Influences of supercritical carbon dioxide fluid on pore morphology of various rank coals: A review

Supercritical carbon dioxide is known to change the pore structure of coals and thus affect their carbon dioxide sequestration capacity. In this study, supercritical carbon dioxide dependence of pore morphology of coals was reviewed. Results indicated that the micropore surface area and volume of dry coals varied between –20% and 20% after exposure to supercritical carbon dioxide. Changes in the micropore size distribution of dry coals after supercritical carbon dioxide exposure were not found to be significant; however, the change in meso- and macropores with diameter of 2–8 nm was observed to be significant. Supercritical carbon dioxide and H2O exposure mainly influenced pores with diameters of 0.4–0.7, 0.7–0.9 and 2–8 nm. The variation in the pore fractal dimensions of the coals ranged from –0.5% to 0.5% after supercritical carbon dioxide exposure. Furthermore, the dependence of supercritical carbon dioxide on the pore structure of coals relies on the coal rank. The change in the pore structure of the coals after supercritical carbon dioxide exposure was observed to be related to the following aspects. First, supercritical carbon dioxide induced swelling in coal matrix, thus reducing the pore surface area and volume of the coal matrix and compressing the cleat system. Next, the extraction of supercritical carbon dioxide mobilised the small organic molecules dispersed in the coal matrix; this increased the pore volume, particularly of micropores. Finally, the mineral dissolution/precipitation also changed the pore structure of the coals. To further examine supercritical carbon dioxide dependence of coal pore morphology, the following studies should be performed. The characterisation of the chemical and pore structure of coals should be combined with existing coal structure models to account for the mechanism of supercritical carbon dioxide changing the pore structure of coals. Combination of physical experiments and numerical simulations is recommended to predict the changes in porosity and permeability of coals due to long-term carbon dioxide sequestration.


Introduction
Carbon dioxide (CO 2 ) capture and sequestration (CCS) has great potential for mitigating anthropogenic greenhouse CO 2 emissions. It has been estimated that globally, CCS can reduce 20% of CO 2 emitted from the energy industry (Haszeldine, 2009). Onshore geologic options for CO 2 sequestration mainly include depleted oil and gas reservoirs, oil reservoirs, salt aquifers and unmineable coal seams (Tao et al., 2019). Among these, CO 2 sequestration in coal seams (CO 2 -ECBM) has been considered to not only mitigate CO 2 emissions, but also enhance coalbed methane (CH 4 ) recovery (Orr, 2009). The data published by the International Energy Agency indicate that worldwide, CO 2 storage capacity and amount of CH 4 recovered from coal seams could reach up to 4.88 Â 10 11 billion tons and 5 Â 10 13 Nm 3 , respectively (Godec et al., 2014).
The CO 2 -ECBM process incorporates complex interactions between multiple fluids, including CO 2 , CH 4 , H 2 O and coals (Wang et al., 2015d;Zhang et al., 2019d); they have received a lot of interest (Li et al., 2017;Liu et al., 2018;Perra et al., 2011;Zhang et al., 2013aZhang et al., , 2017aZhang et al., , 2019c. The coal matrix has been found to have a good adsorption capability, which mainly accounts for CO 2 sequestration and CH 4 occurrence in the coal seams (Pillalamarry et al., 2011). Results have shown that the volume fractions of in-situ adsorbed CH 4 and CO 2 in the coal seams are 85% (Chang et al., 2015) and 95-98% (Busch and Gensterblum, 2011;De Silva and Ranjith, 2012;Topolnicki et al., 2013), respectively. The adsorption, diffusion and flow of CH 4 and CO 2 within coal reservoirs have mainly been determined by the pore structure of coals (Liu et al., 2010;Zhang et al., 2017b). The surface area, volume, pore size distribution and roughness of the pore surface have been observed to have a particularly significant effect on CO 2 storage and CH 4 production (Arami-Niya et al., 2016;Liu et al., 2015Liu et al., , 2017. The estimated optimum depth for CO 2 geologic storage is 800-1000 m (Orr, 2009), where coal reservoir temperature and pressure have been often higher than the critical temperature and pressure of CO 2 , i.e. T c ¼ 31.06 C and P c ¼ 7.38 MPa (Perry and Green, 1997). Therefore, CO 2 exists in a supercritical state that has a high diffusion and dissolution capability, and a low viscosity and surface tension (Nahar et al., 2018). Additionally, the three-dimensional network of coal comprises condensed aromatic and hydroaromatic compounds connected via short alkyl bridges, and ether and thioether linkages (Liu et al., 2019d). Organic compounds with a low molecular weight of <500 amu, including oxy-compounds and hydrocarbons, have been found to disperse in the macromolecular structure of coals. Furthermore, many types of inorganic minerals, such as calcite, dolomite and chlorite