Slug to annular flow pattern transition (SAT) taking place during the upward gas-liquid well transportation is a source of flow instabilities often experienced with conventional gas lifting as well as with unloading operations of water accumulated at the bottom level of gas wells in low-pressure gas or coalbed reservoirs. In order to minimize the pressure drop and gas compression work, gas lifting of relatively large volumes of fluid (oil and water) uses mainly a slug flow pattern while the production of gas with relatively small amounts of condensate or water (unloading operation) uses an annular flow pattern. In both situations, significant decreasing of tubing pressure from perforation to wellhead levels, is associated to significant increase of superficial gas velocity, may induce flow pattern transitions (usually from bubble to slugs and, further from slugs to annular).

This paper uses field data and laboratory measurements to suggest that SAT can be a source of flow instabilities and should be avoided.

Understanding and proper prediction of SAT is particularly essential for developing suitable production operations and for designing effective gas lifting or unloading strategies from low-pressure gas and oil reservoirs (including upward transportation of hot fluids resulting from steam-assisted heavy oil operations).

With depletion of existing gas reservoirs trend the need for effective gas well deliquification is in great demand. Transportation of water produced at the perforation level (usually between 10–60 m3/d) over a vertical depth of 200 to 2000 m under low (often variable) reservoir pressure (< 50 m of water) ask for finding un-conventional and effective artificial lifting strategies. Improving the understanding of gas-liquid upward transportation mechanisms including the avoidance of instabilities induced by flow pattern transitions is essential.

This paper addresses this problem through laboratory measurements of steady and oscillatory components of flow-pressure under a broad range of gas injected rate and simulated reservoir pressures. Comparison of laboratory data with existing STA models is performed first; selected models are then tested for field situations.

Effective field strategies for avoiding the SAT occurrence using either a slug or an annular flow pattern regime under low-pressure and standard (IPR) reservoir conditions are discussed in view of practical field applications and selection of a suitable gas lifting strategy.


Simultaneous production of gas, liquids hydrocarbons and water through vertical wells is a common situation for both land and off-shore production systems. Depletion of conventional oil and gas reservoirs and continuous increase of world oil and gas demands ask for a re-visitation of conventional production strategies. These should include production from extremely low pressure reservoirs as well as off-shore large-volume and large pipe diameters not used for conventional production so far.[1]

Many gas wells are now dormant due to a combined effect of low reservoir pressure and bottomhole accumulation of water. Similar problems encountered with exploitation of deep coalbed gas reserves are asking for economic and practical production methods. Selection of a particular well unloading strategy which may include ESP, PC, rod and jet pumps, as well as gas lifting or various combinations has to use a location-specific design including the most suitable flow pattern transport. Often dry operation conditions and the presence of free gas considerably increases the maintenance costs with most of conventional mechanical pumping systems and suggests the need for a robust artificial lift strategy without mechanical moving components.

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