Analysis of T echnical S olutions U sing S crew C ompressors to Increase the G as Lift Efficiency [602802]

Analysis of T echnical S olutions U sing S crew C ompressors to Increase the G as Lift Efficiency

Abstract

The paper will present the thermodynamic analysis of the gas lift process applicable to a
production and technical solutions that can be applied under the existing parameters.
The thermodynamics of the gaslift process consists in determining the compressing
requirement in the form of pressure and flow for the gases used in the process. Compression is an
energy intensive process; therefore, it is necessary to choose the compressor that uses the smallest
mechanical compression. The oil injection screw compressor is ideal for this process because the oil injected into the compressor along with the lubrication and sealing technology cools the compressed
gas, which makes the compression process to be much closer to an isothermal process (minimum compression required).
In order to calculate the thermodynamic of the gaslift process, the hydraulic calculation of the
process , in which the extracted production, the amount of gas required, the injection pressures and
the location of the valves , should be performed. These parameters were determined to corresponde to
the oil injection screw compressors manufactured in Romania by INCDT -COMOTI . It can be
emphasized that the results of the study can be used very easily by all specialists using these types of gas compressors in various fields .
This paper presents the e ssential characteristics of those screw compressors that could be used
considering the specificity of the location where the compressor will be mounted, as well as the
functional parameters of the gas lift installation in place and the forecast of the evol ution of the
reservoir.
The paper presents the gas thermo -dynamic calculations made taking into account several gas
compression variants for determining the optimum design solution and the maximum compression efficiency of the gas using an oil injection sc rew compressor.

Keywords: gas, compressor, performance, efficiency, energy

1. Scientific and technical description of processes

Continuous artificial eruption or continuous gaslift involves the continuous injection of
compressed gases directly into the column of fluid produced by the well in order to reduce their
density and, implicitly, the dynamic bottom pressure, allowing the layer to properly flow according
to this pressure.
Continuous artificial eruption installations are divided into two categories depending on the
type of well completion, namely artificial eruption installations for simple completion and artificial
dual-fill eruptions [1].
Gaslift valves are used to discharge the well for its production and continuous gas injection in
order to extract a certain fluid flow.
The flow of gases injected into a continuous artificial eruption well to produce a certain fluid
flow is influenced by a number of factors such as: physical properties of the produced fluids,
impurities, extraction pipes diameter, head pressure, the depth of the injection point , the productivity
index of the well [1].
The layer -well working correlation for a continuous artificial eruption well involves
determi ning the behavior curve of the layer and the behavior curve of the equipment, as well as
establishing co -ordinates of the layer -well working correlation point, situated at the intersection of
the two curves.

Fig. 1. Gaslift installation with automated surface equipment (source Parveen Industries)

In the case of continuous artificial eruption, unlike the natural eruption, the behavior curve of
the equipment is determined considering both the gas flow rate injected and the g as injection pressure
[1].
The maximum flow rate produced by a continuous artificial eruption well depends on a
number of factors such as:
 The pressure in the eruption head that can be considered constant or variable;
 The productivity index of the well, which over a short period of time can be considered
constant, but which varies over time as a result of processes occurring in the reservoir;
 The capacity of the gas source (limited or unlimited);
 Injection line pressure (limited value);
 The diameter of the extraction pipes.
Of all these factors, the most important is the capacity of the gas source. If the gas source is
very big, so as to be considered unlimited, the determination of the maximum flow produced by a continuous artificial eruption well requires the determination of the behavior curve of a continuous artificial eruption well.
2. Case study

The following data is known at a continuous artificial eruption well :
 Well depth : H = 2800 m
 Inner diameter of the tubing : d
i = 63 .5 mm
 Inner diameter of the column: D i = 127 mm
 Surface average temperature: ts = 100C
 Crude Oil density : ρt = 830 kg/m3
 Reservoir water density : ρa = 1100 kg/m3
 Gas relative density : ρrg = 0.75
 Surface tension of crude oil : σț = 30·10-3 N/m
 Surface tension of water : σa = 60·10-3 N/m
 Crude oil viscosity : µț = 2.2·10-3 Pa·s
 Gas viscosity : µg = 0.022·10-3 Pa·s
 Water viscosity : µa = 1·10-3 Pa·s
 The pressure in the eruption head : p2 = 4 bar

