American Oil and Gas Reporter - June 2022 - 74

SpecialReport: Artificial Lift Technology
Technique Optimizes Gas Lift Designs
By Robert Vincent
Because gas lift excels at dealing with
sandy, crooked wells and uses the same
downhole equipment over wide production
ranges (an asset in unconventional plays
with high decline rates), it is becoming
more common. To optimize gas lift's operating
costs and revenue, the compressor
needs to be properly sized. Paying for a
compressor that can achieve unnecessarily
high injection pressures is a waste of investment
dollars, but an undersized compressor
can limit production.
Compressor size matters because available
gas injection pressure is the most
significant factor in determining how
deep gas can be injected into the well.
Ideally, the gas lift system should be capable
of injecting gas at the bottom of
the production string so the gas can
lighten the entire fluid column.
Injecting at the deepest point possible
will yield the lowest producing bottomhole
pressure and enable operators to get
more production with less lift gas and
less expensive equipment. However, it
may be impractical to inject at the bottom
of the production string because the compressor
already in place or available has
pressure or rate limitations, or because
of complications presented by downhole
equipment and wellbore geometry.
Equilibrium Curves' Utility
When designing gas lift systems, the
primary objectives are to determine the
gas injection depth, rate and pressure to
achieve the expected liquid production.
Once these key parameters have been selected,
all that remains are the mandrel
spacing and valve specifications to unload
the well.
To guide these design decisions, Shell
Oil Co. developed the equilibrium curve
concept. A completed equilibrium curve
estimates liquid production at various injection
depths and pressures (Figure 1).
The equilibrium curve can be used to assess
whether a proposed compressor or
one already in place can deliver the
desired production rate.
By combining the equilibrium curve
with additional analysis, gas lift engineers
can predict liquid production rates at var74
THE AMERICAN OIL & GAS REPORTER
ious gas injection rates. Injection rate
and pressure then can be used to determine
the needed compression horsepower. The
most efficient operation will be the gas
injection pressure that yields the lowest
compressor horsepower per barrel of
liquid produced.
Creating Equilibrium Curves
The process to create equilibrium
curves includes plotting the formation
fluid gradient with the producing gas lift
gradients for various injection pressures,
then finding the intercepts. The pressure
gradient curve is determined using a multiphase
flow correlation for a given production
rate, gas-to-liquid ratio (GLR),
water cut, set of fluid properties and conduit
size. This curve shows the pressure
in the production conduit at depth.
Also required is the inflow performance
relationship (IPR) curve. Determined
from well tests and associated bottomhole
flowing pressures, the IPR curve shows
the expected liquid production rate for a
given producing bottom-hole pressure
(or a given drawdown pressure).
The intersection of the pressure gradient
and IPR curves, for the same production
rate, is known as the equilibrium pressure;
FIGURE 1
Completed Equilibrium Curve
Gas Injection Pressure, psi
600 BPD
1,000
900 BPD
2,000
1,200 BPD
3,000
200 psi
4,000
1,500 BPD
5,000
6,000
7,000
8,000
800 psi
400 psi
600 psi
200
400
600
800
1,000
1,200
the pressure is the same at the bottom of
the gas lift production string as it is at
the top of the point of natural flow. By
plotting the equilibrium pressures for a
series of production rates, engineers create
the equilibrium curve, a powerful way to
define the basic parameters needed for a
gas lift design to achieve the desired production
rate: the gas injection depth, pressure
and rate.
The basic steps to create an equilibrium
curve are:
· Develop pressure gradient curves
for various production rates using a constant
gas-to-liquid ratio.
· Develop the actual IPR curve for
the well.
· Determine the reservoir drawdown
pressure for each production rate used in
the pressure gradient curves.
· Determine the slope of the formation
gradient using the formation GLR.
· Plot the formation gradient line
from the drawdown pressure associated
with each production rate selected on the
pressure versus depth plot.
· Mark the intersection of the formation
gradient and the production pressure gradient
for each production rate selected.
· Connect these points of intersection.
Depth, ft
American Oil and Gas Reporter - June 2022

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