Maximum Flowrate Equation 2

[KSA221]

I am a new engineer in the O&G industry - I'm the only engineer for our small company. We currently have several gas wells in a specific area that were drilled several years ago, and we are scheduled to drill a few more in this area in the next year. My boss told me that when the first wells were drilled, 3rd party engineers told him that these wells were going to be "Tubing Limited" with 2 7/8" tubing and so 4" tubing was used. I need to find out the maximum flow rate for both of these tubing strings. I've searched all of my textbooks and can't find anything helpful. Can anyone coach me on where to start and what parameters I'll need? I know this is probably a simple question for some of you, but I've got to start somewhere!
Thanks in advance!

 

[dcasto]

you need to know GOR first. Then you ncan calculate the pressure drop in a 2 1/2 versus 4" tubing. From experience, producers don't understand trhat a 5psig wellhead pressure versus a 25 psig wellhead means nothing, the tubing and inflow from the formation controls the production rate. In otherwords a contract to gather at 50 psig or 25 psig WILL NOT CHAMNGE THE CHECK FROM THE GATHERER.

 

[KSA221]

I have the GOR, so basically I just need to calculate the pressure drop from formation to surface through each diameter tubing? Simple pressure differential?

 

[zdas04]

Not terribly simple. Do you have the SPE Petroleum Engineer's Handbook edited by Bradley? It has the equations for flow up a vertical conduit, they're a touch messy since you have to deal with significant gravity effects. I've had really good luck converting the liquid and gas to mass flow rate, adding them together, determining a mixture density and specific gravity relative to air, and using these numbers with the Bradley equations to determine a flowing bottom-hole pressure. This only works if the flow is above the Critical rate (see Turner Critical Rate in the literature) and slip can be disregarded.

I have to disagree slightly with Dcasto about the surface pressure not mattering. It sometimes matters a lot. There are two important steps in the process, first the gas must flow from the reservoir into the wellbore, then it must flow up the wellbore to the surface. Either of these can be limiting. If the inflow is limiting then lower surface pressure will seldom result in significantly increased flow (lower surface pressure will result in some combination of more flow and lower flowing bottom hole pressure). If the inflow is not significantly limiting then lower surface pressure will result in higher flow rate.

If the well is tubular limited then the only result of lower tubing pressure is increased friction in the tubing and little or no change in flowing bottom hole pressure or flow rate.

This stuff is fairly complex with some difficult interactions. They don't teach most of the practical stuff in college which is why small companies are ill advised to put new engineers in a position of operating a field without access to mentors.

David

 

[KSA221]

David, thanks for your reply. I'll assure you that my company is not placing me in any position where my inexperience will be influencing any decisions. They are giving me things to figure out right now for my own benefit, and leaving critical decisions to 3rd party at the moment. To review your reply, you are saying that the effect of using larger tubing (lower tubing pressure) is increased friction (due to more internal surface area?) and negligible change in flowing BHP and flow rate? This would mean that using 2 7/8" vs. 4" would actually seem more beneficial, due to reduced flowing friction, if there is no great change in flow rate. Am I following this correctly?

Also, no I do not have Bradley's book, however as much as I've seen it mentioned on this website I'm starting to think that I may need to cough up the $700 or so to get it.

Thanks for your help.

Kyle

 

[zdas04]

You have that a bit backwards. Think of friction as shoving too much stuff down too small a line. Going to smaller tubing would increase your pressure drop due to friction for the same flow rate at the same pressure. What really happens is that both pressures and flow rates change to find a new equilibrium. It it a really complex interaction.

Bradley is an overpriced pig of a book with a seriouslly inadequate index and several internal inconsistencies, but I find myself going to it several times a week. I have other references that I go to first, but I often end up there.

David

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[KSA221]

David, thank you for your help. As I delve deeper into this I may be writing back asking more questions.

Thanks again

Kyle

 

[DuhonGATE]

zdas04,

Get a copy of the SPE Monograph of Well Flow. Then contact Dr. James Brill at the U. of Tulsa to get the errata if the SPE doesn't supply it with the errata.