Drilling Fluid: Suspends cuttings When circulation is interrupted, the slip velocities of the cuttings will cause them to fall back to the bottom of the hole unless the drilling fluid can suspend the cuttings with its gel strength.  There are a group of liquids, called "non-newtonian liquids", that change viscosity when they are stirred, shaken, or otherwise agitated.  They becomes more viscous when still and liquefies when agitated. This allows it to suspend rock cutting in a borehole when the drilling process is interrupted.  Most liquids are more viscous at lower temperatures.  Upon resumption of circulation, these fluid reverts back to its fluid state and carries the cuttings to the surface.  This ability of developing gel properties while static and then becoming fluid again when pumped is called the thixotropic property of a drilling fluid.  The ability of thickening at low velocities and thinning at high velocities is called the shear-thinning property of a drilling fluid.  The magnitude of gel strengths and shear-thinning ability of a drilling fluid will depend upon the concentration and quality of clay solids in the fluid system. Drilling fluids must suspend drill cuttings under a wide range of conditions. Drill cuttings that settle during static conditions can cause bridges, fill or develop cutting beds in horizontal bores.  This in turn can cause stuck pipe or lost circulation. Cuttings transport in horizontal bores is more difficult than in vertical bores. The transport velocity as defined for vertical bores is not relevant for deviated holes, since the cuttings settle to the low side of the hole across the fluid’s flow path and not in the direction opposite to the flow of drilling fluid.  In horizontal bores, cuttings accumulate along the bottom side of the borehole, forming cutting beds. These beds restrict flow, increase torque and are difficult to remove. In HDD the critical factor during the pilot bore is hole cleaning.  One approach to use for difficult hole cleaning situations found in horizontal bores is the use of shear-thinning, thixotropic fluids with high Low-Shear-Rate Viscosity ( LSRV ) and laminar flow conditions.  Examples of these fluid types are biopolymer systems and flocculated bentonite slurries like the Mixed Metal Hydroxide ( MMH ) system. Such drilling fluid systems provide a high viscosity with a relatively flat annular velocity profile, cleaning a larger portion of the bore cross section. This approach tends to suspend cuttings in the mud flow path and prevent cuttings from settling to the low side of the bore. Even under static conditions these fluid types are superior to bentonite/polymer systems in suspending cuttings. Such systems have been formulated that will suspend cobbles indefinitely.