are found in coals. The injected supercritical carbon dioxide (ScCO 2 ) has the potential to extract organic compounds with a low molecular weight and dissolve and precipitate some minerals within the coal matrix. In summary, the aforementioned characteristics of both ScCO 2 and coals have resulted in multiple interactions between the fluids and coals, i.e. ScCO 2 -induced coal matrix swelling (Sampath et al., 2019), the extraction of small organic compounds (Andr e et al., 2007;Kolak and Burruss, 2006), and mineral dissolution or precipitation (Li et al., 2017;Mavhengere et al., 2015;Zhang et al., 2016). These interactions have been found to be capable of changing the pore structure of coals (Liu et al., 2019e;Massarotto et al., 2010;Zhang et al., 2017b), thereby, affecting CO 2 storage and CH 4 production within target coal seams. Various studies have suggested that the effects of ScCO 2 on the pore structure of coals are associated with the physicochemical property of coals, including coal rank (R o,max ), volatile matter and minerals, and coal reservoir conditions, such as temperature, pressure and moisture content (Arami-Niya et al., 2016;Radlinski et al., 2004). Although the ScCO 2 exposure dependence of coals has been studied extensively, the review on this area has scarcely been reported. Therefore, this study analysed the influences of ScCO 2 on the pore structure of various rank coals based on literature review. The potential mechanisms accounting for ScCO 2 exposure dependence of the pore structure of the coal were addressed. Future directions of study in this area are also given in this paper. Table 1 shows that the studies on the effects of ScCO 2 fluid on the pore structure of coal began in 2000, and have since received significant interest. The coals tested in these studies included dry and moist samples. In addition, most studies were performed under static exposure conditions. Diverse characterisation methods including probe molecule adsorption, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD) and scanning electron microscope (SEM) were used.
Effects of ScCO 2 on the pore morphology of coals In this study, the pore classification standard proposed by the International Union of Pure and Applied Chemistry was adopted. It classifies micro-, meso-and macropores as those with a diameter <2 nm, with a diameter ranging between 2 and 50 nm and with a diameter larger than 50 nm, respectively (Sing, 1985).

Micropores
The micropores of porous materials have been often found to accommodate the main adsorption space for the adsorbates (Do, 1998;Liu et al., 2019aLiu et al., , 2019c. Thus, micropores mainly determine the adsorption capacity of the adsorbates. With respect to CO 2 -ECBM, it has been widely accepted that the micropore structure of the coal matrix determined the CO 2 storage capacity and in-situ CH 4 reserve of the target coal seams (White et al., 2005). Hence, the alterations in the micropores in the coals, after ScCO 2 exposure, are critical to actual CO 2 -ECBM process. Many options can be used to estimate the micropore structure parameters of coals, including CO 2 adsorption, small angle X-ray scattering (SAXS) and small angle neutron scattering (SANS). Among these, CO 2 adsorption at 273.15 or 298.15 K has been widely adopted to estimate the micropore parameters of coals as CO 2 can easily access micropores and ultramicropores (Amarasekera et al., 1995). Based on the CO 2 adsorption data, the Dubinin-Radushkevic and the Dubinin-Astakhov models have been used to calculate the micropore surface area (S mic ) and volume (V mic ), respectively (Wang et al., 2015c;Zhang et al., 2016). Surface area and volume of the micropore. Previous studies reported that S mic of dry coals decreases after ScCO 2 exposure (Mavhengere et al., 2015;Zhang et al., 2016Zhang et al., , 2017b, and ScCO 2 -induced matrix swelling was considered the primary reason for that. Studies on asreceived coals also reported the same trend as that of the dry coals after ScCO 2 exposure (Kutchko et al., 2013;Mastalerz et al., 2008). Furthermore, H 2 O has been considered to play a critical role in the interactions between ScCO 2 and coals. H 2 O has been verified to enhance the effects of ScCO 2 on coal pores, to produce complex interactions in the ScCO 2 -H 2 O-coals system (Jiang and Yu, 2019;Li et al., 2017). Furthermore, Daning coal (R o,max ¼ 2.86%) collected from Qinshui coal field and Xiegou coal (R o,max ¼ 0.67%) collected from Hedong coal field were observed to exhibit the smallest (-1.1%) and the highest (-8.27%) change in S mic , respectively, after ScCO 2 and H 2 O exposure (Li et al., 2017). However, some studies also reported that ScCO 2 exposure could increase S mic of coals (Jiang and Yu, 2019;Mastalerz et al., 2010). Therefore, to better evaluate the influence of ScCO 2 exposure on the micropores of coals, the parameters f Smic and f Vmic were calculated using equations (1) and (2), respectively, and selected to determine the variation in S mic and V mic of coals after ScCO 2 exposure.
