A New Approach for Gas Lift Method Optimization for an Iranian Oilfield Fateme Mehran 1 * , Kamyar Movagharnejad 1 , Ali Didanloo 2 , 1- Mazandaran University, Faculty of Chemical Engineering 2- Iranian Society of Petroleum Engineers Abstract Optimization and simulation for determining the optimal injection rate in a well operating under gas- lift are based on balancing the buoyancy imp
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  A New Approach for Gas Lift Method Optimization for an Iranian Oilfield Fateme Mehran 1 * ,  Kamyar Movagharnejad 1  ,  Ali Didanloo 2  , 1- Mazandaran University, Faculty of Chemical Engineering 2- Iranian Society of Petroleum Engineers Abstract Optimization and simulation for determining the optimal injection rate in a well operating under gas- lift are based on balancing the buoyancy improvements from the additional gas in the production stream with the corresponding increase in the flowline  back- pressure and its negative impact on the overall deliverability. Such an approach, while useful in determining the maximum production improvement achievable for an individual well, ignores a number of practical and relatively significant aspects that tend to complicate the real- world application. Another major complication arises from the limitations associated with the available compressors. Typically, these facilities are designed to handle the requirements identified when gas- lift is first introduced. As the field depletes, the requirement for lift gas increases. Inevitably, the operator is challenged with the classical gas- lift allocation  problem: how does one allocate limited available injection gas to maximize overall field  production Other factors that influence the economics of a field- wide strategy include the reservoir depletion behavior, the effect gas-lift of varying water- cut, the impact of both capital and operating costs, and the detailed performance characteristics of the compressor units. This paper presents an investigation on different effect of various parameters to enhance production rate with gaslift for one of the Iranian oil field, using a simulation software package for different pressure calculation of two phase in vertical and horizontal stream that is, Pipesim™. The results obtained on Shadegan oil field are  plotted at the end of this paper.  Introduction Continuous gas- lift is one of the more common forms of artificial lift in oil production. Gas- lift is effective over a wide range of operating conditions, is relatively inexpensive and simple to install, and requires less maintenance especially when compared to alternatives such as electrical submersible pumps. The mechanism of gas- lift is straightforward. Gas at a relatively high pressure in the casing is injected into the tubing string to lighten the fluid column by aeration, until the reduction in the flowing bottom hole pressure creates the pressure differential across the sand face needed to achieve the desired production rate This paper first simulate the gaslift system of single-wells then evaluating and comparing the effect of various operating and design parameters on gaslift system for enhancing production rate.  Next, the paper addresses the problem of limited gas allocation. The unavailability of adequate injection gas is a common problem with older gas- lift installations. Original design conditions do not necessarily account for adequate reservoir depletion effects. The net result of this under- design is limiting injection gas capacity as the reservoir depletes, which invariably translates into a gas allocation problem for the operator. The requirement is to maximize production with the limited compression resources by adjusting a controllable set of injection rates to the production wells. Gas-lift Mechanism Gas-lift is one of the method to enhance oil recovery. gas is injected from tubing or annulus and reduces FBHP (flowing bottom hole Pressure) .   Mixing gas and oil cause reduction in specific gravity , gradient and hydrostatic  pressure. finally injected gas shall be separated at surface and can be use for re-injection.   Effective parameter on gas-lift optimization  Reservoir Pressure Injection Depth Tubing & Casing Size Productivity Index (PI) Situation of Gas-lift (Tubing or Casing ) Gas Cost Well Sketch Water Cut % GOR Temperature (Some Times)  Advantages of gas-lift : High rates from high PI wells Flexibility - easy with which rates can be changed    Can handle large volume of solids    Well intervention easy    Gassy wells can be lifted any well configuration , i.e vertical, deviated and horizontal Disadvantages of gas-lift : High initial investment Limit to drawdown applied Availability of lift gas  Not efficient at lifting emulsions and highly viscous crudes Potential for hydrate formation Production casing must be able to withstand lift gas pressures Safety aspects - high pressure gas Cannot effectively produce deep wells to abandonment Case Study : Shadegan Oil field in Iran The Shadegan structure is located in the South West part of the Dezful Embayment and is about 19 km long and 3 to 4 km wide at the water-oil contact (WOC) depth of the Upper Asmari reservoir. The field was discovered in 1968 by drilling of well SG-01.  The Shadegan Field contains two separate reservoirs, the Upper and Lower Asmari,  both of which are undersaturated oil reservoirs. The reservoir pressure decreased before  production in both the Upper and Lower Asmari. This was due to pressure interference with nearby fields, especially with the Ahwaz Asmari reservoir, through a common active aquifer. Shadegan oil production was started in 1989, with an initial maximum production rate of 32 MSTB/D. This rate was increased with the drilling of additional wells and, at the time this project was initiated in August 2001, reservoir oil production rate was about 50 MSTB/D. By March 2002, cumulative production from the Upper and Lower Asmari reservoirs had reached to 97.5 and 114.5 MMSTB respectively. Up to the start of this project in August 2001, a total of 8 wells had been drilled in the Shadegan field, most of them located on the crest of the structure. Of these 8 wells, SG-01 is dually completed as an oil producer in both the upper and lower reservoirs. Well SG-03 has been completed in the upper reservoir as an oil producer and in the lower reservoir as an observation well. Well SG-04 was drilled into the aquifer and is suspended for a future workover. Wells SG-05 and SG-08 are only completed in the upper reservoir while wells SG-02, SG-06 and SG-07 are only completed in the lower reservoir, all as oil producers. Gas-lift System Simulation and Optimization for Shadegan Oilfield Table 1  shows the relevant data for a sample well of Shadegan oil field on gas-lift  producing from a reservoir and flowing into surface separation and treatment facilities. This well will be used for subsequent analysis. After well modeling by Pipesim™, the model must be matched with real state So by existing test data of well and changing the equation of state , correlation , friction and hold up factor in vertical line ,the model obtained closes to real model . These matching data shows in Table 2. Gas- lift performance is based on a balance between improved buoyancy in the tubing string and increased back- pressure in the multiphase flow stream.
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