Experimental Study on the Influence of Real-Time Temperature Cycling on Physical and Mechanical Properties of Granite

Abstract

In this paper, a self-developed multi-functional high-temperature rock triaxial servo control testing machine was used to carry out uniaxial compression tests on the granite after the cooling and heating cycles under real-time temperature. The physical and mechanical properties of two types of granite damaged by hot and cold cycling under real-time temperature were discussed, and the changes in apparent color, longitudinal wave velocity, elastic modulus, uniaxial compressive strength, and damage characteristics of the specimen were revealed. The research results show the following: (1) With the increase in temperature or the increase in number of cycles, the uniaxial compressive strength, longitudinal wave velocity, and elastic modulus of the samples under the two cooling methods all show a decreasing trend, but the decrease in the range is different. The change range of the sample with temperature is greater than that with the number of cycles. (2) Under the dual action of real-time temperature and cold heat cycle damage, the failure form of granite is very random, but it is mainly shear failure, longitudinal splitting failure, and conical failure, and it is accompanied by a high temperature with the increase in the number of cycles, and the degree of crushing of the test piece gradually increases. For example, the sample under 600 °C water cooling for 25 cycles is crushed and destroyed. (3) As the temperature and the number of cycles increase, the surface of the water-cooled sample becomes rougher with the increase in the temperature and the number of cycles and the higher temperature, along with more cracks and debris; the increase in the temperature cycle, no obvious cracks appeared on the surface. The test results in this paper can provide relevant theoretical guidance for the stability and safety of rock in geothermal mining.

1. Introduction

In the context of the current “carbon peaking and carbon neutrality goals”, geothermal heat in hot dry rock, as a promising clean energy, plays an increasingly prominent role in the adjustment of national energy structure [1,2]. At present, the basic principle for the development of hot dry rock is to form a fracture network through stimulation technology such as hydraulic fracturing, and the injected low-temperature fluid is raised to the ground after reservoir heat exchange [3,4], in which the high-temperature rock mass is rapidly cooled. In addition, in the process of geothermal energy development, the well wall rock contacts with normal temperature drilling fluid, and the temperature of the high temperature rock mass will also decrease. Therefore, it is of great significance for deep geothermal energy development to study the evolution law of rock physical and mechanical properties and the mechanism of high temperature action.

This has a negative effect on the wellbore stability in the drilling process of high-temperature rock mass, and plays a beneficial role in promoting the further expansion of the fracture network channel and the generation of new fractures in the thermal reservoir [5,6,7,8,9,10]. In this process, the changes in the physical and mechanical properties of high-temperature rock mass after the alternating action of cold and heat play a crucial role [11]. Therefore, this paper studies the changes in the mechanical properties of high-temperature granite at different temperatures after being subjected to different cold and heat cycles [12]. The regularity and mechanism of the change are revealed, which provides a theoretical basis for wellbore stability and artificial thermal reservoir construction in high-temperature geothermal development.

Numerous studies have been performed to investigate the effect of fracture geometry and aperture [13,14,15,16,17,18], as well as rock mechanical/hydraulic/thermal properties [19,20,21,22,23] on the coupled thermo-hydro-mechanical chemical processes in EGS (enhanced geothermal system). Important insights have been gained to improve the understanding of EGS thermal performance and provide necessary guidance for field operations such as fracture stimulation, well configuration, fluid circulation [24,25,26], and so on.

The above scholars mainly conduct experimental research on mechanical properties under normal temperature after a single temperature treatment or cycling. However, there is little research on the change in mechanical properties under real-time high temperature condition after the cold and hot cycle. In geothermal exploitation, high and low temperatures will lead to changes in its mechanical properties, Therefore, on the basis of existing research results, this paper adopts the self-developed multi-functional high-temperature rock triaxial servo control testing machine to conduct uniaxial compression experiments of granite under different real-time temperatures after experiencing cyclic effects of different temperatures and cooling methods. By comparing the macroscopic and microscopic deformation and failure characteristics and their mechanical properties after natural cooling and water cooling, regularities of stress strain curve, peak strength, peak strain, elastic modulus, and failure characteristics of granite are obtained.

