Gas injection compressors are playing major and vital role in oil and gas facilities for maintaining reservoir pressure and sustain production. However, operating experience of these facilities is still limited. Sour and high pressure gas injection compressors present unique challenges especially during start-up/commissioning. Failure of many start-up attempts can cause endless delays and can lead to production and revenue losses. This paper provides useful insights into these issues and their mitigation.
Gas injection compressors operate in very high pressure "Dense Phase" region. Dense phase is compressible fluid and exhibits properties of both liquid and gas. There is no clear boundary separating dense phase from liquid phase or dense phase from gas phase. Slight change in operating conditions can shift the fluid from dense phase to liquid or transition region resulting in unpredictable behavior in compressor/seal system. This can activate spurious shutdown due to either phase change or measurement errors under highly dynamic start-up conditions. Operating range has been studied to identify the issues and mitigation to avoid such conditions.
Detailed root cause analysis has been carried out to investigate full spectrum of operating and startup conditions for compressor as well as dry gas seal system. These are based on author's experience with startup and commissioning of several gas injection compressors which included spurious shutdowns and failure to start up. These have led to development of a robust procedure as well as selection of proper process conditions to successfully start the compressor that include issues such as:
Design of dry gas seal system to avoid condensation /sulfur deposition and seal damage
Scrubber liquid level measurement and control
Potential compressor flooding due to layout and consequent draining issues
Shutdown, settle-out and liquid handling during depressurization
Preferred inter stage pressure, temperature and operating region
Start-up and commissioning challenges due to above mentioned design issues have been successfully tested using dynamic simulation and implemented for smooth start-up of an existing gas injection compressor operating in dense phase region.
This paper provides useful insights into issues encountered during start up and commissioning of high pressure gas injection compressor operating in dense phase region and their mitigation. The proposed mitigations have been demonstrated using dynamic simulation and tested at site. Design strategy is provided to ensure more robust design in future for such high pressure gas injection compressor.