Page 276 - Introduction to Petroleum Engineering

DEPLETION DRIVE MECHANISMS AND RECOVERY EFFICIENCIES 263

Example 13.6 Depletion Drive Gas Reservoir
Early data from a depletion drive gas reservoir are shown in the following.
Estimate OGIP.
G (Bscf) P (psia) Z P/Z (psia)
p
0.015 1946 0.813 2393
0.123 1934 0.813 2378

Answer
The equation for a straight line is y mxb where m is the slope and b is the
intercept. Comparing the equation for a straight line with Equation 13.25 gives
x G , y ( p Z) . The slope and intercept are m ( pZ) i ( / G)
/
1
/
t
p
psia
138 9 and b ymx 2395 psia. G is the value of G when (p/Z) is
.
Bscf p t
zero. This corresponds to y mxb 0 or G x at y 0 so G ( bm)
/
2395 psia
17.2 Bscf. The actual value of G based on additional
138 9 psia/Bscf
.
data from a longer production period is 16.1 Bscf. Additional data can provide
a more accurate straight line for performing the OGIP analysis.
13.6 DEPLETION DRIVE MECHANISMS AND
RECOVERY EFFICIENCIES
Depletion drive mechanisms can be identified by examining the production
performance of properties such as reservoir pressure and GOR. Reservoir depletion
occurs when production occurs without injection. The removal of fluids from the
reservoir without replacement results in reservoir pressure decline. Figure 13.3
presents production profiles for three depletion drives during primary production of
an oil reservoir.
The solution gas drive shows a significant increase in GOR followed by a decline
in GOR as available gas is produced. An undersaturated oil reservoir consists of oil
and immobile water. When it is produced, only single‐phase oil flows into the well-
bore. The production of oil reduces pressure in the reservoir, and the oil in the reser-
voir expands. The rate of pressure decline when reservoir pressure is above bubble
point pressure depends on the compressibility of the oil and the formation. If produc-
tion continues, eventually reservoir pressure will drop below bubble point pressure.
Gas dissolved in the oil comes out of solution and forms a free gas phase as reservoir
pressure declines. Bubbles of free gas expand and help displace oil to the well. If
enough free gas is present, the gas bubbles coalesce and increase gas saturation.
When gas saturation exceeds critical gas saturation, typically 3–5% of pore volume,
the gas forms a flow path for free gas flow. Production GOR is constant initially until