Gas Lift Optimization of Oil Producing Wells Using Proper Nodal Analysis
CHAPTER ONE
Objectives of study
As discussed earlier, the producing capacities of oil wells reduces as the oil field matures (function of time) due to the combined effects of interrelated factors that affects the well’s performance and economic recovery. To address this issue, optimization of the well (production system ) becomes necessary to maximize the well’s production.
One of the most used techniques for optimizing the oil production systems, considering its verified effectiveness and worldwide level trust worthiness is the Nodal analysis (Beggs et al, 1991). In order to optimize the production system using this technique, it is necessary describing the production system, making emphasis on the components of the production system in order to determine the production capacity of the well. The Nodal analysis allows to evaluate the performance of a completions of production, calculating the relation of the flow of production and the pressure drop that will occur in all its components, allowing to determine the flow of oil or gas that can produce a well bearing in mind the geometry of the perforation and increasing the rate of production to a low cost. Though gas lift optimization will always improve the production capacity, there is a need to identify the gas injection rate and tubing size that will lead to a maximum production from the well.
- The project will aim to identify the factors and parameters that affect the well’s performance.
- The project will aim to apply the Nodal analysis technique in simulating flow and analyzing the production system of a naturally flowing well and identifying the pressure losses in the producing system.
- The project will also aim to design a gas lift system (model) for the naturally flowing well and optimizing the production system to improve the production system via the gas lift system model.
- In addition to these, this work will aim to compare the results in terms of production capacity (production rate) for a naturally flowing well and the optimized results of the gas lifted well.
CHAPTER TWO
LITERATURE REVIEW
Literature review
production system
The production system comprises of:
- Reservoir (inflow performance relationship)
- Wellbore (completions, tubing etc.)
- Surface facilities (flow lines, separator, pipelines, etc.)
Production systems can be very simple to complex:
- Simple- Reservoir, completion, tubing, surface facilities
- Complex – Artificial lift system, water injection and multiple wells.
Movement or transport of reservoir fluid from reservoir to surface requires energy to overcome the frictional losses or the pressure drop. The pressure drop of the fluid at any time would be the initial fluid pressure minus the final fluid pressure.
∆p = PR-Psep
The amount of oil and gas flowing into the well from reservoir depends on pressure drop in the piping system. Piping system pressure drop also depends on amount of fluid flowing through it. The entire production systems must be analyzed as a unit.
Production optimization
Production optimization refers to all the engineering techniques and technologies that aim at maximizing the economic recovery of hydrocarbon resources from underground reservoirs by the optimal use of existing or new production wells and facilities.
A typical objective of a production enhancement study will be to optimize the use of existing wells and facilities for maximum field drainage. The optimization study involves the application of the nodal analysis technique.
Nodal analysis
The deliverability of a well can be severely restricted by the performance of only one component in the system. If the effect of each component on the total system performance can be isolated the system performance can be optimized in a more economical way.
The system analysis method is utilized in considering the whole production system as a single unit. Then a point or node is chosen in the system where input and output pressures are the same. The system analysis method is called “Nodal analysis”.
Nodal analysis method has been applied for many years to analyze the performance of systems comprising interactive components. Complex piping network and centrifugal pumping systems are all analyzed using this method. Its application to well production system was first proposed by Gilbert in 1954 and discussed by Nind in 1964 and Brown in 1978. System analysis models each component using equations or correlations to determine pressure losses (∆p) through the component as a function of flow rate (Q), i.e.
Q = f (∆p)
System analysis comprises flow through the following components (as can be seen from fig 2-1):
- Reservoir (porous media)
- Completion (perforation; stimulations; gravel pack etc.)
- Conduit (tubing or casing) + downhole restrictions (safety valves and chokes)
- Artificial lift systems such as pumps, gas lift valves etc.
- Surface flow line and restrictions + separator.
CHAPTER THREE
METHODOLOGY
INTRODUCTION TO THE METHODOLOGY
This project work is aimed at analyzing in detail the components or nodes in the producing system by the use of Nodal Analysis Technique to optimize production. The nodal / system analysis is used in designing new systems and redesigning old systems.
The methodology employed in this study is more of an analytical approach in which a simulation study of the reservoir and the producing well is carried out. The aim of the simulation study is to ensure accurate prediction of reservoir and well characteristics that are influential to the well’s performance.
This methodology, employs the use of an analytical software, PROSPER (PROduction System PERformance analysis software) used for well modeling, design and performance analysis.
DATA SOURCE AND DATA ANALYSIS
A well was selected from the ABURA field in the Niger Delta for this analysis and optimization scheme. Data for this system analysis were gotten from a company’s well report and the field monthly / yearly summary report file.
In reviewing the data, identification of existing problems are crucial in making the best decision with regards to selecting and prioritizing well candidate.
For each of the selected wells, the productivity ratio (PR) or flow efficiency (FE) is computed as:
CHAPTER FOUR
SYSTEM DATA, RESULTS AND ANALYSIS
In this chapter, results obtained from the PROPER model generated will be analyzed and discussed. The PROSPER main screen is divided into 5 sections. They are:
- Options
- PVT Data
- IPR Data
- Equipment Data
- Analysis Summary
OPTIONS
In this section, the fluid description, well description, artificial lifts method, calculation type, well completion method, and user’s information are inputted. This is shown in fig. 3.6.1.
CHAPTER 5
CONCLUSION
Well X-05 has been successfully calibrated and optimised using PROSPER system analysis software tool. The well’s successful calibration shows the effectiveness of the calibration matching procedure aimed at designing a well model to exactly reproduce the observed well performance with less than 0.02% variation in pressure drop calculation and about 0.5% in flow rate calculation. With the aid of PROSPER we were able to utilize the technique of Nodal analysis to analyse majority of the components in the production system and their effects on the production performance of the well under study. This shows the efficiency of using Nodal Analysis technique in modelling well performance if the configuration and location of all system components is accurately described. If accurate matching/calibration of the well model to fit the real life case scenario of the well is achieved, then with full confidence the sensitivity studies can be performed for the well.
In the course of the project, the following was achieved using the nodal analysis technique:
- The PVT, VLP and VLP were successfully matched.
- In the sensivity analysis study, utilizing the concept of Nodal analysis made it possible to determine the effect of critical factors (well head pressure (WHP), reservoir pressure, water cut and tubing size) on the wells performance in terms of productivity.
- The condition at which the well will seize to produce in terms of water cut was determined. In chapter 4 of the project work, the results obtained can be found in the tables. In terms of reservoir pressure, the reservoir and the well will not provide enough pressure drawdown to produce the oil when the reservoir pressure falls down to 2500 psig ( table 4.5.2). at this condition, the well will not flow naturally, therefore the need for the application of artificial lift technology is required to revive the well back to production.
- From table 4.5.2 (chapter 4),it is observed that an increase in Water Cut impedes production rate. Therefore, the maximum economic water cut is 45% delivering a liquid production rate of 4698.9 stb/d at the current reservoir pressure. This is because the flow rate at the 45% water cut is above the economic limit. At a water-cut value above the economic limit (45%), the well seizes to flow.
- The effect of altering the well head pressure on production rate was achieved (table 4.5.2.1)
- The optimum tubing size that will give the highest or optimum production rate was also determined at the current well and reservoir condition using the nodal analysis technique. The optimum tubing size determined is the 5.100 inches tubing size (table 4.5.2.3) delivering an optimum flow rate of 4755.2 stb/d.Installing a tubing size greater than 5.100 inches results in a “no flow condition” (killing the well).
Nodal analysis proved useful in optimizing the system and predicting well performance with changing reservoir properties. The software approach in using nodal analysis is an unparalled way of accurately predicting well performance so that the response to production system design may be perfectly predicted before implementation.
5.1 RECOMMENDATION-
Comprehensive pressure survey test should be conducted to find out up-to-date values of reservoir parameters.
The current study did not undertake coning studies, decline curve analysis and a gas lift case scenario due to insufficient data.. Should the reservoir fluid contacts be known, it will be required to perform calculations to determine the critical production rate for the well as well as the possible breakthrough time at current production rate and the optimum length and depth of perforation. Production history will enable performing decline curve analysis to determine the well fluid’s production rate decline model and the exact time at which the artificial lift operation should be commenced.
Optimisation studies are better done for entire fields rather than just single wells. Should oilfield data be available, an entire filed development program for the oilfield in order to yield optimum recovery at the minimum possible cost could be drafted out. This will involve the use of reservoir simulation software to describe fluid dynamics with depletion, optimum well placement, dynamic fluid contact level monitoring as well gas lifting of the entire field (since gas lifting is not economically suitable for single wells).
REFERENCES
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- Eikrem, G.O., Imsland, L., and Foss, B. 2003. Stabilization of Gas Lifted Wells Based on State Estimation, Proc. of the 2003 IFAC Intl.
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