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Geofluids
Volume 2019, Article ID 4746917, 9 pages
https://doi.org/10.1155/2019/4746917
Research Article

Time-Dependence of Mechanical Property Alterations on Anthracite Coals Treated by Supercritical Carbon Dioxide

1Key Laboratory of Gas and Fire Control for Coal Mines, Ministry of Education, China University of Mining and Technology, Xuzhou 221116, China
2National Engineering Research Center for Coal Gas Control, China University of Mining and Technology, Xuzhou 221116, China
3Postdoctoral Workstation, Wanbei Coal Electricity Group Limited Liability Company, Suzhou, 234000 Anhui, China
4Periodicals Publishing House, Xi’an University of Science and Technology, Xi’an 710054, China

Correspondence should be addressed to Wei Li; nc.ude.tmuc@ytefasiewl and Yuanping Cheng; nc.ude.tmuc@gnehcpy

Received 13 June 2019; Accepted 9 August 2019; Published 28 October 2019

Academic Editor: Lionel Esteban

Copyright © 2019 Wei Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The injection of supercritical carbon dioxide (Sc-CO2) into coal seams remarkably changes the physical-chemical structures of coal mass and thereby improves coal mechanical properties. In this study, a series of unconfined and triaxial compressive strength tests were performed on anthracite coal samples under the pressure of 8 MPa at the temperature of 35°C for 24 h and 48 h, respectively. Besides, experiments with longer Sc-CO2 treatment time were carried out on coal mass without stress constraint to observe the damage mode of coal. The results show that Sc-CO2 treatment obviously alters time-dependent mechanical properties of anthracite coal. The coal samples treated with Sc-CO2 for 240 h and 960 h show different damage modes from limited fractures to a complicated fracture network. The time-dependence of the mechanical weakening model on Sc-CO2-treated coal was proposed to explain the link between CO2 flow and mechanical weakening effect by means of physical-chemical effects on time scales.

1. Introduction

CO2 sequestration in deep unmineable coal seams, commonly recognized as one of the most promising CO2 mitigation methods [13], can also prominently promote CO2-enhanced coalbed methane (CO2-ECBM) recovery [4]. In particular, CO2 reaches its supercritical state when its pressure and temperature exceed the critical point 7.38 MPa and 31.8°C, respectively. The critical point of CO2 can be achieved at the depth of around 800 m under normal hydrostatic pressure. Physical properties such as density, diffusivity, and viscosity of supercritical carbon dioxide (Sc-CO2) change dramatically near the critical point, thus affecting the interaction between rock (coal) and Sc-CO2 [5, 6]. However, it is necessary to consider certain risks for achieving long-term safe storage of sequestered CO2.

Unlike subcritical CO2, Sc-CO2 exhibits higher adsorption capacity [7] and unique physical properties. According to the extensive present measurements, functional groups and pore structure of each pore phase vary hugely in different coal ranks before and after Sc-CO2 treatment [812]. However, the coal permeability decreases mostly after Sc-CO2 injection due to the dominant role of sorption-induced swelling [13, 14]. Sc-CO2 helps to develop seepage-flow pores (mesopores and macropores). In fact, Sc-CO2 notably alters the development of pores in high-rank and medium-rank coals, whereas it scarcely affects that in low-rank coal. Besides, mineral matters in coal are dissolved and mobilized when they react with Sc-CO2 [15]. This process of CO2-ECBM recovery, which alters CO2-adsorption-induced swelling of coal matrix, influences its hydromechanical properties in turn according to Griffith and Eng [16], and the adsorption effect is able to reduce the strength of materials. The geomechanical properties of CO2-treated coal are closely related to CO2 storage volume and stability. A number of scholars have explored the influence of CO2 adsorption on coal strength from subcritical state to supercritical state on both coal powder and tiny cubes. Studies [17, 18] revealed that the strength of low-rank coal decreased greatly with the increase of CO2 adsorption. In many experiments, considerable reductions in uniaxial compressive strength and Young’s modulus were observed as a result of gaseous CO2 saturation [17, 19, 20]. Sc-CO2 adsorption can weaken all types of coals [21, 22]. Coal mass gets ductile as the coal becomes plasticized in the process of CO2 adsorption, and this phenomenon is more notable under the condition of Sc-CO2 adsorption. It has been put forward that physical and molecule microstructures of coal can be altered by the interaction between CO2 and coal induced by sorption-induced swelling, pore size reset, hydrocarbon extraction, and macromolecular rearrangement [2327]. However, the link between the weakening of coal strength and the microcosmic physical-chemical alterations of Sc-CO2-treated coal is still rarely reported.

