Design and Control of Gas Lift System due to Well Depletion with Levelized Cost Analysis - Politeknik Negeri Padang

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Design and Control of Gas Lift System due to Well Depletion with

Levelized Cost Analysis

Abdul Wahid

#

, Hemi Mauly Kurnianto

*

#_{Sustainable Energy Research Group, Department of Chemical Engineering, Faculty of Engineering, Universitas Indonesia, Depok 16424,} Indonesia, E-mail: [email protected]

*_{Sustainable Energy Research Group, Department of Chemical Engineering, Faculty of Engineering, Universitas Indonesia, Depok 16424,} Indonesia, E-mail: [email protected]

Abstract—Gas lift is required to lift the gas on wells that depleted. In the field of X wells located in the offshore area, the necessary gas lift pressure is 1700 psig to maintain the total gas production by up to 11 MMSCFD, whereas the current well pressure is 650 psig. To raise the necessary gas lift pressure, compressor systems are applied. The objective of this research was to select one of two types of compressors (centrifugal and reciprocating) based on their economics. Proportional-Integral (PI) and Proportional-Integral-Derivative (PID) controller is applied and tuned by open loop Ziegler-Nichols (ZN) and closed loop Tyreus-Luyben (TL) method. Integral of Squared Error (ISE) controller performance criteria is applied for controller’s performance evaluation. This thesis will also analyze the dynamic design process with levelized cost method. The result is open loop ZN tuning has a smaller ISE up to 99.33% on a centrifugal compressor configuration and 98.65% for reciprocating compressor configuration than TL method. Reciprocating compressor configuration with a PID controller and ZN tuning able to reduce 22.96% of energy, compared with the centrifugal compressor configuration and tuning PI controller TL tuning.

Keywords—Control System, Centrifugal Compressor, Surge, Reciprocating.

I. INTRODUCTION

Oil and gas wells generally have high initial pressure and flow rate, then depleted. Therefore, gas lift required to be able to lift the gas in the well properly.

X offshore field there has 4 wells; X-24, X-29, X-44 and X-48 with total gas pressure of 1700 psig is required to main-tain total gas production of up to 11 MMSCFD, while the well pressure is 650 psig. The entire gas passing through the com-pressor is 710 ACFM or 47 MMSCFD. Gas is injected con-tinuously into the production conduit at a maximum depth that depends upon the injection-gas pressure and well depth (Walas, 2005).

To increase the pressure is required compressor system. Based on Fig. 1, a centrifugal compressor or reciprocating compressor may be selected at this operating condition. Cen-trifugal compressors are widely used in industrial applications such as natural gas extraction, cooling, gas processing and gas transportation (Cortinovis et al., 2015). While reciprocating compressors are typically used to improve for gas compres-sion with higher pressure ratios but with low flow rates and usually consist of more than one train.

Gas compression requires energy and cost, so it is im-portant to operate a gas compressor in a safe and efficient way to save resources. When operating condition of a gas com-pression system very close to the surge line, there is the po-tential for a surge. Surge on a dynamic compressor causes the compressor to stop operating for a moment and cause a loud rumble followed by a very high vibration. This will cause a fatal compressor damage (Li et al., 2015) resulting in eco-nomic losses to the gas field management company.

Fig. 1 Compressor Selection Curve (Branan, 2005)

In this research, the overall system control of gas lift sys-tem will be designed, especially related to the possibility of reduced flow rate of compressor suction due to the decrease of gas production from the well. The controllers used are PI and PID controllers, because those controls is most widely used in the industry (Aström et al., 2002). The investment needs of the gas lift system along with the necessary control economic will also be analyzed.

The objectives of this research are:

1. Obtain the design of the gas lift system in order to maintain the gas production control variables of exist-ing wells in X field by comparexist-ing the gas lift system with the configuration of the centrifugal compressor and the reciprocating compressor.

