Gas Lift Overview

Introduction

Gas lift is an artificial lift method that injects high-pressure gas into the production tubing to reduce the hydrostatic gradient, thereby decreasing bottomhole flowing pressure and increasing production rate. It is the second most common artificial lift method after rod pumping.

Gas lift is preferred when:

High GOR wells — natural gas handling capability
Offshore platforms — compact, no moving downhole parts
Deviated/horizontal wells — no restrictions on well trajectory
Sandy or corrosive conditions — no close-tolerance downhole equipment
Variable rates — easily adjustable via injection rate
High volume — can handle 1,000-30,000+ STB/d

System Components

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Gas Lift Types

Type	Description	Application
Continuous	Steady gas injection to lighten fluid column	High PI wells, moderate to high rates
Intermittent	Periodic gas slugs to push liquid to surface	Low PI wells, low rates (< 200 STB/d)
Chamber lift	Accumulation chamber at bottom for slug efficiency	Very low rates, deep wells

This documentation focuses on continuous gas lift, the most common type.

Valve Mechanics

Injection Pressure Operated (IPO) Valves

IPO valves open and close based on the injection (casing) pressure. The valve contains a nitrogen-charged bellows that provides a closing force:

Opening pressure (at depth):

$P_{vo} = \frac{P_d + P_t \cdot R}{1 - R}$

Closing pressure (at depth):

$P_{vc} = \frac{P_d}{1 - R}$

Where:

$P_d$ = dome pressure at valve depth temperature
$P_t$ = tubing pressure at valve depth
$R$ = port-to-bellows area ratio ( $A_p/A_b$ )

Valve Spread

The difference between opening and closing pressures:

$\text{Spread} = P_{vo} - P_{vc} = \frac{P_t \cdot R}{1 - R}$

Spread increases with tubing pressure and port size. Smaller ports give less spread and tighter control.

📖 Full Documentation: Valve Mechanics

Gas Injection System

Gas Gradient in Annulus

Injection gas pressure increases with depth due to the gas column weight:

$P_{inj}(D) = P_{surface} \times \exp\left(\frac{0.01875 \gamma_g D}{T_{avg} \bar{z}}\right)$

Where:

$P_{surface}$ = surface injection pressure (psia)
$\gamma_g$ = injection gas specific gravity
$D$ = depth (ft)
$T_{avg}$ = average temperature (°R)
$\bar{z}$ = average gas compressibility factor

Injection Rate

The gas throughput through a gas lift valve follows the Thornhill-Craver equation, which models the valve as a fixed orifice:

$q_g = f(P_1, P_2, d_{port}, T, \gamma_g, C_d)$

Where $P_1$ = upstream pressure, $P_2$ = downstream pressure, and $C_d$ = discharge coefficient.

Design Workflow

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Performance Optimization

GLR vs. Production Rate

There is an optimum GLR beyond which additional gas injection decreases production:

Below optimum: more gas reduces hydrostatic gradient → higher rate
Above optimum: friction increase from excess gas exceeds hydrostatic benefit

Available Injection Pressure

Higher injection pressure allows:

Deeper point of injection
Better unloading
Higher total GLR
But requires larger compressor investment

Gas Lift Details

Valve Mechanics — Dome pressure, opening/closing, throughput

VFP Overview — Multiphase flow correlations for tubing performance
IPR Overview — Inflow performance for lift design
RP Overview — Alternative artificial lift for lower rates
ESP Overview — Alternative artificial lift for high rates

References

Brown, K.E. (1980). The Technology of Artificial Lift Methods, Vol. 2a. PennWell Books.
Takacs, G. (2005). Gas Lift Manual. PennWell Books.
Winkler, H.W. and Smith, S.S. (1962). Gas Lift Manual. CAMCO Inc.
API Recommended Practice 11V6 (2008). "Design of Continuous Flow Gas Lift Installations Using Injection Pressure Operated Valves." American Petroleum Institute.
Economides, M.J., Hill, A.D., Ehlig-Economides, C., and Zhu, D. (2013). Petroleum Production Systems, 2nd Edition. Prentice Hall.