Structural Characterization and Hydraulic Fracturing Responses of Deep-Buried Geomaterials

Topic Information

Dear Colleagues,

After hundreds of years of exploitation, the more accessible shallow resources and energy are being depleted, and some have now been completely exhausted. This means that the economic exploitation of more of the earth’s deeper resources and energy is now required in order to meet society’s growing demand for resources and energy. In addition, due to the requirements of energy conservation and environmental protection, unconventional energy sources such as oil, gas, and geothermal in deep reservoirs have become the focus of current and future development and utilization. Deep reservoirs are mostly buried within 1500−4000 m, while some tight sandstone, shale, and hot dry rock reservoirs are buried over 5000 m or even nearly 10000 m. Deep geomaterials have the characteristics of deep burial, complex pore structure, and low porosity and permeability. Hydraulic fracturing technology is the main means to increase the permeability and productivity of low-permeability reservoirs. In the process of deep-hole hydraulic fracturing, the fracture mechanism of low-permeability rock mass is very complicated due to many factors, such as the properties and occurrence environment of the rock mass, the properties of fracture fluid, flow characteristics, the interaction between the rock mass and the fluid, and phase change. Moreover, the deep in situ stress state is very complex, and the lack of accurate and effective on-site monitoring means makes it impossible to directly observe the hydraulic fracturing effect in deep geomaterials. Hence, it is of great significance to clarify and master the structure characteristics and hydraulic fracturing responses of deep geological materials for hydraulic fracturing mining design, reservoir reconstruction, and improving the recovery ratio of resources and energy. This Special Issue aims to build a broad platform for global scholars to report, exchange, and discuss their ideas, conclusions, and insights regarding the structural characterization and hydraulic fracturing responses of deep geomaterials in engineering practice and Earth science research. We encourage the scholars to reveal and interpret the possible structure characteristics, mechanical properties, and hydraulic fracturing behaviors for deep geomaterials from macro-, meso-, and microscales. The characteristics include but are not limited to the pore structure characterization, permeability characteristics, crack initiation and propagation criteria, mathematical and mechanical models, in situ stress prediction methods, and electromagnetic–acoustic–optical characteristics for deep geomaterials are expected to be explored. Reviews, experimental and numerical work, as well as in situ research, are all welcome for consideration. Potential topics include but are not limited to the following themes:

• Structural characterization methods/techniques for deep geomaterials;
• Hydraulic fracturing tests of large-scale deep geomaterials under true triaxial conditions;
• Impacts of pressurization rate on the mechanical behaviors of deep geomaterials;
• Macroscopic and microscopic observations of hydraulic pressure crack initiation and propagation during hydraulic fracturing;
• Influences of high temperature and high pore pressure on the hydraulic fracturing process;
• Application of non-destructive inspections in failure prediction of deep geomaterials during hydraulic fracturing;
• Impacts of crustal stresses on fracture pressure and hydraulic fracture;
• Chemical reagent tracking in the hydraulic fracturing process;
• Failure criterion for hydraulic fracturing in deep geomaterials;
• Numerical simulation of internal structural changes in deep geomaterials during hydraulic fracturing;
• Numerical modeling for deep geomaterials during hydraulic fracturing;
• Fault reactivation and earthquake mechanisms induced by hydraulic fracturing;
• In situ stress prediction model of hydraulic fracturing in deep geomaterials;
• New development of hydraulic fracturing technique for in situ stress measurement at great depths;
• New theories and equipment for on-site hydraulic fracturing tests.

Dr. Peng Li
Dr. Zhengyang Song
Dr. Mostafa Gorjian
Topic Editors

Keywords

Participating Journals

Journal Name	Impact Factor	CiteScore	Launched Year	First Decision (median)	APC
Geosciences geosciences	2.7	5.2	2011	23.6 Days	CHF 1800
Materials materials	3.4	5.2	2008	13.9 Days	CHF 2600
Minerals minerals	2.5	3.9	2011	18.7 Days	CHF 2400
Energies energies	3.2	5.5	2008	16.1 Days	CHF 2600

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Published Papers (2 papers)

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14 pages, 3847 KiB  

Open AccessEditor’s ChoiceArticle

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

by Peng Guo, Xiao Li, Shouding Li and Tianqiao Mao

Energies 2023, 16(21), 7270; https://doi.org/10.3390/en16217270 - 26 Oct 2023

Viewed by 781

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 [...] Read more.

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. Full article

(This article belongs to the Topic Structural Characterization and Hydraulic Fracturing Responses of Deep-Buried Geomaterials)

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24 pages, 8862 KiB  

Open AccessArticle

Study on Hydro-Mechanical Coupling Failure and Permeability Enhancement Mechanisms for Sandstone with T-Shaped Fractures

by Ying Zhang, Kun Bi, Jiliang Pan, Xun Xi, Dongsheng Zhang, Shengjun Miao and Meifeng Cai

Materials 2023, 16(8), 3118; https://doi.org/10.3390/ma16083118 - 15 Apr 2023

Viewed by 982

Abstract

The rise in the connectivity of the fractures is a key task in oil/gas and geothermal exploitation systems. Natural fractures widely exist in underground reservoir sandstone, while the mechanical behavior of rock with fractures subjected to hydro-mechanical coupling loads is far from clear. [...] Read more.

The rise in the connectivity of the fractures is a key task in oil/gas and geothermal exploitation systems. Natural fractures widely exist in underground reservoir sandstone, while the mechanical behavior of rock with fractures subjected to hydro-mechanical coupling loads is far from clear. This paper employed comprehensive experiments and numerical simulations to investigate the failure mechanism and permeability law for sandstone specimens with T-shaped faces subjected to hydro-mechanical coupling loads. The effects of crack closure stress, crack initiation stress, strength, and axial strain stiffness of the specimens under different fracture inclination angles are discussed, and the evolution processes of permeability are obtained. The results show that secondary fractures are created around the pre-existing T-shaped fractures through tensile, shear, or mixed modes. The fracture network causes an increase in the permeability of the specimen. T-shaped fractures have a more significant effect on the strength of the specimens than water. The peak strengths of T-shaped specimens decreased by 34.89%, 33.79%, 46.09%, 39.32%, 47.23%, 42.76%, and 36.02%, respectively, compared with intact specimen without water pressure. With the increase in deviatoric stress, the permeability of T-shaped sandstone specimens decreases first, then increases, reaching its maximum value when macroscopic fractures are formed, after which the stress suddenly decreases. When the prefabricated T-shaped fracture angle is 75°, the corresponding permeability of the sample at failure is maximum, with a value of 15.84 × 10⁻¹⁶ m². The failure process of the rock is reproduced through numerical simulations, in which the influence of damage and macroscopic fractures on permeability is discussed. Full article

(This article belongs to the Topic Structural Characterization and Hydraulic Fracturing Responses of Deep-Buried Geomaterials)

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