Gas-lift Instability Resulted Production Loss and Its Remedy by Feedback Control: Dynamical Simulation Results

Abstract

Abstract Gas-lift wells can be unstable due to various reasons. The fluctuating, and sometimes chaotic unstable production behavior affects many offshore fields, particularly at their decline stages. The hazardousness of the flucuation to operation safety and smoothness has been awared. But the production reduction due to the instabilities has not been widely addressed. In this paper, a commercial available dynamical multiphase flow simulator is used to conduct investigation on the production loss. Two types of instabilities, casing heading and density wave instability, which both can result in production loss are simulated. Two hypothetic wells, abstracted from typical North Sea gas-lift wells, are built within the simulator. Parametric simulation study is then performed by changing the well settings. The average production rate of the unstable well can be obtained from the simulation and compared with the steady state prediction. The difference of the two results gives the production loss due to instability. Furthermore, the paper presents simulation results of applying feedback control to the unstable wells. The production loss is reduced as an effect of stabilization by feedback control. Introduction Gas-lift is one of the most widely used artificial lift method in oil production. Economically optimised gas-lift design requires that gas-lift wells be operated at the up-slope of their lift performance relationship curve[7]. This leads the gravity effect becomes the dominant factor of gas and liquid two-phase flow in the tubing. Two-phase vertical flow under gravity domination is often unstable. This is particularly true in the gas-lift wells. The instabilities of gas-lift can either be marcroscopic or microscopic. By marcroscopic, we mean that the instability has a systematic background. And by microscopic on the other hand, we refer to the local instability such as hydrodynamic slugging, which is not considered as an operation problem in most situations except that it might be amplified by other factors. Normally, it is the marcroscopic instability that results in the severe unstable flow in the wells. For many years, it has been observed that continueous gas-lift wells can be seriously unstable and sometimes even behave as an intermittent gas-lift due to instabilities with systematic background. Large fluctuation in the well flow rate can result in poor separation, limit the production capacity and cause flaring and shutdown. Besides the operation problems, production loss is another important issue for unstable gas-lift wells. The field tail production will be prolonged due to the reduced production rate since it will take more time to get the same recovery. This will tremendously increase the cost of operation. The concentraion of this paper is to investigate the gas-lift instability resulted production loss since this has not been widely addressed and caused enough attention. Particularly, when the downstream receiving facitlity and process can handle the flow fluctuation, one seldom has the motivation to deal with it. Therefore, the production reduction is ignored. This paper emphasizes the notion that production loss is the norm for oscillating wells and the instabilities have to be seriously treated even when they do not cause any operation problem. It also demonstrates that the pragmatic method such as static choking performed by skilled operators can only stabilize the well at a cost of large production reduction. However, dynamical choking based on feedback control can not only stabilize the well but also save part of the production loss. Thus, the advantage of applying feedback control to suitable unstable wells is promoted. Gas-lift Instabilities Two systematic instability mechanisms that can result in serious unstable phonomena are discussed in this paper. One is casing heading, in which the gas injection through gas-lift valve is non-critical and annulus flow dynamics is involved in the instability. Another mechanism is called density wave instability, in which the gas injection is critical and thus flow in annulus is isolated from any instability taking place in the tubing. Since this instability has never been discussed in prtroleum gas-lift application, we borrow this name from the similar phenomenon observed in airlift pumps that are often used in the mining industry[8][12].