where S x,i and S x,f are the pore surface areas of coals, before and after ScCO 2 exposure, respectively; V x,i and V x,f are the pore volumes of coals, before and after ScCO 2 exposure, respectively; and x represents the micropores or meso-and macropores of coals. As shown in Figure 1, both f Smic and f Vmic varied between -20% and 20%. Interestingly, f Smic of coals, after ScCO 2 exposure, exhibited the same pattern as that of f Vmic . Figure 1. Variation range of micropore surface area and volume of coals after ScCO 2 exposure. Source: reproduced with permission from Li et al., 2017;Liu et al., 2019a;Massarotto et al., 2010;Mastalerz et al., 2010;Wang et al., 2015c;Zhang et al., 2016Zhang et al., , 2017a Changes in the micropores of coals are known to mainly stem from the coal matrix swelling, extraction and mineral dissolution and precipitation because of the exposure to ScCO 2 . Among these, coal matrix swelling has been found to be closely related to R o,max , i.e. the swelling ratio of the coal matrix decreases with increasing R o,max (Majewska et al., 2010;Yu et al., 2003). Furthermore, the extraction effect and mineral dissolution and precipitation effects have been found to depend upon the volatile matter yield and ash content, respectively (Zhang et al., 2013a(Zhang et al., , 2019a. Based on this, the changing amplitude of the micropore surface area (|f Smic |) and micropore volume (|f Vmic |) were plotted against R o,max . The normalised pore structure parameters, i.e. |f Vxmic | (equation (3)) and |f Axmic | (equation (4)) were plotted against volatile matter yield and ash content, respectively.
where V is the volatile matter yield, wt%; A is the ash content, wt%; |f V xy | and |f A xy | are the volatile matter yield and ash content normalised pore structure parameter changing amplitude, %, respectively; x represents the pore surface areas or volumes of coals; and y represents the micropores or meso-and macropores of coals. Figure 2(a) and (b) show the U-shaped trend between |f Smic | and |f Vmic | of the dry coals and R o,max . It can be observed that |f Smic | and |f Vmic | decreased as R o,max increased from 0.5% to 2.0%. As R o,max increased further, |f Smic | and |f Vmic | showed an inverse increase. Furthermore, the swelling ratio of the coal matrix was observed to decrease with increasing R o,max . Thus, both the |f Smic | and |f Vmic | of low-rank coals decreased as R o,max increased to approximately 2.0%. However, high-rank coals have developed micropore structure that was expected to enhance the extraction strength of ScCO 2 to promote the extraction of some small organic compounds from the micropores of coals. Therefore, |f Smic | and |f Vmic | were observed to increase with the increasing R o,max . Both |f Smic | and |f Vmic | also increased significantly as the volatile matter yield and ash content increased (Figure 2(c) to (f)). In general, the effects of small organic compound extraction and mineral dissolution and precipitation increased the microporosity of coals after ScCO 2 exposure (Du et al., 2018;Zhang et al., 2013a). The above-mentioned effects depend highly upon the volatile matter yield and ash content of coals, which can account for the increasing trend shown in Figure 2(c) to (f).
Micropore size distribution. Apart from S mic and V mic , the micropore size distribution (PSD) of coals has also been found to be influenced by ScCO 2 exposure (Kowalczyk et al., 2008;Li, 2018;Li et al., 2017;Liu et al., 2019b;Mastalerz et al., 2010;Wang et al., 2015c;Zhang et al., 2017a). The PSD of coal is usually determined from the non-local density functional theory model using CO 2 adsorption data.
The influences of ScCO 2 on the PSD of micropores of coals have been contested. Specifically, for coals, after exposure to ScCO 2 and H 2 O, ScCO 2 exposure has been observed to mainly affect the micropores of coals, with the pore diameter range of 0.4-0.7 nm and 0.7-0.9 nm (Figure 3), which has been attributed to the heterogeneity of physicochemical properties of coals (Balan and Gumrah, 2009;Gensterblum et al., 2014). It is important to note that ScCO 2 can only break the substructure in coals but has been found to be unable to break the spatial structure of coals formed by chemical cross-links (Kutchko et al., 2013;Mastalerz et al., 2010). Generally, the micropore volume has exhibited a decreasing trend after ScCO 2 exposure, when the intensity of the matrix swelling has been observed to be stronger than the other effects. Instead, the micropore volume has been found to increase  (d)) and ash content of coals ((e) and (f)). Source: reproduced with permission from Kutchko et al., 2013;Mavhengere et al., 2015;Wang et al., 2015c;Zhang et al., 2016Zhang et al., , 2017a. (Chen et al., 2017;Zhang et al., 2017a). However, for the dry coals, numerous studies have shown that ScCO 2 exposure cannot significantly change the PSD of the micropores of coals ( Figure 4). Therefore, the change in the PSD of the micropores of coals has been mainly attributed to H 2 O. Based on previous studies, the influences of H 2 O on the physicochemical properties of coal samples have mainly included the following aspects: (1) H 2 O occupies the adsorption space of CO 2 in the coal matrix, thereby weakening the CO 2 storage capacity (Ozdemir and Schroeder, 2009); (2) oxygen-containing functional groups on coal surface act as adsorption sites of CO 2 and H 2 O, thus, creating competitive adsorption exists between CO 2 and H 2 O (Day et al., 2008b(Day et al., , 2011; (3) mineral dissolution and precipitation effects increase the pore surface areas and volumes of coals; and (4) H 2 O adsorbed on the coal matrix enhances the swelling ratio, thus, degrading the pore morphology of coals (Krooss et al., 2002;Zhang et al., 2014).

