Effect of rock brittleness on propagation of hydraulic fractures in shale reservoirs with bedding‐planes

Zhaohui Chong,Chuanjin Tang,Xuehua Li,Liu Zhu,Qiangling Yao

doi:10.1002/ese3.669

Abstract

AbstractHydraulic fracturing forms complex hydraulic fracture networks (HFNs) in shale reservoirs and significantly improves the permeability of shale reservoirs. Although rock brittleness is a major factor in determining whether a shale reservoir can be fractured, the relationship between HFNs and rock brittleness remains unclear. To investigate this relationship in a shale reservoir with bedding planes, this paper presents a series of hydraulic fracturing simulations based on a hydromechanically coupled discrete element model. In addition, we analyzed the sensitivity of the difference in rock brittleness to bedding‐plane angle and density. The parameters used in the model were verified by comparing the simulated results with experimental results and a theoretical equation. The results showed that breakdown pressure and injection pressure increased with increasing rock brittleness. The tensile hydraulic fracture of a shale reservoir (THFSR) was always the most abundant type of hydraulic fracture (HF)—almost 2.5 times the sum of the other three types of HFs. The distribution of areas with higher fluid pressure deviated from the direction of the maximum principal stress when the angle between the bedding plane and maximum principal stress directions was large. Upon increasing this bedding‐plane angle, the breakdown pressure and rock brittleness index first decreased and then increased. However, regardless of bedding angle, the relative proportions of the various types of HFs remained essentially constant, and the seepage area expanded in the direction of the maximum principal stress. Increased bedding‐plane density resulted in a gradual increase in the total number of HFs, with significantly fewer of the THFSR type, and the large seepage areas connected with each other. This study thus provides useful information for preparing strategies for hydraulic fracturing.

Highlights

Major advances in hydraulic fracturing technology have enabled a rapid increase in worldwide shale-gas production,[1,2] exceeding 20 billion m3 in China's fuling gas field alone
Previous studies have shown that the following several factors contribute to creating complex hydraulic fracture networks (HFNs) in shale reservoirs. (a) The viscosity of the fracturing fluid: low-viscosity fracturing fluids, which have relatively less surface tension, can penetrate into micropores, and these fluids spread over wider areas, resulting in more extensive HFNs.[7] (b) The abundancy of bedding planes (BPs) in the reservoir[8]: Fracturing fluids permeate more extensively in reservoirs with abundant BPs, where the pore pressure distribution is such that hydraulic fracture (HF) can initiate and propagate in various directions. (c) The difference between maximum and minimum principal stresses acting on the reservoirs[9]: As the ratio of maximum principal stress to minimum principal stress increases, HFs more traverse BPs, and this extended propagation results in long, narrow HFNs
The parameters of the matrix and the BPs in the shale reservoir were calibrated with respect to experimental data, and the reliability of the fluid parameters of the model was verified by comparing the breakdown pressure obtained from the models with that obtained from theoretical calculations

Summary

Introduction

Major advances in hydraulic fracturing technology have enabled a rapid increase in worldwide shale-gas production,[1,2] exceeding 20 billion m3 in China's fuling gas field alone. Previous studies have shown that the following several factors contribute to creating complex HFNs in shale reservoirs. (c) The difference between maximum and minimum principal stresses acting on the reservoirs[9]: As the ratio of maximum principal stress to minimum principal stress increases, HFs more traverse BPs, and this extended propagation results in long, narrow HFNs. Previous studies have shown that the following several factors contribute to creating complex HFNs in shale reservoirs. (c) The difference between maximum and minimum principal stresses acting on the reservoirs[9]: As the ratio of maximum principal stress to minimum principal stress increases, HFs more traverse BPs, and this extended propagation results in long, narrow HFNs When this ratio is smaller, HFs propagate in various directions, resulting in more complex HFNs. When this ratio is smaller, HFs propagate in various directions, resulting in more complex HFNs. (d) Bedding planes: Some parameters such as BP angle and density can affect propagate track of HFN. (e) Inherent reservoir characteristics[10]: Reservoir characteristics such as the rock brittleness index B affect HFN formation and can be crucial to fracturing efficiency

Methods

Results

Conclusion