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. 2024 Feb 22;9(9):10177–10189. doi: 10.1021/acsomega.3c07028

Pore Structure Evolution and Failure Mechanism of Limestone in the Taiyuan Formation of the Ordos Basin under High Temperature

Lingfeng Kong †,‡, Zhenyong Yin §,∥,*, Yanpeng Chen ⊥, Zhen Dong ⊥, Jiafang Xu †,#
PMCID: PMC10918677  PMID: 38463247

Abstract

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The study on the destruction of the limestone microstructure after high-temperature treatment has a significant value in the airtightness and safety of underground high-temperature geotechnical engineering. In order to truly simulate the influence of the underground high-temperature environment on limestone, taking seven groups of limestones of the Taiyuan Formation in the Ordos Basin as examples, we carried out a high-temperature (25–1200 °C) heating experiment of limestone in an argon atmosphere. The pore structure of limestone after the high temperature is studied based on scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), porosity, and permeability, and the change in the fractal dimension of the limestone pore structure was discussed based on the thermodynamic fractal theory, combined with X-ray diffraction (XRD) and thermogravimetry differential scanning calorimetry (TG-DSC), the variation of mineral composition with temperature is characterized, and the evolution mechanism of the limestone microstructure under high temperature is discussed. The results show that the evaporation of pore water does not destroy the lattice structure of limestone minerals; however, with the increase of temperature, the complete decomposition of dolomite and calcite occurs, along with the tensile fracture of calcite crystals under the effect of swelling stress. Moreover, the new minerals generated by the decomposition products under the effect of temperature severely damage the crystal structure, leading to the rapid increase of porosity and permeability. The comprehensive results show that the decomposition, expansion, and recrystallization of calcite and dolomite minerals after 800 °C led to the development of limestone macropores and fissures, increased the pore throat radius, enhanced the pore connectivity, simplified the pore structure, and sharply increased the permeability; thus, 800 °C can be used as the critical temperature to change the limestone pores and fractures. The research results can provide data support for subsurface high-temperature geotechnical engineering.

1. Introduction

In recent years, under the dual pressure of energy and environment, vigorously developing clean energy has become the consensus of all countries in the world; hence, underground coal gasification,1,2 heavy oil thermal recovery,3 geothermal resource development,4 storage of nuclear waste,5 and other clean projects have become the focus of research. However, all of these projects will be affected by high temperature; due to the different properties and structures of various minerals in the rock, cracks will appear between and inside rock mineral particles under the action of high temperature, resulting in the destruction of the rock pore structure.6−8 This will affect not only the mechanical properties but also the seepage and transmission capacity of rocks.9−12 In order to ensure the smooth progress of underground engineering, understanding the pore structure change law of limestone under a high temperature has become increasingly important. On the one hand, it is important to prevent the intensification of high-temperature thermal fracture of limestone to solve the problems of surrounding rock stability and tightness in underground coal gasification and other projects.9,13,14 On the other hand, thermal action can promote the pyrolysis of organic matter, leading to the generation of hydrocarbon, the formation and expansion of rock fractures, and the improvement of rock permeability, thus solving the problem of improving the exploitation efficiency of oil and gas resources.12,15,16 Therefore, it is of great theoretical and practical significance to study the changes in the pore structure and failure mechanisms of limestone under high temperatures.

The rock pore structure refers to the type, size, distribution, and interconnection of pores and throat in rocks. The pore is the reservoir space where a fluid exists in a rock, and the throat is an important channel to control fluid seepage in a rock.17,18 Limestone is a porous rock widely used in heavy oil thermal recovery, underground storage of nuclear waste, underground coal gasification, and other projects.19−21 The mineral composition, pore, and throat of limestone undergo complex changes under the action of high temperature. In order to explore their changes, some studies have been conducted. Sklenářová et al.22 explored the variation of mineral composition with temperature based on X-ray diffraction (XRD). Meng et al.23 analyzed the mineral composition change and pore distribution of limestone within the temperature range of 25–800 °C based on XRD and mercury intrusion porosimetry (MIP). Feng et al.24 compared the evolution law of pore distribution in limestone (450–750 °C) under an air atmosphere and approximate vacuum based on a scanning electron microscope (SEM) and nuclear magnetic resonance (NMR). Zhang et al.7 analyzed the changes of pore distribution and porosity of limestone at 25–600 °C based on MIP; the analysis showed that 340 °C is a critical temperature, and several parameters change greatly at this temperature. Liu et al.25 reconstructed a three-dimensional image of limestone at 25–1000 °C based on X-ray computed tomography (X-CT) to obtain the spatial distribution states of pores, fractures, and other parameters. Zhao et al.26 discussed the pore size distribution, porosity, and permeability of limestone at different temperatures (25–700 °C) based on NMR.

