Wet Gas Compressor Testing – Performance Uncertainty

The prospects for wet gas compression of well streams provide a great opportunity for oil and gas producers when compared with traditional top side compression. Lower CAPEX due to simplified process design (no scrubber and pump necessary), increased oil recovery, and an energy efficient solution due to lower flow velocities and therefore less pressure losses in the transportation pipes.

Regardless of the possible upsides for well stream compression by means of centrifugal compressors there are several challenges that need to be addressed before this technology can be considered proven. These include robustness to wet gas in relation to rotor dynamics, erosion, surge and any utility systems that are subjected to wet gas. Furthermore, much work remains in the evaluation of wet gas performance. For dry gas compressors design and testing follow an established route in which the customer provides compressor suction conditions and the requirement for the discharge pressure, and the vendor match impellers to reach this target. Dry gas performance testing is typically agreed to be performed in accordance to established international standards such as ISO 5389 or ASME PTC10. However, for wet gas compression no such international standard exists, and the vendors do not jet possess the knowledge for how to predict wet gas performance in the design phase.

Based on experience from wet gas compressor testing at NTNU (low pressure air water fluid) and K-Lab full scale testing (Normal operating conditions, high pressure and hydrocarbon fluids) this paper document important aspects relating to the uncertainty evaluation of wet gas compressor performance test results;

Accurately determining the correct fluid composition and proper selection of equation of state are of immense importance for how the fluid is characterized, as this directly affect the fluid properties.                  

Uncertainties of physical properties originating from the thermodynamic simulation, shows that gas density should be determined from a direct measurement and not from thermodynamic simulations.

A total composition originating from gas a chromatograph is prone to uncertainties especially for richer gases. The highly accurate GERG equation of state has uncertainty less than 0.1% on gas density if used within given ranges. However, since composition has uncertainties and may be outside the limits of the equation, experience shows that uncertainties will be fare higher than 0.1%.

Monte Carlo method for evaluation of the uncertainty on a wet gas compressor system is shown. Resulting uncertainties on the compressor performance is presented.

Sensitivity analysis has been performed for a wet gas compressor system, in order to access the major contributors. Specially the water impact on the performance has been exposed.