Characterization of Reservoir Fluids Using Body Wave Velocities

Abstract

Improved fluid detection and lithology discrimination using rock properties and attributes cross plots have been attempted using 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. Compressional wave velocity (V ) and Shear wave velocity(V )logs were derived from the inverse of interval transit times of sonic and dipole shear logs respectively, acoustic impedance and porosity were derived from V , V and density logs using appropriate relations. The identified depth of reservoir of interest (A2000) for Well A and B ranges from 1781m to 1818m and 1766m to 1809m respectively. The ratio of compressional wave to shear wave velocity also known as Poisson's ratio(V /V ) is a key parameter in characterizing reservoir fluids. The properties cross plotted comprise: V against V V /V against Ip, V /V against Porosity, V /V against Density and V against Density. V against Density cross plot revealed that the reservoir consists of sand lithology with intercalated shale. V against V shows a linear relationship and does not discriminate fluid in the reservoir. V /V against I distinguish A2000 into hydrocarbon, brine and shale zones. V /V against density and porosity cross plots distinguishes the A2000 into gas, oil, brine and shale zones. The analysis validates the fact that V /V and their combinations cross plots are more sensitive and robust for fluid discrimination. It also reveals that the ratio of V /V is more sensitive to change of fluid type than the use of V or V separately.