Summary
In this paper, we present a simple nonlinear dynamic model that is shown to
capture the essential dynamics of the casing-heading instability in gas lift
wells despite the complex nature of two-phase flow. Using the model, stability
maps are generated showing regions of stable and unstable settings for the
production valve governing the flow of produced oil and gas from the tubing.
Optimal steady-state production is shown to lie well within the unstable
region, corresponding to an oil-production rate that cannot be sustained
without automatic control. Three simple control structures are suggested that
successfully stabilize the casing-heading instability in simulations, and more
importantly in laboratory experiments.
Introduction
Artificial lift is a common technique to increase tail-end production from
mature fields, and injection of gas is among the most widely used methods. Gas
is injected into the tubing as deep as possible and mixes with the fluid from
the reservoir (see Fig. 1). Because the gas has lower density than the
reservoir fluid, the density of the fluid in the tubing and, consequently, the
downhole pressure decrease. As the downhole pressure decreases, the production
from the reservoir increases. The lift gas is routed from the surface and into
the annulus, which is the volume between the casing and the tubing, and enters
the tubing through a valve, or an injection orifice. Backflow from the tubing
into the annulus is not permitted by this valve. Gas lift can induce severe
production flow oscillations because of casing-heading instability, a
phenomenon that originates from dynamic interaction between injection gas in
the casing and the multiphase fluid in the tubing. The fluctuating flow
typically has an oscillation period of a few hours and is distinctly different
from short-term oscillations caused by hydrodynamic slugging. The
casing-heading instability introduces two production-related challenges.
Average production is decreased as compared to a stable-flow regime and the
highly oscillatory flow puts strain on downstream equipment. Reports from
industry as well as academia suggest that automatic control (feedback control)
is a powerful tool to eliminate casing-heading instability and increase
production from gas lift wells (Kinderen and Dunham 1998; Jansen et al. 1999;
Dalsmo et al. 2002; Boisard et al. 2002; Hu and Golan 2003; Eikrem et al. 2006;
Aamo et al. 2005).
Understanding and predicting conditions under which a gas lift well will
exhibit flow instability is important in every production-planning situation.
This problem has been adressed by several authors by constructing stability
maps, [i.e., a 2D diagram that shows the regions of stable and unstable
production of a well (Eikrem et al. 2006; Poblano et al. 2005; Fairuzov et al.
2004)]. The axes may define the operating conditions in terms of gas-injection
rate and production-choke opening or wellhead pressure.
In this paper, we present three different control structures for stabilizing
casing-heading instability in gas lift wells. Stability is analyzed for each
controller, and it is shown how feedback control stabilizes performance, at
least locally, around some operating point. The performance of the controllers
is demonstrated in simulations, but more importantly, stabilization is also
achieved in laboratory experiments.
The paper is organized as follows: A desription of the laboratory facilities
that are used in this work is given. Thereafter, the dynamics of casing-heading
instability are discussed, and suitable models for analysis and design are
proposed. The proposed control structures are presented along with stability
analysis, closed-loop simulations, and experimental results. The paper ends
with conclusions.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
4 January 2006
- Revised manuscript received:
14 November 2006
- Manuscript approved:
2 July 2007
- Version of record:
20 May 2008
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