Comparative experimental study on porosity, mechanical and CO₂ adsorption characteristics of coal and shale

Abstract

To compare the pore structure, mechanical and CO₂ adsorption properties of coal and shale, a series of experiments were carried out using nuclear magnetic resonance (NMR), uniaxial compression, Brazilian splitting, and high-pressure CO₂ adsorption. The results show that the total porosity of coal is 7.51 times that of shale, and shale is dominated by adsorption pores, while adsorption pores and seepage pores in coal are equally important. Moreover, it is found that the micropores in shale are more advantageous, while meso-macropore in coal are more significant. The adsorption pore surface of coal is rougher than that of shale, and the seepage pore structure of shale is more complex. The uniaxial compressive strength, elastic modulus and absorption energy of shale are 2.01 times, 2.85 times, and 1.27 times that of coal, respectively, indicating that shale has higher compressive capacity and resistance to elastic deformation than coal. The average tensile strength, Brazilian splitting modulus, absorbed energy and brittleness index of shale are 7.92 times, 6.68 times, 10.78 times, and 4.37 times that of coal, respectively, indicating that shale has higher tensile strength and brittleness, but lower ductility, compared with coal. The performed analyses show that under the same conditions, the CO₂ adsorption capacity of coal is greater than that of shale. The present article can provide a theoretical basis to implement CO₂-enhanced coalbed methane (CBM)/shale gas extraction.

Graphical abstract

Keywords

coal / shale / NMR / mechanical properties / adsorption characteristic

1 Introduction

Coalbed methane and shale gas, as two forms of unconventional natural gas, which can provide a clean and efficient energy supply (Wang et al., 2018b; Liu et al., 2022b; Wang et al., 2022b). Recently, remarkable achievements have been made regarding this abundant source of energy. In the southern Qinshui Basin, China, the development of coalbed methane has entered the stage of commercial exploitation, and at the same time, exploitation of the shale gas in the Sichuan Basin has reached its historical peak (Chen et al., 2010; Wang et al., 2011). Coal and shale are similar porous media that can store methane in their porous structures (Wang et al., 2018a; Wang et al., 2021a; Liu et al., 2022c; Yang et al., 2022a). Therefore, it is necessary to investigate the differences between their pore structure to deeply understand the gas occurrence characteristics. With the development of technical measures such as hydraulic fracturing in coalbed methane and shale gas extraction, the mechanical properties of coal and shale can be studied to optimize extraction techniques. Moreover, the study of the adsorption capacity of coal and shale for CO₂ can provide a theoretical basis to implement CO₂-enhanced CBM/shale gas extraction.

A review of the literature indicates that the characterization techniques for the pore structure of porous media are relatively mature. In this regard, numerous methods, including irradiation methods (e.g. scanning electron microscopy (SEM), micro-computer tomography (CT), and nuclear magnetic resonance (NMR)), and fluid intrusion methods (e.g. mercury intrusion methods and low-pressure N₂/CO₂ adsorption methods) have been proposed (Gao et al., 2022; Wang et al., 2022a; Wang et al., 2020; Zheng et al., 2022). Cao et al. (2015) adopted the low-pressure N₂ adsorption method and mercury intrusion method and used a scanning electron microscope to analyze the difference between the pore structure of coal and shale. The results showed that the pores of coal are mainly smaller than 17.11 nm, while the pores of shale are mainly micro-scale. Wu et al. (2020) compared the pore structures of coal and shale by analyzing the adsorption of low-pressure N₂ and CO₂ gases and using the mercury intrusion method. It was found that mesopores of shale are more developed, while the micropores of coal are more developed. Based on low-pressure adsorption of N₂, Zhang et al. (2017) analyzed the pore differences between high-rank coal and shale in the Shanxi Formation and found that micro-scale pores are dominant in shale, while small pores are dominant in coal. Although remarkable results were achieved in the reviewed investigations, the pore characterization method is applicable only to test granular samples, and the intrusion of high-pressure mercury destroys the original pore structure of samples. Comparatively, the NMR test causes less damage, has faster detection, and can test larger samples (Wang et al., 2019). Accordingly, NMR can be applied to conduct more comprehensive tests on the pores of coal and shale. Among different methods for determining the pore structure of cylindrical coal samples, CT is limited by its resolution, so it is more suitable to analyze micron-scale pores. Meanwhile, variations of the coal pore are typically very small before and after CO₂ adsorption, while CT technology relies on threshold segmentation in image post-processing so the occurrence of large errors is undeniable. Therefore, the NMR method is applied in the present study to characterize the pore structure of coal and shale.

The mechanical properties of coal and shale are significantly different. Although numerous investigations have been carried out on the mechanical properties of coal and shale, there is no comparative report. Wang et al. (2022c) analyzed the mechanical properties of coal with different bedding angles by Brazil splitting test and showed that the peak loading force of coal is concentrated at 2.06−3.01 kN. Moreover, Zhang et al. (2019) performed uniaxial compression experiments and tested the mechanical properties of coal. It was found that the uniaxial compression strength (UCS) and elastic modulus of coal are 46.07 MPa and 3.37 GPa, respectively. Yang et al. (2022b) carried out experiments and showed that UCS and elastic modulus of shale are 256.1 MPa and 25.27 GPa, respectively. Considering the significance of coal and shale in unconventional gas extraction, the difference between the mechanical properties of coal and shale should be further investigated.

Studies show that coal and shale have different pore structures, so they have significantly different gas adsorption capacities (Wang et al., 2021c). In this regard, Wang et al. (2021b) analyzed the CO₂ adsorption in primary and structural coal and found that since the pores of structural coal are more developed than primary coal, they have a greater CO₂ adsorption capacity that may reach 70 mL/g. Zhou et al. (2018) analyzed the CO₂ adsorption capacity of shale with different maturity levels and concluded that the adsorption capacity of shale for CO₂ varies in the range of 6−8 mL/g.

In the present study, it is intended to study the difference between the pore structures of coal and shale using NMR. Then, uniaxial compression tests and Brazilian splitting experiments are carried out to compare the mechanical properties of coal and shale. Meanwhile, CO₂ adsorption experiments are performed to analyze the difference in adsorption capacities of coal and shale. The results of this study provide a theoretical basis to improve the occurrence mechanism of coalbed methane and shale gas, and a guideline to improve mining technology.

2 Experimental work

2.1 Sampling

The shale samples were collected from the Qiongzhusi Formation on the west bank of Dianchi Lake, south-west of Kunming City, Yunnan Province, and the coal samples were selected from the Yuwang Coal Mine in Yunnan Province. The collected bulk samples were sent to the Chongqing University Coal Mine Hazard Dynamics and Control Laboratory, where samples were cut into 100 mm height and 50 mm diameter cylinders for NMR and uniaxial compression tests. Moreover, 25 mm height and 50 mm diameter discs were prepared for Brazilian splitting tests. The prepared samples are shown in Fig.1. The chemical compositions of shale and coal samples determined by an X-ray diffractometer are listed in Tab.1. It is observed that quartz is the dominant component in shale and coal, followed by illite in shale and calcite in coal.

2.2 Experimental methods

2.2.1 NMR

NMR is an effective method to determine the pore size with little damage to the pores over a wide range covering pores from 0.1 nm to 100000 nm. In the present study, the NMR testing system (Suzhou Newmark Analytical Instrument Co., Ltd. MacroMR12-150H-I, China) and a pulse sequence test (Using Carr-Purcell-Meiboom-Gill) are used in the experiments. The attenuation signal of spin echo string, which reflects the pore size, and the distribution curve of transverse relaxation time (T₂), can be obtained on samples with saturated water and curve fitting. The relationship between T₂ and aperture r can be expressed as follows (Song et al., 2020; Su et al., 2020):

(1)\frac{1}{T_2}\text{=}\rho \times \frac{S}{V}\text{=}{F}_{\text{s}}\text{(}\frac{\rho }{r}\text{)},

where T₂ is the transverse relaxation time, ms; ρ is the transverse surface relaxation strength, μm/ms; S is pore surface area, cm²; V is the pore volume, cm³; F_s is the shape factor of pores; r is the pore radius, m.

The surface irregularity of porous media is measured using the Fractal dimension method, which can characterize the surface complexity and surface roughness of pores in coal and shale. The performed analyses proved that the pore network of coal and shale has fractal characteristics (Chandra et al., 2020; Yang et al., 2020; Geng et al., 2022). Therefore, the NMR method and fractal theory are combined to achieve an effective way to describe the pore structure of coal and shale. In this regard, Chen et al. (2019) established the following spectrum fractal dimension equation through NMR:

(2)\mathrm{l}\mathrm{g}(w)=(3-D_N)\mathrm{l}\mathrm{g}(T_2)+(D_N-3)\mathrm{l}\mathrm{g}(T_{2max}),

where w is the cumulative porosity percentage when transverse relaxation time is less than T₂; D_N is the fractal dimension; T_2max is the maximum transverse relaxation time, ms.

According to Eq. (2), the slope of the straight line obtained by linear fitting the scatter plot drawn from the relationship between lg(w)–lg(T₂) can be used to calculate the fractal dimension D_N of coal and shale.

2.2.2 Uniaxial compression test

A hydraulic servo mechanical system (MTS 815, MTS, USA) with a maximum axial load of 2800 kN is used to control force, displacement, axial strain, and transverse strain. During the experiments, the displacement control loading mode with a loading rate of 0.1 mm/min was adopted in the uniaxial tests. The elastic modulus (E) of coal and shale was calculated using the elastic segment of the full stress-strain curve of the sample.

The deformation and destruction of coal and shale are accompanied by energy absorption, accumulation, and dissipation. The absorbed energy (U) of the rock failure process was used to investigate the mechanical behavior of the rock. This parameter can be calculated by the area between the curve and the abscissa in the force-displacement curve through the following expression:

(3)U={\int }_0^{l_i}{P}_i\text{d}l,

where U is absorbed energy, mJ; l_i is displacement of step i, mm; P_i is load of step i, N.

2.2.3 Brazil splitting test

An IS material testing machine (AG-250kN, Shimadzu Company, Japan) with a maximum axial load of 250 kN, displacement measurement accuracy of ±0.1%, and a loading rate range of 0.0005−1000 mm/s was used to perform the Brazilian splitting test. Load control and displacement control loading methods can be adopted in the test. In this article, the Brazilian split tests were carried out by displacement-controlled loading, and the loading rate was set to 0.1 mm/min. Considering bedding characteristics of coal samples, 7 bedding angles (0°, 15°, 30°, 45°, 60°, 75°, and 90°) were considered and the tests were labeled 1# to 7#, respectively. The shale studied in this paper had no obvious bedding characteristics, so seven shale samples (Nos. 1#−7#) were considered.

Brazilian splitting tests reflect the tensile failure characteristics of the samples through the following expression (Feng et al., 2019; Liu et al., 2022a):

(4){\sigma }_t=\frac{2P_{max}}{\text{π}Dh},

where σ_t is tensile strength, MPa; P_max is peak load, N; D is diameter of sample, mm; h is thickness of sample, mm.

In this test, the Brazilian splitting modulus (E_B) reflects the resistance of samples to elastic deformation, which is helpful to understand the splitting failure characteristics of samples. Yu and Wang (2004) divided the ordinate in the force-displacement curve by the area of the sample meridian plane and the abscissa divided by the sample diameter. In the mapped curve, the slope of the new curve is E_B. The Brazilian fracture absorption energy (U_B) is the same as that of the uniaxial compression test.

The brittleness of samples is a key parameter to evaluate geomechanics. In this regard, Hou et al. (2022) showed that the rock brittleness index (BI) can be calculated by the peak characteristic of the force-displacement curve in the form below:

(5)BI=\frac{{\sigma }_t}{{\epsilon }_t},

where BI is brittleness index; ε_t is axial strain at P_max point of peak load.

2.2.4 CO₂ adsorption experiment

According to MT/T 752-1997 standard and the static volume theory (Sing, 1982), HCA high-pressure volume equipment (Chongqing Research Institute of CCTEG, China) was used to conduct high-pressure CO₂ adsorption experiments (Fig.2). To this end, 50 g of the coal/shale sample (six different particle size ranges: < 0.074, 0.074−0.2, 0.2−0.25, 0.25−0.5, 0.5−1, and 1−3 mm) was placed in a sample container immersed in a 60°C water bath and evacuated. Then the dead space volume of the container was measured using the method proposed by Jin et al. (2016). Finally, the adsorption volume and methane adsorption isotherm were obtained when the methane pressure reached 5 MPa.

The adsorption isotherm data of coal and shale can be fitted by the Lagmuir model, which is widely used in coal and shale fields (Langmuir, 1917; Joubert et al., 1973; Satya et al., 2006):

(6)V=\frac{abp}{1+bp},

where V is the volume of adsorbed gas, mL/g; a is the Langmuir constant, mL/g; b is the Langmuir constant, MPa⁻¹; p is the equilibrium pressure, MPa.

