Page 327 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

494 Reservoir Engineering

Table 5-43
Stiles Method of Calculating Waterflood
Performance in Stratified Reservoirs

Ikjh, +IC, - c,)l
R=
klht
R = fraction of recoverable oil that has been produced
ct = total capacity of formation (md-ft)
9 = mid-ft which have been completely flooded with water
h, = total net thickness of formation (ft)
h, = total net thlckness flooded (ft)
5 = permeability of layer just flooded out
Reservolr conditions at I? W. Surface conditions
Mcj
f, =
[Mc, + (ct - c,)l
f , = fractional flow of water

8, = oil formation volume factor
B, = water formation volume factor

From Reference 17.

(5-229)

where q, is the flow rate in B/D at surface conditions, P, is the flowing
bottomhole pressure in psi, and P, is the external pressure in psi. Some engineers
express injectivity in terms of upw so that when injectivity is given, the reader
is cautioned to understand what base pressure was intended. By dividing I by
reservoir thickness, a specific injectivity index (specific to one well) can be
obtained in B/D/psi/ft. In addition to expressing injectivity in terms of fluid
injection rate in B/D, injectivity also is given as B/D/ac-ft and B/D/net ft of
producing interval. Values of injectivity depend on properties of the reservoir
rock, well spacing, injection water quality, fluid-rock interactions, and pressure
drop in the reservoir. Typical values of injectivity are in the range of 8-15 B/
D/net ft or 0.75-1.0 B/D/net ac-ft. In waterflooding operations, water injection
may begin into a reservoir produced by solution-gas-drive in which a mobile gas
saturation exists, or injection may begin prior to the development of a mobile
gas saturation. In the latter case, the system can be considered filled with liquid.
InJectivltles for Varlous Flood Patterns. Analytical expressions for liquid-filled
patterns were given by Muskat [25] and Deppe [SlS] for a mobility ratio of one
(see Table 5-44). While these exact analytical solutions can be developed for
steady-state pressure distributions, the equations in Table 5-44 cannot be used
directly if the mobility is not one. However, the equations are useful in estimat-
ing injectivity in limiting conditions. For example, if k and p are selected for