2. Obtain energy requirements in each compressor con-figuration and compare them.

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II. RESEARCH METHODOLOGY

The research was conducted with Unisim software to ana-lyze the operation of centrifugal compressors and reciprocat-ing compressor. Stages of research used to analyze gas pro-duction at this well refers based on well development data FEED at field of X. Fig. 2 illustrates the flowchart of method-ology for this research.

Fig. 2 Flow Diagram of Research

Unisim simulation software including Unisim's dynamic simulation package has become the main tool for the evalua-tion and predicevalua-tion of steady state and dynamic systems.

The flow basis of simulation is shown in the Table I.

TABLEI FLOW BASIS OF SIMULATION

Well X-12 X-29 X-44 X-48 To-tal Associated Gas

(MMSCFD) 0.05 4.2 2.6 4.9 11.75 Gas Lift

(MMSCFD) 9 9 9 9 36

The sizing of centrifugal compressor configuration equip-ment is shown on the Table II.

TABLEII

CENTRIFUGAL COMPRESSOR CONFIGURATION SIZING

Equipment Equipment Diameter

(Inch)

Length T/T

(ft) NLL (%)

Suction

Scrubber 70 11.83 17

Discharge

Scrubber 60 10 20

Valve Valve Controller CV

(USGPM) Valve Type

VLV-101 FIC-101 107.02 Linear

LV-77100 LIC-77100 0.16911 Quick Opening LV-77102 LIC-77102 0.00508 Quick Opening

The sizing of reciprocating compressor configuration equipment is shown on the Table III.

TABLEIII

RECIPROCATING COMPRESSOR CONFIGURATION SIZING

Equipment Equipment Diameter

(Inch)

Length T/T

(ft) NLL (%)

Suction

Scrubber 50 8 25

Discharge

Scrubber 36 7 28

Valve Valve Controller CV

(USGPM)

Valve Type

LV-77100A/B LIC-77100 0.08456 Quick Opening LV-77102A/B LIC-77102 0.00254 Quick

Opening Control System of centrifugal compressor consists of: 1. Flow Control on each well.

2. Level control on suction scrubber and discharge scrubber.

3. The temperature controller comes out after cooler to 120°F.

4. Suction pressure control on the compressor, which serves to maintain suction compressor pressure. 5. Anti-surge control on the centrifugal compressor,

which serves to prevent surge on the compressor. Reciprocating compressor package will consist of two trains each equipped with the following control systems:

1. Flow Control on each well.

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292 3. The temperature controller comes out after cooler to

120°F.

4. Suction pressure controller on the compressor, which serves to keep the suction compressor pressure by con-trolling the compressor speed on both trains.

The control scheme on the PID Controller involves three parameter values of the PID constant to obtain the manipulate variable (MV) according to Equation 1.

𝑢(𝑡) = 𝑀𝑉(𝑡) = 𝐾𝑝𝑒(𝑡) + 𝐾𝑖∫ 𝑒₀𝑡 (𝜏)𝑑𝜏 + 𝐾𝑑_𝑑𝑡𝑑𝑒(𝑡) (1)

Where,

u(t), controller output

Kp, proportional gain parameter Ki, integral gain parameter Kd, derivative gain parameter e, error = SP – PV

t, time

τ, integration variable

Controller tuning is performed to obtain the optimum value for the achieved control response by open loop Ziegler-Nich-ols tuning method and closed loop Tyreus-Luyben tuning method (Marlin, 2000; Luyben, 1996).

The evaluation of control system performed by Integral of squared error (ISE) method that can be used independently as an indicator of process control performance (Wahid et al, 2015). The calculation of ISE shown in Equation 2.

𝐼𝑆𝐸 = ∫ [|𝑆𝑃(𝑡) − 𝐶𝑉(𝑡)|]∞ 2_𝑑𝑡

0 (2)

Schematic of centrifugal compressor configurations shown in Fig. 3. While the reciprocating compressor configuration is shown in Fig. 4.

