducing risks. The novelty in this study lies in its ability to combine the sophisticated fluid identification templates by employing different parameterization approaches including the use of bulk modulus of the rock matrix, dry rock, fluid, and saturated rock frame. It also involved the utilization of reservoir porosity, saturated rigidity modulus, dry-rock shear modulus and densities. Elastic logs were generated from the P-wave (Vp), S-wave (Vs) and density (ρ) logs and was followed by cross plotting to determine the fluid zones within the field. Castagna and Biot-Gassmann equations were utilised for the replacement mechanism to identify the nature of fluid present in each of the reservoirs. Findings from the study revealed the presence of hydrocarbon within the field as indicated by the low values of elastic logs such P-impedance (Zp), S-impedance (Zs), Vp/Vs ratio, Lambda-Rho (λρ) and Mu-Rho (μρ) and their cross plots. The hydrocarbon-bearing intervals were isolated in the cross plot space while the reservoir saturated with brine and shaly units formed the background effect. Also, the result of fluid replacement techniques revealed that the reservoirs with hydrocarbon as insitu fluid developed a new log response tracking the original insitu log when the insitu fluid was replaced with hydrocarbon except at the water bearing intervals where the log departed from the path of tracking each other. But the new log deviated from the insitu original log when the insitu fluid was replaced with brine at the oil and gas zones. Again when the reservoir insitu fluid was brine and substituted with brine, the new modified log also tracked the original insitu log except at hydrocarbon intervals where it deviated. The FRM categorically characterised the fluids and confirmed the interpretation from both elastic log and cross plot and all the results corroborated well. The significance of the study is the delineation of hydrocarbon-charged intervals and characterization of various reservoir fluids Therefore, elastic logs, cross plot and FRM served as important means of fluid isolation and discrimination in AbdulBaari-Field. The techiques holds the potential to solve similar problems in any hydrocarbon field in different parts of the world