Currently, experiments on mechanical comparison before and after Sc-CO2 did not take time effect into consideration. The time-dependent Sc-CO2-treated coals provide a good proof for understanding the interaction between Sc-CO2 and coal, which could also provide insights for coalbed CO2 storage and Sc-CO2-treated coal fracturing. Therefore, this study is aimed at investigating the time-dependent mechanical response of anthracite coal samples treated with Sc-CO2. In addition to uniaxial compression, triaxial compression was applied to coals at different confining pressures to simulate the field stress environment. Then, coal cylinders were treated with Sc-CO2 for 240 h and 960 h with nonconfining pressure to observe their damage levels. Finally, the time-dependent mechanical weakening effect was explained by introducing the coal cube model and combining the experimental results.

2. Materials and Experiments

2.1. Materials

Experimental coal samples were taken from Daning Coal Mine of Qinshui Basin Coalfield located in the south of Shanxi Province, China. The results of proximate analysis of the coal are presented in Table 1.

Table 1: Proximate analysis of the coal.

In order to conduct uniaxial and triaxial tests, a total of 14 coal samples perpendicular to the bedding were cored from large coal lumps. The samples were 50 mm in diameter and 100 mm in height. It is noteworthy that only homogeneous samples with no visible cleat were used for the mechanical property measurement, so that the effects of anisotropy and heterogeneity on experimental results could be minimized.

2.2. Experimental Procedure

A customized high-pressure reactor that could provide a maximum saturation pressure of 32 MPa was adopted to ensure saturation conditions (Figure 1). This reactor was equipped with an advanced temperature regulation system that could limit the fluctuation of set temperature to within 0.1°C. In this study, coal samples were saturated in CO2 at a temperature of 35°C under a pressure of 8 MPa for 12 h or 48 h. Afterwards, the pressure chamber was gradually depressurized at a rate of 0.02 MPa/min, so that physical structure of the coal was unlikely to be damaged as a result of sudden changes in pressure. Then, to avoid the effects of oxidation and humidity, the samples were immediately put into a small sealed bag filled with helium and sent for the mechanical test in the first time.

Figure 1: Principle diagram of high-temperature and high-pressure visual reactor. (1) Gas supply; (2) reducing valve; (3) valve; (4) gas booster pump; (5) reducing valve; (6) pressure regulating valve; (7) air compressor; (8) temperature and pressure transducers; (9) temperature and pressure indicator; (10) Sc-CO2 high-pressure geochemical reactor; (11) safety valve; (12) vacuum pump; (13) vacuum gauge.

The mechanical tests were conducted by using an adsorption-permeation-mechanics coupled device whose maximum axial loading and maximum confining loading capacity could reach 600 KN. Axial and radial strain sensors, with maximum test ranges of 8 mm and 4 mm, respectively, were equipped for the sample deformation tests during stress loading, as exhibited in Figure 2. In this study, two main test scenarios were applied. First, the uniaxial and triaxial experiments were performed on raw coal samples under confining pressures of 0, 2, 4, and 6 MPa and under an axial loading rate of 0.3 MPa/min until the samples failed. During the experiments, the peak strength, axial strain, and radial strain were observed and recorded. Second, under the same confining pressure conditions, the procedures of uniaxial and triaxial tests were repeated on Sc-CO2-treated samples for 12 h and 48 h.

Figure 2: Triaxial mechanical device for raw and Sc-CO2-treated coal samples.

3. Results

3.1. Strength Evolution of Time-Dependent Sc-CO2 Treatment on Coal

After the uniaxial and triaxial experiments, the deviatoric stress- (σ1-σ3) strain behaviors for the raw and Sc-CO2-treated samples under different confining pressures were obtained, as illustrated in Figure 3. The axial strain (), radial strain (), and volumetric strain () vary inconsistently with the deviatoric stress. In fact, three stages, namely, volume compression, constant volume, and volume dilatation, can be observed in the deviatoric stress-strain curves. As can be found in Figure 3, despite the small values of slopes of axial and radial strain curves, the axial and radial strains rise rapidly as the confining pressure () goes up. In this process, brittle strain is gradually turned into plastic deformation, and higher confining pressure corresponds to higher peak strength.

Figure 3: Deviatoric stress-strain relation curve of raw and Sc-CO2-treated coal samples (black line = raw sample, red line = 12 h treated, and blue line = 48 h treated) under different confining pressures ((a) confining pressure 0 MPa, (b) confining pressure 2 MPa, (c) confining pressure 4 MPa, (d) confining pressure 6 MPa).

Compared with the raw coal samples, the samples treated with Sc-CO2 for 12 h and 48 h have smaller values of peak strength and strain under the same confining pressure, and the values decreased significantly. Under uniaxial compression, strengths of the samples saturated for 12 h are reduced by up to 4.49 MPa, corresponding to an unconfined compressive strength reduction of 25.6% under initial conditions, and the reduction reaches 45.15% after Sc-CO2 treatment for 48 h.

As can be observed in Figure 4, higher confining pressure corresponds to higher compressive strength when a coal sample fails, and the compressive strength decreases with the Sc-CO2 treatment time under the same confining pressure. Under 4 MPa confining pressure, the compressive strength of raw coal is 47.8 MPa, a decrease of 29.16% and 47.09% compared with those of samples treated with Sc-CO2 for 12 h and 48 h, respectively. The compressive strength exhibits the same variation tendency under 2 MPa and 6 MPa confining pressures. The Mohr-Coulomb strength criterion could be used to determine the failure of coal samples under constant confining pressure, as shown below. where is the cohesion and is the internal friction angle. The cohesion and internal friction angle could be obtained in light of the criterion, as shown in Table 2.

Figure 4: Relationship between maximum and minimum principal stresses of coal under the same treatment conditions.
Table 2: Cohesion and internal friction angle of coal samples under different conditions.

Both cohesion and internal friction angle of coal samples undergo a decrease after Sc-CO2 treatment. The cohesions of samples decrease by 8.9% and 27.4% after Sc-CO2 treatment for 12 h and 48 h, respectively. The internal friction angles of samples drop by 8.36° and 13.71°, respectively, accounting for 17.53% and 28.75% of that of raw coal. Cohesion is the mutual attraction formed by molecular forces in the adjacent parts of coal, and frictional force and chain effect of coal matrix are expressed by internal frictional angle. A larger internal frictional angle implies higher mechanical strength. The Sc-CO2 treatment obviously weakens the strength of coal samples, and the weakening effect is more obvious with a longer Sc-CO2 treatment time.

3.2. Strain Evolution of Time-Dependent Sc-CO2 Treatment on Coal

Coal deformation, which is reflected by Young’s modulus and Poisson’s ratio, could be calculated according to the deviatoric stress and strain obtained in the uniaxial and triaxial experiments (Table 3). The experimental results reveal that during SC-CO2 treatment, SC-CO2 saturation influences Young’s modulus and Poisson’s ratio in addition to the strength of anthracite coal. Young’s moduli drop by 25% and 40%, while Poisson’s ratios jump by 19.23% and 30.76% on average after Sc-CO2 treatment for 12 h and 48 h (8 MPa), respectively. As the confining pressure increases from 0 to 6 MPa, the Young’s modulus grows from 2716 MPa to 3422 MPa during the test of 12 h Sc-CO2 treatment. Meanwhile, Poisson’s ratio shows the same tendency, that is, it increases by a factor of 1.09-1.125 for the raw and treated coal samples. It can be concluded that the confining pressure and the Sc-CO2 treatment share a competitive relationship in terms of Young’s modulus.

Table 3: Experimental results of Young’s modulus and Poisson’s ratio obtained from each test.

Coal is a highly brittle material that rarely moves due to its high-energy structure in a glassy state. The CO2 adsorbed into coal matrix can improve its polymer structure and thus expand the free volume. As a result, the ductile properties of coal mass get promoted, which reduces Young’s modulus and raises Poisson’s ratio [28, 29].

3.3. Pore and Fracture Distribution of Sc-CO2-Treated Coal

The cumulative pore volume curves of raw and Sc-CO2-treated coal samples were analyzed by means of mercury intrusion porosimetry (MIP), as displayed in Figure 5(a). The pore characteristics of coal samples can be explored according to the widths of hysteresis loops and the volumetric difference between mercury injection and extrusion curves. As can be seen from Figure 5, the cumulative mercury intrusion volume rises slowly when the pressure is lower than 20 MPa, while it increases sharply after the pressure exceeds 20 MPa. Compared with the raw coal samples, the mercury intrusion curves of Sc-CO2-treated coal samples increase slower, indicating that Sc-CO2-treated coal samples boast more uniform pore size distribution. Moreover, the cumulative mercury intrusion (0.302 cm3/g) of Sc-CO2-treated coal samples is 10.22% higher than that (0.274 cm3/g) of raw coal samples. The pore size in the Sc-CO2-treated samples alters, and the total pore volume expands. Besides, the hysteresis loop of Sc-CO2-treated samples increases from 0.0028 cm3/g to 0.0043 cm3/g, which may result from the decrease in semiclosed pores, the formation of more open pores, and the development of pore network. This is probably because Sc-CO2 dissolves and mobilizes hydrocarbons, leading to adsorption-induced deformation and physical constriction [31, 32].

Figure 5: Pore volume (a) and Pore size distribution (b) of raw and Sc-CO2-treated coal samples by means of MIP [30].

Pore size distribution can be seen more intuitively from Figure 5(b). The mercury intrusion volume of mesopores and macropores in coal samples increases significantly by 0.0037 cm3/g after Sc-CO2 treatment, while that of transition pores and micropores decreases slightly by 0.0009 cm3/g. Firstly, the CO2 molecules were injected into the coal seam flow through the mesopores and macropores in the form of laminar flow or turbulent flow. The channel of gas flow expands when the volume of mesopores and macropores increases. Previous studies [29] also revealed that Sc-CO2 treatment could result in smoother pore surface of coal samples and better connectivity of pores, which conduces to the diffusion and seepage of gas molecules. Eventually, after the Sc-CO2 treatment, mesopores and macropores develop faster and better in coal, which may help gas to desorb and diffuse from coal matrix and transport in coal fracture.

3.4. Time-Dependent Sc-CO2 Treatment on Coal Damage

Two sets of coal cylinders were treated with Sc-CO2 (9.65 MPa, 35°C) without stress constraint for 240 h and 960 h, respectively. Sc-CO2 was released at the rate of 0.015 MPa/min to prevent the rapid decrease in gas pressure. Figure 6 displays the damage modes of Sc-CO2-treated samples in different time periods.

Figure 6: Damage appearance of Sc-CO2-treated coal samples under different saturated times: (a) and (b) are a comparison before and after treatments for 240 h and 960 h, respectively.

The coal samples saturated with Sc-CO2 for 240 h present limited fractures and cracks compared with raw coal samples. However, after Sc-CO2 treatment for 960 h, the coal samples, which have a more complicated fracture network, lose the main mechanical strength and become looser with poor cohesion. In fact, it can be easily stripped by hand. Judging by the crack shapes, the failure belongs to tensile failure due to compression that may be caused by coal matrix swelling. This is consistent with Zhang et al.’s observation [33]. From the appearance, there are some assumptions that the coal damage is attributed to the time-dependent Sc-CO2 transport and adsorption-induced swelling, and it has been recently observed that the unswelling phase is due to the induced swelling stresses [34]. These stress fields are highly anisotropic; the maximum effective stresses concentrate on the mineral surface in most cases; the Von Mises stresses continuously rise with the increase of coal matrix swelling until the coal fails [33]. CO2 saturation alters the unconfined compressive stress failure modes of the shear-dominant failure mechanism in the raw coal into the failure along the cleat, the tensile failure plane, and the splitting failure plane system [22, 29, 35]. This is mainly because of the generation of secondary cracks resulted from CO2 adsorption-induced heterogeneous swelling and local swelling stress.

4. Discussion

4.1. Time-Dependence of Mechanical Weakening Effect on Sc-CO2 Treatment

The coal cube model, which is widely used to describe gas transport and permeability evolution from coal matrix to fractures [36, 37], also matches well with Sc-CO2 injection and coal strength evolution with time. Hence, the model is introduced to reveal the time-dependence of coal mechanical property variation after Sc-CO2 injection (Figure 7). The two parts of fracture and matrix strength weakening effects are attributed to stress-strain evolution with time.

Figure 7: Time-dependence of mechanical weakening on Sc-CO2 treatment with coal cube model.

Based on the above observations, the Sc-CO2 treatment not only causes a decrease in the mechanical properties (peak strength, elastic modulus, cohesion, and internal frictional angle) of coal in triaxial or uniaxial stress environments but also modifies the properties of coal mass, leading to a transition from brittle failure to ductile failure. The coal treated with Sc-CO2 for 48 h undergoes a more obvious decrease in mechanical properties than the coal treated for 12 h. Other experiments suggest that the mechanical properties of Sc-CO2-treated coal will not decrease continuously over time and can be presented using a simple Langmuir type equation [38]. Flow of Sc-CO2 through the cleats is driven by pressure and can be described using Darcy’s law at first. The adsorption-induced swelling and the reduction in fracture surface energy opens the fractures, hence reducing tensile strength and facilitating crack formation. As the Sc-CO2 treatment leads to the dissolution of minerals, fractures become smoother and apertures expand. When the stress of adsorption-induced swelling exceeds the matrix cohesion, a new crack surface is formed [16]. Sc-CO2 can enhance the coal permeability by promoting the development of micropores, mesopores, and macropores in coal. In this way, fracture morphology is transformed through mineral dissolution, desorption shrinkage, and new minor crack generation, which all contributes to raising the coal porosity and lowering the coal strength.

However, the flow through the coal matrix, which is modeled by Fick’s law of diffusion, is assumed to be driven by the concentration difference and needs a long time to reach full Sc-CO2 saturation. The coal strength further declines as the Sc-CO2 diffuses into coal matrix after long-term treatment. A covalently cross-linked, three-dimensional macromolecular model, together with an intra- and intermolecular model [39, 40], has been widely recognized to describe the organic matrix. Former research of the authors suggested that the injection of Sc-CO2 into coal can lead to the swelling effect, namely, the viscoelastic relaxation of its highly cross-linked macromolecular structure. Sc-CO2 is capable of mobilizing small hydrocarbon molecules from the coal matrix [41] (Figure 7). That is, Sc-CO2 can reduce the intermolecular force in coal atoms. From a macro perspective, the effect is manifested by the decreases in cohesion and internal friction angle with the decrease in coal strength and deformation. The time-dependent model of coal mechanical property weakening contributes to the time-dependent Sc-CO2 transport through coal mass fracture and matrix and physical-chemical reactions in the corresponding position.

4.2. Implication for In Situ Sc-CO2 Coalbed Injection and Storage

The preferable coal seams for CO2 sequestration exist at depths of greater than 800 m. The interaction between coal and Sc-CO2 and the mechanical strength evolution have been widely studied in recent years [22, 29, 38]. The time-dependent triaxial experiments (up to 6 MPa) indicate that the longer the treatment time is, the more significantly the peak strength decreases. The failure mode of coal transits from brittle failure to ductile failure with the rise of confining pressure. In addition, the stress magnitude is much higher than the value obtained in in situ experiments at depths of over 800 m, implying that the further decrease in coal permeability is dominantly controlled by the swelling induced by Sc-CO2 sorption [42]. The ductile deformation of Sc-CO2-treated coal causes the fractures to close and the CO2 to desorb under high effective stress, which also lowers coal permeability to hinder further transport of CO2. It is also found that the CO2 injection volume drops in a short period of time due to the sorption-induced swelling as well as high stress in the field [43, 44]. According to above findings, the coal seam might not be a suitable layer for long-term CO2 storage despite its great adsorption affinity. Nowadays, Sc-CO2 fracturing is applied to enhancing CBM/shale or increasing permeability to improve gas recovery [45]. The injected Sc-CO2 should crack the coal seam as quickly as possible to prevent plastic deformation of fully saturated coal. The fractures generated during hydrofracturing heal with the release of pressure because of the plastic deformation nature of CO2-saturated coal, creating unfavorable formation properties. These observations imply that the ductile behavior of Sc-CO2-adsorbed coal is significantly enhanced with the plasticization of coal, and the surface energy reduces critically at the beginning of CO2 sequestration. However, the effect of Sc-CO2 is reduced over time. This laboratory tests provide a better vision for the effects of CO2 sequestration or ECBM in coal seams in a controlled environment.

5. Conclusion

Sc-CO2 treatment significantly alters time-dependent mechanical properties of anthracite coal. Greater decreases in peak strength were observed after Sc-CO2 treatment for 12 h and 48 h. Higher confining pressure corresponds to greater compressive strength when the coal samples fail. Besides, the strain of coal experiences a transition from brittle to ductile as the confining pressure rises and Sc-CO2 treatment time prolongs. As the treatment time passes, Sc-CO2 saturation reduces cohesion, internal friction angle, and Young’s modulus and raises Poisson’s ratio, which also controls the evolution of coal strain and failure. Sc-CO2 treatment facilitates the development of mesopores and macropores in coal by dissolving and mobilizing hydrocarbons. The coal samples treated with Sc-CO2 for 240 h and 960 h show a different damage appearance, as it alters from limited fractures to a complicated fracture network. A time-dependent Sc-CO2 flow through the coal matrix and the physical-chemical reaction model was proposed to build the link between coal fracture and matrix mechanical weakening effect from experiments. Sc-CO2 interacts with coal in a complicated way after CO2 flows and diffuses from particle coal to massive coal through fractures and coal matrix. Moreover, the saturation of CO2 can lead to the swelling, dissolution, and surface energy reduction of coal with the passage of time, which in turn has a large influence on coal strength and fracture development. The time-dependence of Sc-CO2 transport and interaction with coal may have profound meanings for coalbed CO2 storage and Sc-CO2 organic rock fracturing.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This research was financially supported by the National Natural Science Foundation of China (No. 51874295), the Fundamental Research Funds for the Central Universities (grant No. 2015XKMS006), and a project funded by the priority academic development program of Jiangsu Higher Education Institutions. The authors would like to thank the China Scholarship Council for the financial support.

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