There are many ways to drill domestic water wells. One of the most popular methods is called “mud rotary”. This brief summary will explain in detail whether this is the best or desired method of drilling your well.
Both water and air can be used to extract cuttings from the borehole bottom as the driller drills for your water well.
Drilling with compressed air has the advantage of letting you know when groundwater is present and giving you an indication of how much water your borehole is producing. The mud rotary method uses water drilling. The driller must rely upon his interpretation of borehole cuttings as well as any changes in the recirculating fluid. Borehole geophysical tools can be used by mud rotary drillers to determine which zones are productive enough for your water well.
Mud rotary well drilling is considered closed-loop. The mud is removed from its cuttings, and then recirculated down the borehole. This drilling method is sometimes called “mud”, but it has been used in the industry for many decades and most people know what it means.
Mixing the water with a product called mud is not a good idea because it is highly refined clay product, bentonite. It is added, mixed and closely monitored during the drilling process.
A bentonite addition to water is used to create a thin film on borehole walls to seal them and prevent water loss during drilling. The hydraulic pressure of the bentonite mix pressing against the borehole walls causes the wall to cave in or sluff. The fluid mixture’s purpose is to transport cuttings from the borehole to the surface. Once they are at the surface, the fluid will filter them out and allow it to be pumped down again.
The driller must have a sump or tank to store a few thousand gallons recirculating fluid when using the mud-rotary method. They must also have a mud processing device that removes the sands or gravels from the mix. This is known as a “shale shaker”.
The fluid mixture should have sufficient gel strength to hold marble-sized gravels and sand on the surface. After the cuttings are carried to the surface, and the fluid velocity has slowed down, the fluid allows the gravel and sand to fall out.
Drillers don’t want to send fine sand back through the borehole and through the pump. The shale shaker uses vibrating screen of different sizes and disbanding cones in order to remove the sand from the fluid as it flows through the shaker. This allows the fluid to be reused again.
Once the borehole has reached the desired depth, and evidence has been found that the formation has yielded enough water, it is time to turn the borehole into an actual well.
The recirculating fluid is thinned slowly by adding water to the fluid. This is because the fluid does not need to support sand or gravel. The fluid will be circulated from the bottom of the borehole by the driller, adding clear water to reduce the fluid’s viscosity. After the fluid has been sufficiently thinned the casing, screens and annular space are gravel packed.
The well casing’s outside and borehole walls are separated by gravel pack. This acts as a filter and helps to prevent sand from entering the borehole walls. The thin layer of bentonite clay, which prevented the drilling fluid water from leaking out of the recirculating system, was used to pack the well.
This is where well development takes place to remove the “wall cake”, or thin layer of bentonite. There are many ways to get rid of the wall cake and make the well as productive as possible.
Drillers may use compressed air to blow out the well. They start at the top and work their way down to the bottom. This helps to remove all water from the drill pipe. Then, the driller allows the well to recover and then blows off any remaining sand. After several cycles of airlift pumping, recovery and drilling, the driller can no longer find any sand in water. It is time to put in a well-development pump.
A development pump can be used to develop the well at a higher capacity level than the final installation pump. The development pump can be used at various flow rates until it reaches its maximum capacity, just as the airlift pumping of the hole. The well should not pump sand if the development pump is able to operate briefly at 50% more than the permanent pump.
There are a few economic factors that you might need to consider before making a decision about the mud-rotary drilling method.
- The cost of the bentonite product and water supply.
- The cost and need for geophysical logging of the borehole
- It is possible to dig a mud pit in the ground to store the cuttings
- How much water from the well can be pumped to the ground?
- When the well has been completed, dispose of all drilling mud and any cuttings
- It takes longer to drill the well than other methods.
For decades, mud rotary well drillers have been able to find ways to make this system work for drilling and constructing domestic water wells. It’s ideal for certain areas because of the geologic formations, but it is not recommended in all areas.
Some drilling rigs can drill with either method. The contractor must decide which method is best for you, your well, and your time.
Click the video below to learn more about the differences between air and mud rotary drills. This video is part of the “NGWA – Industry Connected” YouTube series .
About The Author
Gary Hix, a registered professional geologist in Arizona, is specializing in hydrogeology. He was The Groundwater Foundation’s 2019 William A. McElhanney distinguished lecturer. He was a licensed water well Drilling Contractor in Los Angeles in the past and is still actively involved with the Arizona Water Well Association and National Ground Water Association.
In2Wells.com has more information about Gary’s work. The eBooks he wrote, “Domestic Water Wells Arizona: A Guide For Realtors and Mortgage Ledgers”, and “Shared Water Wells Arizona”, are on Amazon .
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