Meso-and macropores
The meso-and macropores of coals are known to often accommodate storage space for free gas and are also known to act as a seepage channel (Wang et al., 2015a). Thus, the mesoand macropores often partially determine the storage capacity and adsorption and diffusion rate of CO 2 (White et al., 2005). N 2 adsorption and desorption method has been widely adopted to determine the meso-and macropores of coals. The meso-and macropore surface areas (S meso/mac ) of coals have been calculated using the Brunauer-Emmet-Teller equation, based on the N 2 adsorption branch falling in the P/P 0 range of 0.05-0.30 (Brunauer et al., 1940). The total pore volume has been estimated using the Barrett-Joyner-Halenda (BJH) model, based on the desorption branch of the isotherm. Furthermore, the meso-and macropore PSD profiles of coals have also been estimated using the BJH model, based on the desorption isotherm (Barrett et al., 1951).
Meso-and macropore surface area and volume. Previous studies have reported the influences of ScCO 2 exposure on meso-and macropores of coals. Results have indicated that ScCO 2 exposure always decreased S meso/mac of many dry coals; however, in some cases, it increased S meso/mac of some of the dry coals having certain metamorphism (Zhang et al., 2013a). Contrarily, S meso/mac of coals has been found to significantly increase with regard to ScCO 2 -H 2 O-coal system (Du et al., 2018;Liu et al., 2018). Based on these studies, it has been considered that H 2 O also plays an essential role in changing the meso-and macropores of coals. Particularly, H 2 O can significantly enhance the mineral dissolution and precipitation ability exerted by ScCO 2 , thus, increase the pore volumes and surface areas of coals. Besides, the effects of ScCO 2 on meso-and macropore structure of coals have also been associated with ScCO 2 -induced matrix swelling and small organic compounds extraction (Du et al., 2018;Gathitu et al., 2009;Li et al., 2017;Liu et al., 2018;Zhang et al., 2013a). Thus, the changing amplitude of meso-and macropore surface areas (|f Smeso/mac |) and volumes (|f Vmeso/mac |) of coals, after ScCO 2 exposure, has been related to R o,max , volatile matter yield and ash content, respectively. However, a discrete correlation has been found between |f Smeso/mac | and |f Vmeso/mac | and R o,max (Figure 5(a) and (b)), indicating that the extraction and mineral dissolution and precipitation, besides the coal matrix swelling, also affect the meso-and macropore structure of coals. In general, matrix swelling has been considered to decrease the meso-and macropore space of coals. Furthermore, the extraction of small organic compound by ScCO 2 has been considered to increase the quantity of mesoand macropores of coals. However, the occurrence rule of minerals within coals is extremely complicated. Thus, effect of mineral dissolution and precipitation of ScCO 2 on meso-and macropores of various ranks of coals is complex. To summarise, the aforementioned effects of ScCO 2 exposure on meso-and macropores of coals can account for the discrete relationship shown in Figure 5(a) and (b). Figure 5(c) to (f) indicates that both |f Smeso/mac | and |f Vmeso/mac | significantly increased with increasing volatile matter yield and ash content of coals, which was identical to the trend of |f Smic | and |f Vmic |, as shown in Figure 2(c) to (f). However, both |f Smeso/mac | and |f Vmeso/mac | of coals were larger than |f Smic | and |f Vmic |, which was associated with following aspects. While the meso-and macropores of coals are abundantly found in the minerals (White et al., 2005), the throat diameters of meso-and macropores have been found to be larger than that of the micropores (White et al., 2005). These two factors enhance the strength of extraction and mineral dissolution and precipitation effects of ScCO 2 on the meso-and macropores of coals, thereby enlarging the changing amplitude of the meso-and macropores of coals after ScCO 2 exposure.
Meso-and macropore size distribution. As displayed in Figure 6, many studies have found that ScCO 2 exposure significantly changes the pores with diameters of 2-8 nm (Gathitu et al., 2009;Li et al., 2017;Zhang et al., 2017aZhang et al., , 2016. The micropores of coals have been found to Figure 5. Relationship between |f Smeso/mac | ((a), (c) and (e)) and |f Vmeso/mac | ((b), (d) and (f)) and R o,max (%) ((a) and (b)), volatile matter yield ((c) and (d)) and ash content ((e) and (f)). Source: reproduced with permission from Wang et al., 2015c;Zhang et al., 2013aZhang et al., , 2016Zhang et al., , 2017a determine the adsorption capacity of CH 4 and CO 2 (Cai et al., 2013;Do, 1998;Liu et al., 2019c), whereas, some studies have also shown that the initial uptake of CO 2 is dominated by the mesopores of coals, particularly, the pores with diameters of 4-10 nm (Li et al., 2017;Mastalerz et al., 2008). Moreover, the bonded and adsorbed water has been found to be mainly present in pores with diameters of 2-10 nm of the low-rank coals, thereby, inducing stronger swelling effect. For high-rank coals, the mesopore walls consist of molecular orientation domains (Prinz and Littke, 2005), which contain some functional groups, such as C ¼ O groups (Zhang et al., 2016). When CO 2 and H 2 O are adsorbed on these functional groups on the coal surface, they alter the mesopores. Therefore, a noticeable change is found in the PSD of meso-and macropores with diameters of 2-8 nm of coals, after ScCO 2 exposure.

