A study of CO2 flooding on wave velocities in the Naharkatiya oil reservoir of Upper Assam Basin

This paper studies the compressional-wave and shear-wave velocities in the laboratory in six conventional core plugs. These plugs were obtained
from a depth of more than 3000 m from the producing horizons of Naharkatiya oil reservoir of Upper Assam Basin, India. The porosities of the
conventional core plugs were from 9.67 to 25.8% and that of unconsolidated sand pack was 47%. These plugs and sand pack were saturated with
n-hexadecane before CO2 flooding. It was observed that during flooding compressional-wave velocities decreased more than the shear wave
velocities. These decreases in wave velocity depend on confining pressure, pore pressure, porosity and temperature of the plugs. Increasing pore
pressure at constant confining pressure not only keeps the pores and cracks open but also reduces the confining pressure effect and increases the
CO2 density. Higher pore pressures causes larger decrease in both compressional and shear wave velocities. In case of conventional core plugs
which are consolidated, having lower porosities tends to decrease the CO2 effect. In unconsolidated sand pack the flooding effect is large even
though porosity is high because the bulk modulus of the sand is low. The experimental and the theoretical analyses in this paper show that the
decrease in compressional-wave velocities caused by CO2 flooding makes it possible to track CO2 front movements and monitor CO2 flooding
process in the reservoir.