【FAQ】1.4 引起可压缩性和/或粘度为负的警告的原因

乔聪颖

1.4 What causes the warnings of negative compressibility and/or viscosity
1.4 引起可压缩性和/或粘度为负的警告的原因

PVT tables should cover completely the pressure range reached in all blocks of the model during initialization and calculation. If the pressure gets outside of the covered range, the simulator will have to resort to extrapolation over the nearest available points. This can produce physically impossible values of certain parameters.
PVT表应完全覆盖初始化和计算过程中模型所有网格所达到的压力范围。如果压力超出覆盖范围，则模拟器将不得不对最近的可用点进行外插。这可能产生某些参数在物理上不可能的值。
When reading PVT tables, the simulator calculates compressibility of oil, gas and gas-oil mixture for the parameters specified in the tables (pressure, volume coefficients, gas saturation, etc). Afterwards it checks for negative values of the compressibilities. The checks are run, for example, in the range of the maximum Pbub from PVTO (see 13.5.4) and the maximum pressure from PVDG (see 13.5.6), taking into account the intermediate values. It may so happen that the maximum pressure in the gas table is much higher than the Pbub in the oil table. Then the extrapolated values of Bo and Rs will be used to calculate the compressibility of the oil:
读取PVT表时，模拟器会根据表中指定的参数（压力，体积系数，气体饱和度等）来计算油、气和气油混合物的压缩系数。之后，它检查可压缩系数是否为负值。例如，在考虑中间值的情况下，在PVTO中的最大Pbub（请参见13.5.4）和PVDG中的最大压力（请参见13.5.6）的范围内进行检查。气表中的最大压力可能比油表中的Pbub高得多。然后，将Bo和Rs的外推值用于计算油的压缩系数：

The result may turn out negative. In this case a warning will be issued:
Warning: negative oil compressibility! In range of pressure <...> and <...> for saturated line in region <number>.
结果可能为负值。在这种情况下，将发出警告：
Warning: negative oil compressibility! In range of pressure <...> and <...> for saturated line in region <number>.
For similar reasons, negative viscosity may be encountered during calculations:
At <calculation step> extrapolation of <PVDG|PVTG|...> table was the cause of the negative value of gas viscosity in <amount> blocks, it has been reset to small value.
由于类似的原因，在计算过程中可能会遇到负粘度：
At <calculation step> extrapolation of <PVDG|PVTG|...> table was the cause of the negative value of gas viscosity in <amount> blocks, it has been reset to small value.
In both cases, the simulator will substitute near-zero positive values (1e-12) instead of negative, which nominally allows to continue the calculation. However, at this point the model becomes physically unrealistic. Hence it is recommended to stop the calculations and revise the PVT tables for the regions of interest.
在这两种情况下，模拟器都会用接近零的正值（1e-12）代替负值，这名义上可以继续进行计算。但是，此时模型在物理上变得不切合实际。因此，建议停止计算并修改感兴趣区域的PVT表。
The keyword EXTRAPMS (see 13.1.8) allows to manage warning messages about PVT table extrapolation in a log file.
关键字EXTRAPMS（请参见13.1.8）允许管理在日志文件中有关PVT表外推的警告消息。

（注：上面介绍的是数据外插原因，也有可能包含其他原因，如数据不光滑，内插时也可能出现类似警告信息（请参考阳光石油论坛上的另外两个帖子：帖子1-帖子2）。总之，尽可能在数据合理的情况下保证模型数据在物理方面的真实性和数值模拟方面的有效性。--qcy）