2. Experimental Process

2.1. Test Equipment

(1) Heating equipment and ultrasonic tester

The test heating equipment is intelligent muffle furnace, the maximum temperature of the equipment can reach 1000 °C; SYC-2 ultrasonic wave velocity tester was used. Before the test, coupling agent (Vaseline) was applied on both ends of the sample to reduce the attenuation rate of the sample during propagation.

(2) Uniaxial compression test equipment

The self-developed multi-function servo control testing machine is adopted, as shown in Figure 1. The equipment is composed of axial loading system, high temperature heating system, circulating cooling system, test console, and data acquisition instrument. The uniaxial compression test bench is mainly composed of heating furnace, uniaxial compression furnace, and holding furnace. The maximum temperature in this test can reach 600 °C. The maximum allowable loading stress of the equipment is 100 MPa.

2.2. Sample Preparation

The granite selected for this test is from Wendeng, Shandong province. The main mineral composition is plagioclase 28%, quartz 45.6%, potassium feldspar 19%, mica 6.5%, and other mineral content 8.9%, and the density is 2.14 g/cm. In order to reduce the test error, the same granite is selected according to the method recommended by the International Rock Mechanics Test Code ISRM [27]. Granite is processed into Φ 50 mm × 100 mm standard sample (as shown in Figure 2) using two-end 30 purpose fine sand paper to burnish its level. The error shall not exceed 0.02 mm.

2.3. Test Method

Cold and hot cycle damage test scheme: The damage mode can be divided into two types: water cooled and natural cooled. Six groups were divided according to the temperature gradient level (100~600 °C), and five groups according to the number of cycles (5~25 times). Each group of tests under the same variable consisted of three samples totaling 183 pieces.

Next, take 100 °C as an example to describe the test steps in detail:

(1)

Before heating, the test pieces were first tested for wave velocity, and then the 30 pieces were grouped (15 pieces for natural cooling and 15 pieces for water cooled). Put them into muffle furnace in order of the number of cycles, and heat them to 100 °C at a heating rate of 4 °C/min. Maintain the heat for 8 h to ensure uniform heating inside the sample.

(2)

Conduct two types of cooling damage treatment on the specimens uniformly heated and take out 15 of them. The other 15 specimens are cooled to room temperature in muffle furnace.

(3)

When the muffle furnace is naturally cooled to room temperature, the 15 specimens cooled in water are put into the muffle furnace and heated to 100 °C at the same heating rate for 8 h. Then, the specimens are taken out and cooled by water; this process is one cycle.

(4)

Repeat the above steps. After 5 cycles of hot and cold, take out a batch of specimens and number them. The specimens cooled by water are numbered as W100 °C-5-X (W indicates water-cooled at a temperature of 100 °C and cycled 5 times; X is the number of specimens as 1, 2, and 3, respectively). When the muffle furnace temperature is reduced to room temperature, the three samples with natural cooling are taken out and numbered as N100 °C-5-Y (N stands for natural cooling, the temperature is 100 °C, the cycle is performed 5 times, and Y is 1, 2, and 3, in turn. The wave velocities of the cycled samples are measured again to obtain the wave velocities of the cycled samples at the corresponding temperature.

(5)

Repeat the steps (1–4) to force the granite to undergo damage treatment 5 to 25 times by water contact and natural cold and hot cycle, respectively, at 100~600 °C; then, uniaxial compression tests at corresponding real-time temperature are carried out for all specimens.

Real-time uniaxial temperature test process:

(1) Place the processed sample W100 °C-5 times in the middle of the test bench. A spherical seat is placed above the sample to adjust the roughness of the sample, and then the sample is in good contact with the spherical seat with 600 N prestress. Then, install the heating protection device for heating, and put the insulation cotton on the empty position above the heating furnace, to ensure that the temperature will not lose during the heating process;

(2) The heating rate of 4 °C/min was used to heat the samples to 100 °C, and the thermocouple feedback control was used to keep the samples warm for 2 h, so as to ensure uniform heating inside the samples;

(3) After the insulation is completed, the sample is loaded with a high temperature universal testing machine (as shown in Figure 1) at a loading rate of 0.1 mm/min. The real-time stress–strain curve under the action of temperature is recorded automatically by the data acquisition system.

(4) After the sample is destroyed, it is cooled to room temperature (20 °C) to take photos and record and conduct the next sample. The experiment process (1–4) was repeated for all other samples.

