Page 23 - Introduction to Petroleum Engineering

LIFE CYCLE OF A RESERVOIR 7

Oil production rate Discovery well Appraisal well First Plateau Decline
Buildup

oil
Economic Abandonment
limit
Time

FIGURE 1.2 Typical production profile.
Petroleum engineers provide input to decision makers in management to help
determine suitable optimization criteria. The optimization criteria are expected to
abide by government regulations. Fields produced over a period of years or decades
may be operated using optimization criteria that change during the life of the reser-
voir. Changes in optimization criteria occur for a variety of reason, including changes
in technology, changes in economic factors, and the analysis of new information
obtained during earlier stages of production.
Traditionally, production stages were identified by chronological order as
primary, secondary, and tertiary production. Primary production is the first stage
of production and relies entirely on natural energy sources to drive reservoir fluids
to the production well. The reduction of pressure during primary production is
often referred to as primary depletion. Oil recovery can be increased in many cases
by slowing the decline in pressure. This can be achieved by supplementing natural
reservoir energy. The supplemental energy is provided using an external energy
source, such as water injection or gas injection. The injection of water or natural
gas may be referred to as pressure maintenance or secondary production. Pressure
maintenance is often introduced early in the production life of some modern
reservoirs. In this case the reservoir is not subjected to a conventional primary
production phase.
Historically, primary production was followed by secondary production and then
tertiary production (Figure 1.3). Notice that the production plateau shown in
Figure 1.2 does not have to appear if all of the production can be handled by surface
facilities. Secondary production occurs after primary production and includes the
injection of a fluid such as water or gas. The injection of water is referred to as water
flooding, while the injection of a gas is called gas flooding. Typical injection gases
include methane, carbon dioxide, or nitrogen. Gas flooding is considered a secondary
production process if the gas is injected at a pressure that is too low to allow the
injected gas to be miscible with the oil phase. A miscible process occurs when the gas
injection pressure is high enough that the interface between gas and oil phases disap-
pears. In the miscible case, injected gas mixes with oil and the process is considered
an enhanced oil recovery (EOR) process.