What is Two-Phase Flow and Why is it Important to Avoid in Groundwater Wells?

There are three phases in substances: solid, liquid, and gas. If a mixture of gas and liquid phase flow is encountered in an aquifer, that is known as two-phase flow. Where two-phase flow happens in an aquifer, trouble ensues. There are a number of ways that two-phase flow can develop in aquifers, beginning with the completion and development of a well and continuing with the operation of the well.

When a well is completed, the drilling process creates significant disturbance to the formation that is being drilled, such that the well has to be developed, i.e., cleaned of drilling fluids and formation material (see LWS February 16, 2021 blog). One common method of developing wells is by airlifting. While this can be an effective method, care needs to be exercised to not have the process result in air being introduced into the formation in large quantities.

Dual-pipe airlifting should always be used to minimize the potential for infiltrating air into the formation. It is also important that the airlifting pipes not be set all the way at the bottom of the well right from the start of the development process, as the compressed air may not be able to lift the heavy muds in the outer pipe and instead the air could go out the bottom of the outer pipe and into the formation instead of being discharged up the outer pipe. This situation may cause the compressed air to become trapped in the aquifer creating a two-phase flow issue that could either restrict, or prevent, water flow into the well. This two-phase flow issue, therefore, could be created before the well ever goes into production.

Similarly, once a well does go into production, drawdown from pumping can cause water levels to decline into the formation where there are well screens, particularly in confined aquifers where air can’t be released into the unsaturated zone. This drawdown during pumping has the potential of introducing air back into the formation. When air is introduced into the pore space of aquifers it can cause the restriction and/or blockage of groundwater flow paths, resulting in the reduction in production potential of a well. This phenomenon is illustrated in Figure 1 for entrained air in the formation, while Figure 2 (below) schematically illustrates air locking.

Figure 1

Figure 2

It is also possible that some of the air is produced from the formation as part of the water being pumped. In this case there would be entrained air in the water entering the pump. Therefore, the pump is not solely producing water but, rather, a combination of water and air. This two-phase flow issue can result in the cavitation of the pump. Cavitation occurs due to the pump operating close to the vapor pressure of the water and, if the pressure drops further, the air bubbles entrained in the water will collapse. These implosions of air bubbles will, over time, cause damage to the pump impellers by pitting the surface of the impellers. Cavitation damage then decreases both the efficiency of the pump as well as the useful life of the pump.

Two-phase flow has the potential to be severely debilitating to the production capability of a well, as well as the pump in the well. Not only can it cause operational issues with the pump but, more importantly, it can cause air locking of the formation if the problem becomes severe enough. Care during the development of wells and paying attention to operational aspects of wells that are susceptible to having air introduced into the formation during pumping can help to minimize the potential for two-phase flow to create issues in production wells.

If you have a need for a water resources engineering firm for designing and observing well drilling, construction, and development of the well, as well as operational aspects of production wells, please reach out to any of the people below and we will be happy to put together a plan for your needs. LWS can also be reached by phone, 303-350-4090.

Bruce Lytle, President of LWS: bruce@lytlewater.com

Chris Fehn, Senior Project Engineer: chris@lytlewater.com

Ben Bader, Project Hydrogeologist: ben@lytlewater.com

Anna Elgqvist, Senior Engineer: anna@lytlewater.com

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