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 p vsV s, V p /V s vsI p , V p /V s vs Porosity, V p /V s vs Density and V p vs Density. V p vs Density cross plot revealed that the reservoir consists of sand lithology with intercalated shale. V p vsV 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.

Fluid and Lithology Discrimination for Reservoir Characterization of HAX Field, Offshore Niger Delta

“…Vp/Vs ratio against Ip distinguishes reservoir of interest A2 into hydrocarbon, brine, and shale zones. Vp/Vs ratio against density and porosity cross-plots distinguishes the reservoir of interest A2 into gas, oil, brine, and shale zones" [11]. "Their analyses validate the fact that Vp/Vs ratio and their combinations cross plots are more sensitive and robust for fluid discrimination.…”

“…The properties cross plotted were Vp against Vs, Vp/Vs against Ip, Vp/Vs against Porosity, Vp/Vs against Density, and Vp against Density. The result of Vp against Density cross plot revealed that the reservoir consists of sand lithology with intercalated shale" [11]. "Vp against Vs shows a linear relationship and does not discriminate fluid in the reservoir.…”

“…"In Niger Delta, the rock types are always sand-shale sequence and the fluid types are brine and hydrocarbon (oil and gas) which constitute the major fluids preserved in the Niger Delta reservoir basin" [10]. "Reservoirs are porous, permeable, well-sealed/trapped, and thick lithologic units that can harbour these fluids" [10,11]. "The fluids which are majorly brine and hydrocarbon saturations usually occur in different proportions because the levels of saturation are not constant across reservoirs.…”

“…According to [11], "rock properties and attributes were extracted using empirical rock physics models on well logs and were used to validate their potentials as pore fluid discriminants". "Then cross plotted to determine their sensitivity to fluid and lithology in cross plot space.…”

“…"Their analyses validate 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 the change of fluid type than the use of Vp or Vs separately" [11].…”