Gas jet compressors or ejectors

Compression

James F. LeaJr, Lynn Rowlan, in Gas Well Deliquification (Third Edition), 2019

5.16 Gas jet compressors or ejectors

Gas jet compressors, or ejectors, are classified as thermocompressors and are in the same family as jet pumps, sand blasters, and air ejectors. They use a high-pressure gas for motive power. Ejectors using gas can impart up to two compression ratios; using liquid they can generate higher ratios if cavitation can be avoided

The ejector, or gas jet compressor, operates on the Bernoulli principle as illustrated in Fig. 5.10. The high-pressure motive fluid enters the nozzle and is accelerated to a high velocity/low pressure at the nozzle exit. The wellhead is exposed to the low pressure at the nozzle exit through the suction ports and is mixed with the motive fluid at the entrance to the throat. Momentum transfer between the motive and produced fluids in the throat and velocity decrease in the diffuser increases the pressure to the discharge pressure.

Figure 5.10. Principles of gas jet compressor (ejector).

Eductors have potential advantages including:

•

No moving parts.

•

Low maintenance/high reliability.

•

Easy to install, operate, and control.

•

Can handle liquid slugs.

•

Low initial cost/payback time usually short.

•

Nozzle sizes can be changed to meet changing well conditions.

Fig. 5.10 shows an ejector in actual field service. One successful configuration uses a flooded screw compressor to pull the tubing/casing annulus down to 8–10 psig. A portion of the gas discharged by the compressor is used to drive an ejector to pull the tubing down to 1–5 psig. The exhaust of the ejector is combined with the casing gas and sent to the compressor (Fig. 5.11).

Figure 5.11. Ejector installed on wellhead from Gas Well Deliquification 2nd Edition, Compression Chapter by D Simpson.

If high-pressure fluid is available (e.g., from a nearby high-pressure gas well) to power the ejector, then it is advantageous to utilize this wasted energy with an eductor to lower surface pressure on a lower rate well to prevent liquid loading.

The principle disadvantage of eductors is that they have a higher hp/MMCF requirement than other technologies (i.e., they have lower mechanical efficiency). This lower efficiency can often be offset by extremely low capital cost. For example, a well was limited to 600 MCF/d with 9 psig wellhead pressure by the two-stage reciprocating compressor installed. The compressor had plenty of horsepower to move more gas but the piping and cylinder configuration did not allow lower pressures. Replacing the 300 hp compressor with a different machine would be very expensive so an ejector was added between the wellhead and the compressor to compress the full stream—basically adding a compression stage. This ejector lowered the wellhead pressure to –5 psig with 9 psig discharge pressure (atmospheric pressure at this sited is 11 psia so the ejector developed 3.3 compression ratios) and increased the well’s production to 900 MCF/d. The efficiency of the ejector is only 46%, but it reduced the capital outlay required by more than an order of magnitude.