Dry Gas Reservoir: 2005 #49 (similar)

The gas MBE is very simple; see 13 RES 2:
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You are usually asked to solve for water influx, since this is the only term you don't know from production volumes. For example, given:

G = 10,000 MMCF, Bgi 1 M RB/MMCF
Gp 1,000 MMCF, Bg 1.3 M RB/MMCF
2,200 MSTB water, Bw = 1 RB/STB

G(Bg-Bgi) = GpBg + WpBw - We

10,000 MMCF(1.3 - 1 M RB/MMCF) =
1,000 MMCF(1.3 M RB/MMCF) + 2,200 M STB(1 M RB/M STB) - We

3,000 M RB = 1,300 M RB + 2,200 M RB - We
3,000 M RB = 3,500 M RB - We

We = 500 M RB

With gas it's all about the units.