Copyright 1983 by ASME CONTROL ASPECTS OF A COMPRESSOR STATION FOR GAS LIFT H. SAADAWI

Transcription

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47 St., New York, N.Y GT-100 The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published In an ASME Journal. Released for general publication upon presentation. Full credit should be given to ASME, the Technical Division, and the author(s). Papers are available from ASME for nine months after the meeting. Printed in USA. Copyright 1983 by ASME CONTROL ASPECTS OF A COMPRESSOR STATION FOR GAS LIFT H. SAADAWI ABU DHABI COMPANY FOR ONSHORE OIL OPERATIONS (ADCO) ABU DHABI, UNITED ARAB EMIRATES Member ASME ABSTRACT The type and characteristics of the control system to be used for a centrifugal compressor station depend on several factors such as the compressor driver, process requirements, and the conditions under which the compressor will be operated. Designing a compressor control system for gas lift applications present different types of problems than those of conventional pipeline applications. This paper describes the control philosophy of a compressor station used for lifting water in a closedrotative gas lift installation in Bu Hasa field, Abu Dhabi. COMP/ TURBINE GAS o r GAS LIFT WELL 900 OIL PRODUCING WELL WATER INJECT ION WE LL Fig.l Water produced by gas lift is used for reservoir pressure maintenance

2 1. INTRODUCTION Located some 60 kilometers onshore from the Arabian Gulf, Bu Hasa Field is the largest oil field in the United Arab Emirates. In order to maintain the reservoir pressure, a four station gas lift/water injection system has presently been installed. Early operating experience with the gas lift compressor stations has been described elsewhere [1]. This paper addresses the main aspects of the station control system. A brief description of the installations follows. A conceptual schematic of Bu Hasa Gas Lift/Water Injection System is illustrated in Figure 1 and a gas lift station is shown in Figure 2. The gas lift station was designed to produce 250,000 b/d (39,750 m 3 /d) of injection water. The water, from overlying acquifers, is lifted by a closed loop gas lift system. High pressure gas for lifting the water is distributed through six inch gas injection lines from the station compressor to four supply wells surrounding the station. The watergas mixture from the wells then flows to a separator. The water flows from the separator into a flow suction tank and is pumped into a ring main for injection in the oil strata. The gas is returned to the compressor for recycling. A flow diagram of the gas lift station is given in Figure 3. This type of gas lift installation is known as a "closed-rotative system", because the gas "rotates" from the wells to the compressor, back to the wells and to the compressor again. Compression is done in a 10,000 h.p. (7.5 MW), 3- section centrifugal compressor with intercooling and aftercooling (Figure 4). The compressor train is driven by an aircraft-derivative gas generator aerodynamically coupled to a single stage industrial gas turbine. An industrial gas turbine, site-rated at 1200 h.p. (0.9 MW) drives an alternator which provides the necessary power for the staiton auxiliaries. The make-up line provides treated gas which fuels both gas turbines. This make-up gas also blankets the flow tank (Figure 3). 2. GAS LIFT WELLS PERFORMANCE The compressor is an integral part of the gas lift station and as such must be matched to the requirements dictated by the gas lift process. A basic understanding of gas lift wells performance is, therefore, essential. In Bu Hasa Gas Lift Wells, gas is injected into 31" tubing and flows via the gas lift valves into the annulus of the 13 3/8" casing where it aerates the water. The water/gas mixture is produced through the annulus. This is shown in Figure 5. The volume and pressure of injection gas is monitored by means of a two-pen recorder at the wellhead. The gas lift valves are of the Nitrogen charged type. They are set to operate at a certain gas pressure in the tubing. As far as the compressor is concerned, the gas lift valves are essentially back-pressure regulators. For each well, the water production rate depends on several parameters. Although both injection gas pressure and flow rate affect the water production rate, there is an important difference in the effect of these two variables. A gas lift installation is designed to operate at a certain injection pressure. If the pressure falls below this value, the well will stop flowing. The gas flow rate however can be varied within the producing range of the well. A typical performance curve of one Bu Hasa water supply well is shown in Figure 6. The curve can be calculated using a computer simulation model and is verified in the field by conducting a well test. Such a curve is used to optimise the water production [2]. As shown in the figure, a continued increase in gas volumes leads to a corresponding increase in the water production up to a point (B) at which increasing the gas injection rate does not lead to an increase in the amount of water produced. The flow at point (A) corresponds to the minimum amount of gas needed to maintain continuous flow. If the gas flow falls below this amount, the well dies or begins to intermit. Similar performance curves are plotted for the other wells. The compressor through-put at normal operating conditions is the sum of injection flow rates that correspond to the optimum point on the performance curve of the four wells. If, due to operational reasons, it is required to reduce the amount of water production, this can be done by reducing the amount of gas flow rate Fig.2 Gas lift station 2

