Discrimination of reservoir fluid contacts using compressional and shear wave velocity
Improved fluid detection and lithology discrimination using rock properties and attributes cross plots have been attempted using well log data in an Onshore Niger Delta field. Rock properties and attributes were extracted using empirical rock physics models on well logs and used to validate their potentials as pore fluid discriminants. These rock properties and attributes were cross plotted for the primary purpose of investigating their sensitivity to fluid and lithology in cross plot space. Vp and Vs logs were derived from the inverse of interval transit times of sonic and dipole shear logs respectively. Vp/Vs ratio, acoustic impedance and porosity were derived from Vp, Vs and density logs using appropriate relations. The identified depth of reservoir of interest (A2) for Well A and B ranges from 5842 ft to 5964 ft and 5795 ft to 5936 ft respectively. The properties cross plotted comprise VpvsVs, Vp/VsvsIp, Vp/Vsvs Porosity, Vp/Vsvs Density and Vpvs Density. Vpvs Density cross plot revealed that the reservoir consists of sand lithology with intercalated shale. VpvsVs shows a linear relationship and does not discriminate fluid in the reservoir. Vp/Vs ratio vsIp distinguish A2 into hydrocarbon, brine and shale zones. Vp/Vs ratio vs density and porosity crossplots distinguishes the A2 into gas, oil, brine and shale zones. The analysis validates the fact that Vp/Vs ratio and their combinations cross plots are more sensitive and robust for fluid discrimination. It also reveals that the ratio of Vp/Vs is more sensitive to change of fluid type than the use of Vp or Vs separately.
Keywords: Crossplot, Reservoir, Porosity, Acoustic Impedance, Density