Gas Lift Optimisation
Gas lift is an artificial lift method where compressed gas is injected into the production tubing to reduce the hydrostatic head of the fluid column, allowing reservoir pressure to push more oil to the surface. It is one of the most widely used artificial lift methods, especially in offshore and high-water-cut fields.
How Gas Lift Works
Gas Compression
Gas (usually produced gas recycled from the separator) is compressed at a central facility and distributed to individual wells via a gas lift header.
Injection via Gas Lift Valves
Compressed gas enters the casing-tubing annulus and passes through gas lift valves (GLVs) at designed depths, aerating the fluid column in the tubing.
Reduced Hydrostatic Head
The injected gas lightens the fluid column, reducing the bottomhole flowing pressure (Pwf). This increases the drawdown (Pr - Pwf), which increases the inflow from the reservoir.
The Gas Lift Performance Curve
The relationship between gas injection rate and oil production follows a characteristic curve: as you inject more gas, oil production increases - but only up to a point. Beyond the optimal gas injection rate, additional gas provides diminishing returns and can actually reduce production due to excessive friction in the tubing.
Example: Well-B12 produces 800 bbl/d with 1.5 MMscf/d of lift gas. Increasing to 2.0 MMscf/d raises production to 950 bbl/d. But increasing further to 3.0 MMscf/d only yields 970 bbl/d - the extra 1.0 MMscf/d of gas gains only 20 bbl/d. That gas is better allocated to another well where it will have a bigger impact.
Field-Wide Optimisation
In a digital oilfield, the total available lift gas is limited by compressor capacity. The optimisation problem is: how do you allocate a fixed volume of gas across N wells to maximise total field oil production?
Equal Allocation (Naive)
Give every well the same gas rate. Simple but wasteful - some wells respond better to gas lift than others.
Optimal Allocation (Digital)
Use each well's performance curve to allocate gas where the marginal oil gain per unit of gas is highest. Solved using gradient-based or LP optimisation.