Gas Lift Valve Mechanics

Overview

Gas lift valves are the critical control elements in a gas lift system. They regulate the flow of injection gas from the annulus into the tubing at specific depths. Understanding valve mechanics is essential for proper valve spacing, pressure setting, and troubleshooting.

The most common type is the Injection Pressure Operated (IPO) valve, which opens and closes based on casing (injection) pressure.

IPO Valve Construction

An IPO valve consists of:

Component	Function
Bellows	Pressure-sensing element (nitrogen-charged)
Dome	Contains nitrogen charge at calibrated pressure
Stem	Connects bellows to port, opens/closes flow path
Port	Orifice through which gas flows
Check valve	Prevents backflow from tubing to annulus

The bellows area ( $A_b$ ) is larger than the port area ( $A_p$ ). The ratio $R = A_p/A_b$ is a key valve parameter, typically 0.05-0.20.

Dome Pressure

Temperature Effect

The nitrogen dome is charged at surface (test rack) conditions but operates at downhole temperature. The dome pressure at depth differs from the test rack setting:

$P_d = P_{TRO} \times C_T$

Where:

$P_d$ = dome pressure at valve temperature
$P_{TRO}$ = test rack opening pressure (at 60°F)
$C_T$ = temperature correction factor

The correction factor accounts for nitrogen compressibility at different temperatures. For nitrogen:

$C_T > 1$ when valve temperature > 60°F (dome pressure increases)
$C_T < 1$ when valve temperature < 60°F

Opening and Closing Pressures

Force Balance

At the moment of opening, the forces on the valve stem balance:

Closing force = Dome pressure × bellows area $F_{close} = P_d \times A_b$

Opening force = Casing pressure on bellows + Tubing pressure on port $F_{open} = P_c \times (A_b - A_p) + P_t \times A_p$

Opening Pressure

Setting $F_{open} = F_{close}$ and solving for $P_c$ :

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

Or equivalently:

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

The sign convention depends on whether $P_d$ is referenced to the net bellows area or total area. The functions use the convention matching standard gas lift design practice.

Closing Pressure

When the valve closes, only casing pressure acts on the bellows (port pressure is zero):

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

Valve Spread

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

Key observations:

Spread depends on tubing pressure and port-to-bellows ratio $R$
Larger port → larger $R$ → larger spread → less precise control
Higher tubing pressure → larger spread

Gas Throughput

Thornhill-Craver Equation

The gas flow rate through a gas lift valve operating in subcritical flow:

$q_{sc} = 155.5 \times C_d \times A_p \times P_1 \times \sqrt{\frac{2g}{k-1} \times \frac{M}{RT_1} \times \left[\left(\frac{P_2}{P_1}\right)^{2/k} - \left(\frac{P_2}{P_1}\right)^{(k+1)/k}\right]}$

Where:

$q_{sc}$ = gas rate at standard conditions (Mscf/d)
$C_d$ = discharge coefficient (0.60-0.85)
$A_p$ = port area (in²)
$P_1$ = upstream pressure (psia)
$P_2$ = downstream pressure (psia)
$k$ = specific heat ratio ( $c_p/c_v$ )
$M$ = gas molecular weight
$T_1$ = upstream temperature (°R)

Critical Flow

When the pressure ratio drops below the critical ratio:

$\frac{P_2}{P_1} \le \left(\frac{2}{k+1}\right)^{k/(k-1)}$

The flow becomes sonic (critical), and increasing the downstream pressure drop no longer increases flow rate. For natural gas ( $k \approx 1.28$ ), the critical ratio is approximately 0.55.

Production Pressure Effect (PPE)

Concept

In practice, the tubing (production) pressure at valve depth affects both valve opening and gas throughput. The PPE factor quantifies this influence:

$PPE = \frac{P_t \times R}{1 - R}$

Higher PPE means:

Higher opening pressure required
Wider spread between opening and closing
More sensitive to tubing pressure changes

Design Implications

$R$ Value	PPE	Control	Throughput
0.05	Low	Precise	Low
0.10	Moderate	Good	Moderate
0.15	Moderate-High	Fair	Good
0.20	High	Poor	High

Valve Spacing

Principles

Top valve depth set by kickoff pressure available
Subsequent valves spaced to maintain pressure differential for unloading
Operating valve is the deepest valve — injection point for steady-state production
Each valve must close before the next one is uncovered

Pressure Drop Per Valve

Each successive valve requires slightly lower opening pressure to ensure sequential operation:

$\Delta P_{valve} = 15-30 \text{ psi per valve}$

This drop accounts for:

Dome pressure setting tolerances
Ensuring upper valves close before lower ones open
Providing a margin of safety

Troubleshooting

Symptom	Possible Cause	Diagnostic
Well won't unload	Insufficient injection pressure	Check surface pressure vs. valve depths
Multipoint injection	Upper valve not closing	Compare casing pressure to valve closing pressures
Erratic production	Valve cycling	Check if operating near valve opening/closing point
Low injection rate	Port too small or valve partially open	Thornhill-Craver calculation vs. actual rate

Gas Lift Overview — System design and optimization
VFP Overview — Tubing pressure calculations for lift design
IPR Overview — Inflow performance at operating conditions

References

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