Production-system-analysis methods have been used and are currently used for gas lift design and troubleshooting. These methods, however, have been found to be inadequate in several cases for achieving optimum design or troubleshooting problems. To make up for the inadequacy, a new approach has been applied that uses the Valve Performance Clearinghouse (VPC) database. The new approach joins well performance from production-system analysis and true valve performance from the VPC database to form an integrated system performance tool.
Introduction
Qatar Petroleum’s Dukhan field is onshore Qatar approximately 80 km west of Doha. The number of wells in this field requiring artificial-lift assistance has increased over the past few years, and the trend is likely to continue because of reservoir-pressure decline and rising water cut. The availability of gas coupled with compression and distribution-facility availability has made gas lift a natural choice among various artificial-lift methods. The increasing number of wells using gas lift makes gas lift design and troubleshooting key factors in achieving crude-oil-production targets.
Production-System-Analysis-Based Procedure
Production-system analysis determines a well’s inflow and outflow performance and combines them at a solution node to obtain the well’s system performance. The purpose of gas lift is to lighten the flowing production gradient by injecting gas into the production string of a well. Gas injection into the production string improves the well’s outflow performance by increasing the gas/liquid ratio (GLR) of the production stream from the formation GLR to an objective GLR (OGLR). There are two phases to gas lift operation: unloading and production.
Unloading. The objective of the unloading phase is to leverage gas-injection pressure by sequentially injecting gas through unloading valves (positioned shallower than the operating valve) until the operating valve is uncovered.
Production. The objective of the production phase is to maintain single-point injection through the operating gas lift valve over a range of gas-injection rates while achieving the target production rate.
Inadequacies. A production-system-analysis-based procedure makes some assumptions that could be incorrect or inaccurate.
One assumption is that gas lift valves snap open and snap closed at their opening and closing casing pressures, respectively. For determining valve port size to pass a required volume of gas, first the valve port is considered to be fully open and, second, an unobstructed flow path is assumed during the gas passage. These assumptions could be incorrect, depending on the way the valve is constructed and the pressure being applied to the valve bellows. In most cases, a gas lift valve is rarely fully open while passing gas. Moreover, the valve stem, downstream restrictions, and the reverse-flow check usually obstruct the gas flow to some degree.
For determining valve-operating pressure, a static-force-balance equation is used. This equation is good only when the valve is closed and is considerably inaccurate when the valve is open. This is because the throttling effect of the bellows spring force is not considered.
Gas lift valves, therefore, show behavior that is markedly different from the assumptions made in the production-system-analysis-based design procedure. As a result, port sizes and operating pressures determined from such procedures turn out to be inadequate in some cases for achieving proper unloading, optimum production, or successful troubleshooting.
True Valve Performance
True valve performance refers to accurate prediction of gas passage through a gas lift valve at pressure and temperature conditions similar to those encountered during unloading and production phases of a gas lift well. This is possible as a result of correlations developed from actual dynamic tests and by use of a property of gas lift valves called “load rate.” Load rate is a measure of a valve stem’s resistance to movement and relates to the amount of opening the valve will attain for a given annulus and tubing pressure across the valve. True valve performance is made available in the form of a database and application program through license from the VPC. The VPC is a not-for-profit joint industry project sharing expenses to establish gas lift valve performance data and correlations.
The Integrated Approach. The integrated approach extends the concept of the operating point for a well to an operating point for the integrated system of well and gas lift valve. An operating point is defined as the point at which the ability of the inflow mechanism to deliver fluid to a specific place is matched by the ability of the outflow mechanism to remove it. The operating point for a well is based on inflow and outflow performance. Similarly, the operating point for the integrated system is defined on the basis of well and valve performances.
Well Performance. The operating point for a well corresponds to the liquid-flow rate and tubing pressure at which the outflow performance matches inflow performance. This operating point (liquid-flow rate and tubing pressure) can be obtained at the position of an active gas lift valve for a given well condition through production-system analysis. Because gas injection through a gas lift valve improves outflow performance of the well by increasing GLR above the point of gas injection, production-system analysis is used to obtain different operating points (or no operating point) at the position of active lift valves with changing gas-injection rates.
