Fracking Gradient

Published: December 31, 2019 | Last updated: July 5, 2023

What Does Fracking Gradient Mean?

The fracking gradient or fracture gradient (FG) is the pressure gradient at which the formation fractures. It's the pressure required to fracture a rock at a given depth.

FG is an important parameter to calculate the expected bottom hole treating pressure (BHTP) before starting a fracking job. FG is measured in pounds per square inch, per square foot or bars.

The FG is also used to establish the upper limit of the mud weight which is a primary well control barrier. Mud loss is frequently encountered while drilling and happens when the mud weight essential to maintain wellbore stability and well control exceeds the fracture resistance of the formation, especially in depleted reservoirs.


Trenchlesspedia Explains Fracking Gradient

Many horizontal wells are drilled in the direction parallel to the minimum horizontal stress in the formation, allowing for multiple transverse fractures that are created hydraulically and enhance production. When the pressure in the fracture is less than the fracture closure pressure, the fracture closes.

Hydraulic fractures propagate perpendicular to the least principal stress, which in some formations is the overburden stress, resulting in a hydraulic fracture in the horizontal direction.

Fractures always form perpendicular to the minimum in-situ stress and in almost all cases, the vertical stress equals the weight of the overburden per unit area. In some cases, higher sub-surface vertical stresses are created by upward forces greater than the overburden weight. At shallow depth, the minimum stress is the vertical stress, causing fractures in the horizontal direction.

Fracture gradient (FG) can be calculated using the formula:

FG = (ISIP + Phyd) / TVD

where, ISIP is the instantaneous shut-in pressure in psi

Phyd is the hydrostatic pressure in psi, and

TVD is the true vertical depth


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