The term "ISIP", meaning instantaneous shut-in pressure, is often used to represent the fracture extension pressure (FEP), and to determine the reported fracture extension gradient. In vertical wells, with minimal near-well friction, this equivalence was often acceptable. In horizontal wells, the difference between an "instantaneous" ISIP and FEP can exceed 1000 psi. Misinterpretation of the FEP and fracture extension gradient can lead to critical misunderstandings regarding fracture orientation and geometry. This paper explains the difference between ISIP, as often misinterpreted, and fracture extension pressure (FEP) and presents a proven method for determining the correct FEP, extension gradient, and near-well frictional pressure.

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