1. 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

2. 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

3. 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 172271 · Experimental Investigation of Gas Lift Instability and Dynamic Regulation to Control It · Christer Andre Larsen

4. 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 172271 · Experimental Investigation of Gas Lift Instability and Dynamic Regulation to Control It · Christer Andre Larsen

5. Gas lift instability, cont. Slide 5 172271 · Experimental Investigation of Gas Lift Instability and Dynamic Regulation to Control It · Christer Andre Larsen Typical production during Casing Heading instability with the laboratory rig:

6. 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 172271 · Experimental Investigation of Gas Lift Instability and Dynamic Regulation to Control It · Christer Andre Larsen

7. 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 172271 · Experimental Investigation of Gas Lift Instability and Dynamic Regulation to Control It · Christer Andre Larsen

8. 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 172271 · Experimental Investigation of Gas Lift Instability and Dynamic Regulation to Control It · Christer Andre Larsen

9. 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 172271 · Experimental Investigation of Gas Lift Instability and Dynamic Regulation to Control It · Christer Andre Larsen

10. Slide 10 0.1 Hz closing freq. 0.3 Hz closing freq. 0.7 Hz closing freq.

11. 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 172271 · Experimental Investigation of Gas Lift Instability and Dynamic Regulation to Control It · Christer Andre Larsen

12. 0.1 Hz closing freq. 0.5 Hz closing freq. 0.7 Hz closing freq.

13. 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 172271 · Experimental Investigation of Gas Lift Instability and Dynamic Regulation to Control It · Christer Andre Larsen

14. 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 172271 · Experimental Investigation of Gas Lift Instability and Dynamic Regulation to Control It · Christer Andre Larsen

15. 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