Numerical Analysis on the Formation of Fracture Network during the Hydraulic Fracturing of Shale with Pre-Existing Fractures

Jianming He,Zhaobin Zhang,Xiao Li

doi:10.3390/en10060736

Abstract

In this paper, configurations of pre-existing fractures in cubic rock blocks were investigated and reconstructed for the modeling of experimental hydraulic fracturing. The fluid-rock coupling process of hydraulic fracturing was simulated based on the displacement discontinuities method. The numerical model was validated against the related laboratory experiments. The stimulated fracture configurations under different conditions can be clearly shown using the validated numerical model. First, a dominated fracture along the maximum principle stress direction is always formed when the stress difference is large enough. Second, there are less reopened pre-existing fractures, more newly formed fractures and less shear fractures with the increase of the cohesion value of pre-existing fractures. Third, the length of the stimulated shear fracture decreases rapidly with the increase of the friction coefficient, while the length of the tensile fracture has no correlation to the fiction coefficient. Finally, the increase of the fluid injection rate is favorable to the formation of a fracture network. The unfavorable effects of the large stress difference and the large cohesion of pre-existing fractures can be partly suppressed by an increase of the injection rate in the hydraulic fracturing treatment. The results of this paper are useful for understanding fracture propagation behaviors during the hydraulic fracturing of shale reservoirs with pre-existing fractures.

Highlights

Fracture propagation behavior during the hydraulic fracturing treatment of shale reservoirs has been investigated by many researchers
The formation of the fracture network during hydraulic fracturing of reservoir rock with pre-existing fractures is investigated using a numerical model based on the displacement discontinuities method
The numerical modeling is carried out based on the reconstruction of the pre-existing fracture networks generated from repeated heating and cooling processes in cubic rock blocks

Summary

Introduction

Fracture propagation behavior during the hydraulic fracturing treatment of shale reservoirs has been investigated by many researchers. Zoback et al [1] implemented both laboratory experiments and numerical modeling to demonstrate that the slow slip on pre-existing fractures is important to the effectiveness of slick-water hydraulic fracturing. Nagel et al [2] found that a higher injection rate favors the creation of tensile failure, while a lower injection rate and lower viscosity favor the creation of shear fracture. Riahi and Damjanac [3] performed a series of comparative studies to establish the effects of injection rates, connectivity, and size distribution of natural fractures on the stimulated area. Fu et al [4] simulated the fracturing process with dozens of natural fractures to investigate the effects of principal stress orientation and stress anisotropy. McClure and Horne [5]

Methods

Results

Conclusion