In summary, scholars have conducted some research on the changes in mineral composition and pore structure of limestone after heat treatment through different experimental testing methods. A series of physicochemical changes occur in the minerals of limestone at different temperatures, leading to changes in the mineral composition and crystal structure; however, research on the thermal destruction mechanism of limestone from the mineralogical point of view still needs to be further conducted. Although some scholars have qualitatively analyzed the changes of mineral compositions in limestone at high temperatures by XRD experiments, quantitative analysis of mineral compositions and their mineral evolution process as well as the effects on the pore structure parameters of limestone still require a lot of research work. Moreover, as the temperature increases, the pore structure becomes more complex. Previously, it was difficult for researchers to effectively characterize the pore complexity of high-temperature limestone through simple experimental methods,27,28 and a combination of experimental testing and mathematical analysis must be used for joint determination. SEM can effectively and intuitively determine the changes in the microstructure of rock surfaces, MIP can well reflect the seepage characteristics and pore throat combination relationship of rocks; and thermodynamic models can effectively characterize the complexity of pore throats. Based on this, the combination of SEM, MIP, and thermodynamic fractal theory can effectively identify and reflect the pore structure and seepage capacity of limestone, improving the reliability and completeness of the pore structure analysis. Meanwhile, combining XRD and thermogravimetry differential scanning calorimetry (TG-DSC) analysis, we can quantitatively analyze the mineral compositions and their evolution process at high temperatures, explore the influence of the mineral evolution of limestone on the pore structure of limestone at high temperatures, and then analyze the thermal failure mechanism of limestone at high temperatures.

Therefore, this paper considers the limestone in the Ordos Basin as an example to simulate the thermal fracture behavior of limestone at 25–1200 °C, and the experimental atmosphere was argon. The variation trends of mineral composition, pore structure, porosity, permeability, and fractal dimension with temperature are quantitatively characterized by XRD, TG-DSC, gas permeability, SEM, MIP, thermodynamic fractal theory, and other technical means, and the evolutionary mechanism of the limestone pore structure during heating is comprehensively discussed. The research results can provide data support for the efficient development of deep mineral resources and the safe implementation of high-temperature underground engineering.

2. Samples and Methods

2.1. Limestone Sample Preparation

The limestone samples were collected from the Baode mining area, Ordos Basin (Figure 1). Limestone is widely developed in the study area, serving as the roof of the coal seam as well as oil and geothermal reservoirs.29,30 In order to understand the influence of high temperature on limestone, fresh limestone in this area was selected as the research object, and seven groups of samples were prepared for a high-temperature heating experiment.

Figure 1.

Figure 1

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Map of sampling of limestone.31

The sample preparation is shown in Figure 2. First, the limestone sample is divided into seven cuboids of 4 cm × 4 cm × 8 cm by wire cutting technology. Then the sample was heated to the target temperature (200, 400, 600, 800, 1000, 1200 °C) in a tubular atmosphere furnace at a heating rate of 10 °C/min, kept warm for 2 h at the target temperature to make it completely heated, and then cooled naturally. In order to eliminate the influence of oxygen, the whole process was carried out in an argon atmosphere from the beginning of the experiment to the end of the experiment. Predecessors tested the difference between the central temperature and the surface of the sample at a constant temperature of 600 °C for 2, 6, and 10 h, finding it to be only about 3%, and the variation range of its mechanical parameters was controlled within 8%;32,33 we also tested the variation range of the physical parameters of limestone samples at a constant temperature of 600 °C for 2, 4, 6, and 10 h, and it was found to be controlled within 4%, which meets the test requirements. The heated limestone sample was cut into a φ2.5 × 7 cm cylinder by wire cutting technology; the top 0.5 cm residual sample was selected for the XRD experiment and the TG-DSC experiment, and the bottom 0.5 cm sample was selected for the SEM experiment. The upper 1 cm cylinder was cut for MIP, and the middle 5 cm part was cut for porosity and permeability experiments.

Figure 2.

Figure 2

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Sample preparation and experimental flowchart.

2.2. Experimental Instruments and Procedure

2.2.1. SEM Experiment

The bottom residual sample was selected for the SEM experiment; the experimental instrument was the HITACHIS-3000N. First, the sample was fixed on a small disk with conductive electric glue, and then a gold spraying instrument was used to coat the sample under high vacuum. The coated sample was placed on a stage with pliers, and then the sandstone surface was observed with an FEI field emission scanning electron microscope.

2.2.2. MIP Experiment

The sample with a size of φ 25 mm × 10 mm was selected for mercury intrusion experiments. The experimental instrument is the American AutoPore IV 9505 mercury porosimeter, with a maximum injection pressure of 200 MPa, test range of 3 nm to 1000 μm, and volume accuracy of less than 0.0001 mL. The samples were dried at 105 °C to constant weight before the test. During the test, the mercury enters the pores of limestone samples under pressure; the greater the external force applied, the greater the amount of mercury entering the pores of limestone samples, i.e., there is a fixed relationship between the pressure and the pore size that mercury can enter.34,35 According to the pressure and mercury input in the mercury injection experiment, the radius and connectivity of the pore throat can be calculated.

