Viscosity can be a misunderstood drilling fluid property when horizontal directional drilling (HDD) is concerned. Many drillers use viscosity as the only criterion to judge drilling fluid. Some feel that the thicker the fluid, the better. Some give it no thought at all.

Many would define viscosity as thickness. However, for use in drilling fluids, viscosity is defined as “resistance to flow.” HDD is primarily concerned with two different viscosity measurements: fluid viscosity and slurry viscosity. Fluid viscosity is just what the name implies — the viscosity of the drilling fluid that is being pumped downhole.

Fluid viscosity can come from one of two sources. It can come from the drilling fluid products being mixed, or, if the fluid is recirculated or recycled, viscosity can come from fine drilled solids that become entrained in the fluid.

Slurry viscosity, on the other hand, can be created by the reamer when it mixes the drilled cuttings with the fluid. A number of factors can influence the viscosity of the slurry, including solids content, reactivity of drilled solids, and initial fluid viscosity. Excess fluid and slurry viscosity can cause several problems.

One of the major problems caused by excess slurry viscosity is downhole pressure. Again, viscosity is defined as “resistance to flow.” The problem is that with higher slurry viscosity, more pressure is needed to get the slurry to flow. High slurry viscosity, fragile formations, shallow depths and longer bores can combine to cause problems that can result in inadvertent frac-outs.

Solids removal equipment does not work efficiently with excess viscosity. The drilling fluid can be working to retain solids while the solids removal equipment is trying to remove them. This competition can result in inefficiency in the solids removal system and a buildup of fine solids in the drilling fluid. High slurry solids content that results in excess slurry viscosity can also reduce the efficiency of solids removal equipment. In general, recycling systems do not work efficiently at solids contents above 20 percent.

Pumps, Bentonite and Viscosity

Pumps lose efficiency as viscosity increases, especially if they are not precharged with a centrifugal pump. A 100-gpm pump can become a 50- or 60-gpm pump when high viscosity fluids are pumped. Additionally, cavitation can result, causing premature pump failure.

The desirable drilling fluid properties in a bentonite fluid are gel strength for suspension, and filtrate control/filter cake development to maintain borehole stability. One way to help improve these properties is to add more bentonite. However, increasing the bentonite concentration also increases viscosity. However, alternatives can be used to achieve the desirable properties while maintaining lower viscosity.

Certain bentonite products are designed to help provide a lower-viscosity enhanced gel strength and filtration control/filter cake development. Additives can enhance gel strength for suspension without creating an appreciable increase in viscosity. Other additives can provide filtration control with minimal increases in viscosity. Recently, additives have been introduced that carry a low viscosity (LV) designation. These materials can enhance the desirable properties of the drilling fluid with very little increase in viscosity. When fluid viscosity is building because a reactive formation is being drilled, small amounts of chemical dispersants and thinners can be used to control viscosity.

Slurry viscosity is a function of the drilling fluid coupled with the volume of fluid being pumped and the type of formation being drilled. Extremely reactive formations can demand higher fluid-to-soil ratios to maintain flow capability. In certain highly reactive formations, as little as 5 percent drilled solids can increase the slurry viscosity to the point that flow is lost. In sand situations, higher solids contents can be tolerated because the sand is not reactive. Longer bores also require higher fluid volume to soil ratios simply because the slurry has to flow longer distances and requires more flow capability.

Normally, Marsh Funnel viscosities for the drilling fluid fall into the range of 45 to 70 seconds per quart. However, we are not usually targeting a specific viscosity number. We are more concerned with gel strengths and filtration control/filter cake quality. We can have a fluid with high viscosity that doesn’t have these properties and that just compounds the potential problems.

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