Also, following the calibration of the well, the following data have result :
 Impurit ies: i = 30%
 Bottom hole pressure : pd = 60 bar
 Static pressure: pc = 90 bar
 Liquid flow rate : Ql = 50 m3/day
 Well gas flow rate: Qg = 2500 Nm3/day
 Injected gas flow rate: Qinj= 30000; 40000; 50000; 60000; 70000; 80000; 90000 Nm3/day

2.1. Thermodinamic analysys of the gas lift process

The gases used in the gas lift process are associated gases that are collected from the separator,
which usually have low pressure. In order to be used, they must be compressed and inserted into the well column. Hence the gases are introduced through 7 val ves, automatically into the tubing. They
expand , mix with the oil and manage to raise it to the surface.
From the thermodynamic point of view, we identify the following processes:
A. Associated gas compressio n – process requiring an energy consumption whos e parameters
will be calculated.
B. The introduction of gas from the column into the tubing is accomplished by means of
valves. The corresponding thermodynamic process is an adiabatic expansion process. The
compressed gas energy must be sufficient to be inserted into the tube.

2.2. Compression calculations

The compressor used as a model is an oil -injected volumetric screw compressor (CU128GM).
The oil injection, besides the technological role, influences the thermodynamic compression process in that the oil cools the compressed gases. For this reason, it will be considered a polytropic process whose exponent will be determined from the experimental data of the compressor [2, 7] .
Parameters to which the compression process is to be dimensioned result from the design of
the gas lift process. The results of the calculation, made with a specialized software, are:
 MAXIMUM LOAD [100%]
 The polytropic exponent = 1 .04207
 Gas flow rate [Sm
3/zi] =50000 Gas flow rate [kg/s] = 0 .4918
 Input parameters :
p1[bar] = 5 .00 V1[m3] = 0 .12414775 T1 [grd.C] = 20.0
 Output parameters :
p2[bar] = 38 .00 V2[m3] = 0 .01772902 T2 [grd.C] = 45.0
 Technical mechanics of compression [J /kg] =131192.682
 Compression power needed [kW] = 50326.038
 Cooling heat Qr [kW] = 47681.930
From the analysis of the results one can conclude that due to the technology specific to the
oil-injected screw compressor, the polytropic compression process is approaching to the isothermal
process in that the value of the polytropic exponent is close to 1. This makes the compression power
required minimum [3, 6] .

2.3. Valves process calculation

The compressed gas is introduced into the well column and from there it automatically enters
the tubing according to the gas lift process. When passing from the column to the tubing the gases
suffer an adiabatic expansion. With each valve, depending on the depth and set pressure, the gas enters the column into the tubing.
This requires a consumption of the mechanical work made by the gas that expand from the
column pressure to the tubing pressure. The graphical results are shown in Figure 2. The results of

the thermodynamic expansion calculation for each valve are shown in Table 1.

Fig. 2. Gas injection mechanism through valves

Tabel 1. Thermodynamic calculation results
Valve
no. p_col
[bar] p_tub
[bar] Specific expansion
mechanical work
[kJ/kg] Generated power fronm gas
expansion through the valve
[kW]
1 34.5 8.5 30919 .92 15206 .42
2 33 11.2 23695 .42 11653 .41
3 31 14 17330 .49 8523 .13
4 29 16.3 12504 .78 6149 .85
5 27 18 8770 .16 4313 .16
6 25 19 5920 .32 2911 .61
7 22.7 19.8 2940 .41 1446 .09