2.2.3. Porosity and Permeability Experiment

We selected a columnar sample with the size of φ 25 mm × 50 mm for porosity and permeability tests, an instrument type KXD 11. The sample was sealed in the holder, and dry helium was allowed to pass steadily through the sample. The inlet and outlet pressures as well as the flow rate of helium were measured, and the permeability was calculated according to Darcy’s law. The rock sample was placed in a core cup, a certain pressure was applied, and the porosity of the rock sample was calculated according to Boyle’s law.36

2.2.4. X-ray Diffraction

The top residual sample was selected for the XRD test. The sample was ground into powder of less than 0.075 mm; the rock powder was tested with the Japan Science TTR III multifunctional X-ray diffractometer, and the diffraction pattern was recorded. The higher the intensity of the crystal diffraction peak, the higher the content.37−41

2.2.5. TG-DSC Experiment

The top sample was selected for the TG-DSC test. The sample was ground into a powder of less than 0.05 mm; then, the STA449C comprehensive thermogravimetric analyzer produced in Germany was used for the test, keeping the heating rate controlled at 5 °C/min, the argon flow rate controlled at 10 mL/min, and the reaction temperature range controlled at 25–1200 °C.

2.2.6. Fractal Dimension Calculation

The internal pore structure of high-temperature-treated limestone is very complex and irregular. Fractal theory, as a discipline that describes the complexity and irregularity of pores, can quantitatively calculate the complexity of the pore distribution and effectively characterize the morphology of irregular geometries. The pore construction method of a thermodynamic fractal model is used to analyze the fractal characteristics of a pore structure with the following expressions (expression 1–3)

2.2.6. 1
2.2.6. 2
2.2.6. 3

where P̅i is the pressure at the ith injection, MPa; ΔVi is the amount of mercury injected at the ith injection, ml/cm3; n is the number of injections; rn is the pore radius at the nth injection, nm; Vn is the cumulative amount of mercury injected, mL; C is a constant; and D is the fractal dimension. According to the experimental data of mercury compression, taking Inline graphic as the Y-axis and ln Qn as the X-axis, the fractal dimensions of tuff samples at different temperatures can be obtained by linear fitting with EXCEL (Figure 7), which reflects the complexity of the pore space within the tuffs. Usually, D refers to the range between 2 and 3, and the bigger the value is, the more complex is the pore space structure.42,43

Figure 7.

Figure 7

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Variation law of limestone porosity and permeability with temperature.

2.2.7. Correlation Coefficient Calculation

Changes in the pore structure during high temperatures will cause changes in porosity permeability. The correlation coefficient, as a quantitative description of the statistical correlation between two sets of random variables, can quantitatively characterize the effect of pore structure parameters on porosity permeability, and the expression for the correlation analysis based on Pearson’s correlation coefficient is (eq 4)

2.2.7. 4

where x̅ and y̅ are the averages of x and y, respectively. R takes the value of [−1,1]: when R > 0, it indicates a positive correlation; and when R < 0, it indicates a negative correlation. The closer |R| is to 1, the better is the correlation, and the closer |R| is to 0, the worse is the correlation.44,45

3. Results

3.1. Pore Structure Analysis

3.1.1. Analysis of the SEM Microstructure Morphology

The surface morphology of the limestone microstructure is shown in Figure 3. In the initial state, the surface of the limestone is flat, and a small number of primary pores are developed. With the increase of temperature, the surface of the chert began to become rough, and the pore fractures gradually developed.

Figure 3.

Figure 3

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Changes of SEM images of samples at different temperatures.

In the range of 25–400 °C, the surface morphology of limestone has no obvious change: it still maintains a relatively flat surface, the mineral grains and boundaries are relatively clear, and the fracture is relatively flat. At 600 °C, some debris particles begin to appear on the surface of limestone, and the size and number of microfractures are small, mainly developing some pores smaller than 0.1 μm, with a fracture width of about 0.064 μm.

At 800 °C, the most obvious phenomenon was that the mineral morphology changed significantly, from massive to loose rounded particles; at this time, a large number of honeycomb pores were generated, and the width and size of the fissures began to increase, with the width of the fractures reaching 0.87 μm, and a large number of pores with size less than 1 μm appeared on the surface of the greywacke.

At 1000 °C, under the action of high temperature, the mineral morphology of limestone changed again, from loose rounded particles to massive, indicating the emergence of new minerals, while the honeycomb pores further developed into larger pores, with the pore size ranging from 1.97 to 4.23 μm, the size of the fracture increased further, the width of the fracture reached 4.72 μm at 1200 °C, the fracture gradually expanded and connected, and the fracture width on the surface of limestone reached 0.87 μm. At 1200 °C, the cracks gradually expanded and connected, forming a network of fractures on the graywacke surface, and the width of the fractures reached 6.12 μm.

3.1.2. Variation of Pore Throat Characteristics with Temperature

According to the Hodot classification,46 the pore fracture system of limestone is divided into small pores (<0.1 μm), mesopores (0.1–1 μm), and macropores and fissures (>1 μm). Based on the piezomercury curves, the parameters indicating the pore permeability characteristics, such as mercury inflow, discharge pressure, mercury withdrawal efficiency, and median radius, can be calculated for different limestone samples (Table 1), and the pore volume, distribution, pore characteristics, and complexity of limestone samples at different temperatures can be evaluated by using these parameters. The mercury removal efficiency of rocks is affected by various factors such as the degree of homogeneity of pore throat and pore size, the average number of pore throats in the connected pores, and the pore roar morphology. The discharge pressure mainly reflects the permeability of coal samples: the lower the discharge pressure, the larger the maximum connected pore roar of the sample, and the better the permeability.

Table 1. Pore Throat Distribution of the Limestone Sample by the Mercury Injection Methoda.
            proportion of pore distribution (%)
sample no. temperature (°C) MWE (%) DP (MPa) APT (μm) Vt (mL/cm3) V1/Vt V2/Vt V3/Vt
L1 25 26.95 62.02 0.005 0.0032 98.34 0.95 0.71
L2 200 27.59 48.22 0.006 0.0038 97.77 1.65 0.58
L3 400 29.51 44.36 0.007 0.0043 96.57 1.82 1.47
L4 600 36.42 11.02 0.023 0.0240 95.40 1.34 2.26
L5 800 59.03 2.74 0.098 0.1866 38.85 56.72 4.44
L6 1000 67.17 0.27 1.611 0.3092 1.00 15.47 83.52
L7 1200 69.46 0.14 6.139 0.3986 1.72 3.52 94.76
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a

Vt, total mercury injection; MWE, mercury withdrawal efficiency; DP, displacement pressure; APT, average pore throat; V1, small throat volume (<0.1 μm); V2, mesopore volume (0.1–1 μm); V3, macropore volume (>1 μm).

It can be seen from Table 1 that within the temperature range of 25–400 °C, the drainage pressure is between 44.36 and 62.02 MPa, the mercury removal efficiency is between 26.95 and 29.51%, and the average pore throat radius is between 0.0005 and 0.0007 μm. The pore throat radius of limestone is small, and the connectivity is poor. At 600 °C, the displacement pressure is 11.02 MPa, the mercury removal efficiency is 36.42%, and the average pore throat radius is 0.023 μm. The size and quantity of the pore throat are slightly increased, and the connectivity of the limestone pore throat is slightly enhanced. At 800 °C, the displacement pressure is 2.74 MPa, the mercury removal efficiency is 59.03%, the average pore throat radius is 0.098 μm, and the connectivity of limestone is further enhanced. At 1000 °C, the displacement pressure is 0.27 MPa, and the average pore throat radius is within 1.611 μm; at this time, it is mainly composed of a mesopore throat and a small pore throat. At 1200 °C, the displacement pressure is 0.14 MPa, the mercury removal efficiency is 69.46%, and the average pore throat radius is 6.139 μm; at this time, the mesopore throat and the macropore throat are mainly developed, with good connectivity. The comprehensive analysis shows that within the temperature range of 25–1200 °C, the average pore throat radius increases, and the displacement pressure shows a decreasing trend, indicating that the difficulty of mercury entering the pores decreases, the mercury removal efficiency gradually increases, and the connectivity of limestone becomes better.

3.1.3. Variation of Pore Throat Volume with Temperature

It can be seen from Figure 4 that with the increase of pressure, the cumulative mercury input of chert samples at different temperatures shows an increasing trend. In the temperature range of 25–400 °C, the cumulative mercury input of limestone does not change much, and the cumulative volume of mercury input ranges from 0.0032 to 0.0043 mL/m3. When the temperature was 600 °C, the unit volume of mercury input was 0.187 mL/cm3, and at 1000 °C, the unit volume of mercury input was 0.309 mL/cm3. At 1200 °C, the unit volume of mercury input was 0.399 mL/cm3, and the comprehensive analysis showed that the cumulative mercury input per unit volume of chert tended to increase in the temperature range of 25–1200 °C.

Figure 4.

Figure 4

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Variation of the mercury-in/mercury-out curve shape of limestone with temperature. (a) 25–1200 °C, (b) 25–600 °C.

