An experimental and theoretical study on the influence of stress gradients on the propagation of hydraulic fractures

Abstract

Hydraulic fracture growth is significantly influenced by the minimum horizontal principal stress gradient and the fracturing fluid pressure gradient. However, these gradients are often neglected in scaled physical modeling experiments due to difficulties in reproducing them. This study uses centrifugal hypergravity to simulate both gradients and investigate their effects on fracture propagation. Artificial mortar specimens (ø200 mm×400 mm) are fractured under 1g (normal gravity), 50g, and 100g. Results show that compared to 1g, fractures under 50g and 100g exhibit increasingly uneven propagation, with higher g-values leading to greater asymmetry. To interpret this, a theoretical analysis based on fracture mechanics is conducted. When the fluid pressure gradient exceeds the stress gradient, a positive net gradient is generated, increasing net pressure at the lower fracture tip. This raises the stress intensity factor at the lower tip, promoting downward growth. As g increases, the disparity becomes more significant, resulting in greater fracture deviation. In conclusion, this study, for the first time, has verified and explained that the net gradient can change the propagation of hydraulic fractures, providing important guidance for wellbore placement under stress gradients.

Keywords

Stress gradients / Hydraulic fracture propagation / Centrifugal experiments / Theoretical analysis

Acknowledgements

The authors wish to acknowledge the financial supports of Basic Science Center Program for Multiphase Evolution in Hyper-gravity of the National Natural Science Foundation of China (No. 51988101), National Natural Science Foundation of China (Nos. 52109138 and 52122403), Young Elite Scientists Sponsorship Pro-gram by CAST (No. 2023QNRC001).

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