Effect of thermal cycling-dependent cracks on physical and mechanical properties of granite for enhanced geothermal system

doi:10.1016/j.ijrmms.2020.104476

International Journal of Rock Mechanics and Mining Sciences

Volume 134, October 2020, 104476

https://doi.org/10.1016/j.ijrmms.2020.104476 Get rights and content

Abstract

Thermal cycling induced micro-cracks can change the physical and mechanical properties of geothermal energy reservoir, which may influence the heat and mass transfer performance as well as the stability of the reservoir. In this study, thermal cycling treatment was carried out on granite over the temperature range of 20–300 °C with 1–20 thermal cycles. The results show that thermal cycling promotes the initiation and propagation of intergranular and intragranular cracks, which are evenly distributed in all directions. With increase in the number of thermal cycles, the crack density (P_l) increases, resulting in increased permeability (K). The path of seepage passage is mainly between the mineral particles. Moreover, the critical crack propagation radius (r_c) of rock decreases with increasing cracking degree, which leads to the decrease in rock fracture resistance. In particular, the fracture toughness (K_eff) of granite decreases most when it is subjected to 1–5 thermal cycles. Water-cooling thermal cycling and cooling rate can significantly affect the micro-crack evolution, permeability and ability of granite to resist fracture. The changes in mechanical and physical properties observed in this work can provide basic theoretical reference for the rational selection of geothermal energy mining methods and process parameters, as well as the study of reservoir stability evaluation.

Introduction

During the process of geological evolution of planet Earth, huge reserves of fossil fuel, such as coal, oil, shale gas, have been formed. However, combustion of fossil fuel produces CO₂, which causes greenhouse effect globally. Geothermal energy as an unconventional resource can be harnessed from the hot dry rock (HDR) that lies deep inside the Earth's crust.¹ At a depth of 6.5 km from the Earth's surface, the thermal reserve is of the order of 1.1 × 10⁶ EJ.² An assessment of enhanced geothermal systems (EGS) for commercial utilization of geothermal heat showed tremendous potential for electricity generation.³ In the mainland of China, the geothermal energy that can be extracted and utilized from the depth range of 3.5–7.5 km and the temperature range of 150–250 °C is 5300 times of the total primary energy consumption in China in 2010.⁴ An EGS consists of two or more well systems. Using various methods, an artificial reservoir can be formed in which fluid can flow in the system. Hydraulic fracturing is a powerful technology for stimulating fluid production from reservoirs.⁵ In practice, it has been found that the output value of thermal energy extracted in the early stage of production changes significantly, which indicates that the fracture occurs again on the basis of large-scale fracture, resulting in the formation of new cracks and growth of existing cracks. This leads to the change in stability of artificial reservoir, thus affecting the mining efficiency of geothermal energy.

Using water as the heat transfer medium, EGS extracts geothermal energy by injecting water into the artificial reservoir. The water turns into hot steam due to the high temperature inside the Earth's crust. The steam then diffuses into the production wells, and the thermal energy is extracted. On the one hand, the temperature of the region where water is injected into the artificial reservoir decreases, and then it is heated up by the heat source inside the Earth's crust, resulting in a repeated cooling and heating of the rock. On the other hand, the wall of the injection well also experiences a repeated cooling and heating effect. The stability of the well wall is necessary to ensure the smooth circulation of hydraulic fracturing and heat transfer fluid. The process of heating and cooling of a rock is called “thermal cycle”. In other cases, such as in the case of fire, monuments or rock buildings are often heated to high temperature, and then cooled by water, which causes the physical and mechanical properties of rocks to change.

These changes are mainly reflected in the macroscopic physical and mechanical parameters, microstructure and cracks of rock. The uniaxial compressive strength, tensile strength and elastic wave velocity of granite and tuff decrease with increasing number of thermal cycles.⁶ The reduction in specific heat capacity and porosity of rocks due to thermal cycling is due to mineral dehydration, decarburization and quartz transformation.⁷ The characteristic stress level and Young's modulus of marble and granite decrease with increasing number of thermal cycles, while the peak strain and maximum volume strain increase.⁸ The weakening of the macroscopic properties of rock is largely due to the generation of the internal grain boundary and micro-cracks of rock caused by the applied thermal stress.⁸

An earlier study showed that new cracks can be produced even at low temperature at a heating rate of 5 °C/min9. In granite, the differential expansion between adjacent quartz and feldspar particles plays a leading role in the development of thermal cracks below the transition temperature of α-quartz to β-quartz. At higher temperature, the different thermal expansion of adjacent feldspar particles causes continuous cracking, resulting in a “loose” structure.¹⁰ The permanent strain on the rock leads to the development of new micro-cracks and opening of existing micro-cracks.¹¹ The rock micro-structure of changes significantly with temperature, resulting in various physical and mineralogical changes in the rock matrix. After the rock cools to room temperature, the change in the thermal cracks is irreversible to some extent.¹² At a temperature of 600 °C and above, the density of micro-cracks in granite is almost constant. Therefore, the evolution of the physical and mechanical properties of granite is mainly due to the expansion of existing micro-cracks.¹³

