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Copyright 2007, Society of Petroleum Engineers
This paper was prepared for presentation at the 2007 SPE
Production and OperationsSymposium held in Oklahoma City, Oklahoma,
U.S.A., 31 March3 April 2007.
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AbstractGas lift is a good artificial lift for those high
gas/oil ratio
(GOR) wells. But the positive displacement pump is used
on many special conditionssuchas the limitation of high
pressure gas, large equipment investment and low
remaining recoverable reserve.To decrease the gas into
the pump, the gas would generally be separated atbottom hole,
then enter the annulus between the tubing
and the casing (tubing-casing annulus), eventually flow
in air or enter oil pipeline by many control valves at
wellhead. In this system, the gas is a potential damage
factor to the pump. So the pump efficiency is low and the
frequency of pump checkout is high.
In this paper, a new approach is presented. One or
two gas lift valves are installed on the tubing near the
wellhead. The other equipments are same as the
conventional system. While the casing pressure reachesthe aimed
value, high pressure gas in the annulus
automatically enter the tubing. This lightens the weight of
the fluid in the tubing above the working valve, which
assists pumping the liquid.
This approach is applied in sucker rod pump system in
Wendong oilfield. Dynamometer cards from 2 typical
wells indicate that the pump efficiency is improved and
the daily fluid production rate is enhanced. Then a
similar approach is applied in Electric Submersible Pump
system (ESP) and the same good result is obtained. This
paper presents the description of this new approach and
reports the results obtained in those field trials. The
energy-saving theory is analyzed and the design
procedure is given.
Introduction
Wendong oilfield is a multiple-zone and fault-block
reservoir with the feature of deep wells, abnormally high
temperature, abnormally high reservoir pressure and
high GOR. Gas lift has become the main artificial lift
method since 1991. With water cut and lift height rising,
the slippage of the liquid would increase and the lift
efficiency of gas lift system would drop. In order to get
the equal daily fluid production rate, more high pressure
gas should be injected than before. No new compressor
units are put into service after large equipment
investment and low remaining recoverable reserves areconsidered.
Some gas lift wells are obliged to reinstall
the sucker rod pump system for good production
management. The average pump efficiency in 15 wells is
24%. Compared to the initial gas lift system, the total
daily fluid production rate is reduced by 156.6t/d. And
another 79 wells also adopt the sucker rod pump
system, among which GOR over 300m3/t is 32 wells.
The average pump efficiency is 18.2%. The main reason
is high casing pressure, which makes much gas in
tubing-casing annulus return into pump system. So itskey how to
automatically drain the gas in the annulus to
drop the casing pressure. In this paper, this work is
finished by one or two gas lift valves. The field test in
sucker rod pump system and ESP system is reported.
The energy-saving theory is analyzed and the design
procedure is given.
Gas lift assisting pump system
In this system, only one or two gas lift valve is added
compared to the conventional sucker rod pump system.
The flow path of the gas in the annulus is changed and
SPE 107041
An Approach To Apply the Gas in the Annulus To Assist Pumping
Oil in High-GORWellsLiu Yonghui and Li Yingchuan Southwest
Petroleum U.; Li Xiaoqi, Zhongyuan Oilfield, S inopec; and Tang
Guilin,Southwest Oil & Gas Co. of PetroChina
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2 SPE 107041
the high pressure gas is effectively used to improve the
pump efficiency. This technology mainly includes two
processes (see figure 1).
(1) Gas anchor is installed at pump depth or a special
pump is used to prevent the gas into the pump. The
mixture from the reservoir would be separated at pump
depth. Then the gas flow in tubing-casing annulus, while
the liquid and a little gas is pumped into the tubing.
Thats same as the conventional sucker rod pump
system.
(2) One or two gas lift valves are installed near the
surface. For the control casing head is shut, the gas is
gathered in tubing-casing annulus and the casing
pressure increase gradually. While the casing pressure
reach the test rack opening pressure of gas lift valve,
high pressure gas in the annulus would return into the
tubing via the gas lift valve. The density of the mixture in
the tubing upon the working valve is lightened. The lower
lifting pressure differential and a little gas in the
annulus
would much improve working condition of sucker rod
pump. Consequently high pump efficiency is obtained.
