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European Journal of Applied Sciences – Vol. 10, No. 2
Publication Date: April 25, 2022
DOI:10.14738/aivp.102.12067.
Adukwu, O. (2022). Effects of Gas/Oil Ratio (GOR), Lift Gas Injection rate and percentage choke opening on Casing-Heading
Instability in a Gas-Lifted System. European Journal of Applied Sciences, 10(2). 339-350.
Services for Science and Education – United Kingdom
Effects of Gas/Oil Ratio (GOR), Lift Gas Injection Rate and
Percentage Choke Opening on Casing-Heading Instability in a
Gas-Lifted System
Ojonugwa Adukwu
ABSTRACT
This research investigated the effects of gas/oil ratio, lift gas injection rate and
percentage choke opening on casing-heading instability in gas lift system. The gas
lift models and parameters used were first presented and injection valve flow rate
at no oscillation was shown. The severity was monitored on the flow rate of gas
through the injection valve. As the GOR decreases, chances of casing heading
instability increases from damped oscillation to mild oscillation till severe
oscillation takes place. On the other hand, increasing the flow of lift gas into the
annulus reduces the chance of oscillation. The percent valve opening increase leads
to increase in chances of instability. This knowledge helps in providing information
needed to reduce or eliminate casing-heading instability. The study is important as
casing heading instability leads to reduced average oil production which means the
gas lift system is operated suboptimally.
Keywords: Gas lift, Casing-Heading Instability, Gas Oil Ratio, Gas Lift Valve, Optimisation
INTRODUCTION
Hydrocarbons are usually located deep down beneath the earth surface. Naturally, the reservoir
pressure provides the energy required to lift the crude oil from the reservoir into the tubing for
onward delivery to the downstream facilities. But as production increases, this reservoir
pressure declines to the point where it can no longer lift the crude oil as desired. Artificial lift
methods is then require to augment the natural reservoir pressure [1]. These artificial means
include sucker-rod pump [2], electric submersible pump [3], and gas lift [4] among others. Gas
lift is used mostly in oil fields with dense oil well and the cost of running the gas lift is extremely
high hence there is the need to run it optimally. Many problems affect the optimal operation of
the gas lift. These problems include faults, weather and casing-heading instability among
others.
Figure (1) is a gas lift system. It is made up of two concentric cylinder-like structure with the
outer one called the annulus and the inner one, the tubing. The lift gas is delivered into the
system from the compressor station (not shown) through the gas lift choke. The annulus holds
the lift gas and delivers it to the tubing through the injection valve. There are many injection
valves in a typical gas lift system and they are located at different heights of the well. The tubing
contains the oil and the gas from the annulus and the reservoir respectively. Produced mixture
is sent to the platform through the production choke. Produced mixtures from other wells in
the field are gathered at the platforms for delivery to the separator through a common riser [5].
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European Journal of Applied Sciences (EJAS) Vol. 10, Issue 2, April-2022
Services for Science and Education – United Kingdom
Casing-heading instability is described here briefly. As production increases, reservoir pressure
becomes insufficient for lifting the oil out of the reservoir due to the hydrostatic pressure of the
oil being too high for the reservoir pressure. Gas from the annulus then mixes with the oil in the
tubing reducing the density of the mixture hence the bottom hole pressure to the point where
the reservoir pressure can then lift the mixture. Production increases and the pressure
decreases further encouraging the flow of more gas into the well from the annulus. This leads
to a decrease in annulus pressure until the annulus pressure becomes lower than the well
pressure. Gas no longer flows from annulus into the tubing. Oil begins to accumulate in the
tubing while gas begins to accumulate in the annulus. This continues until the annulus pressure
becomes larger than the tubing pressure again and flow from annulus to tubing starts again.
This behavior causes the variation of the states and variables of the gas lift system, a situation
called casing-heading instability [6,7].
Figure (1) A single well gas lift system
Casing-heading instability is a phenomenon that makes the variables and states of the gas lift
system exhibit oscillatory behavior. This oscillation reduces average oil production hence must
be minimized or removed [8]. Various factors affect the casing-heading instability including
natural, design and lift gas usage, control among others. In this paper we simulate a gas lift
system using field scale gas lift data to investigate how the GOR, �!" and input valve opening
affect the casing heading instability.
This paper is organized as follows; Chapter 2 discusses various factors that affect the casing
heading instability. Chapter 3 presents the gas lift models that is commonly used in oil industry
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341
Adukwu, O. (2022). Effects of Gas/Oil Ratio (GOR), Lift Gas Injection rate and percentage choke opening on Casing-Heading Instability in a Gas- Lifted System. European Journal of Applied Sciences, 10(2). 339-350.
URL: http://dx.doi.org/10.14738/aivp.102.12067
while chapter 4 simulates the gas lift system showing how the factors affect the instability.
Chapter 5 concludes the paper.
CONTROL OF CASING-HEADING INSTABILITY
As noted earlier, gas lift is used because the bottom hole pressure in the tubing is too large for
the reservoir pressure to lift the oil in the tubing. Mixing the oil with gas reduces the density
hence the bottom hole pressure. The more gas in the tubing, the lower the bottom hole pressure
and the more chance there is a continuous production of oil through the choke. But as the gas
in the well increases, the mass of the gas can be significant enough in the well leading to increase
in bottom hole pressure again. Many natural means therefore affect the gas lift system. The
Gas/oil ratio describes the ratio of the mass of oil to the mass of gas from the reservoir. This
implies that higher GOR increases gas supply naturally to the tubing hence reducing casing
heading instability [8]. Reservoir pressure is the natural source of energy for lifting oil from the
reservoir. The higher the reservoir pressure, the more the flow hence the lower the chances of
casing heading instability.
Casing heading instability can be examined during design too. The pressure in the annulus at
the injection point of the gas into the tubing is called annulus pressure here while the pressure
at the same point in the tubing side is called well pressure. If annulus pressure is held higher
than the well pressure in the tubing, there will be continuous flow of gas from annulus into the
tubing and casing-heading instability will be eliminated. The annulus volume increases the
annulus pressure while the tubing pressure increases the bottom hole pressure. Higher annulus
pressure therefore reduces chances of casing-heading instability while higher tubing pressure
increases the chances.
The amount of gas in the annulus determines the annulus pressure. The mass of gas in the
annulus is determined by flow rate of the gas into the annulus and the flow rate of the gas
through the injection valve into the tubing. Higher flow rate of the injection gas reduces the
chances of casing heading instability. But increasing this gas increases oil production until a
point at which further increase leads to decrease in production as shown in figure 2.
Figure 2 - Oil production variation with injection gas flow rate
In figure 2, the rate of oil production increases with flow rate of the injection gas from Point A.
At point B, the oil production reaches its maximum and further increase in injection gas leads
to a decrease in oil production. Point B therefore is the optimum gas supply beyond which the