The change in the micro-structure of a rock is caused by the physical and chemical reactions of the mineral components in the rock. When a rock is subjected to thermal action, micro-cracks are formed inside the rock. When the micro-cracks expand, they become macro-cracks, and as the macro-cracks become larger, they can cause rock failure. The ability of materials with cracks to resist fracture, the evolution of crack propagation and the fracture criterion are important research areas in the field of fracture mechanics. Fracture toughness is an important index to measure the fracture resistance of a cracked rock, which is often affected by thermal action. The fracture resistance of rock is key parameter to measure and evaluate the formation of a fracture network structure, which is an important reference for selecting a site to be used as a geothermal reservoir. However, the research on the effect of multiple thermal cycles on the micro-cracks and fracture resistance of a rock is lacking. In addition, it is necessary to explore the law of propagation and evolution of micro-cracks, so as to establish the relationship between micro-cracks and the physical and mechanical properties of rocks subjected to thermal cycles.

Therefore, in order to evaluate the operational stability of the reservoir, it is necessary to study the fracture characteristics of the artificial reservoir granite. In this study, the thermal cycling treatment of granite at 20–300 °C was carried out. Optical microscopic observation, P-wave velocity detection, permeability test and three-point bending fracture experiment were also carried out for granite samples. Further, the evolution characteristics of micro-cracks, the change rules of physical and mechanical properties and the correlation between them were investigated quantitatively. The results of this study are of great significance for the construction of artificial reservoir and the stability evaluation of operation under the action of geothermal cycle.

Section snippets

Characteristics of granite samples and thermal cycling treatment

The granite used in this study is from Suizhou, China (Fig. 1). Through visual identification and wave velocity detection, the granite samples with good integrity were selected. This type of granite mainly contains plagioclase, quartzite and potash feldspar, a small amount of biotite, and some other indistinguishable minerals.

Firstly, a large number of granite cylindrical specimens (100 mm high × 50 mm in diameter) are drilled in the project site. Then, a part of the cylindrical specimen is

P-wave velocity

In geotechnical engineering, the propagation of P-wave is highly dependent on rock matrix, and the propagation speed is different in different rock matrices. This is mainly due to the influence of the elastic modulus and density of rock. Based on the measurement of P-wave velocity (V_p), the internal integrity of a rock can be evaluated. Therefore, the P-wave velocities of granite subjected to thermal cycling were measured. The wave velocity is 1604 m/s after one water-cooling thermal cycle, and

Fracture mechanical property of granite

Fracture toughness and fracture criterion are important research aspects of fracture mechanics. The fracture mechanical properties of granite samples were studied after water-cooling thermal cycling experiment.

Relationship between macro-properties and micro-crack characteristics

The physical and mechanical properties of a rock are determined by its material type and micro-structure. For layered rocks, bedding has significant effect on the mechanical properties of rocks.⁵¹ Granite is a type of magmatic rock and is relatively homogeneous. Thermal cycling causes damage to the granite matrix, resulting in a change in its micro-structure, i.e., the matrix becomes softer, and cracks and pores appear inside. The evolution of the cracks with number of cycles can be obtained

Conclusions

In this study, water-cooling thermal cycling experiments of granite in the temperature range of 20–300 °C were carried out for 1, 5, 10 and 20 cycles. The relationship between the micro-crack characteristics and the physical and mechanical properties of granite was quantitatively investigated. Using optical microscopy, the initiation, propagation and evolution of cracks in granite subjected to thermal cycling were investigated. Using acoustic wave velocimeter and low permeability test device,

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figures and Tables
Fig. 1.	Granite from Suizhou, China (The size of plagioclase mineral particles in the granite is generally 0.2–0.5 mm, some 0.5–1 mm, a few 1–2 mm. The particle size of K-feldspar is generally 0.2–0.5 mm, and a few are 0.5~1 mm. The particle size of quartz mineral is generally 0.2–0.5 mm, part of it is

Acknowledgement

This study was supported by the National Natural Science Foundation of China (No. 51827901, 51804204).

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Effect of thermal cycling-dependent cracks on physical and mechanical properties of granite for enhanced geothermal system

Abstract

Introduction

Section snippets

Characteristics of granite samples and thermal cycling treatment

P-wave velocity

Fracture mechanical property of granite

Relationship between macro-properties and micro-crack characteristics

Conclusions

Declaration of competing interest

Acknowledgement

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