Gas lift

Gas lift is the increasing of the oil production flow by injecting gas down a well to mingle with the oil, thus decreasing the average density of fluid and increasing flow rate. The gas can then be recovered and re-injected.

Gas lift advantages

• Operation of gas lift valves is unaffected by produced solids (sand etc.)
• Gas lift operation is unaffected by deviated or crooked holes.
• Use of side pocket mandrels allows easy wireline replacements of (inexpensive) gas lift valves when deviation <65⁰.
• Provides full bore tubing access for coiled tubing or other well service work.
• High gas oil ratio improves lift performance rather than presenting problems as with other artificial lift methods.
• Low surface profile important for offshore & urban locations.
• Tubing & annular subsurface safety valves available when required by safety regulations.
• Gas lift tolerates "bad" design - though "good" design is more difficult.
• Gas lift has a low initial (downhole) equipment cost.
• Gas lift has a low operational and maintenance costs. Major workovers are infrequent when wireline servicing is possible.
• Well completions are relatively simple. This can be important in remote areas.
• Gas lift operation independent of bottom hole temperature.
• Flexible:

- can produce from a wide range depths & flow rates.
- uses the same well equipment from 100-10,000bpd production rates.
- copes with uncertainties and changes in reservoir performance,reservoir pressure, water cut & production index over the well life.

Gas lift limitations

• High tubing intake pressure due to fluid in the tubing restricts production e.g. Gas lifting a well with a Productivity Index of 1 bpd/psi from 10,000 ft with a static bottom hole pressure of 1000 psi is difficult.
• Flowing bottom hole pressure is greater than with e.g. Electric Submersible Pumps. This leads to a potential loss of reserves in depletion type reservoirs.
• Gas lift is inefficient in energy terms (typically 15-20%).
• Gas compressors have a high capital cost. They require expensive maintenance & require skilled operations staff. However, they may already be required for gas sales.
• Annulus full of high pressure gas represents a safety hazard.
• High installation cost can result from top sides modifications to existing platforms e.g. Compressor installation.
• Gas lifting of viscous crude (<15⁰ API) is even less efficient.
• Wax precipitation problems may increase due to cooling from (cold) gas injection & subsequent expansion.
• Hydrate blocking of surface gas injection lines can occur during cold weather if gas is not adequately dried.
• Lifting of low fluid volumes is inefficient due to gas slippage.
• Good data management and complete production/gas lift network modelling is required for efficient/maximum profitability operation.
• Adequate gas supply required throughout project life:

- Decreasing BHP, increasing water cut etc.
- Sufficient gas to start up FIRST well.
- Slow start up after facility shut down.
- Increased gas handling requirements in facilities.