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# Gaslift in simple words

### More By Petro Engineer

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1

Think about it as a force balance equation.

1. Your well is at certain depth.
2. Your reservoir has a certain pressure.
3. Your produced fluid (oil, water, gas) has a certain density, often described as a fluid gradient using psi/ft of depth.

If your reservoir pressure is greater than the pressure of the vertical column of fluid in your well (fluid column height ft x fluid gradient psi/ft) then your well flows naturally.

If the pressure of the vertical fluid column is greater than the reservoir pressure, the well won’t flow naturally. So what do you do? You inject gas into the well to lighten the density of your fluid column until the reservoir pressure is higher than weight of the fluid column and the well starts flowing. The deeper in the well you can inject the gas, the more of the fluid column you lighten, and the less pressure it will exert on the reservoir, and the higher the flow rate.

So why do gas lift valves exist?

Conventional compression has traditionally been limited to roughly 1,200 psi discharge pressure. Depending on how high your reservoir pressure is, that 1,200 psi might only be enough pressure to inject 1/4 or 1/3 or 1/2 way down the well.

This is where gas lift valves come into play. You place your first gas lift valve roughly at the depth you can inject, and then space them the rest of the way down the wellbore so that as your bottom hole pressure drops and your fluid level drops, then you can inject deeper and deeper as time goes on.

The opening and closing pressure of the gas lift valves are set based on the estimated pressure and temperature at the depths they are installed. The concept is that when the gas lift valve is in fluid it’s open, and when the fluid level drops below the gas lift valve it closes and forces the gas down to the next valve.

The port size is designed based on how much gas you need to inject into the tubing (500, 1,000, 1,500 MCFD, etc).

Nodal analysis helps you marry your inflow and outflow curves (IPR vs. VLP) and helps understand gas lift performance sensitivities to help you design your system, like what does production and bottom hole pressure look like at different injection depths and injection rates, flowing wellhead pressures, etc.