Stability Maps for Continuous Gas-Lift Wells: A New Approach to Solving an Old Problem

Abstract

Abstract Continuous flow gas-lift wells are susceptible to hydrodynamic instability (heading), which may cause cyclic variations of the wellhead pressure, oil and gas flow rates. Gas-lift instability is a reason of many operational problems, for example, compressor shutdowns caused by pressure and liquid flow rate surges, difficulties in the operation of low pressure separators, and excessive gas consumption. Stability problems in complex multiphase systems can be solved using stability maps. A stability map is a plane (2D) diagram that shows the regions of stable and unstable operation of the system, as well as its operating limits. In this paper theoretical and experimental stability maps for gas-lift wells are presented. Fild test were conducted to investigate the flow stability in a deep offshore gas-lift well. Different gas-lift stability criteria proposed in the literature are compared. Based on this study, recommendations on the selection of gas-lift stability criteria were developed. Examples of gas-lift stability map applications are also given. Introduction Heading is a common problem in the operation of continuous flow gas-lift installations. This circumstance is taken into account in the existing design rules for gas-lift wells. The size of the orifice valve is selected so that the pressure drop across the orifice be of 100 psi for a given gas injection rate. For many gas-lift wells, this will ensure stable operational conditions. This recommendation is based on experience gained from the operation of gas-lift installations and sometimes it can be too conservative. Flow stability becomes of particular interest in designing the gas-lift systems used for large scale oil production from offshore fields. The associated gas with a high percentage of H2S is usually processed onshore and then is transported to the production platforms through long pipelines. An increase in the pressure drop across the orifice valve results in the increase in the pressure in the pipelines carrying the lift gas and compressor horsepower. This, in turn, may lead to a significant increase in both CAPEX (due to the increase of compressor size and sometimes pipe wall thickness) and OPEX (due to the increase of gas compression costs). In this case it is very important to predict accurately the limits of stable operation of gas-lift wells to reduce costs. Several techniques have been proposed to analyze gas-lift stability. Grupping et al.1,2 developed a transient model of the gas-lift system which can be used to perform non-linear stability analysis. The model predicts variations of principal parameters of the system (pressures in the tubing and casing, and the gas injection rate) in time. However, it is not convenient to use such an approach at the design stage when it is necessary to analyze the flow stability for different operating conditions. Numerical simulations need to be performed for all possible combinations of operating and design parameters. This procedure requires a significant amount of time. Asheim3, Alhanati et al.4 and Blick et al.5 proposed simple criteria that can be used to check the stability of the gas-lift system under a given operating condition. The operating state of the gas-lift system is generally specified by many parameters: fluid (crude oil and lift gas) properties, average reservoir pressure, inflow performance, well completion, wellhead pressure, gas-lift valve size, injection depth and the lift gas rate. Most of these parameters may vary within certain limits. The designer has to perform many calculations (the sensitivity analysis) to assure stable flow. Although the stability criteria are algebraic inequalities, the sensitivity analysis also may require a significant amount of time in designing gas-lift installations. Recently, Poblano et al.6 based on the Asheim3 and Alhanati et al.4 stability criteria developed stability maps for gas lift wells. A stability map is a plane (2D) diagram that shows the regions of stable and unstable operation of the system, as well as its operability limits. Stability maps have been widely used in the analysis of two-phase flow instabilities in nuclear-power systems7. This approach may also be useful in understanding of gas-lift system behavior.