Thats the process of applying high pressure gas in the
annulus to assist pumping the fluid. Then the casing
pressure would drops with high pressure gas entering
the tubing. When the casing pressure is equal to the
closing pressure of gas lift valve, the working valve
would automatically close. Then the check valve in gaslift valve
starts to work and prevents the fluid in the
tubing from entering the casing. A new cycle starts.
To use this system in high GOR wells, high pressure
gas in the annulus is automatically drained into the
tubing, which makes the system in dynamic balance.
Those problems in conventional system such as low
pump efficiency and the control of casing pressure are
effectively solved. And the production cost would add a
little for one or two gas lift valves are added in the new
system with comparison to the conventional pump
system.
Enhance the pump efficiency in gas lift assisting
pump system
The pump efficiency of the sucker rod pump system
includes three parts1: (1) elastic deformation of rod string
and tubing string in upstroke and downstroke process;
(2) pump fillage; (3) the leakage. Here how to enhance
the pump efficiency in the new system would be
analyzed considering the former two factors.
In conventional sucker rod pump system, the elastic
deformation of tubing string can be solved by one tubing
anchor, while that of rod string cant be solved for the
polished rod load is varied with sinusoidal law. Its elastic
deformation fits Hookes law. To drop the elastic loss of
the tubing is to lower the rod load for the rod material fit
well in oil well condition. In the new system, the high
pressure gas in the tubing-casing annulus is injected into
the tubing to lighten the density of the mixture upon the
working valve. Compared to the conventional system,
the lifting pressure differential is lower. The polished rod
load drops to shorten the rod stretch. The pump stroke is
longer than that in the conventional system.
The influencing factor of the pump fillage includes
two parts: (1) gas influence; (2) submersible depth.
According to the casing pressure, the gas in the annulus
flows in air in the conventional sucker rod pump system,
which is finished by the production worker. It is often to
get high casing pressure for bad management. The
producing fluid level would drops and too much gas in
the annulus make the gas-liquid separator in lower
efficiency. Some gas enters the pump. The pump fillage
drops for the gas compressibility and low submersible
depth. In the new system, gas drainage is directly in
control of gas lift valve. Once the casing pressure reach
the test rack opening pressure of gas lift valve, gas lift
valve would automatically open and then high pressure
gas continue flowing into the tubing. Consequently thehigh
casing pressure wouldnt appear. And low casing
pressure would increase the producing fluid level. Its
easy for the fluid to flow into the pump. The pump fillage
increases.
Design gas lift assisting pump system
To apply this new system, it should have enough gas in
the annulus or high casing pressure. Its good that the
above situations exist in one pump wells at the same
time. High casing pressure can ensure to open the gas
lift valve, while enough gas get a good lift effect. Here
this approach is mainly used to solve those problem
wells, which too much gas in the annulus results in lower
pump efficiency.
In this new system, operation parameters of sucker
rod pump system are designed as the conventional
system2 and available daily fluid production rate
enhanced in the new system is considered. The lifting
pressure differential provided by pump system is much
more than that from the gas lift assisting system for
lower injection depth and small gas volume. A reduction
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SPE 107041 3
of polished rod load is small, which has a little effect on
the sucker rod pump system. And even if the gas lift
valve stops working, the conventional system could work
well and the daily fluid production rate is equal.
The key is to design gas lift assisting system. The
casing pressure is one key parameter. Its named as
injection pressure in gas lift system. The high casing
pressure would add the depth of working valve, and then
the gas lift assisting system would make full use. On the
other hand, the producing fluid level drops and the pump
depth would increase to get the equal daily fluid
production rate. Consequently the polished rod load
increases. So there exists a reasonable range of casing
pressure. Here according to the production feature of
sucker rod pump system such as daily fluid production
rate, pump depth and gas-to-liquid ratio, the range of
casing pressure is 1.0~1.5MPa. The other parameter of
gas lift valve is design as the conventional gas lift
system3.
The check valve in gas lift valve is a key tool in this
system. In conventional gas lift system, the gas leakage
of check valve would results in multi-point gas injection,
unstable production and low lift efficiency. But in the new
system, the fluid in the tubing would flow in the casing for
the casing pressure is cyclical, which is called tubing
leakage in conventional sucker rod pump system. The
leakage of gas lift valve is tested by experiment. And itseasily
to be destroyed for the check valve with rubber
seal. Here the metal seal is recommended.
