2014 37th Annual ASME/ALRDC International Gas-Lift Workshop
Houston, Texas, USA, on Feb. 3 - 7, 2014.
Gas-Lift Workshop
Technical Sessions
Session IV --- Gas-Lift Field Reviews, Experiences, Cases
Paper Proposal
Authors:
Bordalo, Sergio N. – UNICAMP (University of Campinas, Brazil)
Barreto Fo., Manuel A. – PETROBRAS (Brazil)
Pestana, Tiago – formerly at Unicamp
Ochoa Lara, Ismael O.– formerly at Unicamp
Title:
“A Combined Experimental & Numerical Research Program to Develop a Computer Simulator for Intermittent Gas-lift”
Abstract:
Currently, hundreds of petroleum wells, mainly in the northeast region of Brazil, are equipped with Intermittent Gas-Lift systems (IGL), due to the high number of mature fields with low reservoir static pressure.
This paper presents a research program to develop a computer simulator for intermittent gas-lift (IGL); it is a partnership between Petrobras and Unicamp. The research program stands on two legs; one is a computer code, the other is a lab facility. The computer code is a numerical algorithm that attempts to simulate an intermittent gas-lift cycle; the gas-lift cycle is broken down into stages, each one is described by a set of equations that represents the evolution of the physical variables of the cycle along time. The goal of the research is to improve the code to the point that it becomes a reliable simulator of the behavior of the gas-lift cycle, to be employed as a tool for the design of a gas-lift system and to pre-configure its operational parameters; the code should also be used to analyze an existing gas-lift installation in order to diagnose its behavior and to optimize its performance. Depuration of the computer code would involve comparing the simulator outputs to real cycle data obtained under controlled conditions. Since it may be difficult to do that with real wells, a lab facility was built to serve as a testing ground for the numerical simulator. As the research proceeds, the simulator should also be tested against field data.
Over the years, mathematical models based on integral analysis were developed to predict the production of IGL wells. Although these models have evolved, they do not include, in their current forms, models for some important parts and components of the well installation. This research extends the IGL mathematical model, which was first laid out by T. Liao (1991) and later developed by Carvalho Filho (2004), including specific formulations for topics such as the throttling flow regime for the gas-lift valve, the two-phase flow on the production line, the behavior of the pressure upstream of the motor valve during the injection stage, the behavior of the flowing bottom-hole pressure while the standing valve is closed and evaluation of the gas velocity during the decompression stage.
A mathematical model for the conventional IGL was built, based on Liao’s and Carvalho’s models, including the aforementioned new formulations. This new model was used as the core of a computer program, which was extended to simulate various other variants of the IGL commonly employed by the petroleum producers in Brazil: the Gas-Lift with Plunger, the Inverted Gas-Lift and the Gas-Lift with Chamber. An algorithm was written for each variant and simulations were carried out using hypothetical and real data to test the code and the models. A graphical user interface was also designed and built, making it easier for the users to input the well completion parameters and to read and display the results of the IGL simulations.
Despite the advances achieved so far, some important aspects of the operation of the conventional intermittent gas-lift (IGL) remain unexplored or poorly studied, as for instance, the concurrency of cycle stages, as presented by Carvalho (2004), or the stability of cycles due to motor valve timing and adjustment of the gas-lift valve. At the same time, the Zadson pneumatic pump (ZPP) has presented good results in Brazilian fields, motivating studies to develop computational simulators for the design of this method. In the course of the present work, a physical simulator was built for the IGL and ZPP, to validate numerical simulators proposed by previous authors. The laboratory apparatus consists of three operational sets, the first set represents the well-reservoir coupling, the second set is the injection system of compressed gas and the third set is a collection of columns of production of the conventional IGL and ZPP. For measurement, there are pressure transducers located at various points of the laboratory apparatus and tanks for measuring the produced volume. The system of data acquisition and actuation of the gas-lift and motor valves are controlled by a microcontroller board. Tests were performed by changing the operating variables. As conclusions of the study the concurrent stages during the IGL cycle were identified, the fallback was rated for various operating conditions, the stability of the IGL cycles was studied, and an operating map of the conditions of a stable IGL cycle was drawn. In addition, three operating modes of the ZPP were tested.
Further work topics are listed in the paper, since the overall goal of this research is to achieve maximum control of the IGL systems.
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