Definition - What does Underpressure mean?
Underpressure is a situation whereby the formation pressure is less than the corresponding fluid hydrostatic pressure at a given depth in a rock formation. Oil and gas creation takes place in rock formations deep beneath the earth's surface over the course of millions of years. These fluids are contained within the rock formations under pressure. The fluids exert pressure as a result of density and height of the fluid column. Formation pressure is the pressure of the fluids within the pore spaces of the formation rock.
Petropedia explains Underpressure
The fluids contained within the pore spaces of a formation rock exert pressure which can be affected by the weight of the rock layers above the formation. At the same time, hydrostatic pressure is exerted on the fluid depending on the depth of the formation rock from the surface. The formation pressure can be higher, equal or less than the hydrostatic pressure of the formation fluid. An underpressured system occurs when the formation pore pressure is less than the hydrostatic gradient, 0.45 psi/ft. Underpressured formations can be developed when the rock layers above the formation are stripped away and the formation is exposed to the surface. It can also be induced due to depletion of formation fluids.