Field test
This new system is firstly applied in typical problem wells
with the sucker rod pump system. The sucker rod pump
system is reinstalled in well 13-286 for the high pressure
gas is often stolen in the initial gas lift system. The
casing pressure varies from 1.5MPa to 5MPa. As a
result, the daily fluid production rate drops by 29.4t/d
with comparison to the gas lift system. The working
parameters of gas lift assisting pump system is listed in
table one. The parameters of the sucker rod pump
system are same as before. The daily fluid production
rate in the new system is 49.9t/d, which is near to that of
gas lift system. The pump efficiency increases by 30.6%.
And the dynamometer cards from two systems indicate
that the working condition of the rod pump is much
improved (see figure 2, 3). The gas lift assisting pump
system is installed in another high GOR well. The GOR
in well 13-69 is 830m3/m3and water cut is 84.7%. The
working valve is installed at depth of 300m. The high
pressure gas in the annulus continues entering the
tubing. The GOR drops by 540m3/m
3, which much
lowers the gas volume into the rod pumps. The pump
efficiency increases from 25.6% to 46%. The
dynamometer cards also show the pump efficiency is
much improved (Figure 4, 5). Then 22 oil wells with the
similar problem adopt this new system.Figure 6 lists the
pump efficiency comparison. The average pump
efficiency increases from 17.4% to 40.5%. And the
worse the sucker rod pump system works, the better
results is attained.
The ESP system is initially installed in 3 wells in
Wendong oilfield and the high GOR results in lower
pump efficiency. The gas lift assisting system is used.
The same good results are obtained. The average daily
fluid production rate of each well is increased by 20t/d.
Conclusions
1. The gas lift assisting pump system could be an
effective method for pumping oil in high GOR wells
both in sucker rod pump system and in ESP system.
2. The check valve in gas lift valve is a key tool in this
new system. The metal seal is recommended and
the leakage is tested by experiment.
3. The reasonable range of casing pressure is a key
parameter to make full use of the high pressure gasin the
annulus and get the sucker rod pump system
working well.
Acknowledgments
The authors would like to thank the management of No.1
oil plant of Zhongyuan oilfield for granting permission to
publish this paper. A special thanks to the supports of
the National Natural Science Foundation of China.
Renferences
1. Li Yingchuan: Oil Production Engineering,
Petroleum Industry Press, Beijing, (2002).
2. API RP 11L, Third Edition, Feb(1997) API
Recommended Practice for Calculation Sucker Rod
Pumping System(Convention Units).
3. Liao Tian LuContinuous Gas-Lift Installation
Design SimulationMS. Thesis. The University of
Tulsa(1988).
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4 SPE 107041
Figure 1. Underground structures of gas lift assisting pump
system
Table 1. The working parameters of well 13-286 with the gas lift
assisting pump system
Sucker rod pump system Gas lift assisting system
Pump depth 1540m The range of casing pressure 1~1.5MPa
Pump size 57mm Depth of the working valve 150m
Surface stroke 4.8m The temperature at the allocation of the
working valve 58
Port diameter of gas lift valve 3.2mm
Opening pressure 1.1MPaStrokes per minute 5
The leakage
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SPE 107041 5
0
10
20
30
40
50
60
70
80
0 1 2 3 4 5
Position(m)
Load(kN)
Surface dynamometer card
Downhole dynamometer card
Figure 3. The dynamometer card of well 13-286 with the gas lift
assisting pump system
-10
0
10
20
30
40
50
60
70
80
90
0 0.5 1 1.5 2 2.5 3 3.5 4
Position(m)
Load(kN) Surface dynamometer card
Downhole dynamometer card
Figure 4. The dynamometer card of well 13-96 with the
conventional sucker rod pump system
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6 SPE 107041
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0
10
20
30
40
50
60
70
80
90
0 1 2 3 4 5
Position(m)
Load(kN)
Surface dynamometer card
Downhole dynamometer card
Figure 5. The dynamometer card of well 13-96 with the gas lift
assisting pump system
0
10
20
30
40
50
60
70
80
90
13-52
13-69
13-286
13-68
96-4
13-303
14-39
96-4
101-52 56
-1
101-55
10-16
13-52
13-397
13-68
215-7
13-122
13-345
92-89
92-76
16-22
N25
-13
w ell number
pumpefficiency,%
conventional sucker rod pump systemgas lift assisting pump
system
Figure 6. Comparison of pump efficiency between gas lift
assisting pump system and conventional system