3 to one or more of the four wells. The well will continue to flow as long as the amount of the injection gas rate is anywhere between points (A) and (B) along the well performance curve shown in Figure CONTROL SYSTEM PHILOSOPHY Before defining the basic control objectives of the Gas Lift Stations, operational requirements and manning philosophy of the stations are first discussed. 3.1 Operational Requirements The desired station water production rate at any given time is a decision based on water injection requirements, water production from other stations, conditions of facilities, etc. The operator can control the amount of water production by controlling the injection gas as will be explained later. The Control System was designed so that the station is manned 24 hours a day with two men per shift. A fourwheel drive vehicle with 2-way radio is available at the station so as to enable one of the operators to open-up or shut-down wells as required. When the station control system was designed in the seventies, the possibility of using a computer-based control scheme was considered. However, a scheme where full time manning is required was selected over a fully automated scheme due to the following reasons: - Greater potential for trouble from equipment failure. With the hot climate in Abu Dhabi, there is the possibility of failure as a consequence of high temperature. The Gas Lift Stations are located in remote TURBINE/ ALTERNATOR A FUEL GAS KI =MIN= COMPRESSOR/TURBINE K2 K3 T 'CONTROL SYSTEM MAKE UP AIR COOLERS ott GAS WATER INJECTION RING ) 2 PHASE ( SEPARATOR FLOW WATER V TO OTHER GAS LIFT WELLS PUMP FLOW TANK GAS LIFT WELL Fig.3 Flow schematic of a gas lift station 3

4 Fig.4 Gas Compressor desert areas on the periphery of Bu Hasa Field. Any computer equipment would be installed in an air-conditioned room. Failure of the air conditioning in a summer day would result in the ambient rapidly rising above the maximum permissible operating temperature. Even if the equipment is shut-down, there is the possibility that the maximum permissible temperature 2 _PHAS E could also be exceeded. F LOW - Even if the station control system is automatic, manual operation is still necessary to "kick-off" the gas lift/water supply wells. This is due to the nature of the process. When a well is "kicked-off", i.e. being started, the two-phase gas/water mixture is first flown to the desert to stabilize the flow and "clean" the well. A milliport test is carried out at site to determine the total suspended solids (TSS) in the wat- N er. Only when the TSS contents fall below 2 ppm, 2 water is flown to the station and injected into the CHARGED oil reservoir. This is to prevent the suspended sol- GAS LIFT ids in the water from blocking the reservoir voids. Depending upon the condition of the well, it may be VALVE necessary to flow the initial well production to the desert for several hours before flowing the well to the station. 3.2 Control Objectives For convenience, the gas lift station control system can be divided into three general categories: WATER (a) (b) Station/compressor performance control Compressor antisurge control (c) Safety and alarm system These are dealt with in the remaining sections of the paper. Fig.5 Gas is injected in the tubing and the two-phase flow is produced in the casing 4

5 INJ. GAS PRESS=1100 PSIG Another factor to consider when calculating system resistance curves and rating compressors for gas lift applications is the change in composition of the recycled gas. As the gas is injected into the wells, separated and recycled through the compressor, its composition and molecular weight change. This is due to the mixing of the injection gas with the gas evolving from the reservoir fluid. The final molecular weight of the gas should be used for computing the design point performance of both the system and the compressor. 4.2 Discharge pressure versus flow control The primary concern in a compressor control system is the process variable; the variable in the compressor performance that is controlled to meet the process objectives. A centrifugal control system can be designed to maintain a desired discharge pressure or a desired flow to a process; it cannot be designed to maintain both [4]. The question now arises: which of these two variables is suitable for gas lift compressors? Fig.6 Performance curve of a typical gas lift well 4. STATION/COMPRESSOR PERFORMANCE CONTROL Performance control means providing the lift gas to the wells at the required pressure and volume as determined by operational needs. 4.1 System resistance curve The compressor operating point is determined by the intersection of the compressor head-capacity curve with the system head curve. It is, therefore, essential to fully understand the system characteristics. The system head curve for a gas lift installation differs from that of a conventional pipeline application For a conventional pipeline, the head developed by the compressor is mainly used to overcome the friction losses in the pipeline. In general, such a resistance curve is of a parabolic nature. Figure 7a shows the head curve for a typical pipeline application. The centrifugal compressor natural characteristics match the normal pipeline operating characteristics. This is because the locus of the maximum efficiency points nearly parallels the normal operating line of the pipeline. The selection of a compressor to match certain pipeline applications is discussed in reference [3]. On the other hand, the resistance curve of a typical gas lift installation is basically a flat one (Figure 7b). As discussed earlier, the gas lift valves are essentially back pressure regulators and a certain operating pressure at the wellhead should be maintained at all times in order to keep the valve in the open position and allow the gas to flow from the tubing to the casing. Due to the flat characteristics of the gas lift system, it is important when selecting a compressor with a gas turbine driver to specify the compressor characteristics with a minimum rise from design to surge. It is clear from Section 2, that the design injection gas pressure must be available at the wellhead at all times. If it is not, the well will die. Therefore, compressor performance is controlled by maintaining constant discharge pressure at all times. This is achieved by speed control as shown in Figure 3. If it is required to decrease the water production from the station, then the corresponding reduction in the volume of gas can be calculated from the wells performance curves (Figure 6). This reduction in gas demand from the compressor is met by decreasing the compressor speed as will be explained in the next section. The new speed can be approximately calculated by using the affinity laws. It is important, therefore, when selecting a gas turbine for a gas lift compressor to make sure that the power turbine "turn-down" ratio is suited to the particular system to ensure that the various expected operating conditions can be met by speed variations. 4.3 Speed control Control of the compressor speed is through a closed loop discharge pressure controller. A block diagram of the compressor/turbine control system is shown in Figure 8. The pressure signal from the injection gas manifold feeds the pressure controller. The controller compares the signal to the set point and issues a corrective signal which is used to bias the power turbine speed (N2) channel. The least gate (shown in Figure 8) denotes that the function N1, N2 or T4 calling for the least a- mount of fuel flow is the controlling factor. Once having decided on the control variable, the signal is converted into a speed demand and subsequently a speed change at the compressor. This is effected by regulating the gas generator output. The servo or COP limiter (Compressor Discharge Pressure) converts the signal received from the actuator into movement of the fuel gas valve by which fuel flow to the gas generator is controlled. 5. COMPRESSOR ANTISURGE CONTROL In addition to meeting the process requirements, the other major objective of the compressor control system is to prevent the machine from surging. 5

