Page 273 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

242 Reservoir Engineering

pressure. (Note: if the formation volume factors are expressed in ft5/scf, 7,758
should be replaced with 43,560 ft3 per acre-ft.
If Equation 5-145 is rearranged in terms of the initial gas in place:

(5-170)

The recovery efficiency, En, which is the volume of gas recovered divided by
the volume of gas initially in place, can be found from:

B, - B,
E, = (5-1 71)
B#

For volumetric gas reservoirs (which assumes no change in Sw) recoveries may
range from 80% to 90% of the initial gas in place.
For gas reservoirs under water drive, recovery efficiency is:

(5-172)

where SBt is the residual gas saturation and the other terms axe as defined above.

Recovery of 011
Solution-Gas or Depletion Drlvr. Oil recovery by depletion or solution-gas
drive is:

(I-% -s*>
(l-Sw)
N, = 7,758 Ah@[ - 1 (5-179)
-
B, BO
where B . is the initial oil formation volume factor and Bo is the oil formation
volume &tor at abandonment, and the other terms are as previously defined.
The ultimate free gas saturation is often estimated from the old data of Arps
[245] and Craze and Buckley [241] in which the average Sg was 30% for a
2.2 cp oil with 400 fts/bbl of solution gas. A general rule suggests that for each
doubling of solution gas, S8 should be increased 3%; and for each doubling of
oil viscosity, Sg should be decreased by 3% [14]. As a first approximation,
Sgr can be taken to be about 0.25 [254]
For a volumetric, undersaturated reservoir,

(5-1 74)

where N, is the oil produced in stock tank barrels, N is the initial oil in place
in STB and the other terms are as already given. In this case, the oil saturation
after any stage of primary production, Sa, can be obtained from: