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GAS LIFT OPTIMIZATION OF OIL PRODUCING WELLS USING PROPER NODAL ANALYSIS

CHAPTER ONE

INTRODUCTION

1.1 Background to the study.

Before a well can produce oil or gas, a borehole is drilled from the surface to the oil and gas pool or reservoir rock. The borehole must be stabilized with casings cemented in place. A small diameter tubing string is centered in the wellbore and is sometimes held in place with packers. This tubing will carry the oil and gas from the reservoir to the surface. Reservoirs are typically at elevated pressure because of the underground forces that surrounds them. The driving force which causes these fluids to move out of the reservoir comes from the compression of the fluids that are stored in the reservoir. The actual energy that causes a well to produce oil results from a reduction in pressure between the reservoir and the producing facilities on the surface.  Early in its production life, the underground pressure will often push the hydrocarbons all the way to the surface facilities.

Depending on reservoir conditions, this “natural flow” may continue for many years. The production capacity of an oil well is a  key surveillance factor in monitoring the well’s performance  which is dependent on a number of other factors which include; fluid properties and composition of the oil itself, gas oil ratio, water cut, reservoir characteristics and completion strategy/design (Beggs et al, 1991).  The well performance consists of the inflow performance relationship (IPR) which involves flow from the reservoir into the wellbore and the outflow performance which involves flow from the wellbore up the tubing to the surface production facilities. Constant monitoring of the well performance is of paramount importance in the oil and gas industry as it is critical to the crude oil production obtainable from such wells and the equivalent profit margins that will be generated from the production (Hernandez et al, 2001). Monitoring of the well’s performance also help the engineer to determine the economic limit and amount of oil production recoverable from such wells. Once a well is produced down to its economic limit due to pressure depletion, increase in water-cut the pressure differential which is the driving energy becomes insufficient in producing the fluid (oil) to the production facilities (Beggs et al, 1991). 

Gas lift method once applied at the required injection gas rate can supplement the reservoir energy to drive the oil to the surface. The introduction of lift gas to a non-producing or low producing well is a common method of artificial lift. Naturalgas is injected at high pressure from the casing into the wellbore and mixes with the produced fluids from the reservoir . The continuous aeration process lowers the effective density and therefore the hydrostatic pressure of the fluid column, leading to a lower flowing bottom-hole pressure (Pbh). The increased pressure differential induced  across the sand face from the in situ reservoir pressure (Pr), given by (Pr − Pbh), assists in flowing the produced fluid to the surface. The method is easy to install, economically viable, robust, and effective over a large range of conditions, but does assume a steady supply of lift gas. At a certain point, however, the benefit of increased production due to decreased static head pressure is overcome by the increase in frictional pressure loss from the large gas quantity present. This has the effect of increasing the bottom-hole pressure andlowering fluid production.  

1.2 statement of problem

Movement or transport of oil to the production facilities requires energy to overcome friction losses in the systems and to lift the products to the surface. The production system can be relatively simple or can include many components in which energy or pressure losses occur. The production rate or deliverability of a well can often be severely restricted by the performance of only one component in the system due to pressure losses. As mentioned earlier, within the life of a reservoir, there is a time when the available reservoir pressure is unable to lift the produced fluid to the surface. This is mainly as a result of pressure depletion , increased water production (water cut). Also, the need may arise for producing companies of oil and gas to maximize the production of oil and gas at the current installation facilities and reservoir condition. Most times, the efforts of the companies are directed to a medium and long term project to maximize the factor of recovery (production of oil to the minor possible cost), and in the short term to accelerate the recovery of the recoverable reservations. These efforts are usually realized during the economic limit period of the well in which the energy for production (pressure draw down) is insufficient for the required production rate. It becomes necessary to optimize the production system of the well to increase the economic recovery and to meet up with the increasing global energy demands.  

1.3 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.  

1.4 Scope of study

The study involves the collection of well, reservoir, completion and well test data (and other relevant data) to build a well model in order to characterize the well.  The following points highlight the scope of this study:

-  Designing of a well model that will serve the purpose of this study using relevant input data.

-  Appropriate selection of the best correlations available in the Prosper nodal analysis software that will aid in a more accurate performance prediction and matching of the well data.

-  Generating the inflow performance relationship (IPR) of the well model to characterize the well’s performance.

-  Generating the vertical lift performance (VLP) of the well’s model to characterize the outflow performance of the well.

-  Matching the IPR and VLP curve to determine the optimum flow condition of the well at the current reservoir pressure.

-  Validating the well model with the help of available well test data.

-  Using the validated well model to run a sensitivity analysis and prediction to investigate the effects of different parameters on the well performance.

-  Designing a gas lift system for the well model.

-  Perform a sensitivity analysis and prediction study as part of the optimization process.

-  Comparing the optimized results derived from the gas lifted well model and the natural flowing well.  This will help meet the objectives of this study and also examine the results of optimizing production from the well. The predictions of different parameters that are critical to the performance of the reservoir system will also be analyzed.

1.5 Limitations of the study

During the course of this study, some limitations were highlighted and assumptions made. This was done so as to achieve the goal of this project by idealizing the study/model. These limitations include:

-  Multi phase fluid flow problems along the tubing.

-  Ability of the well to deliver a stable flow rate which can impose additional restrictions on the achievable flow rate.