Additional Material Balance Plots
To distinct waterdrive and depletion drive in gas reservoirs (Source: Clarke et al, 2017)
p/z plot to identify waterdrive in gas reservoirs (discussion about straight curve and curving upward, See Unit 9). (Source: Clarke et al, 2017)
In a volumetric reservoir producing due to depletion drive only, production is balanced by the oil and water/formation expansion and the original oil-in-place is constant. If a plot of cumulative oil production versus the net withdrawal over expansion is created with this reservoir type's data, the points will remain along a horizontal line.
If a gas cap is present, there will be a gas expansion component in the reservoir's production. As production continues and the reservoir pressure decreased, the gas expansion term increases with an increasing gas formation volume factor. To balance this, the withdrawal over oil/water/formation expansion term must also continue to increase. Thus in the case of gas cap drive, the Dake plot will show a continually increasing trend.
Similarly, if water drive is present the withdrawal over oil/water/formation expansion term must increase to balance the water influx. With a very strong aquifer the water influx may continue to increase with time, while a limited or small aquifer may have an initial increase in water influx that eventually decreases.
(Source: Fekete website)
The Campbell plot is a very similar diagnostic to Dake, with the exception that it incorporates a gas cap if required. In the Campbell plot, the withdrawal is plotted against withdrawal over total expansion, while the water influx term is neglected. If there is no water influx, the data will plot as a horizontal line. If there is water influx into the reservoir, the withdrawal over total expansion term will increase proportionally to the water influx over total expansion. The Campbell plot can be more sensitive to the strength of the aquifer. In this version of the material balance, using only ET neglects the water and formation compressibility (compaction) term.
(Source: Fekete website)
More Material Balance Plots in Sylvester Okotie's Reservoir Engineering Springer Textbook (p. 245-288)
Gas cap material balance with indicator of aquifer magnitude