Pore Pressure Gradient
Compute a formation's pore pressure gradient by dividing the pore pressure by the vertical depth, in psi/ft (or kPa/m). The normal saltwater gradient is ~0.465 psi/ft; higher values indicate overpressure (dangerous, can cause kicks and blowouts) and lower ones, underpressure. It is a critical drilling-safety parameter, since it sets the mud weight needed to balance the formation. Enter the pore pressure and the vertical depth.
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Pore pressure gradient
Pore pressure is the pressure of the fluid held in the pore space of the rock, and its gradient (pressure ÷ depth, in psi/ft) is one of the most closely watched numbers in drilling — well safety hangs on it. The normal gradient, that of a salt water column connected to surface, is about 0.465 psi/ft. When the gradient climbs above that value, the formation is overpressured: the fluid has been trapped and compressed (by rapid burial, hydrocarbon generation, or geological seals), and that is dangerous — if the drilling mud weight fails to balance the pressure, the formation 'kicks' fluid into the wellbore, and a kick left uncontrolled turns into a blowout, as at Deepwater Horizon. Below the normal value there is underpressure, with a risk of lost circulation. That is why the drilling engineer works out the pore pressure gradient to set the mud weight and the casing seat depths — keeping wellbore pressure inside a safe window between the pore pressure gradient and the fracture gradient. Enter the pore pressure and the true vertical depth.
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