Well Site – Entrainment Velocity
A well that produces 100,000 SCFD of gas as a minimum, but periodically reaches a peak production rate of 300,000 SCFD once a day, is continuously loading and unloading liquids. The sequence of events are:
• The velocity of gas flowing up through the tubing is insufficient to entrain liquids out of the tubing to the surface.
• Liquids accumulate (load) in the tubing.
• The weight of liquid increases the pressure differential between the wellhead tree and the bottom of the hole, as per equation 2.
• The gas flow from the well drops, as per equation 3.
• Gas flow continues to bubble-up through the tubing; but at a rate insufficient to entrain liquids out of the tubing.
• The gas pressure inside the tubing at the bottom of the well, and also in the sand formation surrounding the perforations continues to build as the gas flow diminishes.
• At some point, the well reaches a condition of instability. For example, a small reduction in the wellhead pressure due to a downstream pressure reduction causes a small increase in gas flow. This promotes a small amount of liquid unloading from the tubing. The resulting decrease in average fluid density in the tubing drops the bottom hole pressure. Gas is now sucked out of the sand formation, and through the perforations, at an accelerated rate.
• A chain reaction has been set in motion. Accelerated gas flow speeds liquid unloading; which in turn drops the bottom hole pressure, and progressively increases the rate of gas production.
An atomic bomb is detonated by creating a critical mass of plutonium. A gas well is unloaded by reaching the well’s entrainment velocity; a point encountered suddenly and in a dramatic fashion. The sound of slugs of brine and condensate blasting through the wellhead tree and surface equipment is quite audible. Typically, both the wellhead pressure and the gas flow will increase as the slugs of liquid “hit” the surface piping with increasing frequency.
Once the liquid is cleared out of the tubing (this takes 30 minutes to a few hours), the flow stabilizes for several hours and then slips away as the pressure in the sand formation around the casing perforations
is dissipated. Once the velocity through the tubing drops below that needed to continue entraining the liquids, gas production drops rapidly, and the cycle, as shown in Figure 1—2 is repeated.