IDENTIFICATION OF TYPE OF RESERVOIR FLUID - Petroleum Reservoir Fluid Property Correlations - page 191

5
IDENTIFICATION OF TYPE OF RESERVOIR FLUID
McCain gave guidelines for determining reservoir fluid type from
field data. 1 Additional data have shown that some of these guidelines
are incorrect.
Stock-Tank Liquid Gravity
as an Indicator of Reservoir Fluid Type
Table 1 from McCain (1994) gives some guidelines for using
stock-tank liquid gravities in degrees API for determining reservoir fluid
type. 2 The numbers in the table are incorrect, and in fact, stock-tank
liquid gravities are not valid indicators of reservoir fluid type. Figure
5-1 presents stock-tank liquid gravities and initial producing gas-liquid
ratios for 2,828 different reservoir fluid studies or production tests.
Both oils and gas condensates are represented on the graph.
There is obviously a trend in the gravities of the stock-tank liquids;
however, the scatter is too large for the use of stock-tank liquid gravities
in determining reservoir fluid type. The black oils have stock-tank
liquid gravities with values ranging from less than 10ºAPI to more than
60ºAPI. The volatile oils have stock-tank liquid gravities from 30ºAPI to
more than 60ºAPI. The gas condensates have stock-tank liquid gravities
from slightly more than 30ºAPI to more than 70ºAPI. The overlaps in
stock-tank liquid gravities among the three fluids are just too great for
stock-tank liquid gravities to be useful in determining reservoir fluid
type. One exception to this statement might be that if stock-tank oil
gravity is less than 30ºAPI, the reservoir fluid is most likely a black oil.