Combined Effect of In Situ Stress Level and Bedding Anisotropy on Hydraulic Fracture Vertical Growth in Deep Marine Shale Revealed via CT Scans and Acoustic Emission

Abstract

The economic exploitation of unconventional gas and oil in deep shale relies closely on effective hydraulic fracturing stimulations. However, the fracturing operations of deep shale reservoirs face challenges of insufficient fracture growth and a rapid decline in productivity due to the increasing in situ stress level. In addition, the shale strata on the margin of the Sichuan Basin are frequently folded and faulted, and the change in bedding inclinations significantly complicates the process of hydraulic fracturing. The investigation of the combined effect of the in situ stress level and bedding anisotropy on the hydraulic fracture configuration is vital for fracturing engineering design. To analyze this, we conducted hydraulic fracturing tests on shale cores to simulate the hydraulic fracture initiation and growth from a horizontally positioned perforation. By using acoustic emission detection and CT scans, the influence of natural stress levels and the angle of the shale’s bedding on the process of hydraulic fracturing in shale and the resulting fracture geometry were analyzed. The results showed that the area of hydraulic fracture under a higher stress level (σ1 = 50 MPa, σ3 = 40 MPa) was about 13%~23% smaller than that created under the lower stress level (σ1 = 30 MPa, σ3 = 20 MPa) when the bedding angle was smaller than 60°. With the increase in bedding angle, the curves of the fracture area and fracture network index under two different stress levels presented similar decreasing trends. Also, the time from micro-crack generation to sample breakdown was significantly reduced when the bedding orientation changed from the horizontal to vertical position. The increasing stress level significantly increased the breakdown pressure. In particular, the fracturing of shale samples with bedding angles of 0° and 30° required a higher fluid pressure and released more energy than samples with larger bedding inclinations. Additionally, the measurement of the sample radial deformation indicated that the hydraulic fracture opening extent was reduced by about 46%~81% with the increasing stress level.

Experimental Study on Hydraulic Fracturing with Acoustic Emission under Tri-Axial Stress Conditions

“…AE is high-frequency elastic sudden signals created by material loading and emitted from that. The AE signal during the hydraulic fracturing process of sample is monitored by using AE equipment; the AE activities are revealed during the initiation and propagation of generated fractures (Guo et al, 2023;L. Li et al, 2019).…”

“…The hydraulic fracturing characteristics of shale with different bedding angles under high hydrostatic pressure have been studied (Zhao et al, 2022). Guo et al analyzed the influence of natural stress levels and the angle of the shale's bedding on the process of hydraulic fracturing in shale by using acoustic emission detection and CT scans and the resulting fracture geometry also investigated (Guo et al, 2023). In addition to the researches mentioned above, other scholars have performed the hydraulic fracturing experiment using AE monitoring technology including the following: Chitrala et al (2013) (Chitrala et al, 2013), Chen et al (2014) (Chen et al, 2015), Guo et al (2014) (Guo et al, 2014), , Stoeckhert et al (2015) (Stoeckhert et al, 2015, Ning et al ( 2018) (Ning et al, 2018), Liang et al (2017) (Liang et al, 2017), Hampton et al (2018) (Hampton et al, 2018), Li et al (2018) (Li et al, 2018), Naoi et al (2018) (Naoi et al, 2018), Zhuang et al ( 2019) (Zhuang et al, 2019), Xing et al (2019) (Xing et al, 2019), Ha et al (2020) (Ha & Yun, 2020), Zhai et al (2020) (Zhai et al, 2020), and Tan et al (2021) (Tan et al, 2021).…”