Roughness of the pore structure
The pore structure of coals is highly complex and heterogeneous, making its quantitative analysis more difficult. Fractal theory has been widely acknowledged as a useful tool to estimate the complexity of the pore structure of coals. The key parameter regarding fractal theory is the fractal dimension (D), which indicates the roughness of the pore structure. Experimental methods, including physical adsorption, SAXS and SANS can generate D of coals. Among these, physical adsorption using N 2 as a probe molecule is considered simply to study the fractal property of the coal pore surface. Based on the N 2 adsorption data, various mathematical models such as Avnir model (Avnir and Jaroniec, 1989), Frenkel-Halsey-Hill (FHH) (Li et al., 2015;Zhu et al., 2016), modified FHH (Yao et al., 2008), Neimark (Neimark et al., 1993), Sierpinski (Jiang et al., 2013) and Menger (Song et al., 2018) summarised in Table 2 can be used to calculate the D of coals.
By combining the low-temperature N 2 adsorption data with the Avnir model, D of coals, before and after ScCO 2 exposure, can be estimated, as summarised in Figure 7. It has been found that D of all the coal samples varied from 2.4 to 2.7, proving that the pore morphology of coals was irregular and fractal. D of coals was observed to vary between -0.5% and 0.5%, due to ScCO 2 exposure. Generally, with increasing D, the pore surface of coals was observed to get rougher. Instead, the pore surface of coals becomes smoother (Gathitu et al., 2009;Wang et al., 2015c;Zhang et al., 2013aZhang et al., , 2016Zhang et al., , 2017a.

Model Form Description References
Avnir V/Vm ¼ K[ln(P 0 /P)] Àr D ¼ 3Àr Good linearity only with relative pressure (P/P 0 ) <0.5 Avnir and Jaroniec, 1989 Applicable to describe heterogeneous pore with 8-217 nm in-situ coal Zhu et al., 2016 Modified The fractal theoretic system consists of single-fractal theory and multi-fractal theory. The single-fractal model is known to only generate apparent information of the pore structure of coals (Wang et al., 2015d). However, multi-fractal theory can be used to divide a fractal material into many sub-regions with different degrees of singularity. The analysis of the fractal features of different sub-regions can reveal more fractal information about the pore structure of coals, including capacity dimension (D 0 ), information dimension (Shannon entropy) (D 1 ), correlation dimension (D 2 ) and correlation parameters of the pore unit, within different pore sizes (H). Thus, future investigation on multi-fractal can gain more information regarding the effects of ScCO 2 on the pore morphology of coals.
Long-term ScCO 2 exposure dependence of the pore morphology of coals CO 2 geologic sequestration in the coal seams is a long-term process. It can be observed from Table 1 that very few studies have investigated the ScCO 2 exposure dependence of the pore morphology of coals under relatively longer time scale (120-4320 h). However, numerous studies have carried out investigations under short time scale ( 72 h). Therefore, the change in the pore morphology of coals, due to ScCO 2 exposure, during longer time scale has gained interest in recent years. Guo et al. (2018) examined the interaction between ScCO 2 , H 2 O and coals for 5-6 months and found that the average pore diameter, porosity and microfracture of coals increased due to dissolution of calcite and dolomite. Particularly, S meso/mac and V meso/mac of all the coal samples were observed to increase by approximately 30-73% and 40-54%, respectively. In addition, Xiao et al. (2009) also demonstrated the positive effect of mineral dissolution on the porosity of the reservoirs. They adopted the reactive transport modelling to study the spatial and temporal evolution of the injected CO 2 and associated gas-fluid-rock interactions, and predicted that calcite dissolution can increase the porosity of carbonate reservoirs containing the formation water from 30% to 41% in 10,000 years.  Wang et al., 2015c;Zhang et al., 2013aZhang et al., , 2016 It should be noted that H 2 O also plays an essential role in long-term interaction between ScCO 2 and coals. Kutchko et al. (2013) exposed two dry unconfined cores (L Â W Â H: 25.4 Â 25.4 Â 25.4 mm) of the bituminous coals to ScCO 2 (15.3 MPa and 55 C) for 104 days. Results from both field-emission SEM and N 2 /CO 2 adsorption indicated that no significant changes were observed in the pore morphology of all the coal samples. Furthermore, Mavhengere et al. (2015) studied the pore structure characteristics of a vitrinite-rich dry coal after exposure to ScCO 2 (12.5 MPa and 35 C) for six months and found that ScCO 2 significantly decreased the surface area and number of micropores of the coal sample. They attributed the decreasing pore structure parameters to vitrinite swelling induced by CO 2 . In conclusion, the existing studies show that long-term ScCO 2 exposure can enhance the porosity of coals via mineral dissolution with the aid of H 2 O. Under other conditions, long-term exposure can decrease or barely change the porosity of coals.
