Science Of Mud, Systems In HDD Ops

March 2012, Vol. 67 No. 3

The key to maintaining the integrity of the walls of the bore is bentonite, the primary component to mud mixes. Bentonite is a mineral found in clay beds. Its properties are such that when it’s mixed with water, it forms a filter cake much like the layers of a biscuit. This filter cake helps seal the walls of the bore tunnel allowing the drilling fluid to remain in the hole, which helps mechanically support the structure of the bore hole.

Water is one of the critical elements to the mud mix. Water can vary depending on its source, whether it is extracted from a creek or river or sourced from the city. Water with a high concentration of sand or clay will weaken the binders in the mix and compromise the effectiveness of the drilling fluid. Contaminates in the water could also damage, or at the very least shorten, the life of your mud pump, says Heinen.

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Other elements and additives in the mix help with fluid viscosity and gel strength, which determine how the fluid can suspend particles and cuttings and carry them out of the tunnel. The ground conditions and type of soil you are drilling into will affect the viscosity and gel strength of the drilling fluid. That is why HDD contractors are constantly tweaking the formula, especially when soil conditions change as the bore progresses.

Rise of mud recycling
Rising costs of mud disposal and the limited availability of disposal sites are driving some HDD contractors to reuse their drilling fluid, which requires a reclaimer.

“Environmental concerns are also a factor so we are seeing more reclaimers required in drilling specs,” says Heinen. “Also mud disposal is cost prohibitive and we are seeing an increase in mud recycling requirements on job sites.”

Cost factors in mud disposal include hauling of used fluid, whether it is trucking or rail cars, sometimes out of state. Dumping costs are expensive; certain situations may require additional additives to be mixed with the used fluid that have come into contact with hazardous materials underground.

It is estimated that about 20 percent of what comes out of the exit pit is solids and cuttings. Mud recycling is simply using a reclaimer instead of a standard mixing system — it’s a combination of a mixing system and a system to clean dirty fluid, and then recirculate the cleaned fluid.