计算天然气偏差因子的新方法

A new method to calculate the Z factor of natural gases

天然气偏差因子是定量描述真实气体(天然气)与理想气体偏差程度大小的系数,是天然气其他物性及天然气藏地质储量计算、管道天然气产量设计的一项重要参数。过去天然气偏差因子是根据Standing-Katz偏差因子图版得到。但这种方法使用不便,且精度也难得到保证。通过对一些相关经验方法的分析表明,天然气偏差因子的计算仍然存在进一步提高计算精度的空间。基于Strobridge修正的BWR(Benedict-Webb-Rubin)状态方程,建立了一种计算天然气偏差因子的新方法,并在等压线和等温线构成的二维平面上对计算结果的相对误差进行分析。误差分析表明,该方法可以将计算值的相对误差提高到1%以内,有效地改善天然气偏差因子的计算精度。

The Z factor of natural gases is a coefficient that quantitatively describes the deviation extent between the actual gas（natural gases） and the ideal gas.It serves as a very important parameter to measure other properties of natural gases,calculate geological reserves of gas reservoirs,and design production pipelines of natural gases.Traditionally,the Z factor was read directly from the Standing-Katz deviation-factor plate,this way,however,is inconvenient in practice and can not guarantee the accuracy.In order to improve the accuracy,a variety of relevant empirical methods have been proposed successively.Through careful analyses,we found these methods could be further improved in the accuracy.Based on the BWR（Benedict-Webb-Rubin） state equation modified by Strobridge,we developed a new approach to calculate the Z factor of natural gases and analyzed relative errors of calculated results on the two-dimensional plane composed by isobars and isotherms.Error analyses showed that the proposed new approach could keep relative errors below 1% and effectively improve the calculation accuracy of the Z factor of natural gases.