Abstract
Most gas wells produce either condensed water or liquid hydrocarbons, which over time build up in the wellbore and eventually kill the well – a phenomenon called "liquid loading". Industry operatives have used a criterion based on Stokes's Law (the Turner criterion, 1969) for predicting when liquid loading will begin. However, this approach does not adequately account for multiphase flow effects in the wellbore, causing the Turner criterion to give inconsistent results when applied in the field. In addition, there are anecdotal observations suggesting that Turner's results are more accurate in medium-sized tubing, but become less accurate for diameters smaller than 2.0″.
This research was performed by recreating and observing the conditions leading to liquid loading in a controlled laboratory setting. Two series of experiments were conducted in 2.0″ and 1.0″ inner diameter flow tubes of the Montana Tech Vertical Flow Loop to observe morphology changes as the multiphase flow system transitioned from stable annular mist flow (gas dominant) into churn flow and ultimately slug flow (liquid dominant). The two fluids in the multiphase system were air and water, acting as proxies for produced natural gas and natural gas liquids. The water and air rates were varied to cause the flow to move from stable annular mist flow into churn flow and ultimately slug flow.
The experiments were recorded using slow motion video taken at three locations on the flow loop to observe and quantify changes in flow characteristics over a range of air and water flow rates. The slow-motion video was then visually evaluated to qualitatively describe the flow behavior at the droplet and wave level. These data were then used to define a four-stage flow behavior progression based on the fluid morphology changes leading to liquid loading, and ultimately enables a more accurate prediction of when liquid loading will occur in small diameter pipes.
This research supports previous assertions that Turner's critical velocity model lacks the complex multiphase flow behaviors necessary to accurately predict the onset of liquid loading in small diameter pipes. Unusual slugging behavior was observed in the 1.0″ pipe, suggesting that the effectiveness of small diameter "siphon strings" might be tied to the "liquid sweeping" properties of the liquid slugs.
