Discrimination of reservoir fluid contacts using compressional and shear wave velocity

Abstract

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. V_p and V_s logs were derived from the inverse of interval transit times of sonic and dipole shear logs respectively. V_p/V_s ratio, acoustic impedance and porosity were derived from V_p, V_s 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 V_pvsV_s, V_p/V_svsI_p, V_p/V_svs Porosity, V_p/V_svs Density and V_pvs Density. V_pvs Density cross plot revealed that the reservoir consists of sand lithology with intercalated shale. V_pvsV_s shows a linear relationship and does not discriminate fluid in the reservoir. V_p/V_s ratio vsI_p distinguish A2 into hydrocarbon, brine and shale zones. V_p/V_s ratio vs density and porosity crossplots distinguishes the A2 into gas, oil, brine and shale zones. The analysis validates the fact that V_p/V_s ratio and their combinations cross plots are more sensitive and robust for fluid discrimination. It also reveals that the ratio of V_p/V_s is more sensitive to change of fluid type than the use of V_p or V_s separately.

Keywords: Crossplot, Reservoir, Porosity, Acoustic Impedance, Density