It is worth noting that the coal matrix swelling, extraction and mineral dissolution and precipitation associated with ScCO 2 exposure are very slow. Thus, the sole experimental method cannot reveal the real impact of long-term ScCO 2 exposure on the porosity of coals. Instead, the combination of physical experiments and numerical simulations is recommended to address this issue. Specifically, the numerical simulations, such as Reactive Transport Modelling (Pruess et al., 2001;Xiao et al., 2009), are a promising way to predict the changes in porosity and permeability of coals due to mineral dissolution and precipitation associated with long-term CO 2 sequestration, as well as the influence of mineral heterogeneity of coal reservoirs on CO 2 injectivity and storage security.

Mechanism of ScCO 2 on the pore morphology of coals
Change in the pore structure of coals, after ScCO 2 exposure, mainly arises due to multiple effects, including matrix swelling (Figure 8(a)) (Wang et al., 2015c), small organic compounds extraction (Figure 8(b)) and mineral dissolution and precipitation (Figure 8(c)).

Matrix swelling
Coal matrix has been found to not only adsorb the injected ScCO 2 , but also dissolve it, thus, inducing coal matrix swelling (Cai et al., 2018). It has been reported that coal matrix swelling can compress the cleat space of the coal seams, thereby, reducing the permeability of the coal seams. Thus, coal matrix swelling adversely affects the migration of CO 2 and CH 4 within the target coal seams.
Studies have shown that coal matrix swelling induced by ScCO 2 reduces the pore surface area and volume of coals (Day et al., 2011;Karacan, 2003;Li et al., 2012). Coal matrix swelling has been found to be associated with the fluid type and metamorphic degree of coals (Wang et al., 2015b). Under same operating temperature and pressure, the ratio of coal matrix swelling has been found to decrease in the order of CO 2 > CH 4 > N 2 (Mazzotti et al., 2009;Pini et al., 2009;Van Bergen et al., 2011), which was found to be consistent with the order of adsorption strength among these fluids. Helium is a non-adsorbing gas (Mohammad et al., 2009), thus, the coal matrix swelling induced by He is almost negligible. These information suggests that injecting mixed gas, such as CO 2 and N 2 , instead of pure CO 2 , can mitigate the significant decreasing permeability of the coals seams due to coal matrix swelling induced by pure CO 2 . Furthermore, many studies have confirmed that the swelling ratio of the coal matrix increases with decreasing R o,max (Day et al., 2012;Ferreiro et al., 2010). Thus, a change in the pore structure of low-rank coals is more pronounced than that of high-rank coals.
The reversibility of the coal matrix swelling has been a pending subject of analysis. Based on the pore structure analysis or ATR-FTIR, some studies have considered that ScCO 2induced swelling of coal matrix is an irreversible process (Goodman et al., 2005;Zhang et al., 2013a). Contrarily, other studies have used the optical method or tensometric measurement to find that the process of coal matrix swelling is completely reversible (Ceglarska-Stefa nska and Czapli nski, 1993; Day et al., 2008a). In addition, there has never been a consensus among the researchers about the mechanism of coal matrix swelling. It has been reported that ScCO 2 dissolved in the coal matrix can work as a plasticiser, thereby, creating additional free volume of coals, inducing molecular motion and rearranging the coal physical structure (Goodman et al., 2005;Larsen et al., 1997). However, the coal matrix swelling has also been explained by the chemical interactions between ScCO 2 and coals through hydrogen bond and/or electron transfer, thus, reducing the crosslink degree of macromolecular structure of coals (Walker et al., 1988). The disagreement regarding the reversibility and mechanism of coal matrix swelling induced by ScCO 2 is significantly close due to the complexity and heterogeneity of the coal structure. Hence, a deep resolution of the coal structure can help in clarifying the mechanism of coal matrix swelling induced by ScCO 2 . First, various methods such as nuclear magnetic resonance, laser Raman spectroscopy, FTIR, XRD, high-resolution transmission electron microscopy, are recommended to determine the aromaticity, aliphaticity, functional groups and species of the main elements of coals.Furthermore, due to the complex and heterogeneous structure of coals, the experimental methods cannot fully elucidate the coal structure. Thus, it is necessary to apply the tools based on quantum chemistry theory, such as VASP, Gaussian, DMol and PWSCF to calculate and optimise the coal macromolecule structure. Next, the experimental and theoretical results need to be combined with the exiting coal models such as Heredy and Wender, Given, Wiser, Shinn and Hirsch to accurately study the coal structure for efficient examination of the coal matrix swelling process. Finally, advanced imaging technology, such as atomic force microscope, can be used since it provides the visual information of the intermolecular hydrogen bond (Zhang et al., 2013b). Usage of these methods can also help in further studying the mechanism of ScCO 2 -induced coal matrix swelling.

