Sand Screen Selection

How to determine a flow rate through a screen

"What size screen do I need for my production rate?"

That's a very common question, and it's really a reasonable one. But the answer is more problematic than first assumed. Let's look at the open area in a perforated pup joint (tubing sub). The open area in a perforated pup joint is 4.5" of cross-sectional area when you add up all the hole intake on a normal perforated 4-foot sub. The normal open area of the tubing itself is approximately 3.5", so as you can see you have greater opening in the perforated part of the sub that in the internal flow of the tubing.

Since this is a greater restriction than the open area, it will determine your maximum flow rate.

A similar conclusion occurs when you look at our sand screens and tubing screens. For example, a 1" x 8' 12-slot screen has approximately 51" of opening in the filtration membrane, where the cross-sectional area of the pump connection has approximately 1 square inch of opening. As you can see, with a brand new filter membrane, you're getting 51x greater flow than can pass through the base pipe itself!

That having been said, the fluid volume should be considered, but the real heart of the question has more to do with the percentage of trash the filter membrane is trying to separate. The larger the amount of trash, the greater the total filtration membrane area needs to be in order to achieve the longest possible run time. In most wells, this is an unknown factor. We've found some difficulty in trying to determine filter length and slot size in brand new applications.

The best process (and probably the most in-depth, length process) is take a look at the filter when it comes out of service. At that point, we can tell a lot about our downhole trash size as well as the volume of trash or sand in the hole. And just like in a football game, halftime adjustments need to be made at this point in order to maximize investments.

Mesh to Slot Conversion Table

Running Purge Valves with a Tubing Screen

We are commonly asked whether or not the user should run a purge valve at the bottom of a tubing screen. What we've found is that a bull plug or a solid jointed tubing with a bull plug is ideal. Usually, the solids that are passed through the screen are small enough to stay suspended and will lift to the surface.

There is very little accumulation of solids on the inside of our tubing screens. However, we have found a decreased run life when operated in conjunction with a purge valve. As you well know, whenever a purge valve allows solids out, it allows unfiltered liquids in. Most of that unfiltered liquid will then bypass the screen altogether, which renders a tubing screen useless.

This information was provided by Odessa Separator, Inc.'s Technical Support. For further inquiries, please contact REP Sales (contact info on the left) or Odessa Separator (432-580-7111).