Page 274 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Material Balance and Volumetric Analysis 243

(5-175)

If calculations are made at the abandonment of primary production, So will
represent the oil saturation after primary. The gas saturation at any time, Sg,
can be found from:
s =1-so-sw (5-176)
!3
For a solution-gas drive reservoir with water encroachment [73]:

Bo - Boi W
(5-177)

where We is the cumulative water influx in barrels and all other terms are
consistent with the prior definitions.
Water-Drive Reservoir. Recovery from a waterdrive reservoir (which assumes
no appreciable decline in pressure) can be calculated from [14]:

(5-178)

where Sor is the residual oil saturation as a fraction of pore volume. Residual
oil saturation can be obtained from cores taken with water base mud or from
logs as described earlier. Methods of estimating residual oil saturation after water
injection are discussed later. In the absence of data, residual oil saturations are
sometimes obtained from the empirical data of Arps [245] and Craze and
Buckley [241] which are given in Table 5-32. However, the caution given earlier
is repeated here: from a more recent study [239] no statistically valid correlation
was found between oil recovery and definable reservoir parameters.
The recovery efficiency is the case of water influx is given by:

(5-179)

Gravity Dralnage. For segregation drive reservoirs, Equation 5-178 can be used.
Residual oil saturations for gravity drainage reservoirs tend to be low (possibly
on the order of 0.10) [14].

Gas-Cap Drive. For a gascap drive reservoir with no water influx, the oil satura-
tion can be estimated from [73]:

N-N 1 1
'0 = ( mOi s)" -sw)[ 1 - m(B, - B,)/B, (5-180)

where m is the ratio of original gas zone to original oil zone in the reservoir
and all other quantities have been defined.