Steady control and no oscillations are the main objectives of the process of setting control parameters. The Ziegler-Nichols and Tyreus-Luyben tuning methods is going to be tested. To observe the influence of type of compressor, con-troller and tuning method, 8 cases is simulated:

1. Case 1: Centrifugal compressor configuration with PI controller and Ziegler-Nichols tuning method. 2. Case 2: Centrifugal compressor configuration with PI

controller and Tyreus-Luyben tuning method. 3. Case 3: Centrifugal compressor configuration with

PID controller and Ziegler-Nichols tuning method. 4. Case 4: Centrifugal compressor configuration with

PID controller and Tyreus-Luyben tuning method. 5. Case 5: Reciprocating compressor configuration with

PI controller and Ziegler-Nichols tuning method. 6. Case 6: Reciprocating compressor configuration with

PI controller and Tyreus-Luyben tuning method. 7. Case 7: Reciprocating compressor configuration with

PID controller and Ziegler-Nichols tuning method. 8. Case 8: Reciprocating compressor configuration with

PID controller and Tyreus-Luyben tuning method.

Energy output data from the control system evaluation ei-ther by Ziegler-Nichols or Tyreus-Luyben tuning methods are analyzed to calculate the economy with Equation 3.

𝐿𝑒𝑣𝑒𝑙𝑖𝑧𝑒𝑑 𝐶𝑜𝑠𝑡𝑎= 𝐶𝐴𝑃𝐸𝑋𝑎+ 𝑂&𝑀𝑎+ 𝐹𝑢𝑒𝑙 𝐶𝑜𝑠𝑡𝑎

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In accordance with Equation 3 the component to establish the levelized cost is as follows:

1. The price of the total equipment cost will be used as CAPEXinitial.

2. The total fuel demand comes from the compressor fuel and the cooling cost of the heat exchanger.

3. Operation and Maintenance Cost is taken 5% from

wellhead price 5 USD/MMBTu, which is 0.25 USD/MMBTu.

Profit is derived from the wellhead price less the annual cost from levelized cost method.

III.RESULTS AND DISCUSSION

A train of compressor package is designed in centrifugal compressor configuration, whereas in the reciprocating com-pressor, two train compressors are installed.

In the centrifugal compressor configuration there are 9 con-trolled variables shown in Table IV:

TABLEIV

CONTROLLER OF CENTRIFUGAL COMPRESSOR CONFIGURATION

Control Control

Objective

Controlled Variable

Manipulated

Variable Set Point

Anti

surge Safety

Flow inlet compressor

Anti-surge

valve opening -

FIC-101 Smooth Operation

PIC-100 Product Quality

Manifold Pressure

Compressor

Speed 644 psig

TIC-100 Safety Temperature

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Fig. 3 Simulation of Gas Lift System Control System with Centrifugal Compressor Configuration

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294 In the reciprocating compressor configuration there are 11 controlled variables shown in Table V.

TABLEV

CONTROLLER OF RECIPROCATING COMPRESSOR CONFIGURATION

Control _ObjectiveControl Controlled _Variable

Manipu-lated

Varia-ble

Set Point

77101A Equipment Protection

after cooler A

Duty Cooler

A 120°F

after cooler B

Duty Cooler

B 120°F

PIC-100 Product Quality

Manifold pressure

Compressor

A /B speed 644 psig

A. Tuning Result

The control system acts as the nervous system for the plant. It provides sensing, analysis, and control of the physical pro-cess. When a control system is at properly tuned, the process variability is reduced, efficiency is maximized and energy costs are minimized.

In all of the gas lift system configurations Ziegler-Nichols open loop tuning method and Tyreus-Luyben tuning method were applied, the PI and PID parameters are obtained.

Result of centrifugal compressor tuning is shown in Table VI and Table VII.

The result of reciprocating compressor tuning is shown in Table VIII and Table IX.

TABLEVIII

B. Controller Performance

In all cases, a scenario is performed to test the performance of the controller by the following disturbance:

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295 2. At the 70th_{minute, X-29 is shutdown, so that the}

flow rate was reduced to 46.60%.

3. At the 130th_{minute, X-44 is shutdown, so that the} flow rate is reduced to 70.89%.

From the 4 cases that have been simulated for the centrifu-gal compressor obtained the results shown in Fig. 5.

