Tips for Preventing Borehole Collapse
The goal of water well drilling is to make a borehole that will create a long-lasting well. Without the proper support, the borehole can collapse in on itself. Here are a few things to keep an eye out for during the drilling process to ensure your borehole has the best chance of success.
The Right Soil Conditions
Borehole failure or collapse can be the result of drilling in the wrong soil conditions. If soil starts to collapse into the borehole, the gauge size or structural integrity is compromised. Unconsolidated or poorly consolidated sediments, shales, naturally fractured reservoirs and other poor soil conditions can lead to borehole collapse. Other problematic soil conditions include silty or heavy clay, fine sand, soil with many large rocks or an absence of topsoil.
The ideal soil conditions are deep, well-balanced soils free of rocks with a middling to high water table. Loose soil allows high water yield, yet it needs to be dense enough to filter rainwater safely. The deeper the topsoil and higher the water table, the less bedrock there will be to drill through to reach the water.
Choosing ideal soil conditions can lower the risk for borehole collapse and make the drilling project easier all around. However, this isn’t the only consideration.
Unbonded Soil Formations
Unbonded soil formations feature rock, pebbles or boulders that drilling mud can’t support. The drilling mud flows into the rock formation, then sand or gravel falls into the borehole and packs the drill pipe. This type of borehole instability can happen gradually or suddenly.
The topography of the land near the drilling site gives insights into what sort of soil or rock formations might crop up throughout the drilling process.
Maintaining Drilling Mud
Drilling mud is the key ingredient for borehole stability throughout mud rotary drilling. The drilling mud is a heavy, viscous mixture of water and bentonite that’s pumped into the borehole through the drill bit. When the drilling mud circulates through the borehole, the pressure and weight keep the borehole from collapsing. The mud needs to be denser than the surrounding earth and, over time, will pack itself into the soil to stabilize it.
If the mud is too dense or under too much pressure, the walls of the borehole will collapse or narrow. Thick mud might also cause borehole enlargement or a fracture in the borehole walls. To avoid this, constantly monitor the drilling mud. If the mud is too thick, add more water to the mud pit to thin the fluid. Learn more about monitoring drilling mud here.
The opposite is also true. If the drilling mud becomes too thin, the borehole can start to narrow or collapse in on itself, trapping the drilling auger and halting the process. This is another instance where monitoring the drilling mud comes into play. If the viscosity is too low, add more bentonite mixture to thicken the mud so it can support the borehole walls. There are, however, some instances where the drilling mud just can’t fight against a collapse.
What To Do If the Borehole Does Collapse
Most drilling experts suggest drilling a new borehole if the original is blocked or collapses. The original signs of an imminent collapse can be fixed with the proper steps: choosing a stable location for the borehole, increasing or decreasing the viscosity of the drilling mud and monitoring drilling pressure. Sometimes, it is possible to dislodge the bit and salvage the project. However, in worst-case scenarios, you may need to leave the bit and some of the pipe in the ground, abandon the well and choose a new location.
The best way to ensure your borehole doesn’t collapse is to train your crew properly. Lone Star Drills offers a half-day training session with the purchase of a water well drill. Our partner organizations, like Equip International, offer additional training and instructional information on their websites. If you have questions or are curious about additional training, contact us today.