Page 278 - Introduction to Petroleum Engineering

DEPLETION DRIVE MECHANISMS AND RECOVERY EFFICIENCIES 265
TABLE 13.5 Recovery Efficiencies for Different Depletion Drive Mechanisms
Depletion Drive Mechanisms Recovery Efficiency (% OOIP)

Water drive 35–75
Gas cap drive 20–40
Solution gas drive 5–30

Source: Data from Ahmed (2000).

West East

East Texas
field
Woodbine outcrop
Wilcox SS
Austin LS
Taylor LS
Wachita LS
Woodbine SS

FIgURE 13.4 Cross section of the East Texas Basin. (Source: Adapted from Halbouty
(2000).)

Austin and Wichita limestones sketched in Figure 13.4. The section of the Woodbine
sand from the outcrop to the East Texas oil field is an aquifer that provided pressure
support to the oil field. The outcrop made it possible to recharge the aquifer when
rain and snow fell. Both gravity drainage and water drive functioned as natural drive
mechanisms during primary depletion.
A combination drive is active when two or more natural drive mechanisms are
functioning at the same time during production of an oil reservoir. Primary produc-
tion by gravity drainage and water drive from the East Texas oil field is an example
of a combination drive. The relative importance of drive mechanisms can be deter-
mined using drive indices calculated from the general material balance equation. The
production profile of a combination drive reservoir depends on which drive is domi-
nant at different points during the primary production period.
Natural reservoir energy can be supplemented by injecting fluids into the reser-
voir. Water and gas are typical injection fluids. The injected fluids reduce the rate of
pressure decline that would have occurred during primary depletion. Water drive can
be an effective means of displacing oil to production wells regardless of the source of
water. Thus, if a reservoir does not have significant aquifer support, injection wells
can be used to supplement existing natural resources. Water breakthrough occurs