Experimental study of supercritical CO2-H2O-coal interactions and the effect on coal permeability

Fuel 253 (2019) 369–382

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Full Length Article

Experimental study of supercritical CO2-H2O-coal interactions and the effect on coal permeability

T

⁎

Yi Dua,b, Shuxun Sangb, , Zhejun Panc, Wenfeng Wangb, Shiqi Liud, Changqing Fue, ⁎ Yongchun Zhaoa, Junying Zhanga, a

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science & Technology, Wuhan 430074, China Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of Education, School of Mineral Resource and Geoscience, China University of Mining and Technology, Xuzhou 221116, China c CSIRO Energy Business Unit, Private Bag 10, Clayton South, VIC 3169, Australia d Low Carbon Energy Institute, China University of Mining and Technology, Xuzhou 221116, China e College of Geology and Environment, Xi’an University of Science & Technology, Xi’an 710054, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Supercritical CO2-H2O-coal Pore structure Permeability High-rank coal 3-dimentional mode

Transformation of pore and fracture structure of coals with supercritical CO2 (scCO2) –H2O is a key to CO2 injection and CH4 production efficiencies during the CO2 enhanced coalbed methane process. To study the transformation of pores and fractures in the coals with CO2, two reservoir conditions, which simulate1000m (45 °C, 10 MPa) and 2000 m (80 °C, 20 MPa) depths, are applied to four types of high metamorphic coals from Qinshui Basin to study the influences of temperature and pressure on pore volume and pore sized distribution change. Nuclear magnetic resonance, high pressure mercury intrusion, X-ray CT scanning and permeability experiments are performed and the effects of scCO2 on coal permeability and the influencing factors are discussed. The results show that scCO2-H2O has a positive effect on the improvement on the pore fracture system. It could add or expand pores and fractures, leading to the increase in pore number, porosity, pore volume, pore specific surface area, connected pore volume, and pore throat number. And then, increased the permeability which had a positive correlation with the experimental temperature and pressure. The growth of permeability could be as high as 114.10 times, and it was higher in horizontal to bedding direction than that of the vertical to bedding direction. Coal expansion could lead to the addition and enlargement of micro-fractures and enhance the connectivity between seepage pores and fractures. Mineral dissolution could lead to the formation of a large number of effectively connected and non-effectively connected pores, especially the latter, which was positively correlated with simulated temperature and pressure. In addition, the effectively connected pores tend to develop in vertical original micro-fractures. Moreover, the more complete the reaction is, the more favorable it is to increase the pore volume of fractures.

1. Introduction A large amount of CO2 emission into the atmosphere has caused a series of environmental problems such as the greenhouse effect due to the utilization of fossil fuels such as coal [1]. Carbon Dioxide Capture and Storage (CCS) is the most direct and efficient method to remediate climate change by reducing the CO2 emission into the atmosphere, and deep saline aquifer, depleted petroleum reservoir, deep unminable coal seam and salt cavern are effective geological reservoirs [2,3]. It is a prospective technology to inject CO2 into coal seam, especially into the coal seams with coal gas exploitation value. Injection of CO2 could improve the recovery ratio of coal bed gas [4]. Pilot experiments have

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been conducted in the U.S.A., Canada, Poland, Japan and China with demonstrated effectiveness [5–7]. In large scale CO2 storage, CO2 is ideally transmitted through the pipeline to the storage site. From the perspectives of operability, economy and efficiency, in the application of injecting CO2 to improve oil and gas recovery, most of the CO2 pipeline transport adopts supercritical CO2 (scCO2) [8]. As the critical point of CO2 is 31.1 °C and 7.38 MPa [9], CO2 would stay in supercritical state under the condition of high reservoir temperature and pressure after being injected into a deep coal seam [10]. CO2 injected into coal seam has mainly three phases: first, adsorbed onto the surface of pores; second, free gas in the pores and fractures;

Corresponding authors. E-mail addresses: [email protected] (S. Sang), [email protected] (J. Zhang).

https://doi.org/10.1016/j.fuel.2019.04.161 Received 13 February 2019; Received in revised form 25 April 2019; Accepted 30 April 2019 Available online 13 May 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

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Table 1 The macerals and vitrinite reflectance of the prepared coal samples (modified according to [21]). Sample

BF3# XJ3# YW3# XJ15#

Ro (%)

2.83 2.64 2.19 2.68

Total (%)

9.63 11.78 7.7 11.41

Relative content of mineral components (%) Clay

Carbonate

Quartz

Rutile

Aluminum hydroxide

Apatite

Pyrite

73.6 75.14 58.43 72.71

4.12 4.07 9.67 16.81

16.28 20.79 17.74 9.45

2.13 / 3.2 /

3.87 / / /

/ / 14.16 /

/ / / 1.03

Notes: “/” means the undetected mineral; “Total” means the percentage of total mineral content in a coal sample tested by X ray diffractometer.

