A fracture with no pressure gradient along the gap during the flow.
It's a proxy concept as the continuous flow along fracture can not happen without pressure gradient at all.
It assumes that all fluid entering the fracture is instantaneously withdrawn from it in place (and not moving towards a well)
The actual meaning of this concept is that pressure gradient inside fracture is many times smaller than pressure gradient in reservoir adjusting to this fracture.
See Also
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Petroleum Industry / Upstream / Production / Subsurface Production / Well & Reservoir Management (WRM) / Well stimulation / Hydraulic fracture
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