3.1.4. Variation of Pore Throat Distribution with Temperature

Figure 5 shows the pore throat distribution of limestone at different temperatures based on MIP. Through comparison, the pore throat distribution measured by MIP shows an obvious three-segment formula. In stage I (25–600 °C), the limestone samples mainly develop a small pore throat and a small amount of mesopore throat. The pore throat radius is mainly between 0.001 and 0.1 μm, the proportion of the small pore throat is 95.4–98.38%, the proportion of the medium pore throat is 0.95–1.34% (Figure 6), and the proportion of the large pore throat is small, between 0.58 and 3.26%. Within this temperature range, the number of small pore throats increased slowly, the number of mesopore throats and macropore throats changed little, and the proportion of all kinds of throats changed little. In stage II (600–800 °C), the number of small pore throats, mesoporous throats, and macropore throats increased significantly, the pore throat radius is mainly between 0.01 and 1 μm, while the proportion of small pore throats decreased continuously; the proportion of mesoporous throats and macropore throats increased, especially at 800 °C—the proportion of mesoporous throats increased to 56.72%. In stage III (800–1200 °C), the pore throat radius was within 0.1–10 μm, the number and proportion of small pore throats and mesopore throats decreased, and the number of macropore throats increased. At 1200 °C, the proportion of macropore throats reached 94.76%. In the process of temperature increase, the number of pore throats increased, and the small pore throats and mesopore throats gradually evolved into macropore throats, which enhanced the connectivity of limestone.

Figure 5.

Figure 5

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Distribution characteristics of limestone pore throat under different temperatures. (a) 25–1200 °C, (b) 25–600 °C.

Figure 6.

Figure 6

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Variation of limestone pore throat number and percentage with temperature. (a) Variation of pore throat number. (b) Variation of pore throat percentage.

3.1.5. Variation of Porosity and Permeability with Temperature

Porosity and permeability are important indexes to evaluate rock pore development and seepage capacity.12 As the temperature increases, the pores and fissures of limestone undergo complex changes, leading to a change in the porosity and permeability of limestone.

The pore volume of the rock can be characterized by porosity. Figure 6 shows the nuclear magnetic porosity and growth rate of limestone samples at various temperature points. The initial porosity of limestone is 3.87%. The porosity of limestone generally increases with the increase of temperature; the porosity of limestone increases by 9.78 times at 1200 °C, reaching 44.1%. However, within the temperature range of 25–400 °C, the porosity changes little, with the porosity being between 3.87 and 5.74%, and the growth rate is relatively flat. At 400–600 °C, the porosity increases slowly to 10.24%. Within the temperature range of 600–1000 °C, the porosity growth trend is the fastest, with the porosity being 2.65–8.08 times that at 25 °C; the porosity of limestone increases fast after 600 °C, and the porosity growth rate reaches the maximum at 800 °C.

The permeability and growth rate of limestone were measured at each temperature (Figure 7). The permeability of limestone was 0.007 mD at 25 °C. At 1200 °C, the permeability of limestone increased 4687 times to 32.82 mD. Within the temperature range of 25–800 °C, the permeability increases with temperature, and the change trend is not obvious. Combined with the MIP experiment, the number of pores increases within the temperature range of 25–800 °C, but the connectivity is poor. Within the temperature range of 800–1200 °C, the permeability increases the fastest, which is 20.3–4687 times that of the initial condition. It can be seen from Figure 6 that the permeability of limestone increases fast after 800 °C, and the growth rate reaches the maximum at 1000 °C; due to the massive decomposition of calcite after 800 °C, the pores increase rapidly; as the reaction continues, the small pore throat gradually changes to a medium pore throat and a large pore throat (Figure 5), the pore connectivity becomes better, and the permeability begins to increase rapidly.

3.1.6. Variation of Fractal Dimension with Temperature

As can be seen from Figure 8, the linear correlation coefficients R2 of ln Qn and Inline graphic of the experimental data of mercury compression of tuff samples at various temperatures ranged from 0.9906 to 0.9979, which showed a good linear relationship. With the increase of temperature, the fractal dimension of chert pores shows a decreasing trend, from 2.9386 in the initial state to 2.541. It shows that the pore structure gradually tends to be simpler with the increase of temperature. In addition, the fractal dimension of the pore structure of chert at different temperatures shows an obvious stage change with the change of temperature; when the temperature is in the range of 25–600 °C, the fractal dimension of the pore structure of chert changes less, and the change curve is almost a straight line, indicating that the pore complexity changes less in this range of temperatures, and the internal pore structure is still maintained in this complex pore structure. In the range of 600–800 °C, the pore fractal dimension of chert shows a slow decreasing trend, i.e., in this temperature range, the complexity of the internal pore structure of chert begins to decrease, and when the temperature is in the range of 800–1200 °C, the pore fractal dimension of chert shows an obvious decreasing trend and the D value is smaller, which indicates that the pore structure of chert is significantly less complex in this temperature range.

Figure 8.

Figure 8

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Fractal characteristics of MIP in limestone samples at different temperatures.

