172271 Experimental Investigation of Gas Lift Instability and Dynamic Regulation to Control It Author/presenter: Christer Andre Larsen, NTNU Co-author:

172271

Experimental Investigation of Gas Lift Instability and Dynamic Regulation to Control It

Author/presenter: Christer Andre Larsen, NTNU

Co-author: Harald Arne Asheim, NTNU

Outline

• Short introduction to gas lift and gas lift instability

• Description of stabilization procedure

• Description of laboratory model

• Description of the experimental procedure

• Results

• Discussion

• Conclusion

• Acknowledgements

Slide 2

172271 · Experimental Investigation of Gas Lift Instability and Dynamic Regulation to Control It · Christer Andre Larsen

Basics of gas lift

• Pressurized gas is supplied to annulus

• Injected down hole into the production tubing

• The injected gas is mixed with the production fluid, reducing the volumetric density

• Decreased volumetric density increases flow

Slide 3


Gas lift instability

• Casing Heading:

• Small pressure variations lead to a dynamic response from annulus

• Large fluctuations in pressure and flow

• Whole production system contributes to the instability

• Can lead to:

• Safety hazard, Production loss, flaring, process shutdown

Slide 4


Gas lift instability, cont.Slide 5


• Typical production during Casing Heading instability with the laboratory rig:

New approach to reduce the fluctuations

• The pressure variations has a natural frequency, which depends on volume and production rate

• By inducing a new frequency to the system, a destructive interference can occur

• In this case this is accomplished by successive opening and closing of the tubing outlet valve or the gas injection valve at a predetermined frequency

• Its effect is investigated by a laboratory rig

Slide 6


Laboratory facility

• Air lifts water up a 7.2 m long, 4.2 m high tubing with a 25 mm inner diameter

• A 20 L tank acts as the annulus volume

• Different valves and transmitters regulates and measures the flow in real time

• Data is logged for analysis

Slide 7


Experimental procedure

• Water is filled up to a predetermined height

• Supply and injection of gas is started

• Supplied with a constant rate monitored by a rotameter

• Electronic measurement of water inflow rate, annulus pressure and injection rate starts

• Natural frequency of the instability is found by Fourier analysis, and a control of either tubing outlet valve or production valve is started

• Data is logged for analysis throughout the experiment

Slide 8


Results

• Control of tubing outlet valve:

• Natural frequency of 0.034 Hz was found (30 sec. period)

• Controlled by a 60 % reduction in valve opening with 0.1, 0.3, 0,7 Hz closing frequency

• Up to 90 % reduction in oscillations depending on control frequency

• No production decrease after control is initiated

Slide 9


Slide 10

0.1 Hz closing freq.



Results

• Control of injection valve:

• Natural frequency of 0.034 Hz was found as before

• Controlled by a total closing of valve opening with 0.1, 0.5, 0,7 Hz closing frequency

• Up to 80% reduction in oscillations depending on control frequency

• 37% production increase after control is initiated

Slide 11





Discussion

• Scaled down models should be viewed with sobriety

• The laboratory model shows similar trend to reality, giving credibility to the results

• Control of tubing outlet valve most realistic control method

• Both control methods improved stability, with degree of stabilization depending on valve closing frequency

• Best stabilization occurred when the valve control frequency was dominant in the frequency spectrum

Slide 13


Conclusion

• The measurements showed that frequency control may be imposed by manipulation of either the injection valve, or the tubing outlet valve

• Imposing a frequency may reduce pressure- and flow variation up to 80-90% and increase production rate up 37%

• The measurements identified destructive frequencies that eradicated the natural instability

• Highest rate improvement was achieved by inlet valve control. However, as the tubing outlet valve is easier accessible, it may be of equal practical interest.

Slide 14


Acknowledgements / Thank You / Questions

Presented by the authors are grateful to Aage Sivertsen for help in instrumentation. Yuri Ivanov and Alexei Ctovas provided substantial

assistance to the Russian translation.

Slide 15

Documents

172271 Experimental Investigation of Gas Lift Instability and Dynamic Regulation to Control It Author/presenter: Christer Andre Larsen, NTNU Co-author: