A nonlinear hydraulic fracture propagation criterion considering the fracture process zone

Abstract

The linear elastic hydraulic fracture criterion is not applicable to deep reservoirs when nonlinear behavior is present over an extensive zone at the fracture tip. This study aims to develop a criterion for nonlinear hydraulic fracture considering the fracture process zone (FPZ) and seeks to reveal the causes of nonlinearity during fracture propagation in deep reservoirs. A closing stress profile considering the in-situ stress was established by using the cohesive zone model (CZM) to describe the FPZ at the fracture tip. An analytical model for the FPZ length was derived, while the criterion for nonlinear fracture propagation was proposed. The FPZ fully developed and the fracture began to propagate when the apparent stress intensity at the fracture tip reached the apparent fracture toughness or when the in-situ stress intensity reached the in-situ fracture toughness. The proposed criterion can clearly determine the length of the FPZ, accurately predict the breakdown pressure during fracturing operations, and establish a relationship between these two parameters. It addresses the inherent limitations of conventional linear elastic fracture mechanics (LEFM), which often underestimates fracture toughness and neglects the effects of the FPZ. This research is expected to enhance the fracturing design in deep reservoirs.

Keywords

Hydraulic fracturing / Fracture process zone / Nonlinear fracturing / Breakdown pressure

Acknowledgement

This work was supported by the National Natural Science Foun-dation of China (No. 52434001).

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