Jun 16

Data shows: Field personnel struggle most with VRTs and VRUs

We understand that the facility is not the most exciting part of production ops. People generally are a lot more curious about what happens downhole. However, just understanding the artificial lift isn’t enough. Operators, engineers, and managers all need to know how everything in the facility works. With the increased focus on environmental compliance, optimizing the emissions control system is critical to keeping all the wells flowing and to putting the most oil in the tanks.

 From the results we’ve compiled of operators, techs, and engineers who have taken our lease operator training program, we see that our users score lowest on the emissions control equipment in the facility. Read below to see how all these components work together to keep the facility emissions-free.

Typical production facility emissions control system

How the vapor recovery tower works

The purpose of the vapor recovery tower is to flash out as much gas from the oil before the oil reaches the tanks. It is important to keep pressure as low as possible to get the maximum amount of gas to flash from the oil. Additionally, the amount of retention time the oil has in the tower will contribute to the amount of entrained gas that can escape from the liquid. The tower is designed to gravity feed oil into the tanks so the only way it can ‘technically’ overfill is if the oil outlet piping is too small or gets clogged with paraffin. There is disagreement on how much retention time is necessary for all the gas to escape, so we just provided a chart below that shows how the tower liquid capacity changes depending on the residence time.

Assuming 36"x40' tower with oil level at 20'

How the vapor recovery compressor works

Vapor recovery units (VRUs) are typically skidded, electrically-driven compressors that compress gas from a very low pressure source to line pressure. VRUs were historically installed to pull gas from the storage tanks, but they have become more popular off the towers in recent years. This change is primarily due to ease of operation. Installing a VRU to pull off the tanks comes with concerns of oxygen, blanket gas, and overall more unsteady state operations.

It is easier to operate a VRU off the tower as this is a closed system that limits the chance of oxygen entering. It is also just easier to work with setpoints on the variable frequency drive and control panel when installed on the tower. Dealing with pounds instead of ounces gives you more flexibility in your operations. Also the recycle line has an easier time keeping the suction pressure above shut down pressure in tower operations.
Tips to make the VRT / VRU system operate effectively

We’ve included a few best practices we’ve seen used to optimize VRT / VRU operations in the field:

  • The VRU will typically have a pressure transmitter on the skid, but it is more effective to have the skid VFD and control panel use a pressure upstream on the tower gas outlet piping. Having a PT closer to the tower gas outlet allows the compressor to speed up by the time gas actually enters the compressor
  • On the piping it is always helpful to include a future for an additional separator in case a lot of liquids start dropping out of the gas. Rotary screw compressors can handle a little bit of liquid, but a slug of liquids dropping out of some heavy flash can total a compressor. If the compressor starts shutting down on high scrubber liquids, consider an additional separator or insulating the gas line.
  • No dead legs in the piping upstream of the VRU for liquids to collect as this would prevent gas from flowing from the tower to the VRUKeep the VRU on a regular maintenance schedule; this one is pretty obvious!
How can Wellsite LMS help?

If you made it this far in the article, 1) thank you -- you made our day, 2) you would probably be interested in checking out our lease operator training program. We cover all this information and more, so we would love to get in touch!
 
   Vivek
 
   LinkedIn: vivek nadig