2. Samples and methodology

third, dissolved in formation water [11,12]. Most of the CO2 would be adsorbed on the surface of the coal matrix pores when passing through the cleat system through diffusion and flow under the pressure difference [13]. Since the molecular radius of CO2 is less than that of CH4, it is easier to occupy more favorable adsorption sites, thus replacing the adsorbed CH4. Under the action of high ground temperature, CO2 could move faster in coal [14,15], and could make the thermal energy of coal exceed the intermolecular interaction energy [16,17], resulting in a certain plasticity of coal [17,18]. Furthermore, some CO2 could react with water and generate H2CO3 which would react with minerals, so that the structures of the coal pores and fractures could change. Changing the physical structure of coal also plays the role of plastic agent [12,19]. The pore and fracture system in coal reservoir, as the main space for CO2 displacing CH4 and the main channel for gas flow, its development characteristics (porosity, pore size distribution, pore structure and etc.) and connectivity are of critical significance for CO2 injection and displacement of CH4. Therefore, the scCO2-H2O-coal system has received increasing research interest in recent years. It is considered that H2CO3 generated by reaction of CO2 and water has relatively powerful action on the migration of Ca and Mg, resulting in leaching of calcite, dolomite and magnesite. This makes the originally closed or semi-closed pores open and the pore size distribution change, resulting in the change of porosity and permeability [12,13,19–23]. Injection of CO2 could also make the originally filled minerals dissolve, the pores and fractures open and connected and the mechanical properties of the coal changed [9,24–26]. Although the variation characteristics of pore structure and permeability in coal under the action of scCO2 are still controversial, its influence on the pore structure of coal seam has been generally recognized, and it is believed that it plays an important role in the effectiveness of CO2-ECBM. Pores and fractures in coal seam are complex, so it is hard to acquire its change characteristics and connectivity directly and effectively. There are three characterization methods of pore structure: (1) microscopic observation, including optical microscope, electron microscope, etc. (2) ray detection, including nuclear magnetic resonance (NMR), Xray CT, small angle X diffraction and neutron scattering; and (3) adsorption of gas and fluid intrusion methods, such as mercury intrusion porosimetry (MIP) and low temperature liquid N2/CO2 adsorption technique [27]. Both NMR and X-ray CT have the advantage of nondestructive to the sample, so, it would be more accurate to test and observe the same sample before and after the reaction [28,29]. At present, the structural changes of coal reservoir caused by scCO2H2O-coal reaction often adopt a single test method and ignore the impact of the test method on the results. High-rank coals from Qinshui Basin in China were selected for the study. Through NMR, overburden pressure porosimetry-permeametry, MIP and X-ray CT scanning technique, pore structure changing characteristics of high-rank coals before and after scCO2 reaction and its influences on pore connectivity under different temperature and pressure conditions were discussed. Moreover, characteristics and reasons of the changes of pore volumes and permeability of the different size samples were explained further to provide a reference to the study of CO2-ECBM injectivity and the effect of CO2 displacing CH4.

2.1. Samples The samples were collected from the 3# coal seam in the Bofang Mine, Yuwu Mine and Xinjing Mine in Qinshui Basin, Shanxi, and the 15# coal seam in the Xinjing Mine [23]. The samples used in the experiment were all anthracites with a high degree of metamorphism. The lowest and highest vitrinite reflectance, Ro, was 2.19% and 2.83%, respectively. A Leica DM-4500P microscope equipped with a Craic QDI 302™ spectrophotometer was used to determine the vitrinite random reflectance based on ASTM Standard D2798-11a [30]. The minerals in the coals were mainly clay minerals, and a certain amount of carbonate and quartz minerals. Only XJ-15# sample contained sulfide minerals (Table 1). The relative and total content of mineral components in coal was determined by X-ray diffraction using a D/max-2500/PC powder diffractometer with Ni-filtered Cu-Kα radiation and a scintillation detector. XRD patterns were recorded in a 2θ interval of 2.6°–70°, with a step size of 0.01°. The total mineral content is the crystallinity content calculated according to the Rietveld refined XRD curve. BF3# and XJ3# samples were drilled along vertical to stratification direction, while YW3# and XJ15# samples were taken along parallel to stratification direction. Diameters of all the samples were 25 mm. Pressure and temperature conditions to simulate the depth of 1000 m and 2000 m were applied to the samples respectively and the experiments were numbered respectively BF3#-1000 m, BF3#-2000 m, YW3#-1000 m, YW3#-2000 m, XJ3#-1000 m, XJ3#-2000 m, XJ15#1000 m and XJ15#-2000 m for NMR and overburden pressure porosimetry-permeametry before and after the geochemical reaction. In addition, a small core sample with a diameter of 4 mm was drilled from the remained coal of XJ15# for 2,000 m simulation experiment as well as for X-ray CT experiment before and after the reaction. These experimental conditions have also been described in Du et al. [23] for studying mineral change during the reaction. Choose MIP, NMR and CT to analyze the pore structure mainly for comprehensively analyzing the changes of pore structure and permeability. Each test has different advantages. The MIP samples must be small particle samples, but it cannot detect the changes of micropores (< 3 nm) and cannot compare the same particle sample before and after the reaction. NMR can detect full-scale pore and fracture changes and can compare the same sample before and after the reaction, but the sample must be core (diameter: 25 mm). CT test can intuitively build a three-dimensional model and compare the same sample before and after the reaction, but its accuracy is low and is related to the sample size. In order to obtain smaller pore size changes as far as possible and to ensure that the samples are not easily damaged during the reaction, the sample with a diameter of 4 mm was selected for CT analysis. 2.2. Geochemical simulation experiment Geochemical simulation experiment of CO2 on coal was completed in a high-pressure reaction vessel, during which the temperature and pressure were kept constant. In this study, the experimental 370

