Deep Bedrock Wells for Municipal Use: A Series on the Process - #5) Well Development
Welcome to the fifth installment in our well drilling series following the construction of Monument Well 10 by Layne, A Granite Company, and Lytle Water Solutions! In the first blog we talked about how the borehole for the well is drilled. In the second blog we discussed the basics of geophysical logging and well design. In the third blog we discussed the installation of the well itself in the borehole, while the fourth blog discussed the installation of the gravel pack and grout seal. Now we will move on to the development of the well in this fifth blog of the series.
The well is now fully installed, but the the borehole, including the gravel pack, is still filled with drilling mud. The drilling muds need to be removed so the well can produce clean water, but the drilling muds and sands in the well need to initially be removed by a method other than pumping, as pumping all of the suspended material still remaining in the well would be very abrasive on the pump. Luckily, there is a way to remove the drilling muds and larger grain sands from the well with no moving parts below the ground. This development method is called airlifting.
DUAL PIPE AIRLIFTING
To begin the airlifting process, a pipe of roughly 4 inches in diameter is lowered into the well to about 200-300 feet below the level of the water and drilling muds. Then smaller pipe of roughly 1.5 inches in diameter is lowered into the 4-inch pipe until the bottom 1.5-inch pipe is about 20 feet above the bottom of the 4-inch pipe in the well. This method is called dual pipe airlifting since there are two pipes used in the well. Compressed air is then forced down the 1.5-inch diameter pipe. The compressed air forces its way down through the water and drilling muds and out of the bottom of the pipe. Now the compressed air is in the 4-inch pipe, and the air wants to rise back to the surface inside the 4-inch pipe, but there are hundreds of feet of water above it. The air begins to rise in the 4-inch pipe and pushes the water above it to the surface. As this happens, more water from the well rushes into the bottom of the 4-inch pipe and begins to rise. As it passes the 1.5-inch compressed air pipe it mixes with the compressed air and rises to the surface.
The drillers can estimate the water level above the bottom of the 1.5-inch compressed air pipe by measuring the amount of air pressure it takes to push the air into the 4-inch pipe. It is important that the airlifting pipes not be set all the way at the bottom of the well right from the start. If the airlifting pipes were placed at the bottom of the well right from the beginning, the compressed air may not be able to lift the heavy muds in the 4-inch pipe and instead the air could go out the bottom of the 4-inch pipe and into the gravel pack and aquifer. Since there is a grout seal from the top of the gravel pack all the way to the ground surface, the compressed air can become trapped in the aquifer and either restrict or not allow water to flow into the well. This is a two-phase flow issue where air is essentially injected into the saturated water, trapping air in the water, which can potentially result in air-locking a well. This situation is very difficult to fix and it is for this reason the dual pipe airlifting is done in stages: starting higher in the well with less head to lift, airlifting that column of water, and then lowering the pipes and airlifting again to reduce the amount of water and drilling muds that must be lifted by the compressed air at once.
Once the airlifting pipe nears the bottom of the well, the dual pipe airlifting continues until the drilling muds and larger grain sands are mostly cleared from the well. At this point we are ready to move on to the second stage of the well development, single pipe airlifting.
SINGLE PIPE AIRLIFTING
Now that the drilling muds are mostly removed, the 4-inch pipe is taken out, leaving only the 1.5-inch compressed air pipe. The compressed air is then forced down into the well and lifts the water directly up through the 12-inch well casing to the surface. Intermittently, the air is shut off to allow water to fall back into the well. This surging method of airlifting is called single pipe airlifting or “rawhiding”. This method imparts much more energy into the well and breaks loose drilling muds and sands from the gravel pack more effectively that the dual pipe airlifting. This process is run until the water coming to surface begins to clear up. When the water begins to clear with any development method, that is a sign that a higher energy method is needed to continue the development of the well.
After the rawhiding of the well, the drilling muds and large grain sands that can damage down-hole electric pumps are largely removed, so at this point a temporary well pump is installed. The pump can pull much more water from the well than airlifting, and that means more energy. This higher energy begins to pull the smaller silts out of the gravel pack. When the pump is turned on, the water being discharged at the surface is usually very cloudy and brown in color. The discharged water will slowly begin to clear, indicating it is time for a higher energy development method.
SURGING
The next step in the well development is called “surging” the well. While pumping, the pump can either be turned off completely or the pumping rate can be varied to provide a surging effect. With a pump shutdown, the thousand plus feet of water in the discharge pipe going all the way to the surface then falls back down through the pump and out into the well. The water leaving the pump back into the well is moving very fast, and the high velocities break free even more and smaller sediments from the gravel pack and the wall of the aquifer outside the gravel pack. The pump is then turned back on and pumped at a high flow rate to remove the sediments that were broken free by the surging. Surging the well could also involve continuously running the pump but increasing and decreasing the pumping rate to provide a similar surging effect on the well. This surging process is done many times until the discharge water is clear even right after a surge.
JETTING
Using the temporary pump, the engineers estimate the approximate discharge rate the well will be able to produce on a long term basis. If the flow is in line with the expected flow rate for the well, the engineers may elect to move on to the pump testing phase of the well installation process. However, if the flow rate is significantly lower than desired, more energy intensive development methods may be used in an attempt to further develop the well. There are many possible methods, but one of the more popular high energy development methods is called “jetting”. A jetting tool looks like two pressure washing nozzles spraying 180 degrees apart from each other while being rotated on a drillstem. The rotating nozzles spray high pressure jets of water that knock loose sediments from the screens of the well, the gravel pack, and the face of the aquifer material. The jetting tool is lowered down and up in the screened section of the well, slowly cleaning out the more stubborn sediments. To remove the sediment that is knocked loose during this process, the well then subsequently needs to be subsequently pumped to remove the sediments and dirty water dislodged during jetting.
At this point the development of the well is complete. For a deep, large diameter well such as the 1,700-foot Arapahoe aquifer well being completed, the entire development process takes about 3 to 4 days to get to the point where the well is discharging clean and clear water within the range of flow rates expected to produce average and maximum flow rates with the permanent pump. The process takes less time with shallower and smaller wells and more time for larger deeper wells like Monument Well 10.
Next week: We will explain the process of pump testing the well, so stay tuned and we will see you then! And if you have a need for a water resources engineering firm for well drilling, groundwater modeling, water rights cases, or more, please reach out to Lytle Water Solutions, to any of the people below, we will be happy to put together a plan for your needs. We help with projects big and small.
Bruce Lytle, President of LWS: bruce@lytlewater.com
Chris Fehn, Senior Project Engineer: chris@lytlewater.com
Anna Elgqvist, Senior Engineer: anna@lytlewater.com
