Be it vertical rotary mud drilling, horizontal directional drilling or foundation drilling, one commonly overlooked factor in maintaining borehole stability is hydrostatic head pressure. A drilling contractor can have the best drilling fluid or slurry in the world, but if the hydrostatic head pressure is not adequate or is excessive, problems will arise. This article will delve into the effects of hydrostatic head pressure in various drilling applications in order to help drilling contractors use this often overlooked factor to their advantage.
As this author mentioned on a previous article, two things must happen in order to create borehole stability when using a drilling fluid or slurry. The drilling fluid or slurry must create a barrier between the soil and the drilling fluid (filter cake or polymer gel membrane), and we must be able to apply positive pressure against the barrier in order to stabilize the formation. Hydrostatic pressure is providing the positive pressure against the filter cake or polymer gel membrane that keeps the hole stable. Merriam-Webster defines hydrostatic head as “a measure of pressure at a given point in a liquid in terms of the vertical height of a column of the liquid which would produce the same pressure” (Merriam-Webster, 2016). “Groundwater and Wells” (second edition) describes head as “energy contained in a water mass, produced by elevation, pressure, or velocity” (Driscoll, 1995, p.888). To calculate hydrostatic head pressure in pounds per square inch, simply take the weight of the drilling fluid or slurry (in pounds per gallon) times 0.052 times depth.