Extraction effect
Small organic compounds with molecular weight <500 amu, have been found to disperse within the macromolecular structure of coals (Mathews and Chaffee, 2012). The injected CO 2 has been observed to always exist as a supercritical fluid in the coal reservoirs (Orr, 2009). It is known to have excellent dissolution and mass transfer ability, which can aid in the extraction and mobilisation of small organic compounds from the coal matrix. Thus, the extraction effect of CO 2 exposure can change the pore structure of coals.
For R o,max ! 1.5%, the amplitude of the micropore parameter of coals has been observed to change due to the extraction effect of ScCO 2 ; they have been observed to increase with increasing R o,max (Figure 2(b)). This was typically attributed to the developed pore structure of high-rank coals, which promoted the migration of ScCO 2 within coals and finally enhanced the extraction process (Liu et al., 2010). In addition, H 2 O prefers oxygencontaining functional groups on coal surfaces as adsorption sites, thereby preventing CO 2 from entering coal pores. Hence, H 2 O can weaken the extraction effect of ScCO 2 , particularly in low-rank coals (Kolak et al., 2015). Furthermore, ScCO 2 can only extract trace amounts of small organic compounds. Thus, ScCO 2 has a minor impact on meso-and macropores (Kutchko et al., 2013). ScCO 2 extraction via extraction A and B has been reported to have two adverse effects on the pore structure of coals, as illustrated in Figure 8(b) (Zhang et al., 2019b). On the one hand, if ScCO 2 exhibits only slight extractability to some organic compounds in coals, small organic compounds may block the pores and further reduce coal pore space. On the other hand, the pore space of coals will be enlarged if some organic compounds are extracted and escape from the pore throat.
The small organic compounds extracted by ScCO 2 have been found to comprise biologically toxic aliphatic and polycyclic aromatic hydrocarbons (Kolak and Burruss, 2006;Kolak et al., 2015;Zhang et al., 2013a) that can be mobilised by ScCO 2 and enter the groundwater system. Thus, actual CO 2 -ECBM should focus on environmental safety and health issues related to CO 2 sequestration. Furthermore, the temperature and pressure dependencies of the composition of organic compounds extracted from the coal matrix by ScCO 2 and their effects on changing pore structure of coals after ScCO 2 exposure needs to be studied further.

Mineral dissolution and precipitation effects
The actual coal seams have been always found to contain water (Massarotto et al., 2010;Tong et al., 2019), which has been considered to likely react with ScCO 2 and form carbonic acid (H 2 CO 3 ). Thus, the multiple complex chemical reactions summarised in Table 3 have been found to occur between some types of minerals contained in coals and H 2 CO 3 . The minerals of coals typically undergo the following three types of interactions, after CO 2 is injected into the target coal seams, i.e. dissolution of minerals (Credoz et al., 2011;Hedges et al., 2007), transformation of clay minerals (Farquhar et al., 2015) and formation of new minerals (Liu et al., 2015). H 2 CO 3 has been found to leach out and dissolve some minerals in coals, especially, minerals filled in the pores and fractures, such as carbonate and aluminosilicate minerals (Black et al., 2015;Dawson et al., 2015). The dissolution of minerals has been observed to increase the pores and fractures of coals and generate pores in the minerals, which can increase the volume and connectivity of meso-and macropores (Chen et al., 2017;Liu et al., 2018). Thus, the dissolution of minerals helps in increasing the CO 2 adsorption capacity of the target coal seams. Under the conditions of temperature being 60 C and pH being 4, the reaction rates of calcite and dolomite have been found to be 10 À5 -10 À3 and 10 À7 -10 À5 molÁm À2 Ás À1 , respectively (Sonnenthale and Spyoher, 2001). ScCO 2 -H 2 O has been found to exhibit strongest reactivity to calcite, followed by dolomite, chlorite and albite, respectively. However, such reactions have not been found to be significant for illite, quartz and kaolinite (Du et al., 2018). Figure 9 shows that calcite on the coal surface dissolved and almost disappeared (Figure 9(b)) after ScCO 2 exposure. The inner calcite then generated some cone pores (Figure 9(d)) and newly dissolved crystal faces (Figure 9(f)). However, the dissolution of dolomite only generated some rhomboid pores (Figure 9(d)). It has been confirmed that dolomite is more difficult to dissolve in ScCO 2 than calcite. Inorganic compounds, such as silicate and carbonate, have been observed to have high polarity of metal ions (Iwai et al., 2000), which prevent the inorganic compounds of dry coals from being dissolved in ScCO 2 (Iwai et al., 2000;Wang et al., 2015c). However, inorganic minerals have been found to dissolve in ScCO 2 -H 2 O-coal system due to exchange of inorganic minerals with H þ (Du et al., 2018). Thus, the pore structure of coals has been considered to get further enhanced (Li, 2018;Liu et al., 2018). Therefore, injecting CO 2 , H 2 S Source: reproduced with permission from Guo et al., 2018. and SO 2 fluid is considered to enhance the intensity of dissolution effect, thereby increasing the pore volume of coals and to mitigate the hazardous gaseous pollutants emitted into the atmosphere.