3.1.7. Correlation Analysis between Pore Structure and Porosity Permeability

The change of pore structure during the heating process affects the change of porosity and permeability. Based on the Pearson correlation coefficient, the correlation between the pore structure parameters (pore volume, small pore volume, mesopore volume, macroporous volume, and pore fractal dimension) and porosity and permeability during the heating process was calculated, and the results of the calculations are shown in Figure 9. As shown in the figure, in the range of 25–600 °C, the correlation R values among the total volume of the pore, small pore volume, mesopore volume, macroporous volume, and pore fractal dimension correlation R with porosity were 0.97, 0.97, 0.98, 0.98, and −0.89, and the correlation R values with permeability were 0.92, 0.92, 0.95, 0.98, and −0.88, respectively. The correlation R values of the total pore volume, small pore volume, mesopore volume, macropore volume, and pore fractal dimension with porosity and permeability in the range of 600–800 °C were 1, 1, 1, 1, and −1, respectively. In the range of 800–1200 °C, the correlation R values of the total pore volume, small pore volume, mesopore volume, macropore volume, and pore fractal dimension with porosity were 1, −0.92, −0.95, 1, and −0.97, and the correlation R values with permeability were 0.92, −0.63, −0.69, 0.87, and −0.75, respectively. It can be seen that the changes of porosity and permeability have a close relationship with the pore structure parameters. The pore volume during the heating process shows a positive correlation with porosity and permeability. The increase of pore volume promotes the increase of porosity and permeability. The pore fractal dimension shows a negative correlation with porosity and permeability; with the decrease of fractal dimension, the complexity of the pore decreases, and the porosity and permeability show a trend of growth. It is worth noting that when the temperature is less than 800 °C, each pore volume is positively correlated with porosity and permeability and the correlation coefficients are not much different. When the temperature is more than 800 °C, due to the gradual decrease of the small pore volume and mesopore volume and the increase of the macropore volume, there is a negative correlation between the small pore volume and mesopore volume and porosity and permeability, and the macropore volume is positively correlated with porosity and permeability, and the correlation is higher. The increase of macroporous volume in the high-temperature section tuff promotes the increase of porosity and permeability.

Figure 9.

Figure 9

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Correlation analysis of limestone pore structure parameters with porosity and permeability.

3.2. Mineral Composition Analysis

3.2.1. XRD Result Analysis

XRD is an important method to determine the mineral composition and relative content of rocks.23,47 The X-ray diffraction patterns and content changes of limestone samples at different temperatures are shown in Figure 10, and the semiquantitative results are shown in Table 2. For the limestone samples, the mineral components of the initial samples are mainly calcite and dolomite, along with a small amount of clay minerals and quartz.

Figure 10.

Figure 10

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Changes of XRD patterns and mineral composition of samples after heat treatment at different temperatures.

Table 2. Whole Rock and Clay Mineral Data of Limestone Samples at Different Temperaturesa.
    whole rock (%)
 clay (%)
no. T (°C) Qua Pot Cal Dol Per Por Lar Kil Mul Cla Ill Kao
L1 25 9.1 0.7 72.3 12.2           5.7   100
L2 200 8.9 0.8 72.2 12.3           5.8   100
L3 400 8.9 0.7 73.1 11.8           5.5   100
L4 600 7.5   78.6 5.7 4.2         4.0 100  
L5 800 8.0   58.4   4.3 25.2       4.1 100  
L6 1000         4.4   48.5 42.7   4.4 100  
L7 1200         4.2   46.9 43.1 4.6 1.2 100  
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a

Qua, Quartz; Pot, potassium feldspar; Cal, calcite; Dol, dolomite; Per, periclase; Por, portlandite; Lar, larnite; Kil, kilchoanite; Mul, mullite; Cla, clay mineral; Ill, illite; Kao, kaolinite.

Within the temperature range of 25–400 °C, the X-ray diffraction patterns of limestone samples are similar, and the diffraction intensity of each mineral changes little, indicating that in this temperature range, the chemical composition and crystal structure of limestone hardly change, the calcite content is between 72.2 and 73.1%, and the dolomite content is between 11.8 and 12.3%.

Within the temperature range of 400–600 °C, the diffraction intensity of dolomite decreases and its relative content decreases to 6.3%. This is mainly due to the decomposition reaction of dolomite (eq 4).48 The decomposition products are mainly calcite and periclase crystals. At this stage, calcite did not decompose. The X-ray diffraction pattern shows that the diffraction intensity of calcite increases slightly, mainly due to the decomposition of dolomite, the content decreases, and some calcite crystals are produced.

3.2.1. 5

Within the temperature range of 600–800 °C, the diffraction peak of dolomite disappears, indicating that dolomite decomposes completely, and the diffraction peak intensity of calcite shows a decreasing trend. Calcite began to decompose (eq 5), and its percentage content decreased to 58.4%. At this time, a new mineral calcium hydroxide is formed, which is mainly formed by the reaction of CaO generated by calcite decomposition with water in rock (eq 6). Within this temperature range, the content of quartz and clay changes little.49

3.2.1. 6
3.2.1. 7

Within the temperature range of 800–1000 °C, the calcite decomposes rapidly with the further increase of temperature. At 1000 °C, the calcite diffraction peak disappears. At this time, calcite decomposes completely, while SiO2 will react with CaO and MgO to form merwinite (eq 7).50 With the increase of temperature, merwinite will decompose to form larnite (eq 8). At the same time, the SiO2 reacts with CaO to form orthorhombic kilchoanite (eq 9).

