Most drillers have, at one time or another, attended a mud school and have heard the term “hydrostatic pressure.” Hydrostatic pressure plays a critical, yet often overlooked and misunderstood, role in drilling applications like mud rotary and foundation drilled shafts. I want to offer readers some insight into the concept of hydrostatic pressure and illustrate how it relates to keeping a hole open.
Whether vertical or horizontal, mud rotary or drilled shaft, the key to a successful bore lies in keeping the hole open. For a drilling fluid or drilled shaft slurry to maintain borehole integrity/keep the hole open, a driller must create conditions where positive hydrostatic pressure applies evenly against an almost impermeable filter cake or polymer gel membrane on the walls of the borehole. Hydrostatic pressure is the pressure exerted by a fluid at equilibrium at a given point within the fluid due to the force of gravity. Hydrostatic pressure increases in proportion to the depth measured from the surface because of the increasing weight of the fluid exerting force from above.