Equations for the Z-Factor and Compressibility of Nigerian Natural Gas

Abstract

New equations for calculating the Gas Deviation Factor (z) and the Compressibility of Nigerian Natural Gas have been developed. The equation for the Gas Deviation Factor was developed from a plot of the logarithm of the ratio of Pressure to Temperature versus the logarithm of the ratio of the Pressure to the Gas Deviation Factor.. The plot gave a linear relationship that gave the Gas Deviation Factor equation. A differentiation of the Gas Deviation Factor equation produced the new equation for calculating the Gas Compressibility. Values from the new equation for calculating the Gas Deviation Factor were compared with experimental Pressure, Volume, Temperature (PVT) measurements and the following statistical measurements of accuracy were obtained: Average percent error of 3.050, average absolute percent error of 3.649 and standard deviation of 3.910. Over 75% data tested showed error less than 5 %. A big advantage of the new equations is that they are simple and do not require pseudo critical pressure and pseudo critical temperature needed to obtain z from the current popular equations of state used in the Oil Industry. The data used to develop the model in this work came from Gas associated with produced oil in the Niger Delta. API gravity of the oil, ranged from 17.4 to 44. Pressure ranged from 15 psia to 4015 psia and Temperature ranged between 127 and 205 degree Fahrenheit. This range of data covers at least, ninety five percent of data used in the upstream sector of the Nigeria Oil and Gas Industry.