Page 326 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Fluid Movement in Waterflooded Reservoirs 293

where N = oil potentially recoverable by waterflooding, STB
Pj = initial oil in place, STB
NP = oil produced during primary operations, STB
Boi = initial FVF
These equations can be altered to include a residual gas saturation, if present.
The volumetric sweep efficiency can be estimated from one of the correla-
tions given previously or can be obtained from an analogy from similar water-
flood projects.

Prediction Methods

An extensive survey on prediction of waterflood performance was provided
by Craig [133]. Of the methods reviewed, three appeared most promising:
(1) the Higgins-Leighton streamtube model [309], (2) the Craig, Geffen, and
Morse model [282], and (3) the Prats et al. method [283]. Discussion of the
various prediction methods is beyond the scope of this text, and only two very
simple methods will be presented for illustrative purposes. Both the Dykstra-
Parsons [294] and Stiles [310] methods are very cursory and, if used, they are
normally followed by more extensive evaluations, usually by computer simulation.
For either the Dykstra-Parsons or Stiles methods, the permeabilities are
arranged in descending order. For the Dykstra-Parsons method, the permeability
variation is determined as described earlier. Two options are then possible: a
progrim [311] for hand-held calculators can be used, or the graphical technique
presented by Johnson [296] can be used. The fractional recovery, R, (see Figure
5-164 for example) expressed as a fraction of the oil in place when the waterf-
lood is started, muii be multiplied by the areal sweep efficiency (for example
from Figure 5-161) to obtain the waterflood recovery.
For the Stiles technique, a program [312] for hand-held calculators is available
or the procedure summarized in Table 5-43 can be used. A straightforward
presentation of the Stiles method is in the text by Craft and Hawkins [17]. The
fractional recovery obtained with the Stiles method is a fraction of the recover-
able oil (ST - Sm) that has been recovered at a given reservoir water cut. Since
a water-oil ratio (WOR) is measured at surface conditions, the fractional water
cut at reservoir conditions, fw, is obtained (assuming B, = 1.0) from:

WOR
f, = (5-228)
WOR + Bo
where Bo is the oil formation volume factor.

Performance Evaluation
Monitoring waterflood performance is crucial to the success of the flood.
From a reservoir engineering standpoint, the primary concerns are water
injectivity and oil productivity. A few important factors related to these concerns
will be summarized.

Injectivlty and lnjectivlty Index. Whereas productivity index was the ability
of a well to produce hydrocarbons, injectivity index, I, in B/D/psi, is a measure
of the ability of a well to accept fluids [17]: