Reinjection compressors

In order to increase the amount of oil production the enhanced oil recovery (EOR) techniques based on gas re-injection or gas injection are used. The oil and gas mixture produced from oil fields is separated into oil and gas components at the processing facilities. The gas is recompressed and re-injected again and again until as much oil as economically feasible has been produced from the oil field. Alternative option is injection of gas from other sources to increase oil recovery (for oil fields which have limited gas mixed with oil). Today competent vendors are capable of manufacturing re-injection centrifugal compressors of discharge pressures more than 900 Barg for compression of extremely sour gas (above 20% H₂S).

Injection compressor

Vertically split (or barrel type) casings are generally used for re-injection centrifugal compressors except low pressure initial stage which may use horizontal split casing (below 45 Barg). Casing with heavy bolted head is used for medium pressure. Common-in-use configuration is forged casing with shear ring head. Modern re-injection high pressure compressors generally use closed type impellers with three-dimensional blade profile. Both efficiency and pressure coefficient are higher for three dimensional impellers. Impellers are generally manufactured from low alloy medium strength steels to NACE (National Association of Corrosion Engineers) which can accommodate low and moderate corrosive gases. Impellers tip speed limit is around 15-20% lower than common-in-use non-corrosive gas applications. Critical compressor components in contact with the extremely corrosive gases (for example sometimes sour gas containing more than 10% H₂S), are made out of corrosion-resistant alloys.

For high pressure re-injection compressors the thrust generated in machine is much greater than can be handled in a thrust bearing. Common design is back-to-back impellers, and using two balance pistons for cancelling the thrust force. Remaining axial force is the difference between forces which are a magnitude larger and advanced double acting tilting thrust bearing is required.

High percent of all outages of centrifugal compressors are caused by bearing and seal problems. The shaft end seals in modern re-injection compressors are generally dry gas seals. Oil seals were very common in old designs but they are not used today due to high power consumption, low reliability and many operational problems. There are a few dry gas seal sub-suppliers for extremely high pressure sour gas re-injection services. The seal is generally a tandem seal configuration. Special attention required due to possibility of damage upon rapid depressurization and explosive decompression.

Injection train design

More than half of gas re-injection compressor trains are driven by gas turbine. While heavy-duty gas turbine drivers have generally been used for compression trains in onshore applications, the majority of high pressure re-injection offshore installations use aeroderivative compact and light weight gas turbine drivers.

Electric motor drivers are used in small and medium size gas re-injection trains. One common arrangement is two-end-motor configuration. The train consists of a both ends drive motor (output at both ends of the shaft) installed in the center of the train, two speed increasing gears provided at outer ends, and two compressors outside them. By this configuration, optimum speed can be achieved for each casing and the power loss from speed increasing gears is suppressed to a minimum. The compressor can be opened by drawing out the internal cartridge (barrel) in the axial direction, and it is not required to disassemble the compressor casing. Therefore, maintenance of the compressor is possible without disassembling the main gas piping.

Remote injection plant has limited allowance in electric power source capacity considering high power required by injection compressors. Large current flows on direct online start can cause voltage drop and other adverse effects in other power appliances in the district. Usually soft starter is required.

Train energy is high, gas handled and site conditions are very risky and compressor surge (aerodynamic instability in low flow) can lead to disaster. Minimization of gas volume between compressor discharge and anti-surge valve is mandatory. Reliable and modern anti-surge loop design is from compressor discharge to upstream of previous stage cooler.

Dynamic of compressor

Vibration needs to be minimized at all operating conditions by using very rigid shafts. In high pressure re-injection applications, the fluid density becomes very high and compared with normal atmosphere, the natural frequency of the impeller is lowered, and impeller resonance may occur. Compressor rotor may vibrate unstably. For example in a re-injection machine, the natural frequency is lowered to about 60% of that of normal atmosphere. A small flow rate of high density gas is likely to cause rotating stall. To prevent increase of rotor vibration due to rotating stall, the range of continuous operation is usually narrowed. Another major risk is rotor-dynamic instability of the rotating shaft induced by the inter-stage and balancing drum seals.