3.2.1. 8
3.2.1. 9
3.2.1. 10

Within the temperature range of 1000–1200 °C, the contents of kilchoanite and larnite change little, and the content of clay minerals decreases, mainly because the structure of dehydrated illite undergoes amorphous transformation at about 1100 °C. As the temperature continues to increase, a new mineral mullite is formed at 1200 °C. During the whole heating process, the content of quartz gradually decreased until it disappeared, mainly because quartz reacted with CaO and MgO to form larnite and kilchoanite.51,52

3.2.2. TG-DSC Analysis

The thermal analysis curve of the limestone sample in an argon atmosphere is shown in Figure 11. The experiment shows that the heating process of limestone in an argon atmosphere can be roughly divided into three stages: the first stage is dehydration and a small amount of white cloud decomposition, and the corresponding curve is T0–T1. With the increase of the experimental temperature, the water in the limestone sample volatilizes continuously, and there is a small endothermic peak. At this stage, the weight of the limestone sample shows a slightly reduced trend, resulting in a weight loss of 1.16% at 400 °C. As the temperature continues to increase, dolomite begins to decompose; the main reaction is that dolomite is decomposed by heat to produce calcite and CO2. Dolomite is decomposed into an endothermic reaction, and there is an endothermic peak. Due to the escape of CO2 gas, the limestone quality gradually decreases, and the weight loss rate is 3.1% at 600 °C. The second stage is the weight loss stage of calcite decomposition, and the corresponding curve shows the T1–T2 segment. The main reaction in this process is that a large amount of calcite is decomposed by heat to produce calcium oxide and CO2. Calcite is decomposed into an endothermic reaction, and the calcite content in limestone is high. There is a large endothermic peak at about 800 °C, indicating that calcite is decomposed in a large amount at about 800 °C. During this stage, CO2 gas escapes in a large amount, and the quality drops rapidly. The weight loss rate of calcite at 800 °C is 21.64%. The third stage is the mineral stabilization stage, and the corresponding curve diagram shows the T2–T3 section. In this process, the mineral composition tends to be stable gradually and the limestone quality did not change significantly.

Figure 11.

Figure 11

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Thermal analysis curve of the limestone sample in an argon atmosphere.

3.3. Discussion

Limestone is a mixture of calcite, dolomite, and a small amount of quartz and clay minerals. In underground high-temperature geotechnical engineering, the crystal structure, composition, and types of rock minerals change; for different minerals, the change process is different. The main mineral components of limestone minerals are calcite and dolomite as well as a small amount of quartz and clay minerals. With the increase of temperature, the decomposition reaction of dolomite and calcite and the recrystallization of decomposition products will lead to the increase of rock pores and serious destruction of the crystal structure, which will affect the distribution and connectivity of rock pores and fissures.

In stage I (25–600 °C), the content of quartz, calcite, and clay minerals did not change much, the content of dolomite decreased from 11.8 to 5.7%, the mass loss was 1.16 and 3.1%, respectively, in this temperature range (Figure 11), and the main physicochemical reactions that occurred were the evaporation of pore water and the decomposition of a small amount of dolomite. Comparing the SEM images at 25–600 °C (Figure 3), with the physicochemical reactions, the surface of the chert gradually becomes progressively rougher, mainly developing small pore throats with poor connectivity and a small number of microfractures. The proportion of small pores reaches more than 95.4% and the pore fractal dimension decreases from 2.94 to 2.92, which still maintains a relatively complex pore structure. A slight increase in the pore volume and number and a change in the pore structure led to a slow upward trend in the porosity and permeability of the chert, with the porosity increasing from 3.87 to 10.26% in the initial state and the permeability increasing from 0.007 to 0.021 mD. When the temperature is below 600 °C, there is no significant change in the mineral composition and pore structure of limestone. When it is used as a roof for underground coal gasification and an exterior wall for nuclear waste storage, it still maintains good airtight capability. However, when it is used as a reservoir, the changes in porosity and permeability are minimal, and the fracture limit is not reached, making it difficult for oil and gas migration.

In stage II (600–800 °C), the content of quartz and clay minerals did not change much, the content of calcite decreased from 78.6 to 58.4%, dolomite disappeared, with a mass loss of 21.64% in this temperature range (Figure 11), and the main chemical reaction that occurred was the decomposition of dolomite and calcite. Comparing the SEM images at 600 and 800 °C (Figure 3), the mineral morphology changed significantly with the decomposition reaction, a large number of honeycomb pores appeared on the surface of the chert, the number of pores and the size of the cleavage increased, the proportion of small pores decreased from 95.4 to 38.85%, the proportion of mesopores increased from 1.34 to 56.72%, and the fractal dimension of pores decreased from 2.92 to 2.88. The complexity of pores began to decrease slowly, the connectivity of pores was poor, the volume and number of pores increased rapidly, the change of pore structure led to a rapid increase in porosity and a slow increase in permeability of the limestone, the porosity increased from 10.26 to 19.27%, and the permeability increased from 0.021 to 0.142 mD. At this time, despite the increase in the number of pore throats, the pore complexity was high, the connectivity was poor, and with the lower permeability, the limestone remains well sealed.

