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in a net decrease in pore pressure in the reservoir over time, particularly if fluids are not
reinjected to maintain or regain original pore pressure conditions (see also other technol-
ogy descriptions, below, and Chapter 2). This change in pore pressure can cause changes
in the state of stress of the surrounding rock mass and of nearby faults, with the potential
to result in induced seismic events.
Secondary Oil and Gas Recovery
Secondary recovery is the process of injecting water (often described as a “waterflood”)
or gas (also known as pressure maintenance) into a petroleum reservoir. The water or gas
replaces the produced hydrocarbons and water in order to maintain the reservoir pres-
sures and is used to “sweep” an oil reservoir; injected gas may become dissolved in the oil,
reducing the oil's viscosity. Secondary recovery processes drive hydrocarbons trapped in
the rocks from the injection well toward production wells (Shepherd, 2009; Figure 3.9).
Waterflood or pressure maintenance projects can result in recovery of up to 40 percent of
the initial petroleum in the reservoir (DOE, 2011). The number of permitted wells that use
Production Well
Injection Well
Drive
Water
Drive
Water
Oil
Bank
Water
Water
FIGURE 3.9
Diagram illustrating waterflooding method of secondary recovery. SOURCE: NETL (2010).
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