Doing some lunchtime gas lifting today guys. Apologize for not meeting my Friday deadline, didn’t expect to wake up at 10 after Thursday night...

Everyone gather round for a little and hopefully I can wrap this thread up over the next few days

First thing to understand about gas lift is your limitations on the gas injection side. The driving force behind your design will be the maximum surface pressure that you get from your compressor when FLOWING. Every well can pressure up to the static pressure

That a compressor is rated to, but as soon as you start injecting, that Max pressure will drop due to friction and make for a fun little rodeo. ALWAYS DESIGN FOR FRICTION IN GAS LIFT, IF YOU DONT YOU CAN LOOK LIKE A REAL IDIOT (me)

With that diatribe out of the way, let’s talk about gas lift valves and valve design.

*The internet draws this way better than I do* Gas lift valves are internally made up of an intake port (where injection gas goes in), a bellows with a stem and sear, nitrogen dome and the point at which gas enters the fluid stream.

Every gas lift valve dome is charged with nitrogen to a certain pressure. A valve will open when the force from the pressure coming through the intake port is higher than the force acting downwards from the bellows due to the pressure of the nitrogen.

When wells unload and pressures drop, gas lift valves will begin closing and injection will start moving down from the top valve. The purpose of gas lift valves is to allow us to inject higher in the string when bottom hole pressure is higher than compressor discharge

And then as we draw the reservoir pressure down, they close and we can start injecting deeper and getting better drawdown. See in this picture how the pressure vs depth gradient drops as we can get gas in the system - the deeper you inject, the more effect this will have on BHP

So how do we do a valve design on a gas lift well? Using your max pressure AT THE WELL, temperature correlations, wellbore fluid gradients, and an understanding of turner critical rates, you too can get pretty close to the gas lift designs that I use as a degrees engineer.

We’ll start with turner critical rates. The turner critical rate is defined as the minimum gas rate required for steady flow up a given tubular. Liquid needs a certain amount of gas flow velocity to want to flow with the gas, other wise it will fall back and “load” up the well.

In gas lift, you want to live on the right side of this picture, which you can do by knowing your critical gas rate.

Because I don’t want to pontificate here, we’ll call that critical gas rate 1000 MCFD for my 2-7/8” tubing. Between the well and my injection gas I need to make 1000 MCFD to steadily lift fluid.

Based on critical rates I know I need 1000 MCFD. On top of that, from analog wells around the asset, I know this well will make 200 MCFD, and thus I need to inject 800 MCFD to get this well to 1000 MCFD up the tubing.

Now I know my max injection pressure is 1150 PSI, bottom hole temp is 155 degrees F, and the fluid in my wellbore has a gradient of .5 PSI/ft. I have basically all I need to know for a gas lift design!

Using my fluid gradient from surface, and intersecting it with my maximum injection pressure + the weight of the gas in the casing, I know where I have to put my first valve.

The rest of the design is all based on preference and experience, but essentially you then space your valves out from there to the bottom based on experience and some more advanced modeling that I won’t touch on in here. A similar concept is used based on the intercept of

The fluid gradient and injection gas gradients, except you go from the valve above instead of from surface. Luckily this is all automatically done these days and inputs are tweaked to achieve desired pressure drops across valves and to account for changes in production.

The reason you need your temperature at bottom hole is because valves are all pressured up in a shop at 60 degrees. Using correlation, those pressures are converted to in situ conditions - the P on surface needed is derived from the pressure that the design needs downhole to work

We needed our injection rate because the valve ports are sized based on it. Smaller injection rates will use smaller ports so that they exhibit more stable flow, and larger rates will use larger ports because you can’t inject over the speed of sound through a fixed choke point.

I think this should be thorough enough for basic gas lift design, I’ll call this the end for now and work up the next 2 sections over the next few days.