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Nitrogen Lifting

Nitrogen lifting is a well intervention technique that injects nitrogen gas into a wellbore to reduce the weight of the fluid column inside the production tubing, allowing the well to flow when it could not flow on its own. The injected nitrogen displaces or aerates the heavier fluid in the tubing, lowering the bottomhole pressure required to push fluid to surface. The technique is used to kick off newly completed wells, restart wells that have died from fluid loading or pressure depletion, recover from killed-well conditions during workovers, and clean up drilling mud or completion brine from the wellbore before commencing production. Coiled tubing is the most common deployment method, with a CT mast unit running coiled tubing into the well and pumping nitrogen down through the tubing to the desired depth.

Key Takeaways

  • Nitrogen lifting reduces the hydrostatic pressure of the fluid column inside the production tubing by displacing or aerating the fluid with nitrogen gas. The reduced pressure lets the well flow at a lower bottomhole pressure than the unlifted column would require.
  • Nitrogen is the preferred lifting gas because it is inert (does not react with formation fluids or tubing metallurgy), readily available in cryogenic liquid form delivered by truck, and easy to convert from liquid to gas at site using a vaporizer.
  • The most common deployment is coiled tubing with a nitrogen pumping unit on site. The CT runs to a target depth, the nitrogen unit pumps liquid nitrogen through the CT, the nitrogen vaporizes as it warms in the wellbore, and the gas displaces fluid up the annulus between the CT and the production tubing.
  • Major applications include kicking off newly completed wells, recovering wells that have died from liquid loading in gas wells, cleaning up drilling mud or kill fluid after workover, and unloading wells before pressure transient testing.
  • Cost runs from CAD 25,000 for a simple shallow lift on a routine workover to over CAD 250,000 for a deep coiled-tubing lift on a deepwater or HPHT well. The economic case usually depends on the value of the production that the lift restores.

Fast Facts

Liquid nitrogen at minus 196 degrees Celsius expands by a factor of approximately 700 when it vaporizes to gas at room temperature. A single tanker truck of liquid nitrogen (about 8,000 US gallons or 30,000 litres) can produce roughly 21 million standard cubic feet of nitrogen gas, enough to lift a typical onshore well from kill conditions back to producing flow in a few hours. The same vapourization expansion is what makes nitrogen lifting both effective and logistically practical, since the volume of cryogenic liquid that has to be trucked to site is much smaller than the volume of gas required for the lift.

How Nitrogen Lifting Works

Imagine a long vertical drinking straw filled with water, with the bottom of the straw submerged in a glass of water. The water in the straw is held up by atmospheric pressure on the surface of the glass. Now blow air into the straw through a small side tube partway down. The air displaces some of the water, the column of water-and-air mixture is lighter than the original water column, and water from the glass starts flowing up through the straw. The air did not push the water up directly; it just made the column light enough that the existing pressure could.

Nitrogen lifting in a well works the same way. The producing well has a column of fluid inside the tubing. The hydrostatic pressure of that column is set by the fluid density and the height of the column. If the bottomhole pressure of the reservoir is not quite high enough to overcome the fluid column, the well will not flow. Inject nitrogen at some depth in the tubing, and the column above the injection point becomes a mixture of fluid and nitrogen gas. The mixture is much lighter than the original fluid. The hydrostatic pressure drops. The reservoir bottomhole pressure can now overcome the lighter column, and the well starts flowing.

Where Nitrogen Lifting Is Used

Kicking off newly completed wells is the most common application. After a well is completed (perforated, fractured, and tested), the wellbore is full of completion fluid (typically a calcium chloride or potassium chloride brine designed to control formation pressure during the completion). The brine is heavier than the eventual produced fluid, so the well will not flow on its own until the brine is unloaded. A nitrogen lift unloads the brine in a few hours and brings the well online.

Restarting dead gas wells is a major application in mature gas fields. Gas wells often die from "liquid loading": water and condensate accumulate in the tubing faster than the gas velocity can carry them up, the column gets heavier, the well slows down further, and eventually the gas rate drops below the critical lift rate and the well stops flowing entirely. A nitrogen lift unloads the accumulated liquid and restarts the well, sometimes adding years of production life.

Workover recovery is the third common application. After a workover that requires killing the well with weighted mud or brine, the well needs to be unloaded back to producing fluid. A nitrogen lift removes the kill fluid from the tubing and lets the reservoir reestablish flow.

Mature fields in the Western Canadian Sedimentary Basin, the Gulf of Mexico shelf, the Cooper Basin in Australia, the Norwegian Continental Shelf, and onshore Middle East producing regions all run nitrogen lifting as a routine part of their production maintenance programs. The technique is not glamorous but it keeps producing wells producing.

Nitrogen lifting is sometimes called nitrogen kickoff, nitrogen unloading, or N2 lift. The CT-deployed version is called CT nitrogen lift or coiled-tubing nitrogen unloading. Related terms include coiled tubing (the small-diameter continuous reel of pipe used to deploy nitrogen lifting; the most common delivery method for the gas), liquid loading (the gas-well failure mode where accumulating liquid in the tubing chokes off gas flow; one of the most common reasons to perform a nitrogen lift), well intervention (the broader category of work performed on producing wells without a full workover rig; nitrogen lifting is one of the standard intervention techniques), kill fluid (the weighted brine or mud used to control formation pressure during workovers; the fluid that nitrogen lifting often unloads after the workover is complete), and gas lift (a permanent artificial-lift technique that injects gas continuously into the production tubing through gas-lift mandrels; conceptually similar to nitrogen lifting but installed for permanent operation rather than one-time intervention).

Why a Truck of Liquid Nitrogen Restarts a Million-Dollar Well

A Western Canadian gas operator manages a portfolio of mature gas wells in the Western Canadian Sedimentary Basin. Well 47 has been producing 380 thousand cubic feet per day for 8 years. Production starts to fall over a six-month period as water and condensate accumulate in the tubing. By the end of the slowdown, the well has died entirely, sitting at static wellhead pressure with no flow.

The operator schedules a nitrogen lift. A CT mast unit and a nitrogen pumping unit drive to the wellsite on the same morning. The CT runs into the well to a depth of 2,400 metres (just above the producing interval). The nitrogen unit begins pumping liquid nitrogen at controlled rate. The nitrogen vaporizes as it travels down the CT, exits at the bottom, and rises up the annulus carrying the accumulated water and condensate ahead of it.

Three hours into the lift, the wellhead pressure builds and the operator opens the production choke. Gas begins flowing. Liquid water and condensate flow ahead of the gas, gradually clearing as the unloading completes. By the end of the eight-hour job, the well is producing 410 thousand cubic feet per day, slightly above its previous rate.

The cost of the nitrogen lift: about CAD 38,000 in service charges. The recovered production: about 410 mcf/day at CAD 4 per mcf, or roughly CAD 600,000 per year of restored gas revenue. The payback is measured in weeks. The well will eventually slow down again and need another lift in a year or two, but each cycle of production-and-lift extends the well's useful life by another year or more. Nitrogen lifting is one of the most economically reliable interventions in mature gas operations.

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