Casing & Tubing Size 1

[Jriad]

What kinds of factors should be addressed when choosing casing & tubing sizes. It seems that they are chosen on rules of thumb (i.e. casing sizes based on depth) but what details should I consider. How is tubing size determined (amount of fluid to lift in the case of gas?)?

Thanks,

 

[DrillerNic]

You do inflo/ outflow modelling over the completion's design life, using estimates of the PI, water cut and reservoir pressure over the completion's design life to come up with a tubing size.

You then check that the tubing you've chosen can fit inside the casing you've got (or perhaps, if you're in an enlightened company, you're designing the well around the completion). Avoid the common mistake of forgetting that SSSCV's a lot bigger than the tubing. Then you do a tubing stress analysis to check the wall thickness, material and connection type.

Casing sizes are often driven by the number of casing points needed to drill the well, the desired completion size ane things like the rig's BOP size or the wellhead size (in subsea wells, the biggest casing size is usually 30" as that's what fits through the guide base). Ideally you start at eh bottom and work up, but often you start at the top and work down. Casing wall thickness is then driven by the casing stress analysis: 47ppf is the usual thickness for 9-5/8", but 53.5ppf may be specified if you need more strength due to mobile formations like salt, or you've got ESPs which have a high drawdown or it's an HPHT well. the casing stress will also tell you if you need premium connections. Which conenction is often due to what commonly vailable in a particular area: so in the North Sea VAM's conenctions (new VAM, VAM top etc) are used, but in the States Hunting have a greater share of the market

 

[Bayden]

Jriad:

You might want to consider corrosion. In some areas formations are very corrosive and can eat up your pipe quickly. Depending on the life of your project, you might want to "beef" up your design. If companies know that a corrosive formation exists, they try and cement it up. However, cement will leave gaps where corrosive fluids can contact the casing and start "working on it".

Bayden

 

[johnsmith2]

"Ideally you start at eh bottom and work up, but often you start at the top and work down. Casing wall thickness is then driven by the casing stress analysis: 47ppf is the usual thickness for 9-5/8", but 53.5ppf may be specified if you need more strength due to mobile formations like salt, or you've got ESPs which have a high drawdown or it's an HPHT well. ""


If anyone tell me what is ppf?

 

[cmlpetro]

In the case of a gas well tubing size is choosen based on your expected gas rate and if the well will have to lift water.

Different diameter tubing will give a different critical gas rate which can be predicted using Turner or Coleman equation. You will notice that both of these equations are independant of the water-gas ratio. The below link has a critical rate calculator but I suggest you read the original SPE paper by Turner so that you know what situations the equation is valid for.

http://www.peteng.com/jmm/gwl01_process.jsp

Usually in a gas well you will start with a larger tubing such as 60.3mm(2 3/8") and then when the resevoir begins to deplete the gas rate will fall and you may start producing water. Once this occurs you can switch to a smaller tubing diameter such as coil tubing to increase critical velocity and help with liquid lift.

With many unconventional gas wells the expected gas rate is small, only 2 E3m3/d (70 mcf/d). In this case I usually equip the well with steel coil tubing. Some engineers even use smaller diamter plastic coil tubing but I do not have much experience with this.

And as discussed above corrosion can be an issue. In this case "polycore" tubing or stainless steel can be choosen as the material.

If you are planning to use plunger lift you may want to use a larger diamter tubing like 2 7/8" as these plungers usually work better than the smaller diam plungers.

 

[DrillerNic]

Sorry I was using using acronyms- naughty boy!

ppf = pounds per foot; the way API describe casing and tubing wall thicknesses.