Experimental investigation of the effects of heterogeneity and geostress difference on the 3D growth and distribution of hydrofracturing cracks in unconventional reservoir rocks

- 3D hydrofracturing behavior of heterogeneous rock was investigated.
- Effects of material heterogeneity and geostress difference were addressed.
- Mechanisms for fracture initiation and propagation were analytically interpreted.
- Fractal theories were used to characterize 3D hydrofracturing crack network.
- A threshold geostress difference for occurrence of multiple fractures was proposed.

Accurate understanding and characterization of the growth and distribution of hydrofracturing cracks is a pivotal issue in enhancing hydraulic fracturing stimulation of unconventional oil and gas reservoirs. In-situ investigations have been conducted to probe hydrofracturing crack growth and distribution patterns underground. Unfortunately, few are available for providing accurate knowledge of the three-dimensional growth and distribution behavior of hydrofracturing cracks in unconventional heterogeneous rock formations. This study reports an investigation that incorporates model materials, triaxial hydrofracturing tests, CT technology, and numerical tools to probe the effects of material heterogeneity and geostress difference on the crack growth and distribution in three dimensions in heterogeneous rocks. The initiation positions and propagation azimuths of cracks influenced by geostress difference and heterogeneous gravels are analyzed by means of fracture mechanics and finite element methods. The CT technique and the fractal theory are used to characterize the 3D growth and distribution patterns of cracks in the media. The results show that material heterogeneity and horizontal geostress difference greatly influence the 3D initiation, growth and distribution of hydrofracturing cracks. The horizontal geostress ratio 1:1.7 appears to be a threshold value lower than which multiple, twist hydrofracturing cracks emerge in heterogeneous reservoir glutenite.