With respect to secondary minerals, they are not considered to be precipitated, when the saturation index (SI), is less than zero. SI of aluminium hydroxide mineral (Al(OH) 3 ) has been found to be the highest among the minerals contained in coals (Hayashi et al., 1991;Tarkowski et al., 2011), indicating that Al(OH) 3 is most likely to form secondary minerals in coals. In ScCO 2 -H 2 O-coal system, Al(OH) 3 has been found to readily react with H þ , resulting in the concentration of Al 3þ around the minerals, which subsequently combine Figure 9. Changes in carbonate before and after ScCO 2 exposure. Source: reproduced with permission from Du et al., 2018. with the H 4 SiO 4 and Fe ions. The new aluminium silicate minerals are generated due to the low solubility of aluminium silicate. Moreover, the newly formed aluminium silicate minerals comprise a large number of nanoscale pores with surface area greater than that of mineral Al(OH) 3 . Thus, the newly formed aluminium silicate minerals can increase the storage capacity of CO 2 within the target coal seams.
The minerals in coals have been found to comprise primary and secondary minerals, with mass fractions of 1-2%, and < 10%, respectively (Xie, 2012). The type of identifiable inorganic minerals has been observed to reach up to 316 (Yu et al., 2015). Thus, the occurrence rule for the inorganic minerals in coals is extremely complex, which makes further exploration of the interaction mechanism between ScCO 2 exposure and the minerals in coals more difficult. Moreover, with regard to CO 2 -ECBM, sealing of the roof and floor in the coal seams is critical for CO 2 storage in geologic time scale. However, very few the studies have examined the reactions between ScCO 2 and the minerals in the roof and floor, which should be studied in future.

Conclusions
CO 2 -ECBM is a viable option for geological storage of CO 2 . The stored CO 2 is known as a supercritical fluid (ScCO 2 ), with great potential to change the pore structure of coals. Thus, the influences of ScCO 2 on the pore morphology of various rank coals were examined in this study. The main conclusions derived from this study are summarised below.
For the dry coals, the S mic and V mic of coals, due to ScCO 2 exposure, varied between -20% and 20%. ScCO 2 exposure was not observed to significantly change the PSD of the dry coals. However, the meso-and macropores, with the diameters of 2-8 nm of the dry coals evidently changed. For ScCO 2 -H 2 O-coal system, ScCO 2 exposure mainly affected the pores of coals with diameter ranges of 0.4-0.7, 0.7-0.9 and 2-8 nm. The fractal dimension of the coal pore surface was observed to vary between -0.5% and 0.5%. Moreover, it was considered that long-term SCCO 2 exposure could enhance the porosity of coals via mineral dissolution, with the aid of H 2 O. Otherwise, the long-term exposure could decrease or scarcely change the porosity of coals.
The changes in the pore surface of coals, after ScCO 2 exposure, were mainly associated with the coal matrix swelling, small organic compounds extraction and mineral dissolution and precipitation. It has been evidently found that the ScCO 2 -induced matrix swelling decreased the pore surface area and volume of the coal matrix, and further compressed the cleat space of the coal seams. The extraction effect of ScCO 2 could extract or mobilise small organic compounds within the coal matrix, thus, increasing or decreasing the pore surface area and volume of coals. Generally, the extraction had a significant impact on the micropores than the meso-and macropores of coals. Finally, the dissolution and precipitation increased the pore surface area and volume of coals.
To clarify the influences of ScCO 2 on the pore structure of coals during the implementation of CO 2 -ECBM, following studies should be carried out. First, multiple advanced characterisation methods, quantum chemistry simulation as well as existing coal models need to be combined to explore ScCO 2 -induced coal matrix swelling. Furthermore, temperature and pressure dependence of the composition and concentration of the organic compounds extracted from the coal matrix should be studied, and their effects on changing pore structure of coals after ScCO 2 exposure should be examined. Finally, combination of physical experiments and numerical simulations are recommended to predict the changes in porosity and permeability of coals due to mineral dissolution and precipitation associated with long-term CO 2 sequestration, as well as the influence of mineral heterogeneity of coal reservoirs on CO 2 injectivity and storage security.

Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research is sponsored by the funding from National Natural Science