Definition
Hydrostatic Pressure — Hydrostatic pressure is the pressure exerted by the column of drilling fluid in the wellbore due to its density and true vertical depth, and it forms the primary well-control barrier that holds back formation pressure.
Also known as: hydrostatic pressure, mud column pressure, hydrostatic head
What hydrostatic pressure is
Hydrostatic pressure is the static pressure created by the weight of the fluid column in the well. It depends only on the fluid density and the true vertical depth (TVD) — not on hole diameter or the measured length of a deviated well. In field units it is calculated as 0.052 × mud weight (in ppg) × TVD (in feet), giving pressure in psi; in metric units, 0.0981 × density (in kg/L or SG) × TVD (in m) gives pressure in bar.
By keeping the hydrostatic pressure at or slightly above the formation pore pressure, the mud column acts as the primary barrier that prevents formation fluids from entering the wellbore.
Why it governs well control
- Overbalanced — hydrostatic exceeds pore pressure, holding the formation back.
- Balanced — hydrostatic equals pore pressure, the transition point where a well can begin to flow.
- Underbalanced — hydrostatic is below pore pressure, allowing a kick unless managed deliberately.
Because it depends on TVD, hydrostatic pressure must be evaluated at the depth of interest, and swabbing during tripping can momentarily reduce it and induce a kick.
Why it matters for rentals
Maintaining hydrostatic control is the foundation of safe operations, backed up by certified drill-string barriers. See our pressure control equipment and well control application, or contact us.