1. n. [Geology]

The change in pressure per unit of depth, typically in units of psi/ft or kPa/m. Pressure increases predictably with depth in areas of normal pressure. The normal hydrostatic pressure gradient for freshwater is 0.433 psi/ft, or 9.792 kPa/m, and 0.465 psi/ft for water with 100,000 ppm total dissolved solids (a typical Gulf Coast water), or 10.516 kPa/m. Deviations from normal pressure are described as high or low pressure.

See: abnormal pressure,  formation pressure,  geopressure gradient,  lithostatic pressure,  normal pressure,  pore pressure

2. n. [Well Testing]

A change in pressure as a function of distance. This can refer to

• radial change in pore pressure with distance from the well (which can be calculated from well-test analysis results)
• change in pore pressure with depth (which can be measured by formation tests and implies formation fluid density, fluid contacts, or both)
• change in wellbore fluid pressure with depth (which can be measured with production logs and implies wellbore fluid density).

See: formation fluid,  pore pressure,  production log