3. Analysis of Test Results

3.1. Effects of Different Temperature and Cooling and Heating Cycles on Compressional Wave Velocity of Granite

Formula (1) is used to calculate the compressional wave velocity under different temperature and cooling and heating cycles.

V_mp = l/(t − t₀)

(1)

where V_mp is the longitudinal wave velocity of the sample, m/s; l is the length of the sample, mm; t is the excitation time of ultrasonic signal, s; and t₀ is the receiving time of ultrasonic signal, s.

As can be seen from the average wave velocity at different temperatures and under the action of cold and hot cycles in Figure 3, the change in wave velocity presents a downward trend as a whole. The variation rule of wave velocity is obtained from five aspects: the wave velocity decreased with the gradual increase in temperature. With the increase in the number of cycles, the wave velocity of the samples at the same temperature also showed a downward trend, but the downward trend was relatively slow. The wave velocity of the two cooling methods also showed a downward trend with the increase in temperature and cycle times, but the wave velocity of the water-cooled samples was lower than that of the naturally cooled samples. (4) It is particularly noteworthy that when the temperature is greater than 500 °C, the wave velocity under the two cooling methods shows a sudden downward trend. It can be reflected that when the sample reaches 500 °C, the internal damage of the sample is relatively obvious. For example, when W400 °C-5 times is 764.21 m/s, it drops to 374.03 m/s when W500 °C-5 times is 51.5%. The decrease rate from 1012.75 m/s at N400 °C-5 times to 583.79 m/s at N500 °C-5 times was 42.4%. The decreasing range of the sample under water cooling is obviously larger than that under natural cooling. (5) There is also a certain difference in the compressional wave velocity of granite under the same temperature and cycle times: the wave velocity of the sample under water cooling is lower than that under natural cooling, and the wave velocity is smaller; this indicates that the longer the wave velocity passing through the sample, the smaller the density inside the sample, and the more serious the damage inside the rock.

3.2. Analysis of the Variation Law of Granite Stress–Strain Curve under Real-Time Temperature Cycle

Figure 4 shows the stress–strain curve of granite after cold and hot cycle damage under real-time temperature. With the increase in temperature and cycle times, the stress–strain curve of granite changes as follows: the curve trend of samples generally goes through a compaction stage, elastic stage, yield stage, and failure stage.

At the initial stage of loading (compaction stage), their curves reduce with the increase in the number of cycles. It shows that the compression deformation of the sample is caused by the change in load, which is mainly caused by the compaction of the micro-cracks in the sample caused by thermal stress. Then, the stress–strain curve shows a straight-line state in the elastic stage. Finally, the failure stage is entered. After the failure, the stress of the sample decreases rapidly, the overall bearing capacity is lost, and brittle failure occurs to the rock.

The following can also be seen from Figure 5: (1) The uniaxial compressive strength of granite decreases with the increase in temperature; the uniaxial compressive strength of the sample cooled by water at the same temperature is less than that of the sample naturally cooled. (2) As the number of cycles increases, the uniaxial compressive strength of granite decreases gradually, while the peak strain increases gradually, and the slope of the curve gradually decreases with the number of cycles. (3) By comparing the changes in granite under two cooling methods, it can be seen that the uniaxial compressive strength of the samples under water cooling is smaller than that under natural cooling compared with that under the same temperature and cycle times. (4) It is worth noting that the slope of the samples under water cooling is slower than that under natural cooling, and the peak strain is larger than that under natural cooling. The uniaxial compressive strength of samples cooled by water at the same temperature decreases more rapidly with the increase in cycle times than that of samples naturally cooled. For example, W600 °C-5 times is 72.16% of W600 °C-25 times, while N600 °C-5 times is 47.34% of N600 °C-25 times. (5) It can also be seen from Figure 5 that the strength of the sample W600 °C-5 times is higher than that of the sample W500 °C-5 times, which may be caused by the anisotropy of the sample. Due to the different content and structure of the mineral composition inside the sample, individual differences exist in the sample. (6) The peak strain of granite under two cooling methods increases with the increase in cycle times, among which the peak strain under water cooling is larger than that under natural cooling.