Generally various methods are used to eliminate or damp the magnitude and effect of excitation forces. Such special features will normally only be needed at high pressures (let say above 250 bar). These solutions are all mainly based on empirical data, comparison with successful designs and testing at full pressure.

A “shaker test” is required particularly for high pressure stage casings in order to verify the rotor-dynamic behavior and to determine the damping of the rotor at various operating conditions. For this test, an excitation to the rotor that is frequency independent from the rotor speed is required. For this test, usually the end cover of the barrel carrying the rotor is fitted with a temporary extension designed to host an active magnetic bearing around the rotor shaft extension. The magnetic bearing is then operated in such a way to induce vibrations on the rotating shaft. The overall damping of the compressor is determined by evaluating the rotor vibration response to a non-synchronous unbalance produced by the exciter.

Special considerations

Specific attentions are required for the rated head to include a margin covering gas property uncertainties, fouling and performance deterioration in re-injection service. Best vendor shop test package for re-injection compressors is the pressurized shop mechanical run test with nitrogen followed by a shop performance test as per ASME PTC 10 type 1 with natural gas. Generally sweet natural gas with some makeup is used in shop performance test. Some vendors offer ASME PTC 10 type 2 with gas mixture of CO₂ and helium, or other mixtures to simulate actual gas. It is not recommended since ASME PTC 10 type 1 with natural gas better simulates the site conditions. Shop performance test is more useful if real natural gas used and pressures, compressor speed, capacity, power, etc matched as close as possible to site operating conditions. This test can give very valuable information about the likely behavior of machine such as certain aspects of aerodynamic excitations, operation near limits such as surge or choke, rotor stability, rotating stall, impeller resonance, validity of gas properties prediction, realistic effects of seal or bearing on dynamic behavior, etc. These effects are of concern in high pressure re-injection compressors from even the best-known vendors.

Case studies

First case study is a re-injection compressor with discharge pressure around 250 Barg and driver power around 15 MW. This compressor is driven by both ends electric motor installed in the center of the train. Two speed increasing gears (at both ends) and two compressors outside them are installed. This train needs soft starter. Performance is satisfactory. It is an example of successful motor driven medium size re-injection machine for small or medium oil field enhanced oil recovery.

Second case study is a two barrels sour gas high pressure centrifugal compressor. Each compressor train consists a medium-pressure (MP) casing, a high pressure (HP) casing, a 12 MW, eight-pole, fixed-speed, electric motor driver and step-up gearboxes. The MP casing (six-impellers) receives the gas at around 110 Barg and delivers it at around 240 Barg to the HP casing (five-impellers) which compresses sour gas to the final re-injection pressure around 560 Barg. Machine design, fabrication, testing and site job were faced with some delays, however, machine operation and performance are satisfactory. This case study shows that re-injection machine is complex, needs case by case design and each project has unique features. Considerable preparation, study, planning and coordination, in advance, are necessary for a successful, on-budget and on-time re-injection project.

Third case study is a complex gas turbine driven centrifugal compressor for around 650 Barg sour gas (around 20% H₂S) re-injection application. Safety and environmental aspect and also material selection requirements for sour gas handling, impact considerably compressor package design and manufacturing. Each train consists, a large frame gas turbine driver (around 32 MW), a gear unit (speed increase to around 10,000 rpm) and three compressor casings. Sour gas pressure is increased from around 50 barg to around 200 Barg, around 400 Barg and around 650 Barg in three process stages (LP, MP and HP stages respectively). For MP and HP sections, discharge temperatures are limited (below 100^oC) to reduce gas harsh effects considering available material technology and long term reliability required. This case study is a successful example of large gas turbine driven high pressure sour gas re-injection trains.

Extensive reviews of these three case studies show main risk areas as follows:

• Rotor-dynamic (high density fluid), rotating stall, train transient and normal behaviors (torsional, surge, etc).

• Compressor component materials.

• Seals.