Fig. 5 Comparison of Controller Simulation Results in Gas Lift System with Centrifugal Compressor Configuration

From Fig. 5 the anti-surge position that can handle lower flow is close to the surge line, it can be seen in centrifugal compressor configuration with PID controller and Ziegler-Nichols tuning method. X-12, X-29 and X-44 flow controllers (FIC-101, FIC-102 and FIC-103) show similar results and are

difficult to distinguish, this is because those streams are con-trolled in addition also become disturbance variable. While overshoot can be seen clearly at the X-48 well controller (FIC-104), at the second disturbance overshoot peak is greater be-cause the flow loss is also greater. Level controller of the suc-tion scrubber (LIC-77100) and the level controller on the dis-charge scrubber (LIC-77102) shown the smallest overshoot on Ziegler-Nichols tuning method, those configuration reach the steady condition faster. The suction pressure controller (PIC-100) and temperature controller (TIC-100), the smallest overshoot shown in the centrifugal compressor configuration with PID controller and Ziegler-Nichols tuning method.

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Fig. 6. Comparison of Controller Simulation Results in Gas Lift System with Reciprocating Compressor Configuration

From Fig. 6. Flow controllers show similar behavior with centrifugal compressor configuration. The suction pressure controller (PIC-100) is achieve stability fast enough because manipulated variable that is speed of compressor very influ-ential to the pressure at compressor suction. The level variable in suction scrubber A (LIC-77100A) in the second the third disturbance, the smallest deviation precisely on the recipro-cating compressor configuration with PI controller and Zieg-ler-Nichols tuning. This is fair because at the level usually the proportional controller is more dominant and the delay time is usually small, so the PI controller is better used here (Se-borg et al., 2004). At LIC-77100B, after the third disorder tends to be unsteady; this is because the reciprocating B com-pressor package has shutted down at the third disturbance. In discharge scrubber level controller that is LIC-77102A also experience the same with level suction scrubber controller. As for LIC-77102B on the third interruption compressor B has trip, so the level at discharge scrubber B is reduced dras-tically. At the TIC-100A temperature control controller, the least overshoot deviation of the set point is also shown in re-ciprocating compressor configuration with PID control and Ziegler-Nichols tuning method. At temperatures its overshoot deviation tends to be high due to high the delay time (Seborg et al., 2004). While the TIC-100B tends to fall on the third disturbance because there is no gas fluid that passes train B.

C. Economic Analysis

Fuel costs in the gas lift system are focused on providing energy for compressors and providing energy for cooling. At duration of 190 minutes, the simulation of total energy re-quirements is shown in Table X.

TABLEX

ENERGY REQUIREMENTS ON EACH GAS LIFT CONFIGURATION

Gas Lift Configuration

Compressor Energy (MBtu)

Cooling Energy (MBtu)

Total (MBtu)

Centrifugal, PI,

Ziegler-Nichols 17,381 22,946 40,327 Centrifugal, PI,

Tyreus-Luyben 18,224 23,885 42,109 Centrifugal, PID,

Ziegler-Nichols 17,382 22,934 40,316 Centrifugal, PID,

Tyreus-Luyben 17,950 23,595 41,545 Reciprocating, PI,

Ziegler-Nichols 13,244 19,204 32,448 Reciprocating, PI,

Tyreus-Luyben 13,245 19,375 32,620 Reciprocating, PID,

Ziegler-Nichols 13,247 19,195 32,442 Reciprocating, PID,

Tyreus-Luyben 13,245 19,352 32,597 Based on Table XII, the smallest energy needs of the gas lift system is reciprocating compressor configuration with a PID controller and Ziegler-Nichols tuning method amounting to 32,442 MBtu when there are interruptions of wells X-12, X-29 and X-44 respectively.

With maintenance costs equal to 0.25 USD/MMBTu, the total cost can be summed from CAPEX cost, fuel cost for compressor, fuel cost for cooling, maintenance cost. The re-sult as shown in Fig. 7.