3.3. Failure Mode Analysis

Figure 6 is a typical failure diagram of the cold–heat cycle damage of granite under real-time temperature. According to the failure form and mechanism of uniaxial compression in rock mechanics, the failure forms of rock specimens under uniaxial compression are mainly split failure, shear failure, and conical failure. There is no obvious pattern to follow, which is mainly due to the uneven mineral content in the granite. In addition, the density and number of cracks generated in the samples under the action of high temperature are also different, and the differences in the extension direction and the degree of connectivity of the cracks are important reasons for the differences in the compression deformation of the samples. Therefore, the failure forms are irregular, and the failure forms are complex and diverse.

The following can be deduced from Figure 6: (1) The samples are mainly conical failure and splitting failure modes after failure. And by producing a large number of pieces, the sample in the destruction will give out a violent sound. (2) When the sample in the high temperature stage, the sample mainly exhibits shear failure. When the temperature is greater than 500 °C and the number of cycles is higher than 15, the sample will fail at that moment. The impact of the head on the test machine is gradually reduced, and the sound produced during the break is also gradually reduced. This indicates that the strength of the sample at high temperature and high cycle times is lower than that at low temperature and low cycle times. (3) When the temperature is greater than W500 °C, with the increase in cycles, the mode after the failure becomes more and more complex, and the sample after the failure is relatively loose and easy to be broken. (4) It can be seen from the figure that the sample changes without rules after failure under the two cooling methods. It is worth noting that the top end of most samples in Figure 6 mainly exhibits conical failure. This is mainly due to the relatively hard texture of granite, so the strength is higher than other rocks, and the end effect is obviously prominent. The end effect is caused by the unidirectional force of friction inhibition. It may also be the phenomenon of stress concentration caused by thermal stress under real-time temperature.

It can also be seen from Figure 6 that the sample has a blocky shape after failure. At the high temperature stage, the sample will increase with the number of cycles, and the failure surface will gradually become rough. When the temperature reaches above 500 °C, a large number of damage cracks appear on the surface of the sample. The particles attached to the surface of the sample are fall off easily, and the failure mode is complex. This is mainly because the samples cooled by water will undergo physical and chemical changes in the process of repeated immersion. The mechanical properties of the sample are weakened again. Granite is affected by thermal shock, and the impact failure degree of granite changes sharply. A change in temperature creates a tiny thermal strain inside the rock. Thermal failure occurs when the thermal stress exceeds the compressive strength of the rock itself. The water ACTS as a transition between the particles inside the water-cooled rock, reducing the degree of cementation between grains or microcracks in the rock. At the same time, the pore pressure of water also has a certain splitting effect on the micro-cracks. Due to the repeated heating and cooling of the sample, the internal structure of the rock is damaged to a certain extent, and the higher the number of cycles, the more serious the deterioration [28,29,30].

4. Discussion

4.1. Relationship between Uniaxial Compressive Strength of Granite and Temperature and Number of Cycles

As shown in Figure 7, the average curve of uniaxial compressive strength of granite under the effect of real-time temperature in cold and hot cycle damage can be seen. (1) The uniaxial compressive strength decreases with the increase in temperature, but the decrease rate is lower than that of the sample with the number of cycles. (2) Under the action of the same temperature, the uniaxial compressive strength of the sample decreases with the increase in the number of cycles; the uniaxial compressive strength curve declines rapidly under the action of cycling, and the higher the number of cycles, the lower the strength.

The results show that the cyclic action has great influence on uniaxial compressive strength. (3) The uniaxial compressive strength of both cooling methods showed a downward trend. But the drop in water cooling is greater than the drop in natural cooling, Sample W100 °C-5 times under water cooling is 49.9% of that under W600 °C-5 times, while sample N100 °C-5 times under natural cooling is 38.54% of that under N600 °C-5 times. There are mainly two reasons for the difference: (1) The sample under the two cooling methods contains a certain amount of water. However, in the process of heating, the water gradually evaporates; that is, dehydration occurs. The evaporation of water creates cracks between the particles, which can lead to microcracks in the rock and ultimately affect the strength of the sample. The sample under water cooling has more water in it than the sample under natural cooling due to the process of repeated heating and water cooling. In the process of continuous water immersion and evaporation, the cracks in the water-cooled sample are continuously expanded. when the sample is loaded, the rock will expand through microcracks until it is destroyed. The naturally cooled samples were heated and cooled on the basis of the previous one, so the strength of the samples declined with the increase in the number of cycles, but not as much as that of the samples cooled by water. It also shows that the samples have certain influence on the uniaxial compressive strength of rock under the action of cold and hot cycling.

