Propagation of fractures, hydraulic fractures included, is accompanied by growth of a certain process zone or "crack layer" around the fracture. This process or hydraulically fractured zone (HFZ) can be either smaller or larger depending on the properties of formation. The goal of this paper is to determine how the extent of HFZ depends both on rock properties and operational parameters as well. A better understanding of the HFZ is important because growth of hydraulically fractured zone can significantly increase permeability adjacent to the fracture leading to improved hydrocarbon production. More energy is required to induce an HFZ than just creating the main hydraulic fracture itself but a larger extent of HFZ leads to lower elastic energy for the same cumulative volume of pumped fluid. In order to adequately describe the entire hydraulic fracturing process, all main energy contributions should be taken into account. Traditionally, in using linear elastic fracture mechanics (LEFM) to model the hydraulic fracturing process we would predict creation of large planar fracture or even fractures without considering the possibility of developing an HFZ around the main hydraulic fracture. In this paper, a new description of hydraulic fracturing based on the Least Action Principle, is proposed to appropriately account for the potential development of an HFZ. Since hydraulic fracture for Unconventional Resource is induced by injection of large volume of fracturing fluid into tight rock formations, it is therefore convenient to formulate the model in the Rice-Cleary framework where "undrained" rock parameters are used. Elastic interaction of the hydraulically fractured zone with surrounding rock is described using Eshelby inclusion approach. It can be demonstrated that when the model is applied to the propagation of a single hydraulic fracture, the model reduces to the conventional (Geertsma-de Klerk) regime of fracture growth. As a result, the extent of the hydraulically fractured zone and its permeability can be expressed in terms of rock properties and key operational parameters (e.g. pumping rate, etc.). Validity of this model will require further work, especially in the laboratory experimental arena as well as analysis of field observations.

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