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European Journal of Applied Sciences – Vol. 9, No. 6
Publication Date: December 25, 2021
DOI:10.14738/aivp.96.11430. Kosisochukwu, N. C., & Saturday, E. G. (2021). Analysis of Crude Oil Production Using Gas Lift. European Journal of Applied Sciences,
9(6). 598-608.
Services for Science and Education – United Kingdom
Analysis of Crude Oil Production Using Gas Lift
Nwudoh Chisom Kosisochukwu
Offshore Technology Institute, University of Port Harcourt
Ebigenibo Genuine Saturday
Department of Mechanical Engineering, University of Port Harcourt
ABSTRACT
Gas lift is used to increase production by reduction in the density of the oil column
in the tubing, reduction of the flowing bottom-hole pressure which is affected by the
hydrostatic pressure of the fluid column and by providing external energy to the oil.
In this study an oil field in the Niger Delta was used as case study; it has a production
life span of over ten years with characteristics such as high gas-oil ratio (GOR) and
high reservoir pressure which makes gas lift suitable for optimizing the wells. The
aim of this study is to select the gas lift method suitable for optimum production in
the wells using the production modelling softwares PROSPER, and GAP. Simulation
studies were conducted using data from two wells in the oil field. The performance
of continuous and intermittent gas lifted performance on two wells were compared.
The results indicated that continuous gas lift system will be suitable for a well that
has high flow rate and high GOR. Also, intermittent gas lift system will be suitable
for a well that has lower reservoir pressure and low productivity. Sensitivity
analysis of water cuts, gas injection rates and well head pressures were carried out
to investigate how the wells will respond to changing operating conditions such as
increasing water cut, increasing gas injection rates and increasing well head
pressures in the future.
Keywords: Gas lift, Gas-Oil Ratio, Continuous and Intermittent Gas Lift System, Sensitivity
Analysis.
INTRODUCTION
Production of crude oil from the well can be achieved with the natural energy of the well at its
early life, when the natural drive is most active and crude oil is produced from the reservoir at
commercially viable rate. Factors most responsible for this are the gas cap, solution gas, and
water drive. A well produces with a combination of these drives at a particular time;
subsequently as production progresses, the reservoir natural energy depletes leading to
reduced reservoir pressure and bottom hole flowing pressure (BHFP) that accompanies the oil
to the surface, increased water cut with resulting decrease in gas-liquid ratio (GLR). Increased
pressure drop and water cut are often experienced in mature fields that has been in production
over a period of time. Other challenges that could also affect decline in production rate are
inappropriate perforation method, inadequate design of the well production system and inept
management of a well. At this point where the well is unable to produce at its own natural
energy and at economical production rate, a best known option is the application of artificial
lift method for enhanced oil recovery (EOR). Artificial lift systems are designed to assist
reservoirs that may not be able to produce its formation fluids using its own natural drive at
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Kosisochukwu, N. C., & Saturday, E. G. (2021). Analysis of Crude Oil Production Using Gas Lift. European Journal of Applied Sciences, 9(6). 598-608.
URL: http://dx.doi.org/10.14738/aivp.96.11430
economical rates for optimal production. A good percentage of the producing oil wells require
forms of artificial lift for increment in the flow rate of the well fluids where the reservoir has
insufficient energy to produce naturally at economical rates or to achieve an early production
from a well where the well fluid is very viscous. The goal of artificial lift system is to maximize
profit or improve financial performance. Operators of oil wells are required to know the most
suitable artificial lift system to incorporate in their wells to optimize production to ensure
minimal capital and operating cost.
Gas lift system has been identified as the most commonly used artificial lift systems for EOR.
During the gas lift process, very high pressured gas is passed down into the wellbore through
the gas lift valves, which is positioned inside the gas lift mandrels, through the annulus, into the
tubing strings. The gas undergoes expansion in the tubing thereby causing a reduction in the
reservoir pressure, density and column weight of the fluid flowing from the reservoir which
allows the well to flow properly. There are two major types of gas lift namely; continuous and
intermittent gas lift systems. Continuous flow gas lift is commonly selected for wells that
produce at a very high reservoir pressures which is most applicable to the deep water oil wells.
It is a common application in the offshore operations owing to its cost effectiveness, ease of
implementation, effectiveness in varying operating conditions and minimal maintenance [1]
while intermittent gas lift is normally for wells with relatively low reservoir pressures and rate
for effective production. Although high injecting rate can increase production rate in the
reservoir, there is a limit to the amount of gas that can be injected into the well as a pressure
surge in the entire production equipment could result from high injection rate and low
production from too low injection rate, both of which may result in serious operational damage
and increase in the operational cost. To this effect there is need to analyze the production under
the given well conditions with different gas lift methods to determine the optimum gas lift
method and governing factors that affect the selection process. This paper analyzes and
compares oil production between continuous and intermittent gas lift systems based on real
data obtained from two oil wells in a Niger Delta oilfield and to choose the system best suited
to increase the production of the wells.
Many researchers have considered the factors responsible for the selection of the type of
artificial lift system to be employed for a given well [2-4]. The operations of types of gas lift
systems and their applications was studied [1,5]. Factors affecting gas lift optimization that
leads to improved well performance was studied [6-10]. The use of gas lift performance is a
common practice to determine the optimum gas lift rate to be allocated to a well in a gas lift
analysis involving two wells sharing a common flow line [11]. Computer modelling, simulation
and optimization program is required to model, simulate and analyze gas lift optimization for
maximum oil production [11-14]. Most field in the Niger Delta are mostly been produced with
the continuous lift system due to the high GOR and high reservoir pressure. Not every well in
the same field will be compatible with continuous gas lift system, some will be compatible with
intermittent gas lift system. The option of simultaneous usage of both the continuous and
intermittent lift systems on different wells from the same reservoir for Niger Delta fields has
not been really explored, little is also known on how the application of this concept could offer
a boost or a decline to the overall production from a reservoir. All of this would be evaluated
and confirmed through this paper.
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European Journal of Applied Sciences (EJAS) Vol. 9, Issue 6, December-2021
Services for Science and Education – United Kingdom
METHODOLOGY
This paper used commercial software in the Integrated Production Modelling (IPM) suite -
Production and Systems Performance Analysis (PROSPER) and General Allocation Package
(GAP) to model and examine two producing oil wells located in the Niger Delta using nodal
analysis. Production data from two wells operating in the Niger Delta were obtained and the
behavior of the wells were analyzed and gas lift was implemented at a later stage when the
wells were unable to flow naturally. The wells production performances were optimized using
continuous and intermittent gas lift systems. PROSPER was used to design the continuous and
intermittent gas lift systems for the wells and GAP was used to integrate the whole system for
production optimization. This involved an outline description of different methods applied and
the procedures undertaken to effectively arrived at the objectives of the study illustrated in
Figure 1 and 2.
Nodal Analysis
Nodal analysis was carried out to evaluate the performance of production wells and
subsequently evaluate the most viable option for optimizing the wells performance. It involves
specifying a nodal point, usually at the bottom hole or wellhead, and dividing the producing
system into two parts - the inflow and the outflow. The flow behavior, expressed as flow rate
vs. pressure, from the reservoir to the node is called Inflow, and from the node to the outlet,
fixed pressure point at the surface is called Outflow.