Page 277 - Introduction to Petroleum Engineering

264 PRODUCTION PERFORMANCE

Solution gas drive Gas cap drive GOR

GOR
Pressure
Pressure
Oil rate Oil rate

Time Time

Water drive

Pressure
Water cut

GOR
Oil rate
Time

FIgURE 13.3 Production profiles of drive mechanisms.

a dip occurs in the GOR curve. The GOR dip shows the formation of critical gas
saturation when reservoir pressure drops below bubble point pressure. When
gas saturation exceeds critical gas saturation, free gas flows into the well and the
GOR increases.
A gas cap is present in a reservoir when the volume of oil is not large enough to
dissolve all of the gas at original reservoir temperature and pressure. Over geologic
time, free gas migrates to the crest of the reservoir and forms a gas cap. A well com-
pleted in the oil zone will produce single‐phase oil. Gas cap expansion displaces oil
to the well. Production GOR is constant initially until the gas–oil contact reaches the
well. Increasing GOR shows that free gas is being produced.
Water drive is the most efficient drive mechanism for producing oil. Pressure
support and displacement of oil by water helps reduce the rate of pressure depletion.
Recovery efficiencies for the most common depletion drive mechanisms for primary
production are shown in Table 13.5.
In some cases, reservoir dip is so large that gravity drainage is an important natural
drive mechanism during primary depletion of an undersaturated oil reservoir. The
East Texas oil field is an example of a gravity drainage oil field. The giant oil field
was discovered in 1930, and oil production to date exceeds five billion barrels of oil.
The East Texas oil field is the part of the Woodbine sand that is wedged between the