Recently, this author read an article about a horizontal directional drilling contractor who fracked out under a creek. The drilling fluid was referred to as natural gas drilling lubricants and, allegedly, the inadvertent return of 50 gallons of bentonite caused several water wells in the surrounding area to become cloudy. It is easy to see how this sort of thing can get blown out of proportion and even confused with a totally unrelated hotbed issue such as natural gas well fracturing. While there are unseen conditions in which frack-outs can be virtually unavoidable, a large percentage of frack-outs that occur during the horizontal directional drilling process are preventable. This article will delve into the causes of frack-outs, preventive measures to reduce the chances of frack-outs/inadvertent returns, and possible ways to seal off frack-outs.
Frack-outs/inadvertent returns occur when the pressure within the borehole (annular pressure) exceeds what the formation can withstand. There are a multitude of causes for high annular pressures, and one of the most common causes is high viscosities in drilling fluid returns. Viscosity is the resistance of a fluid to flow. Therefore, the higher the viscosity in drilling fluid returns, the higher the resistance to flow, and the more mud pump pressure it takes to push material out of the borehole. The days of mindlessly mixing a super-thick muck of just high-yield bentonite and water and calling it drilling fluid are gone thanks to a better understanding of the ill effects of high annular pressures, along with a better awareness of the benefits in matching drilling fluids to the various soil conditions encountered. Today, we know that the ideal drilling fluid will perform all of the required functions, such as gel-strength to suspend and transport cuttings to the surface, fluid-loss/filtration control to maintain an open borehole, and controlling reactive clays, at the lowest possible viscosity. Failure of a drilling fluid to perform the required functions can lead to high annular pressures and frack-outs, along with many other problems such as stuck pipe or stretched product line.