INTRODUCTION
Oil and gas reservoirs are a formation of rock in which oil and natural gas has accumulated. It is created naturally over the course of hundreds of millions of years when layers of decomposing plants and animals encounter with intense heat from the Earth and pressure from rocks. Oil and gas are subsequently collected in small, connected pore spaces of rock that are trapped within the reservoir by adjacent and overlying, impermeable layers of rock. In order to recover oil and gas, the reservoir production will need to be assessed for further exploration and production.
At early stage of well production, there is typically an adequate amount energy stored within the reservoir to drive high production rate. This case is usually being adopted as a design case for most system installed at topsides. Upon maturity, production operating pressure and rate are bound to deplete due to reduction of reservoir pressure. It has been observed that incoming fluid temperature would become higher than normal temperature due inadequate fluid cooling during production by the end of field life. As a result, it has a direct impact to equipment’s performance specifically towards rotating equipment such as Gas Compression System and Crude Export Pump.
DESCRIPTION
Gas Compression System are typically used to improve gas pressure from production operating pressure to pipeline export pressure at offshore. The package is designed to cater for maximum production gas rate at certain suction pressure. The suction pressure is selected based on lowest possible operating pressure based on reservoir’s production profile and selected topsides’ process scheme. A low compressor suction pressure indicates low flow input to the compressor. These conditions will result compressor operating point closer to compressor’s surge line. In that case, the compressor is expose to risk of mechanical damage. It is because the rotor of the compressor repetitiously shifts back and forth from the active side to inactive side at the surge line which damages the thrust bearing. Further to this, there would be a risk of overheating of the compressor, exceeding the maximum allowable temperature of the unit.
To inhibit compressor from operating at the surge line, compressor skid is usually equipped with anti-surge valve to prevent risk of mechanical damage and overheating. At end of field life, usually the anti-surge valve will be continuously open as to allow parts of compressed gas returned to compressor suction system.
Potential impact of prolonged recycling the gas includes the following:
- Risk of gas leaning
- Waste of energy due to gas pressure reduction
- Waste of energy due to recycle gas cooling
Jet pump is an option on top of conventional anti-surge valve as to move compressor operation point back to design operating point. It is simply by improving the suction pressure deriving from the Joule-Thompson (JT) effect across the jet pump motive nozzle. In essence, jet pump can be utilized as incoming gas cooler.
It is a common scenario for operators to deal with high gas suction temperature from compressor at end of field life. Conventional heat transfer equipment such as air cooler, shell and tube exchanger, etc., are usually selected for gas cooling. Following challenges is required to be carefully addressed when heat exchangers are considered for incoming gas cooling process:
- Space
- Weight
- Utilities Support
- Pressure Drops across the Exchangers
Nonetheless, using a jet pump for incoming gas cooling process benefits the following compressor suction concerns:
- Suction Mass Rate
- Suction Pressure
Concerns pertaining to compressor suction can be addressed by utilizing part of the discharge gas as a motive gas for jet pump, particularly while produced gas from reservoir is routed to jet pump via a jet pump suction nozzle. The discharge gas is then subsequently sent to compressor for further gas compression process as per normal operation.
A minimal facilities modifications being required for jet pump installation. There are three (3) tie-ins connections are required. The connections are as below Note 1:
- Jet Pump motive connection
- Jet Pump suction connection
- Jet Pump discharge connection
Notes 1: No modification to existing compressor skid. All jet pump’s tie-ins will be designed outside of compressor skid as to ensure compressor’s warranty is well kept
Nonetheless, there are prerequisite for installing a jet pump to improve operation of compressor at the end of the field life. It is feasible for the case of the following design considerations are met:
- Availability of motive fluid
- Pressure ratio
- Larger capacity
- Operating envelope
- Ability to provide maintenance space if interchangeable jet pump design is selected
ADVANTAGES
Jet pump offers better solution in improving compressor at end of field life due to the following:
- Minimal space required for jet pump installation
- Minimal additional weight to platform
- No additional utilities are required
- No pressure drops across equipment
- Lower suction temperature
- Improved suction gas molecular weight
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