Experimental Investigation of Fracture Propagation Behavior Induced by Hydraulic Fracturing in Anisotropic Shale Cores

Abstract

Hydraulic fracturing is a key technology for the development of unconventional resources such as shale gas. Due to the existence of numerous bedding planes, shale reservoirs can be considered typical anisotropic materials. In anisotropic shale reservoirs, the complex hydraulic fracture network (HFN) formed by the interaction of hydraulic fracture (HF) and bedding plane (BP) is the key to fracturing treatment. In this paper, considering the anisotropic angle, stress state and injection rate, a series of hydraulic fracturing experiments were conducted to investigate the effect of anisotropic characteristics of shale reservoirs on HFN formation. The results showed that the breakdown pressure increased first and then decreased when the anisotropic angle changed at 0°–90°, while the circumferential displacement had the opposite trend with a small difference. When θ = 0°, fracturing efficiency of shale specimens was much higher than that under other operating conditions. When θ ≤ 15°, the bedding-plane mode is ubiquitous in all shale reservoirs. While θ ranged from 30°–45°, a comprehensive propagation pattern of bedding-plane and crossing is presented. When θ ≥ 60°, the HFN pattern changes from comprehensive mode to crossing mode. The propagation pattern obtained from physical experiments were verified by theoretical analysis. The closure proportion of the circumferential displacement was the highest when the propagation pattern was the bedding-plane mode (θ ≤ 15°), following by crossing. The closure proportion was minimum only when the bedding-plane and crossing mode were simultaneously presented in the HFN. The results can provide some basic data for the design in hydraulic fracturing of tight oil/gas reservoirs.

Anisotropic Mechanical Behaviors of Shale Rock and Their Relation to Hydraulic Fracturing in a Shale Reservoir: A Review

“…The maximum and minimum breakdown pressures occur at 45 • and 90 • , respectively. This variation agrees well with the tendency determined in the research of Chong et al [199], which synchronously considered the effect of confining pressure and under a condition of σ v = 20 MPa, σ c = 10 MPa, and Q inj = 6 mL/min. In the research of Lin et al [40] and Zhang et al [198], the breakdown pressure under triaxial hydraulic fracturing (σ v = 25 MPa, σ c = 20 MPa, Q inj = 12 mL/min) shows a decreasing tendency as the bedding inclination increases.…”

Effect of Pressure and Stress Cycles on Fluid Flow in Hydraulically Fractured, Low-Porosity, Anisotropic Sandstone

“…9, 10) is measured, a slightly lower difference of 3.0 MPa. This observation suggest that mechanical properties of the rock is influenced by confining pressure (Wang et al 2021) and the orientation of the bedding (Chong et al 2019;Guo et al 2021). However, this mechanical anisotropy is not measured at the intermediate pressure of 11 MPa.…”

Three-Dimensional Ultrasonic Imaging and Acoustic Emission Monitoring of Hydraulic Fractures in Tight Sandstone

“…If these key mechanical problems can be grasped and understood, the process of fracture development can be predicted and evaluated, thus providing important references for optimizing hydraulic fracturing plans and improving recovery efficiency. By conducting hydraulic fracturing experiments in the laboratory, the influence of environmental factors can be reduced, and the dynamic development process and distribution ranges of fractures can be obtained using multiple physical means, such as acoustic emission monitoring, ultrasonic velocity, and X-ray tomography [2][3][4][5].…”