Gas Lift

 

Each gas lift control loop can be controlled independently via its Gas Lift Control Mode control parameter. Each control mode is described in detail below. Note that a gas lift control loop must be enabled, and a control mode must also be selected before gas lift will be active. Both settings must be configured.

 

Regardless of the control mode, when gas injection is active, PID control loops will always run once per second, even though the Plunger Lift application itself runs on a 5 second interval. This allows the PID to be more responsive.

 

Except for the Injection Steps control mode, the PID loop will always honor the value of the Injection Control Mode configuration parameter in the associated Injection Control Settings configuration group. The Injection Control Mode configuration parameter allows you to choose between Flow Control and Flow with Pressure Override control.

 

 

When Flow is chosen, the PID loop will ignore the pressure in the injection line. When Flow with Pressure Override is selected, the PID will switch from flow control to pressure control when the pressure in the injection line exceeds the value of the Injection Pressure Set Point control parameter. The system will switch back to flow control if the value of the Injection Flow Set Point control parameter is exceeded when controlling via pressure.

 

In the Injection Steps control mode, pressure control with flow override is automatically implemented. This is only state where the Injection Control Mode setting is not respected.

 

In this mode, gas lift functions are disabled.

 

In this mode, gas is injected continuously at the rate specified by the Flow Rate Set Point configuration parameter. The Injection Control Settings that are associated with the gas lift control loop allow you to tune the PID loop. If the Injection Control Mode is Flow then the feedback element is the measured injection flow rate. If the  Injection Control Mode is Flow With Pressure Override, then the feedback element will either be the measured injection flow rate or the injection pressure.

 

This control mode has no integration with the plunger control algorithm. It will simply continuously inject gas at the specified rate. It's the operators job to make sure that the sales valve is the desired state.

 

 

In this mode, gas is injection continuously at the calculated Critical Flow Rate multiplied by the Critical Flow Multiplier, which can be found in the Gas Lift configuration group. In this state, the feedback element is the flow from the well, not the injection flow, and as such, the sales valve must be open so that the PID can work correctly.

 

In applications before version 0.20.x, because the sales valve must be held open, when the Continuous Critical Flow mode is enabled, plunger control will be interrupted and the plunger control loop will move to the Continuous Critical Injection state.

 

Starting in application version 0.20.0, the gas lift program will no longer interrupt plunger control and will not interact with the sales valve. The user is responsible for controlling the sales valve.

 

 

 

In this mode, gas is injected continuously according to configuration of the associated Injection Step Settings configuration group. The PID control mode is pressure with flow override. The pressure set point is controlled by the automatically by the program and the flow set point is configured via the Flow Rate Limit configuration parameter.

 

The sales valve is held open in this state and plunger control is interrupted. When all injection steps are complete, control will automatically be handed back to the plunger control loop. If plunger control is not enabled, nothing will happen, otherwise normal plunger control will start.

 

 

In this mode, gas lift works with the plunger control algorithm to inject gas during the plunger control cycle.

 

Gas can be injected while the well is shut in using the Shut In Injection configuration parameters in the Gas Lift configuration group. When shut in injection occurs, plunger control will be held off until shut in injection is complete.

 

Gas can be injected after the sales valve is opened, but before the plunger arrives using the Pre-Arrival Injection settings. Pre-arrival injection will always end when the plunger arrives, and won't interrupt the plunger control cycle. The injection set point can be configured based on the type of plunger arrival in the previous cycle, or can be automatically adjusted by the program via the Pre-Arrival Injection Optimization settings.

 

Gas can be injected after the plunger arrives using the the Post Arrival configuration parameters in the Gas Lift configuration group. Post arrival injection will always ends when shut in conditions are met and will not interrupt the plunger control cycle.

 

Details are in the flow chart below.

 

 

 

In this mode, gas is injected continuously at a variable rate that is determined by the difference between the calculated critical flow and the well's actual flow. A PID is used to tune the transfer function that controls the injection flow rate set point. The primary value of this PID is the well's measured flow rate and the set point is the calculated critical flow times a configurable multiplier.

 

This mode is a replacement for the Continuous Critical Injection mode and is available in the Plunger Lift application starting in configuration version 1.3.4.  It allows for more predictable operation because instead of trying to control the well's flow rate via the injection valve, we're using an independent PID to come with an injection flow rate set point. The injection PID only controls the injection valve to set the injection flow rate, which is predicable and works smoothly.

 

The PID that is used to determine the injection flow rate in this mode executes once every 5 seconds.

 

Use the Variable Injection configuration settings to set up continuous variable injection.

 

Each gas lift control loop has an associated control parameter that allows for the current Injection Control Mode to be overridden. Details on how to use this can be found in the Gas Lift Control section.