In stage III (800–1200 °C), as the temperature increases, along with the disappearance of quartz and calcite, new minerals of larnite, kilchoanite, and mullite appear at the same time, the quality of graywacke does not change much, and the main chemical reaction that occurs in this temperature range is the decomposition of a large amount of calcite and the recrystallization of the decomposition products to produce new minerals. Comparing the SEM images at 800–1200 °C (Figure 3), with the chemical reaction, the morphology of the minerals changed significantly, from honeycomb pores to macropores and fractures. With the increase of temperature, these cracks gradually expanded into a network of fractures. The width of the cracks reached 6.12 μm, in which the ratio of small pores decreased from 38.85 to 1.72%, the ratio of mesopores increased from 56.72 to 1.72%, the proportion of macropores increased from 4.44 to 94.76%, and the fractal dimension of pores decreased from 2.87 to 2.54. The complexity of the pores decreased rapidly, the connectivity of the pores was enhanced, and the volume and number of pores increased rapidly. The change in the pore structure resulted in the rapid increase of the porosity and permeability of greywacke. The porosity increased from 19.27 to 40.39%, and the permeability increased from 0.142 to 32.82 mD. When the temperature exceeds 800 °C, the number and size of pore throats increase rapidly, the fracture network develops, the pore complexity decreases, the connectivity is good, and the permeability increases rapidly. At this time, it is unsuitable to be used as the top plate of underground coal gasification, but it is favorable for the transport of oil and gas in the limestone reservoirs. The evolution of limestone mineral composition at high temperatures mainly includes a series of physical and chemical changes, essentially the evaporation of water, decomposition of dolomite and calcite, expansion of calcite crystals, and recrystallization of decomposition products. During the heating process, changes in the mineral composition of limestone also cause changes in the pore structure, porosity, and permeability of the limestone (Figure 12). In stage I (25–600 °C), the main reactions that occur are the evaporation of pore water and the decomposition of a small amount of dolomite, but it does not damage the lattice structure of the limestone. The number of pore throats slightly increases, mainly developing small pore throats, which still maintain a relatively complex pore structure, leading to a slow increase in porosity; moreover, the change in permeability is not significant. As the temperature continues to increase, the main reaction that occurs in stage II (600–800 °C) is the decomposition of dolomite and calcite. The number and size of pore throats begin to increase, mainly developing small pore and mesopore throats. The complexity of pores begins to slowly decrease, and the connectivity of pores is poor, resulting in a rapid increase in porosity and a slow increase in permeability. In stage III (800–1200 °C), the main reactions that occur are the large-scale decomposition of calcite and the recrystallization of decomposition products, which seriously damage the lattice structure of the mineral. At this time, a large number of macropore throats are developed, and the complexity of pore pores decreases rapidly. Macroscopically, both porosity and permeability increase rapidly, and the limestone begins to lose its sealing ability.

Figure 12.

Figure 12

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Evolution mechanism of limestone pore structure change.

4. Conclusions

  • (1)

    The increase in temperature promotes the development of pore throats, especially in the range of 600–800 °C, where the number and size of pore throats increase, the discharge pressure drops to 2.74 MPa, the mercury extraction efficiency increases to 59.03%, and the pore connectivity is enhanced.

  • (2)

    As the temperature increases, the fractal dimension of pores shows a decreasing trend. Comparing the changes in the pore fractal dimension under different temperatures, it is found that in the range of 600–800 °C, the fractal dimension of pores begins to slowly decrease, which can serve as a temperature boundary for the changes in the pore fractal number of limestone.

  • (3)

    The evaporation of pore water will not destroy the lattice structure of limestone minerals and has little effect on the microstructure of rocks. As the temperature continues to increase, dolomite and calcite decompose completely, the calcite crystal appears to be pulling cracks under the action of expansion stress, and the decomposition products produce new minerals under the action of temperature, which seriously destroys the crystal structure and leads to the rapid increase of porosity and permeability at 600–800 °C.

  • (4)

    The mineral composition, pore throat distribution and complexity, porosity, and permeability of limestone vary consistently with temperature. When the temperature exceeds 800 °C, a large number of pores and cracks develop in limestone, and the pore structure is severely damaged. 800 °C can be used as the critical temperature for the change in the pore structure of limestone.

Acknowledgments

The authors acknowledge support from the Major Scientific and Technological Project of China, National Petroleum Corporation (CNPC) [2019 × 10−25].

The authors declare no competing financial interest.

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