3 Results

3.1 Pore structure difference

Based on the obtained results from NMR tests, the nuclear magnetic T₂ spectrum of coal and shale is presented in Fig.3. It is observed that there are three peaks in the coal and shale spectra. The three peaks of coal are significantly larger than those of shale. Coal samples have the characteristics of “high at both ends, low in the middle”, while the peak of shale at 0.1−2 ms is much larger than the other two peaks, and three peaks are decreasing. By analysis of the cumulative porosity test results, the total porosity of coal is 18.199%, while that of shale is only 2.423%. Accordingly, the total porosity of coal is 7.51 times that of shale.

Hodot proposed the pore structure classification of micropores (0−10 nm), minipores (10−100 nm), mesopores (100−1000 nm), and macropores (> 1000 nm), in which micropores and minipores are called adsorption pores, and mesopores and macropores are called seepage pores. According to the NMR experiment, the pore size distributions of coal and shale are presented in Fig.4. It is observed that the proportion of adsorption pores in shale is much larger than that of seepage pores, while the gap between adsorption pores and seepage pores in coal is small. In both adsorption pores, the proportion of adsorption pores of shale is larger than that of coal, while the proportion of seepage pores of shale is smaller than that of coal. Consequently, shale samples mainly contain adsorption pores, while adsorption pores and seepage pores are equally important in coal.

To further analyze the difference between coal and shale in porosity, the proportion of micropores, minipores, mesopores, and macropores are presented in Fig.5. It is observed that the proportion of micropores, minipores, mesopores, and macropores in shale is 50.58%, 45.06%, 3.41%, and 0.95% respectively, while that in coal is 21.85%, 41.54%, 6.79%, and 29.82%, respectively. It is inferred that micropores and minipores in shale are dominant, while the four pores in coal are equally important. Moreover, it is found that the proportion of micropores and minipores in shale is 2.31 times and 1.08 times that of coal, respectively, indicating that micropores in shale are more advantageous than in coal. The proportion of minipores in shale and coal is almost the same. In addition, the proportion of mesopore and macropore in coal is 1.99 times and 31.39 times that of shale respectively, indicating that the proportion of mesopore and macropore in coal is more significant than that of shale. This is especially more pronounced in macropores.

To compare the pore structure between coal and shale in more detail, the fractal dimension of the two is calculated using Eq. (2), and the results are shown in Fig.6. According to the division of adsorption pore and seepage pore, lg(w)−lg(T₂) curve is divided into two parts to obtain the surface fractal dimension D_N1 of coal and shale adsorption pore and the volume fractal dimension D_N2 of seepage pore. Generally, the larger the D_N1, the higher the pore surface roughness; moreover, the larger the D_N2, the more complex the seepage hole structure (Song et al., 2020). According to the fractal dimension results of coal and shale in Fig.5, the curves can be fitted to a linear line, indicating that the fractal dimension can be used to characterize the pore structure. According to the calculation results of the fractal dimension, D_N1 of coal and shale is 2.076 and 1.113, respectively, indicating that the surface roughness of the coal adsorption pore is lower than that of shale. Furthermore, D_N2 of coal and shale is 2.930 and 2.998, respectively, indicating that the seepage pore structure of shale is more complex, but the difference between the two is not obvious. The fractal dimensions of coal and shale show that D_N2 is greater than D_N1, indicating that seepage holes are more complex than adsorption pores (Shao et al., 2017).

3.2 Uniaxial compression characteristic difference

Uniaxial compression tests can well show the difference in mechanical properties of coal and shale samples. The stress-strain curves of coal and shale samples are presented in Fig.7. The mechanical parameters of coal and shale such as UCS, E, and U are listed in Tab.2.

The results show that both coal and shale samples have similar characteristics with rock stress-strain curves, namely the pore compaction stage, elastic deformation stage, stable development stage of microcracks, and rapid crack growth stage. In the first stage, the pores in coal and shale are gradually compressed, resulting in nonlinear deformation. In this stage, shale and coal have similar stress-strain curves that end at about 0.3%. In the elastic deformation stage, however, the stress-strain curve of shale is linear and the duration is long, while the elastic stage of coal is short. In the third and fourth stages, internal microcracks significantly develop in coal and obvious plastic deformations occur, while these two stages of shale are very short and the fracture is brittle.

