Choke - AOF - Max Rate

[wood5896]

I apologize already if this is a dumb question. I am reservoir engineer and I am trying to learn some production engineering basics but don't have a good mentor to ask questions too. I have a well that was just perf and frac and is producing 3 MMcf/d on a 12/64 choke. There is 5000 psi wellhead pressure with 650 psi of line pressure. I understand at the current conditions the flow is critical and any reduction in the backpressure won't increase the rate for the given choke size, but I am wondering if there is a way to know what the rate would be as a function of the orifice size. The production engineer is babying the choke due to worries of bringing in tons of sand, due to this being a very unconsolidated formation, and I want to know what the rate would be if he wasn't babying it. Even if I don't have enough information to solve this I want to know if there are any rules of thumb that the wise old timers would use to get an estimate.

Thanks!

 

[Latexman]

I googled "compressible flow through an orifice" and got 37,300 hits:

http://www.google.com/search?hl=en&...+an+orifice&aq=2&oq=compressible+flow&aqi=g10

You can find on-line calculators too.

Also, most college fluid flow textbooks have what you need.

Perry's Chemical Engineers' Handbook has what you need.



Good luck,
Latexman

 

[wood5896]

I have googled it and I have spreadsheets with all the calculations (from Petroleum Production Engineering, A Computer-Assisted Approach), but all I can do with what I have (pressure inlet and outlet and orifice size) is calculate the maximum rate for the 12/64ths choke. What I wanna know is if I change the orifice size can I solve for the new rate (just as a matter of is it physically or theoretically possible, all I need is a yes or a no) I presume you can't because depending on the reservoir's thickness, perm,and drainage extent you would have different corresponding inflowing pressure responses when you change the orifice size. Rate is a function(orifice size and upstream pressure) knowing just one isn't enough. I just want someone to tell me yes that's right, or no that's wrong and this is what will happen.....

 

[Latexman]

Yes, you can change the orifice size and calculate a new rate. How close you come to reality depends on what you estimate or guess the other parameters in the orifice equation to be. Does the wellhead pressure decrease? Does the pressure downstream of the choke increase? The more science you put into all the parameters, the closer you should get to reality. Just changing the orifice diameter and keeping all other parameters constant will probably give a flow somewhat larger than reality, but it's easy.

Good luck,
Latexman

 

[wood5896]

First, thank you for your help!

To answer your question, I will be changing the orifice and will hold the downstream pressure constant, the wellhead pressure will change and that is where I have been trying to go with this thread but haven't asked very well. Different gas wells have different inflow performances. In the first few weeks of production you don't know how large the "tank is" therefore if it is very large you change the orifice size and the wellhead pressure will only change a little; however, if you have a small "tank" and change the orifice size you will change the wellhead pressure alot. With this being known I didn't know if there were rules of thumb on how to go about this calculation with multiple things changing (orifice size and well head pressure). I think the answer is isochronal well testing. You fix the choke and measure the stabilized rate and wellhead pressure, change the choke multiple times and measure the same thing and solve Q=C(Pr^2-Pwh^2)^n with a semi log plot to get your deliverability equation (Beggs - Gas Production Operations. Is this thinking sound or am I missing anything.

An additional question. How do you do this analysis if you are critical (Pup=5000, Pdown=650 Pup/Pdo=13% well below the 55% needed for critical flow) because you will change the orifice size and you won't get a different wellhead pressure until you are sub critical, correct or incorrect?

I attached the spreadsheet with the data I am speaking about. At the 12/64 I am locked in around 3MM a day.

Thanks again replys are very appreciated!

 

[wood5896]

Just because it's critical doesn't mean you can't get different pressures, just means you can't increase the rate as a result of lowering the downstream pressure, right? You can still increase the rate by opening the choke which will change the wellhead pressure as a result.

Keep the replies coming, your helping me work myself through actually visualizing this.

 

[Latexman]

Right, if flow is sonic, lowering the downstream P does nothing. Increasing orifice area will increase flow, even if upstream pressure decreases and downstream pressure increases (backpressure increases due to more flow in the pipe - more flow = more dP).

Good luck,
Latexman

 

[wood5896]

Ok i'm on the right track finally I think. So utilizing the sonic flow equation it looks like if you take out the constants that aren't a function of choke size.... as you change orifice diameter=(Rate/Wellhead Pressure)^1/2 as shown below:

q=878 x C x A x Pupstream * (big constant in square root)
combing all the constants
q= C x d^2 x P

d = (q/p)^1/2

all = signs should be proportional signs

So if I change the orifice diameter by 2 times lets say from a 12/64 to 24/64 and I held the wellhead pressure constant the rate would quadruple

On the flipside if I change the orifice diameter by 2 times and held the rate constant the well head pressure would be cut by 4. Is this true?

I know you can't assume either of these extreme cases when you go to a large orifice size or choke size but is there a rule of thumb somewhere in the middle like: "If you increase the orifice size by 2 (double it) the rate doubles and pressure is cut in half. The keeps the equation solved, just don't know if that happens in real life.

If there is no real of thumb, is the only way to solve this is to use the isochronal testing as I mentioned a few posts above at different choke size and see the relationship with pressure and rate with respect to choke size?

Thank you very much again for the replies, I promise I won't ask much more if anything.

 

[Latexman]

I have no idea on the well head pressure change. Testing will tell.

Good luck,
Latexman

 

[tamumatt14]

Latexman,
I'm new to this forum and I was wondering if you have any references for the spreadsheet you have created in this thread. Great spreadsheet btw! Thanks!