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It assumes that all fluid entering the fracture is instantaneously withdrawn from it in place (and not moving towards a well).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.
It happens because permeability of the fracture is usually much higher than that of reservoir and the fluid flow velocity along the fracture plane is much higher than in reservoir and corresponding pressure gradient is much lower than in reservoir.
In Pressure Transient Analysis the infinite conductivity fracture is the most popular practical case.
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