(2) As the sample is loaded under the state of thermal stress, the uniaxial compressive strength of the sample is lower than that after cold and hot cycling. Moreover, the higher the temperature is, the more obvious the uniaxial compressive strength degradation degree of the sample is [31]. The cooling rate of the samples under natural cooling is relatively slow compared with water cooling. The shrinkage deformation during cooling is relatively uniform and the number of new cracks is relatively small. However, the non-uniform deformation causes the specimen to produce new cracks or the original cracks to further extend. The higher the temperature is, the more cracks will be produced. At the same time, under the cyclic action of the sample, the irreparable damage accumulation of the sample is aggravated, which makes the pore fracture of the sample expand gradually and the damage degree become more obvious.

4.2. Relationship between Elastic Modulus of Granite and Temperature and Cycle Times

As can be seen from the average elastic modulus curve of the sample in Figure 8, (1) With the increase in temperature, the elastic modulus as a whole shows a downward trend. This is consistent with the research results of [32], which may be because when the temperature rises, the internal non-uniform thermal stress will increase the fracture density inside the sample. As a result, the elastic modulus of the sample decreases continuously, from W100 °C-5 times to 48.76% of W100 °C-25 times. (2) In addition, the elastic modulus of the sample decreases with the increase in cycle times; the change law of the elastic modulus of the samples under the two cooling methods is that the decline in the elastic modulus of the samples after water cooling is faster than that of the samples after natural cooling. Compared with the normal temperature (20 °C), the elastic modulus of water-cooled and natural cooling decreased by 91.79% and 85.24%, respectively. It indicates that the change degree of samples under water cooling is greater than that under natural cooling. When the number of cycles reaches 15, it is the cut-off point of the two cooling methods. The sample larger than 15 times declines significantly. However, this difference may be caused by the fact that the sample cooled by water is subjected to the effect of temperature in the process of water contact, and the sample drops in temperature rapidly, and the surface of the sample drops rapidly. However, due to the slow transfer speed inside and outside the sample, a large temperature difference occurs, which leads to the damage of the rock.

In Figure 8b, the declining trend of the elastic modulus of granite with a temperature cycle under natural cooling can be divided into two stages: (1) When the number of cycles is less than 15, the elastic modulus decreases rapidly with the increase in the number of cycles, from N100 °C-5 times to N100 °C-15 times, by 29.12%. (2) When the number of cycles is greater than 15, the elastic modulus decreases gently with the increase in the number of cycles, from N600 °C-15 times to N600 °C-25 times, by 24.59%.

4.3. Relationship between Peak Strain and Temperature and Cycle Times of Granite

Figure 9 shows the change rule curve of peak strain of granite damage in cold and hot cycle under the action of real-time temperature. As can be seen from the figure as a whole, the peak strain of the sample increases with the increase in temperature. The higher the temperature is, the higher the peak strain of the sample will be.

At the same temperature, the peak strain also increases with the increase in cycle times [33]. The peak strain of the samples under both cooling methods increases with the increase in temperature and cycle times, but the peak strain of the samples under water cooling is greater than that under natural cooling. The peak strain of the sample under water cooling increased from 1.44 × 10⁻² at W100 °C-25 times to 2.05 × 10⁻² at W600 °C-25 times, with an increase of 0.61 × 10⁻² and an increase of 29.75%. The peak strain of the sample under natural cooling increased from 1.35 × 10⁻² at N100 °C-25 times to 1.59 × 10⁻² at N600 °C-25 times by 0.24 × 10⁻², with an increase of 15.1%. This is mainly because there are many pore fractures in the sample cooled by water. In the process of compression, the pore fractures are first compacted and then destroyed. However, naturally cooled samples have a relatively dense interior and few cracks, so the compression time is relatively short, which will lead to the decrease in peak strain at last.

4.4. Microscopic Observation

Figure 10 shows the microsection diagram of granite under two cooling methods of 600 °C cycling times and 15 times. As can be seen from the figure, the number of cracks in the sample increases gradually with the increase in the number of cycles, and the cracks become wider gradually. Fine holes and cracks can also be observed on the surface of the sample. However, with the increase in cycle times, the damage changes in the samples under water cooling are greater than that under natural cooling. This is mainly because the quartz crystal in the mineral content will undergo phase transition when the temperature of granite is 500 ~600 °C. When the temperature of quartz grain reaches 573 °C, it transforms from to. Under the effect of this temperature, the granite changes obviously, and the color gradually turns yellow. The ductility of the sample also increases gradually, and the strength also decreases rapidly.

As can be seen from Figure 10, when the number of cycles is 5, the sample contains a small amount of microcracks in the two cooling methods. As the number of cycles increases, when the number of cycles reaches 20, the fracture degree of the sample becomes more and more serious, and the failure forms are more complex, including both trans-granular failure and trans-granular failure. The failure of rocks is usually the result of a combination of many factors. As the sample is cycled after cooling and heating, this process has caused irreversible damage to the sample. Meanwhile, the sample is loaded under the effect of real-time temperature, so when the sample is in the state of thermal stress, the result of rock damage is more complex. The continuous increase in cracks is due to the further development of primary microcracks into fine cracks after heat treatment until the propagation and penetration form a large crack. Secondary cracks also occur on large cracks, and the formation and propagation of cracks are also related to the size, size, and shape of mineral particles.

In order to accurately evaluate the weakening mechanism of the cold and hot cycle damage of granite under the action of real-time temperature, a microsection analysis was carried out on the samples of granite after real-time temperature failure. The mineral composition did not change, and the weakening mechanism was mainly dehydration reaction. Dehydration changes the microstructure inside the granite. When the rock temperature is 100 °C, the inter-layer water evaporation in the granite sample makes the mineral particles harden and improves the mechanical properties of the rock. When the temperature of rock reaches above 200 °C, thermal cycling will reduce the bonding force between mineral particles, resulting in the decline in mechanical properties. When the rock temperature exceeds 500 °C, the internal cracks of the sample increase gradually, and more particles and slag are produced on the fracture surface, which makes the surface of the sample more broken. The continuous water loss reaction during the cycle will gradually loosen the granite crystal structure, and some mineral particles even start to break. The degree of bonding between particles also gradually weakens, resulting in the gradual increase in cracks, which will lead to the inevitable decrease in the mechanical properties of the internal microstructure of granite.

There are two main reasons why the mechanical properties of granite gradually weaken under the effect of real-time temperature: (1) The effect of temperature. Firstly, the sample is loaded in real time after being subjected to the effect of temperature, so the effect of temperature has a dual nature, and the effect of temperature can be divided into two ways: One is the deformation caused by the thermal expansion of rocks during the heating process. The other is that the rocks shrink and deform during the cooling process, both of which will produce thermal fracture on the rocks. The cooling process makes the rock more prone to micro-fracture. However, after heat treatment, the pores inside the samples are closed under the effect of expansion by heat and contraction by cold, resulting in a higher strength of the samples than those under real-time loading. Specifically, when the temperature is less than 400 °C, the mechanical properties of the sample are less affected, while when the temperature is higher than 500 °C, the mechanical parameters of the sample vary greatly. (2) Cyclic action. After repeated heating and cooling, the sample has been damaged to a certain extent. When the sample is under the action of thermal stress, the thermal fracture of the sample will eventually affect the mechanical strength of the sample.

This paper is mainly based on experiments, and a large number of experiments are used to verify the research results of this paper. It may be better to combine numerical simulation in the following research.

5. Conclusions

Based on the mechanical test of cold and hot cycle damage of granite under real-time temperature, this paper compares and analyzes the difference in granite changes under two cooling methods, and draws the following conclusions:

(1) With the increase in temperature and cycle times, the compressional wave velocity, elastic modulus, and compressive strength of the samples under the two cooling methods gradually decrease.

(2) By comparing the samples under different cooling methods, it can be found that under the action of different temperatures and cycles, the damage degree of the samples under water cooling is significantly greater than that under natural cooling.

(3) Under different temperatures, the compressive strength and elastic modulus of specimens under the two cooling methods decreased with the increase in temperature, while the pressure-tight section, elastic section, and peak strain all increased with the increase in temperature. At the same temperature, the variation in the range of mechanical parameters of the sample under water cooling conditions is larger than that under natural cooling condition.

(4) The failure modes of the specimens under the two cooling methods are mainly columnar splitting, shear failure, and conical failure.