Abstract: The calculation formula for gas reservoir volume factor in petroleum engineering is introduced. The shortage of calculation formula is analyzed. A modified method is proposed for the calculation formula for gas reservoir. The deviation factor is calculated by PR EOS and Gopal (1977) methods widely used in the petroleum engineering. The gas reservoir volume factor calculation is performed for ten gas reservoir fluids with methane content ranging from 50.56 to 97.75 percent and compared with experimental data. The relative errors average 1.325% and 3.173%, respectively, with the modified calculation formula, compared with the average relative errors of 1.867% and 3.701% with available calculation formula. The results show that the modified calculation formula can improve the prediction accuracy for the gas reservoir volume factor.

Key words: gas reservoir, gas volume factor, formula, modification