Gas Lift Optimization with SCADA

Gas Lift Principles and Automation Opportunity

Gas lift is the second most common artificial lift method after rod pumping, widely used in offshore, high-rate onshore, and deviated wells. The principle is simple: compressed gas injected into the tubing-casing annulus enters the tubing through gas lift valves, reducing the fluid column density and allowing reservoir pressure to push fluids to surface. The automation opportunity lies in optimizing the volume of injection gas, which has a non-linear relationship with oil production.

Every gas lift well has an optimal injection rate. Below this rate, the well underproduces because the tubing gradient is too heavy. Above this rate, additional gas provides diminishing returns and eventually decreases production due to excessive friction in the tubing (a condition called "over-injection"). SCADA-based optimization continuously finds and maintains this optimal point despite changing well conditions.

Instrumentation for Gas Lift SCADA

Required Measurements

Effective gas lift optimization requires accurate, real-time measurement of several parameters at each well:

• Gas injection rate: Orifice plate or vortex flow meter on the injection gas line to each well. Accuracy of +/-2% is adequate for optimization. Differential pressure transmitters with square root extraction in the RTU/PLC calculate flow from orifice measurements.
• Casing pressure: Measured upstream of the gas lift injection point. Indicates the pressure available to push gas through the operating gas lift valve.
• Tubing pressure: Measured at the wellhead downstream of the choke. Combined with surface flow rate, this determines the flowing gradient in the tubing.
• Production flow rate: Test separator flow rates (oil, gas, water) measured during well tests. Some operators install continuous multiphase flow meters for real-time production monitoring.
• Gas lift choke position: Motorized choke valves (Fisher, Kimray, or similar) with 4-20mA position feedback allow remote adjustment of injection gas rate to each well.

Control Valve Selection

The gas lift injection control valve is the primary actuator for optimization. Key selection criteria include:

• Rangeability: The valve must provide smooth control from minimum to maximum injection rates, typically 50:1 turndown ratio.
• Erosion resistance: Injection gas often contains small amounts of sand or compressor oil. Tungsten carbide trim resists erosion in high-velocity gas service.
• Fail position: Fail-closed prevents excessive gas injection during controller or communication failures. Fail-open may be preferred if the risk of well loading up (going dead) is greater than the cost of over-injection.
• Actuator type: Pneumatic actuators with I/P transducers are common where instrument air is available. Electric actuators are preferred for remote solar-powered sites.

Gas Lift Performance Curves

The gas lift performance curve (GLPC) is the relationship between injection gas rate (x-axis) and oil production rate (y-axis) for a given well. This curve is typically parabolic, rising steeply at low injection rates, reaching a maximum, and then declining as friction losses from excessive gas dominate. The shape and peak of the GLPC change as reservoir pressure depletes, water cut increases, or well conditions change.

Automated systems build and update GLPCs by correlating injection rate changes with production response measured during well tests or from continuous multiphase meters. Machine learning algorithms can identify the optimal operating point and recommend or automatically implement injection rate adjustments. The most sophisticated systems update GLPC models continuously, eliminating the need for periodic manual well tests.

Multi-Well Gas Lift Allocation

The Allocation Problem

Most gas lift fields have limited compressor capacity, meaning the total available lift gas must be distributed among multiple wells to maximize field-wide production. This is a classic optimization problem: given N wells with different GLPC shapes and a fixed total gas volume, what allocation maximizes total oil production?

The mathematical solution is to equalize the marginal oil gain per unit of injection gas across all wells. In practice, this means directing more gas to wells with steep GLPC slopes (where additional gas produces significant incremental oil) and reducing gas to wells operating near or past their GLPC peak.

SCADA-Based Allocation Optimization

Automated gas lift allocation requires:

• Individual well gas measurement and control: Each well must have its own flow meter and motorized control valve for injection gas.
• Compressor discharge monitoring: Total available gas volume and discharge pressure determine the constraint for the allocation optimization.
• Optimization algorithm: The SCADA system or a connected optimization application solves the allocation problem periodically (typically every 1-4 hours) and adjusts individual well injection rates.
• Production validation: Well test data or continuous meters verify that predicted production matches actual output. Discrepancies trigger GLPC model updates.

Common Gas Lift Problems Detected by SCADA

Real-time monitoring enables early detection of gas lift system problems:

• Valve failure: A gas lift valve stuck open or closed changes the injection point depth, dramatically affecting efficiency. Casing and tubing pressure patterns shift from normal baseline.
• Multi-point injection: Gas entering the tubing through multiple valves instead of the designed operating valve wastes injection gas and reduces lift efficiency. Diagnosed by flowing gradient surveys or pressure pattern analysis.
• Heading or slugging: Periodic surges in production followed by liquid loading. Caused by insufficient injection rate or a malfunctioning deepest valve. Visible as cyclical pressure and flow oscillations on SCADA trends.
• Compressor issues: Declining discharge pressure or capacity reduces gas available for lift, affecting all wells simultaneously. SCADA detects compressor performance degradation through trending discharge pressure, suction pressure, and throughput.

Integration with Compressor SCADA

Gas lift optimization must be coordinated with compressor station operations. The SCADA system monitors compressor suction and discharge pressures, engine/motor load, vibration, oil pressure, and coolant temperatures. When compressor capacity changes (due to maintenance, load shedding, or equipment failure), the gas lift allocation optimizer automatically redistributes available gas among active wells to minimize production impact. This closed-loop integration between compression and artificial lift is a hallmark of mature, well-automated gas lift fields.