Page 295 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

262 Reservoir Engineering

where I is the constant fraction of the produced gas which is reinjected into
the oil reservoir, B, is the oil formation volume factor at bubblepoint con-
ditions, R is the producing gas-oil ratio, the subscripts 1 and 2 refer to the time
increments at p1 and ps, the other standard terms are as already defied, and
Ra,v is (R, + %)/2. As given in Table 5-34, the instantaneous gas-oil ratio, R, is:

(5-202)

where R, is the solution gas4 ratio, B is formation volume factor, k is per-
meability, p is viscosity, and the subscripts o and g refer to oil and gas,
respectively. The relative permeabilities are determined at the total liquid
saturation, S,:
(N-N,) -
Bo
s, = s, +so = s, +(l-S,) (5-203)
N Bob
The simultaneous solution of these three equations will provide estimates of oil
produced at any chosen conditions for a dispersed-gas-drive injection project.
Additional details can be found in Reference 254.
Many of the flow equations and concepts for immiscible gas displacement are
similar to those that will be presented later for waterflooding. Because of the
importance of. water injection processes in U.S. operations, waterflooding
concepts will be emphasized.

Water Injection
Water injection processes may be designed to: (1) dispose of brine water,
(2) conduct a pressure maintenance project to maintain reservoir pressure
when expansion of an aquifer or gas cap is insufficient to maintain pressure,
or (3) implement a water drive or waterflood of oil after primary recovery. As
mentioned before, waterflooding is the dominant secondary recovery process
which accounts for about 50% of the current oil production in U. S. operations.
Because of the importance of waterflooding, fluid displacement in waterflooded
reservoirs is covered as a separate discussion in a later section.

Spacing of Wells and Well Patterns
Spacing of Wells

One section (one sq mile or 5,280 ft by 5,280 ft) is 640 acres. If wells are
drilled evenly such that each well is theoretically assigned to drain 40 acres,
the 16 wells per section would be as spaced as in Figure 5-148. Each 40 acres
(v4 mile by '/4 mile or 1,320 ft by 1,320 ft) would contain 40 x 43,560 or
1,742,400 ft2. The 10-acre region in Figure 5-148 would measure 660 ft by 660
ft and would contain 435,000 ft*. Similarly, a 20-acre region would contain
871,200 ft* and would measure [20 x 871,200]1/4 or 933.4 ft by 933.4 ft.
In many parts of the United States, 40-acre spacing or less is common for oil
wells, and 160-acre or 320-acre spacing is common for gas wells. Because drilling
costs increase considerably with depth, deeper wells may be on larger spacing.