Effects of Varying Choke Size on Water/Gas Well 1

[RCameron]

Hello,

I'm in a training program for the oil and gas industry.

We've been provided with fictionalized flow rates for a hypothetical water/gas well:

Inches Gas Flow Water Flow
8/64 0.5 MSCF 1600 BBL/Day
16/64 0.3 MSCF 2200 BBL/Day
32/64 0.22 MSCF 3000 BBL/Day

Is it reasonable or possible that the flow rate of gas decreases as the choke size increases? If so, what phenomena causes this...it goes against most of what I know of fluid dynamics that I learned in chemical engineering coursework.

 

[RCameron]

Better formatting:

Inches   Gas Flow    Water Flow
8/64     0.5 MSCF    1600 BBL/Day	 	 
16/64    0.3 MSCF    2200 BBL/Day	 	 
32/64    0.22 MSCF   3000 BBL/Day

 

[zdas04]

The flow rate from a gas well is determined by the sum of the flow inefficiencies from deep in the well through the production equipment. Friction, phase change, preferential flow of one product over another (rel perm) all contribute. Most of these are pressure dependent to some lesser or greater extent (reservoir flow is largely isothermal). This means that as pressure goes up (smaller choke), the flow paths can significantly increase in efficiency and give a smaller dP per unit length. The converse is also true.

We almost never have a really clear picture of all of the flow paths, so if you do an experiment like that and find that closing the choke increases production DO NOT call it a measurement error and open the choke back up like one of my colleagues did, close it a bit more and see what happens.

The change in WGR is probably not physical but an artifact of using SCF. As pressure increases, gas takes up less space so there is more room for largely incompressible water.


David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

I agree with Zdas04. The thing you are missing here which might help you and us understand it better is what the wellhead and d/s pressures are and what size tubing it is.

In this case it all depends what you're trying to extract - is it gas or water? As the choke opens, and presuming tat pressure d/s of the choke stays more or less the same, then FTHP will decrease and decrease all the way along the well bore until it meets the reservoir rock. As it does that the gas will expand more and lso increase in velocity. As it increase in velocity ti takes more water with it so to speak. Volume of the water is not affected by pressure to any significant degree so more velocity = more flow. Gas on the other hand is greatly affected and hence although velocity might increase, the density reduces amd friction losses increase due to the higher velocity, hence less mass of gas actually makes it to the top.

It can seem counter intuitive, but until you get the whole picture and can convert some of these standard figures into actual velocities and densities, it is difficult to comprehend sometimes.

I have always been told that one thing you don't want to do is go below the reservoir bubble point in a liquid well otherwise gas starts to form in the rock and from what I've been told this is a bad thing. (I will stand to be corrected here though....) Therefore sometimes less (choke size) can be more...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[zdas04]

The reason that flashing in a flow path is bad is two fold: (1) the energy required for phase change is manifested in a large pressure drop; and (2) multi-phase flow is inherently less efficient than single phase flow, flashing the liquid will change the relative masses of gas and liquid and will wreck any dynamic equilibrium that might form. I've seen a 2-3 psi decrease in FTP reduce oil production 25% in a well that was right on the cusp of flashing.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist