Summary
While casing-heading instability in single gas lift wells has attracted a
lot of attention, gas-distribution instability in dual gas lift wells has not.
In this paper, we present a simple, nonlinear dynamic model that is shown to
capture the essential dynamics of the gas-distribution instability 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 valves
governing the produced flows from the two tubings. Optimal steady-state
production is shown to lie well within the unstable region, corresponding to a
gas distribution between the production tubings that cannot be sustained
without automatic control. A simple control structure is suggested that
successfully stabilizes the gas-distribution 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 (gas lift) rates among the most widely used
of such methods. 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 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 et al. 1998; Jansen et
al. 1999; Dalsmo et al. 2002; Boisard et al. 2002; Hu and Golan 2003; Eikrem et
al. 2003; Aamo et al. 2005). Automatic control may or may not require downhole
measurements. If downhole information is needed by the controller, the use of
soft-sensing techniques may alleviate the need for downhole measurements. In
Aamo et al. (2005), downhole pressure is estimated on line using a simple
dynamic model and measurements at the wellhead only. The estimated pressure is
in turn used in a controller for stabilizing the casing-heading
instability.
Understanding and predicting under which conditions a gas lift well will
exhibit flow instability is important in every production-planning situation.
This problem has been addressed by several authors by constructing stability
maps [i.e., a 2D diagram that shows the regions of stable and unstable
production of a well (Poblano et al. 2005; Fairuzov et al. 2004)]. The axes
define the operating conditions in terms of the gas-injection rate and, for
instance, the production-choke opening or wellhead pressure.
A dual gas lift well is a well with two independent tubings producing from
two different hydrocarbon-bearing layers and sharing a common lift gas supply.
The injection gas is supplied through a common casing and injected into the
tubings through two individual gas lift valves. A sketch of a typical system is
shown in Fig. 1. The dual gas lift well introduces a new instability
phenomenon: the gas-distribution instability. This relates to the fact that
under certain operating conditions, it is impossible to sustain the feed of
injected gas into both tubings. Instead, all the injected gas will eventually
be routed through one of the gas lift valves. As a consequence, the second
tubing produces poorly or not at all, decreasing the total production
substantially. There are few reports, if any, on automatic control of dual gas
lift wells, although Boisard et al. (2002) briefly mentions an application.
In this paper, we present a simple, nonlinear dynamic model that captures
the essential dynamics of the gas-distribution instability. It is an extension
of the model for a single gas lift well presented in Eikrem et al. (2003) and
Aamo et al. (2005). Using the model, we generate a stability map for a
single-point dual gas lift well, and present a control structure for
stabilizing the system at open-loop, unstable setpoints. The performance of the
controller is demonstrated in simulations using the model, but more
importantly, stabilization is also achieved in laboratory experiments.
This paper is organized as follows. In the Mathematical Model section, a
nonlinear dynamic model applicable to dual gas lift wells is presented,
followed by a discussion on instability mechanisms and the generation of
stability maps in the Instability Mechanisms and Control section. The stability
analysis is based on computing eigenvalues for the linearized model,
accompanied by simulations using the nonlinear model. The proposed control
structure is presented in the Automatic Control segment of that section, and
experimental results using a gas lift laboratory located at the U. of
Technology—Delft are shown in the Laboratory Experiments section. The paper
ends with discussion and conclusions in the Conclusions section.
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
23 March 2005
- Revised manuscript received:
17 June 2005
- Manuscript approved:
23 June 2005
- Version of record:
20 May 2006
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