Valve Performance. The operating point for a gas lift valve corresponds to the gas-injection rate and tubing pressure at the active gas lift valve for a given valve type and port size and for given annulus-pressure conditions. The VPC database is used to obtain different operating points for the gas lift valve with different gas lift injection rates. The locus of all such points when plotted as gas-injection rate vs. tubing pressure gives a valve-performance curve. The valve-performance curve has subcritical, near-critical, and throttling regions. The subcritical region represents the stage of a valve’s operation when insufficient pressure differential across the valve does not allow it to pass the volume of gas it is capable of passing. In the throttling region, a valve’s operation is highly sensitive to small changes in tubing pressure, causing obstructed or interrupted gas passage and unstable operation. A valve operating in the near-critical region enables almost steady gas passage and stable operation. Such valve-performance curves can be generated for different valve parameters such as pressure conditions and port sizes.
Integrated-System Performance. The point at which the well-performance curve matches the valve-performance curve provides the operating point for the integrated system of well and gas lift valve. Sensitivity analysis of the integrated system is then performed against changes in well conditions and valve parameters. Such analyses reveal whether the operating point for a given well condition and valve parameter is stable. They also enable adjusting valve parameters (pressure conditions and port sizes) as required to obtain a stable operating point.
Design and Troubleshooting
The integrated-system performance is used in the creation of a new design or in troubleshooting an existing design for both production and unloading phases of gas lift operation.
Unloading. Unloading valves are designed such that a lower unloading valve should use gas-injection pressure approximately 20–50 psi less than the upper valve. This recommendation assumes that the gas-injection rate at surface is less than approximately 80% of the combined total of gas-injection rate through the upper and successive lower unloading valves when the lower valve uncovers and valve transfer operation takes place. If this happens, gas-injection pressure in the annulus drops to closing pressure of the upper valve and the upper valve closes. This allows unloading to switch from upper to lower valve after some moments of simultaneous operation, without surface adjustments. However, if the gas-injection rate through the unloading valves is less than the surface injection rate, then the gas-injection pressure will not drop but will continue to increase. This issue can result in multipointing and an erratic unloading process. The integrated-system performance enables examining such issues and adjusting the unloading-valve design (spacing, port size, and operating pressures) to allow smooth unloading operation initially and during restarts.
Production. Operating-valve depth and required gas-injection rate to achieve OGLR and target liquid-production rate are determined clearly from a production-system-analysis-based procedure. Choices of gas lift valve types are usually set by the available inventory. However, valve port size and operating pressures are determined on the basis of integrated-system performance. Port size and operating pressures should be such that the operating point falls within the near-critical region of the integrated-system performance for stable operation and provides adequate gas-injection rate across the expected range of well conditions. The same approach is used for troubleshooting an existing design of operating valve if its performance is suboptimal.
Adjustment Options. The key objective of designing or troubleshooting gas lift is to ensure that the production and unloading phases of the well are stable and smooth. Sensitivity of integrated-system performance is examined by means of several options to achieve this objective. These options include
- Lowering valve set pressure
- Increasing valve-port size
- Increasing gas-injection pressure (if available)
In addition, the option of using a range of temperatures is also available. This is particularly relevant for unloading-valve design because there remains an uncertainty about the temperature at the valve during the unloading phase. During the initial stage of unloading, when there is overbalance against the formation and only the wellbore fluid is being lifted without any flow from the formation, the temperature at the unloading valve is given by the geothermal gradient. However, when the wellbore fluid is sufficiently unloaded to create underbalance against the formation and if the formation fluid (usually a volume of brine lost into the formation during workover, or formation water in case of restart) flows back, temperature at the valve rises. A range of temperatures, therefore, can be used to examine the unloading-valve performance, depending on the unloading stage.
Conclusions
Design and performance management of gas lift wells are key factors in achieving crude-oil-production targets of Dukhan field. Integrating true valve performance into production-system-analysis-based procedures provides an integrated-system performance for the combined system of well and gas lift valves. Analyzing the integrated-system performance for a range of well conditions and valve parameters enables effective gas lift design and troubleshooting. Application of this approach for Dukhan field has resulted in superior well performance, unlocked incremental oil potential at negligible cost, and extended the producing life of wells.
This article, written by Special Publications Editor Adam Wilson, contains highlights of paper IPTC 16656, “Integrating True Valve Performance Into Production-System-Analysis-Based Gas Lift Design and Troubleshooting for Dukhan Field, Qatar,” by Sanjay K. Singh, James W. Hall, SPE, Eraky Khalil, Balsam Al-Marri, and Reem Al-Abdulla, Qatar Petroleum, prepared for the 2013 International Petroleum Technology Conference, Beijing, 26–28 March. The paper has not been peer reviewed. Copyright 2014 International Petroleum Technology Conference. Reproduced by permission.