As children digging in the dirt, one of the most basic lessons one learns is that in order to make a hole in the ground, dirt must be removed. Kids may accomplish the removal of dirt by means of makeshift tools such as sticks, bare hands, or even utensils pirated from mom’s kitchen — but the concept and goal of removing dirt to make a hole in the ground becomes clear and concise. Horizontal directional drilling (HDD) contractors must keep this basic concept in mind when drilling a bore to install a product line underground. Drilling fluid is not only a tool we use to cool and lubricate the bit and drill string and to stabilize the borehole. It’s also a tool for conveying dirt or drill cuttings out of the ground in order to make a borehole opening large enough to pull product line through. Gel strength is one of the most important characteristics of an HDD drilling fluid, and understanding how to test for gel strength and adjust drilling fluids to varying soil conditions can greatly increase the success rate for HDD contractors.
Drilling fluids are non-Newtonian fluids because viscosity changes with shear/movement as opposed to a Newtonian fluid, like water or oil, where viscosity changes with temperature. The faster a drilling fluid moves, the lower the viscosity, and when a drilling fluid is static, it forms gels to keep drill cuttings in suspension. Gel strength is a measure of the electrochemical forces in a drilling fluid when it’s static, and it is these electrochemical forces that enable drilling fluid to suspend drill cuttings and transport cuttings to the surface. Bentonite platelets have a negative electrostatic charge in the surface of the platelets and a negative charge on the ends of the plates, which creates gel strength and viscosity. Synthetic polymer — like the type commonly used to control/inhibit reactive clays — has little to no gel strength/suspension, which is why bentonite is recommended even when drilling through clay formations.