The comparison of mechanical parameters in coal and shale shows that the UCS of shale is 2.01 times that of coal, indicating that shale has a stronger compressive capacity than coal. Moreover, the E of shale is 2.85 times that of coal, indicating that shale has more resistance to elastic deformation. The absorption energy of shale before fracture is 1.27 times that of coal, indicating that higher levels of energy are required to destruct shale samples, demonstrating that shale has a higher resistance to destruction.

This difference in macroscopic properties of shale and coal is closely related to the internal pore structure of the two materials. According to the results of NMR experiments, the total porosity of coal is much higher than that of shale while the proportion of meso-macropores in coal is much higher than that of shale, which leads to more obvious pore defects in coal. Such a huge difference between micropore structures of coal and shale results in a significant expansion of micro-fractures during coal compression, which significantly affects the mechanical parameters.

3.3 Differences in Brazilian splitting characteristics

To compare the tensile failure characteristics in coal and shale, the Brazilian splitting test results of the two materials are shown in Fig.8. Based on the force-displacement curves, the Brazilian splitting parameters were calculated and the obtained results are listed in Tab.3. It is observed that the Brazilian splitting curves of coal and shale are similar to the uniaxial compression curve and consist of four stages. It is found that the plastic stage of shale is very short and has brittle characteristics, while coal has a significant plastic stage. According to the experimental results in Tab.3, the average tensile strength and Brazilian splitting modulus of shale are 7.92 times and 6.68 times of coal, respectively, indicating that shale has a stronger tensile. Generally, the lower the E_B value of the rock, the weaker its brittleness and resistance to crack propagation, the more severe the damage to the internal structure, and weaken the elastic deformation resistance (Aybar et al., 2014; Gholami et al., 2016). Moreover, the U_B of shale is 10.78 times that of coal, indicating that the resistance of coal to damage is relatively weak, and coal is damaged by absorbing less energy. The BI value of shale is 4.37 times that of coal, indicating that coal has lower brittleness and higher ductility.

3.4 Difference in CO₂ adsorption capacity

The CO₂ adsorption of coal and shale under different particle sizes is shown in Fig.9. It is observed that as the equilibrium pressure increases, the CO₂ adsorption capacity of coal and shale increases. At a constant equilibrium pressure, the CO₂ adsorption capacity increases with a decrease in particle size. The fitting results of the Langmuir constant for coal and shale adsorption data are shown in Tab.4. It is found that a values of coal and shale for a particle size < 0.074 mm are 1.54 and 1.88 times that of 1–3 mm, respectively. In addition, under the same equilibrium pressure and particle size, the adsorption capacity of coal for CO₂ is greater than that of shale, indicating that coal has a higher adsorption capacity than shale. This conclusion can also be obtained from the average a value of coal, which is 6.62 times that of shale.

4 Conclusions

In the present study, the porosity, mechanical parameters, and gas adsorption of coal and shale are compared. Based on the performed analyses and obtained results, the main achievements can be summarized as follows.

1) The total porosity of coal is 7.51 times that of shale, and shale is dominated by adsorption pores, while coal’s adsorption pores and seepage pores are equally important. The micropores and minipores ratios in shale are 2.31 times and 1.08 times that of coal, respectively, while the mesopores and macropores of coal are 1.99 times and 31.39 times that of shale, respectively. The adsorption pore surface of coal is rougher than that of shale, and the seepage pore structure of shale is more complex.

2) Coal has significant stages, in which internal microcracks develop and propagate rapidly and obvious plastic deformations occur, while these two stages are very short in shale and it is obviously brittle. The UCS, E, and U of shale are 2.01 times, 2.85 times, and 1.27 times that of coal, respectively. It is concluded that shale has more compression resistance and resistance to elastic deformation than coal.

3) The average tensile strength and Brazilian splitting modulus of shale are 7.92 times and 6.68 times that of coal, respectively, indicating that shale has stronger tensile capacity. Meanwhile, the U_B of shale is 10.78 times that of coal, which indicates that coal has weak resistance to damage. In addition, the BI value of shale is 4.37 times that of coal, indicating that coal has lower brittleness and higher ductility.

4) The CO₂ adsorption capacities of coal and shale increase with the decrease in particle size. Under the same conditions, the CO₂ adsorption capacity of coal is greater than that of shale, and the average a value of coal is 6.62 times that of shale.

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