If we add up the gas consumed powers when passing through the valves ( 50203.67 kW) the
value is comparable to the one required to compress the gas ( 50326.038 kW), which shows that the
system is balanced and that the power used for compression is consumed to introduce the gas into the pipe.
Considering the station parameters and the types of screw compressors manufactured in
Romania by INCDT -COMOTI, before choosing the optimal solution, two types of compressors can
be selected:
 Screw compressor with oil injection CHP220;
 Screw compressor with oil injection CU128GM.
Both the CHP 220 compressor and the CU128GM compressor show the following possible
applications:
 Replacement of old GKN 10 piston compressors used in PETROM -OMV gas lift stations;
 High pressure compressors for national transport or distr ibution pipelines;
 Various applications required by users in the country or abroad, which require increasing
the pressure and flow of the gas used.
Both compressor can be used in:

 Oil/gas extraction stations;
 Offshore stations due to their relatively small size, relatively low weight and low operating
noise;
 Petrochemical industry or industrial applications;
 Pumping of gas into pipelines or storage tanks;
 Gas supply system for gas turbines (booster);
 Marine platforms exploitation.
From the comparative analy sis of the operating parameters of the compressor types specified
above, it is advisable to choose as the compression unit for the gaslift process of the CU128 GM
compressor. Operating parameters of the CU128GM high pressure discharge screw compressor (see
Figure 3) are as follows:
 Maximum Discharge pres sure: 45 bara
 Suction pres sure: 4.5 bara
 Volumetric ratio : 4.8
 By modulating the output pressure different volumetrical ratio can be obtain: 2.6; 3 .5; 4. 8
and the suction pressure can go to maximum 9 bara.
 Theoretical flow rate :
 Minimum driving rotor speed 1148 – Theoretical flow rate 230 m3/hour
 Maximum driving rotor speed 4591 – Theoretical flow rate 921 m3/hour
 multiplier ratios :
 Electrical engine drive 1500 RPM – multiplier ratios 1.2; 1.2407; 1.2830 …….2.5588
 Electrical engine drive 3000 RPM – multiplier ratios 1.2; 1.2407; 1.2830 …….1.5208
Transmission ratios are obtained by correlating the number of toothed gear teeth on the motor
shaft driven by the electric motor and the driving rotor shaft, and by varying the transmission ratio, the requested flow in the range is ultimately obtained.
Experimental samples for the CU128GM compressor were performed on the COMOTI stand
(figure 3), and after processing the generated data (Figure 4), the performance graph of the compressor (Figure 5) was obtained.

Fig. 3. CU 128 GM Vi = 4.8 compressor ready
for tests Fig. 4. Print screen with the screw compressor
test data

Fig. 5. CU 128 GM Vi= 4 .8 compressor for the stand is tested

3. Conclu sions

To operate a well in gaslift, a quantity of gas is required (typically associated gases collected
from the separator), which is compressed at the process pressure.
Gas compression is the only energy consumption (which translates into costs) for the gaslift
process. For this reason, the dimensioning of the compression process must be rigurosly made.
In this paper a compression process for an oil compressor has been dimensioned which allows
a polytropic process to be performed with a polytropic exponent close to unit, which reduces the
specific mechanical work of compression. The dimensioning of the compression process has been made correctly because the compressing po wer of the gas is equal to the power required to pass it
through the well valves.
Several types of compressors have been studied for such a process and analised for the operating
parameters, the choice as compression unit of the CU128 GM compressor is advisable. It is underlined that this type of compressor will ensure the operation of the compressor station at optimal parameters. Also, one of the reasons why this type of screw compressor can be considered is also the changes that
may occur in the evolutio n of gas -lift conditions, for which the CU128GM compressor would operate
at normal conditions and in these conditions unlike by the CHP220 compressor that would function with much diminished volumetric or adiabatic efficiency.

Acknowledgement
The researches activities was financed by Executive Unit for Financing Higher Education, Research, Development and Innovation (UEFISCDI) within Bridge Grant (Knowledge T ransfer to the
Entreprise), Project code: PN -III-P2-2.1-BG-2016- 0270.

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