It is the pressure gradient inside the pore space of the rock column from the surface of the ground down to the total depth (TD), as compared to the pressure gradient of seawater in deep water.
In drilling engineering, the pore pressure gradient is usually expressed in API-type International Association of Drilling Contractors (IADC) physical units of measurement, namely "psi per foot", whereas in "pure math," the gradient of a scalar function expressed by the math notation grad(f) may not have physical units associated with it.
In the well-known formula taught in almost all petroleum engineering courses worldwide, the mud weight (MW) is expressed in pounds per U.S. gallon, and the true vertical depth (TVD) is expressed in feet, and 0.052 is a commonly used conversion constant that can be derived by dimensional analysis:
For an onshore vertical wellbore with an exposed open hole interval at 21,000 feet with a pore pressure gradient of 19 lb/U.S.
gal, the BHP would be The calculation of a bottom hole pressure and the pressure induced by a static column of fluid are the most important and basic calculations in all well control courses taught worldwide for the prevention of oil and gas well blowouts.
Using the figures above, we can calculate the maximum pressure at various depths in an offshore oil well.
That pressure is reduced at the surface by the weight of oil and gas the riser pipe, but this is only a small percentage of the total.
It takes heavy mud (drilling fluid) inserted at the bottom to control the well when pressures are this high.