6 ca 110 = 100 w PERCENT RATED CAPACITY (a) Pipeline PERCENT RATED CAPACITY (b) Gas lift Fig.7 System resistance curve Two separate antisurge loops exist round the L.P. and H.P. compressors (Figure 3). The problems experienced with the original antisurge control have been discussed in reference [1]. The main problem with the original pneumatic system was one of response. The pneumatic system has been replaced by an electronic one. The new system was designed in-house and the hardware was installed entirely by the company personnel. A drawback of the old antisurge control system was that the flow signal was obtained by measuring the flow at the suction of the first section (K1) of the compressor. Under certain operating conditions, the second section (K2) surges before the first. A new flow element was installed at the suction line of K2. The differential pressure generated across K2 is found by the difference of two pressure measurements. This pressure differential sets a flow controller using the differential across K2. One of the major problems in establishing good surge control is obtaining a good value for the orifice differential pressure. Careful considerations were given to choosing the orifice location at the inlet to K2, since the gas flow near the compressor is usually turbulent. It was necessary to modify the piping configuration and to use straightening vanes in order to ensure that the orifice is installed in properly designed meter runs, in accordance with AGA Standards. 6. SAFETY AND SHUTDOWN SYSTEM There are two control panels in the station control room; one for the compressor/turbine package and the other for the other process equipment (separators, flow tank and booster pumps). The safety and shut-down system include the following: - Alarm - Shut-down - Fire fighting system An anunciator system for both alarm and shut-down is provided. The alarm system for compressor/turbine package monitors the various parameters for the machines and the auxiliary systems which include: - Fuel gas - Gas generator hydraulic and lub oil - Power turbine and compressor lub oil - Compressor seal oil It is outside the scope of this paper to describe all the capabilities of the alarm system. The station shut-down system incorporates venting of the gas compressor to the high pressure flare and closing of the gas/water inlet, to the production and test separators. In case of shut-down, the fuel gas skid is also vented to the high pressure flare and the inlet gas line to the station is isolated. The separators are vented to the vent pit. The injection gas lines to the supply wells are isolated at the station boundary. The gas generator/power turbine enclosure is protected by a Hallon 1211 Fire Extinguishing System. The system is automatically activated by cross-zoned ultraviolet flame sensors and temperature rate-or-rise sensors. Due to the high ambient temperature and inadequate cell ventialtion system, the conditions of the sensors deteriorated rapidly. A modification on the system was carried out by relocating the electronic control box in the control room. 6

7 2 ACTUATOR WF P.T N2 G.B TO GAS LIFT WELLS MIN WF MAX WF SERVO LIMITER P2 N1 Ni Ni SPEED Ni BIAS CHANNEL L EAST T4 I T4 TEMP GATE CHANNEL T4 T4 BIAS N2 N2 N2 DISCHARGE 4 SPEED PRESS CHANNEL CONTROLLER N2 BIAS { AUTO MANUAL Pd Pd BIAS Fig.8 Block diagram of the control system of the gas turbine/compressor package G.G. gas generator Wf fuel gas mass flow rate P.T. power turbine P2 G.G. compressor discharge press (CDP) G.B. gear box N1 G.G. speed COMP gas lift compressor N2 P.T. speed Pd COMP discharge press T4 G.G. exhaust gas temp Nc. COMP speed

8 7. CONCLUSIONS An analysis of the control system of Bu Hasa Field Gas Lift/Water Injection Scheme has been presented in this paper. This analysis can provide guidelines to both users and designers when selecting the control system of gas turbine-driven compressor stations for gas lift applications. The control system discussed here is particularly relevant to closed-rotative gas lift installations where the gas is compressed in a central station and injected in several satellite supply wells. Based on our operating experience with Bu Hasa Lift Project, the following conclusions are drawn: - Gas lift performance should be properly understood by the designer of the compressor station control system. The engineering and control system design need to be done in conjunction; not in isolation from each other. - The system resistance of a closed rotative gas lift installation is basically one with flat characteristics. - Maintaining the minimum design value of injection pressure at the wellhead of a gas lift well is essential for flowing the well. Therefore, for a closed rotative gas lift system, maintaining a constant discharge control should be the primary objective of the compressor control system. - If constant discharge pressure control is to be accomplished by speed variation, then the gas turbine package turndown ration should be high enough to accomodate the compressor speed variation. - The compressor should be selected such that the head rise from design to surge must be of a certain minimum, and adequate flow range for control and operating flexibility. ACKNOWLEDGEMENT The author wishes to thank the management of Abu Dhabi Company for Onshore Oil Operations (ADCO) and the Abu Dhabi Government Department of Petroleum for permission to publish this paper. REFERENCES 1. Saadawi, H., "Oil Field Experience with Gas Turbine Compressor Stations", ASME Paper No. 82-GT-300, presented at the 27th International Gas Turbine Conference, London, April 18-22, Brown, K.E., The Technology of Artificial Lift Methods", Vol. 2a, Pennwell Publishing Company, Tulsa, Heard, T.C., Lang, R.P. and Wright, D.R., "Compressor Selection and Matching", Paper No. GER-3097, General Electric Co., New York, Rollins, J.P., ed., "Compressed Air and Gas Handbood", 4th Edition, Compressed Air and Gas Institute, New York,