Fig. 7 Dynamic Economic Profit Result with Levelized Cost Method on Each Gas Lift System

Based on Fig. 7 the entire gas lift configuration is econom-ically profitable, as evidenced by the absence of negative profit values. But the profit of each configuration shows var-ying results. With only X-48 wells operating is enough to give profit to the entire system.

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297 3.88 USD/MMBTU and the average profit on the reciprocat-ing compressor also rises from 3.62 USD/MMBTU to 3.80 USD/MMBTU This is because the production of the X-12 well is returned more as a gas lift and produced only 0.05 MMSCFD, while the decrease in the fuel cost for compressors and cooling cost far enough, this resulted the profit actually goes up.

As the second disturbance, the profit drops as the amount of gas produced decreases significantly compared to the de-cline in the fuel cost for compressors and cooling cost. The centrifugal compressor profit position decreased from 3.88 USD/MMBTU to 3.54 USD/MMBTU and the average profit on the reciprocating compressor also fell from 3.80 USD/MMBTU to 3.68 USD/MMBTU.

At the third disturbance, the profit for reciprocating com-pressors drops slightly from 3.68 USD/MMBTU to 3.64 USD/MMBTU, while the gas lift system with centrifugal compressor configuration drops drastically from 3.62 USD/MMBTU to 3.12 USD/MMBTU. At the end of simula-tion, precisely the profit of the reciprocating compressor con-figuration is above the concon-figuration of the centrifugal com-pressor because the reciprocating comcom-pressor system has 2 compressors, so when the flow is below half of the compres-sor capacity, train B on the comprescompres-sor is trip so that the sys-tem becomes more reliable, and Profit on this disorder for re-ciprocating compressors to be higher.

D. Overall Configuration Evaluation

In general the best configuration results should have the smallest error value evaluated by the ISE method and the larg-est profit.

Controller evaluated by the ISE method because this con-trol criteria provide large deviations, so there is a significant difference between bad control system and good control sys-tem. Profit response calculate from the area of profit response in each configuration shown in Fig.7.

The ISE and its profit response relationship of each gas lift system configuration shown in Table XI.

TABLEXI Reciprocating PI TL 6525.8

3 3.62

Reciprocating PID ZN 88.33 2.46 Reciprocating PID TL 5732.3

7 3.50

In the same system, the PID controller performance has a smaller ISE than the PI controller. The Ziegler-Nichols tuning has a smaller ISE up to 99.33% on a centrifugal compressor configuration and 98.65% for reciprocating compressor con-figuration than Tyreus-Luyben tuning method.

IV.CONCLUSIONS

This research has studied the behavior of a dynamic gas lift system with centrifugal compressors and reciprocating com-pressors due to well depletion. This research obtained the value of each ISE of each configuration, control and tuning. The conclusions of this research are:

1. The design of the gas lift system is obtained. The Zieg-ler-Nichols tuning has a smaller ISE up to 99.33% on a centrifugal compressor configuration and 98.65% for reciprocating compressor configuration than Tyreus-Luyben tuning method.

2. Energy requirements in each compressor configura-tion is obtained. The smallest energy needs of the gas lift system is reciprocating compressor configuration with a PID controller and Ziegler-Nichols tuning method amounting to 32,442 MBtu or can reduce by 22.96% of the gas lift system configuration with a cen-trifugal compressor with a PI controller and Ziegler- Nichols tuning method.

3. Dynamic response economic result is achieved. The entire configuration of the gas lift system is economi-cally feasible. Gas lift system with reciprocating com-pressor configuration with two trains more reliable to face the well depletion. Profit response behavior for controllers with small ISEs are faster up than control-lers with high ISEs.

ACKNOWLEDGMENTS

We express our gratitude to the Universitas Indonesia which has funded this research through the scheme of Hibah Publikasi Internasional Terindeks untuk Tugas Akhir Maha-siswa (PITTA) No.2068/UN2.R3.1/PPM.00/2017.

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