Extended Walsh-Towler Algorithm to Compute Black Oil PVT Properties for Volatile Oil Reservoir Fluids

Abstract

Walsh and Towler in 1995 presented an algorithm to calculate the modified Black-Oil PVT properties (Bo, Rs, Bg and Rv) of gas condensate reservoir fluids using the experimental data from the Constant Volume Depletion (CVD) test combined with the recovery calculations provided by commercial PVT laboratory reports as part of the PVT study. The purpose of the proposed algorithm is to extend the Walsh and Towler method to volatile oil reservoirs fluids. Walsh-Towler algorithm requires as input parameters the gas compressibility factor, the two- phase compressibility factor, the cumulative produced gas, the cumulative produced oil, the volume fraction of liquid condensate, at each pressure depletion stage of the CVD experiment and the initial gas compressibility factor (Zi). The algorithm requires the initial gas compressibility factor (Zi) to calculate the initial moles of hydrocarbon at the saturation pressure, to calculate the remaining mole fraction of gas at each pressure stage as a fraction of the total moles initially present at the saturation pressure and, combined with the two-phase compressibility factor, for the calculation of the total moles of gas and liquid remaining after each stage of depletion. For volatile oil samples, the initial gas compressibility factor (Zi) is undefined; to overcome this limitation a modified algorithm is proposed that replaces the use of the two-phase compressibility factor by the wellstream produced cumulative from the CVD experiment where the initial gas compressibility factor, for volatile oil, is not required. The proposed algorithm can be applied to calculate the Black-Oil PVT properties (Bo, Rs, Bg and Rv) for both Gas condensate and Volatile oil reservoir fluids. The common practice to calculate the black-oil PVT model is by calibrating an Equation of State and export the modified black-oil PVT properties for both gas condensate and volatile oil reservoir fluids. Walsh and Towler presented a valuable tool, that not requires the use of a calibrated EOS, to calculate the black-oil PVT properties for gas condensate. The proposed algorithm extends the method to volatile oil reservoir fluids. The proposed algorithm is rigorous, direct and simple to use and can be applied to both gas condensate and volatile oil.