Regardless of whether one is drilling a foundation drilled shaft, vertical well or doing a horizontal directional drilling project, contractors require a lot out of a drilling fluid or slurry. Functions of a drilling fluid include cooling and lubricating the hole and bit, cleaning the hole, suspending and transporting cuttings, holding the hole open, stabilizing the hole, controlling fluid loss, loss circulation and frac-outs, reducing torque associated with sticky soils, and controlling sub-surface pressure.

In knowing all of the functions of a drilling fluid or slurry, it is ironic how some contractors simply dump bentonite into untreated water and expect the fluid or slurry to flawlessly perform all of these functions. This article is a focus on borehole stability. Understanding how a drilling fluid or slurry stabilizes a borehole can dramatically increase odds of success for drilling contractors.

Two things have to happen for a drilling fluid or slurry to create borehole stability:

1. A drilling fluid or slurry must create an almost impermeable barrier between the soil and drilling fluid. A bentonite drilling fluid utilizes overlapping bentonite platelets to shingle off against the formation to create a filter-cake that serves as the barrier. Fluid loss is a measure of how effective the filter cake is in serving as a barrier to stabilize the borehole. Polymer slurries, such as those used in foundation drilled shafts, create a polymer gel membrane that serves as the barrier between the slurry and soil.
2. Positive pressure must be applied evenly against the barrier between the drilling fluid and formation in order to stabilize the hole and prevent cave-ins. Hydrostatic head pressure is required to apply this pressure. The pressure does not need to be great; it just has to be positive.

A great example of the two above mentioned factors in creating borehole stability is a vacuum pack of coffee. The wrapper serves as the impermeable barrier and 14.5 pounds of atmospheric pressure is evenly applied against the wrapper, stabilizing the granules of coffee. The coffee granules are packed so tight that the package is almost as hard as a brick, but when one punctures the wrapper, you can hear the rush of air into the package, equalizing the pressure, and the coffee granules suddenly become loose and unstable.

As mentioned earlier, fluid loss is a measure of how well a bentonite drilling fluid will maintain borehole stability, especially in loose, unconsolidated formations. The first step in creating a bentonite drilling fluid with low fluid loss is pre-treating the mix water with soda ash to raise the pH to a range of 8.5 to 9 in order to neutralize calcium. Calcium in mix water will cause bentonite platelets to clump up (flocculate) and create soft, thick, porous filter cake that is ineffective in stabilizing the borehole (hydrostatic head pressure is equalized into the formation instead of evenly applied to the surface of the filter cake). PAC (poly anionic cellulose) polymer is a great fluid loss enhancer that can dramatically improve borehole stability while having almost no impact on viscosity.

The importance of hydrostatic head pressure created by the fluid or slurry in the hole — to provide positive pressure against the filter cake or polymer gel membrane, to stabilize loose unconsolidated formations — cannot be overstated. In foundation drilled shafts where synthetic (polymer) slurry is used, a minimum of 6 feet of hydrostatic head above the water table is recommended to provide adequate hydrostatic head pressure to maintain borehole integrity. In foundation drilling applications where the water table is less than 6 feet below the surface, a top can/casing is utilized to get the necessary hydrostatic head. In horizontal directional drilling, contractors do not have the luxury of adding additional hydrostatic head with a top can; therefore it is important to minimize fluid loss and make sure the borehole is full of drilling fluid when encountering loose, unconsolidated soil conditions. Oftentimes, HDD contractors do not think about the need for hydrostatic head pressure while vacuuming drill spoils out of an exit pit when they may be better off leaving more fluid in the pit.

Borehole stability in loose, unconsolidated soil conditions such as sand or gravel is essential to the overall success of a drilling project. Understanding and implementing the basic principles of how drilling fluid or slurry stabilizes a hole in these types of formations can save a drilling contractor a lot of headaches, drill pipe, down-hole tooling and financial loss.

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