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according to the NMR data (Eq. (2)) [32]. According to the works of Perfect [33], VP can be calculated based on the NMR T2 distribution (Eqs. (3) to (4)).

temperature and pressure were 45 °C, 10 MPa and 80 °C, 20 MPa to simulate depths of 1000 m and 2000 m, respectively, and the reaction duration was 10 days. The samples for the geochemical simulation experiment of CO2 and coal were coal core samples with the diameters of 25 mm and 4 mm, as well as 4–8 mm coal blocks, with the total weight of 150 g and the deionized water of 600 ml. In order not to mix the 4 mm cores with other samples, they were placed in 800-mesh high temperature resistant and corrosion-resistant nylon bag during the geochemical experiment. Specific description of the high-pressure container and the experimental process can be found in our previous study [23]. After the geochemical experiment, the coal samples were placed first in a vacuum drying oven at 50 °C for 24 h and then they were used for subsequent tests.

lg(VP ) = (3 − D)lg(T2) + (D − 3)lg(T2max )

VP = VPi =

Sdi S × Φd − wi × Φw Sd Sw

(3)

(4)

where Vp is the cumulative porosity, T2 and T2max are the transverse relaxation time and the maximal transverse relaxation time, respectively, and D is the fractal dimension of coal pores. Eq. (2) shows a linear relationship between lg(VP) and lg(T2). VPi is the incremental porosity percentage, Sd is the total signal amplitude of dried coal sample, Sw is the total signal amplitude of saturated water coal sample, i is T2, Φd is the measured porosity of the dried coal sample, and Φw is the measured porosity of the saturated water coal sample.

2.3. NMR and permeametry test NMR experiment used a Micro MR12-150H-I NMR equipment manufactured by NIUMAG, with the resonant frequency of 12.952 MHz, the magnet temperature controlled at 32.00 ± 0.02 °C, and the diameter of the probe coil being 70 mm. Other parameters include TW = 1500 ms, TE = 0.07 ms, NECH = 8000, and NS = 32. The water saturation device was a type ZYB- device manufactured by Nantong Huaxing Petroleum Instruments Co., Ltd, the maximum water saturation pressure could reach 60 MPa. The maximum saturation pressure in this experiment was 20 MPa. PDP 200 pulse decay permeability instrument produced by American Core Lab company was adopted for permeability test. The gas used in the experiment was nitrogen. The testing range of the permeability was 0.00001–10 mD. The experimental gas pressure was 700 psi and the confining pressure was 1000 psi.

2.4. MIP The mercury injection experiment was carried out using an AutoPore IV 9500 manufactured by Micrometrics, USA, and the procedures were in accordance with ISO 15901-1:2005. The mercury injection pressure was 1.00-33000 psi, i.e. the upper pressure limit of 227.59 MPa. The pore structure parameters were calculated by Washburn formulae (Eqs. (5) to (6)) [34].

2.3.1. Experimental procedure

d=−

4γ cos θ P

(5)

S =−

PΔV γ cos θ

(6)

where, d is the pore diameter, μm; P is the mercury injection pressure, MPa; γ is the surface tension, normally 0.48 N/m; θ is the contact angle between mercury and material, normally 130°. In the calculation process of pore diameter, the values of related parameters refer to article [22].

(1) Samples before the reaction (coal core Φ25 mm) were dried in a vacuum drying oven at 50 °C for 48 h. The dried samples were weighted. (2) Pore volume and porosity were measured with PDP 200 pulse decay permeability instrument. (3) After the NMR was calibrated with standard samples, place the dried coal samples in to measure the T2 relaxation. This procedure is to obtain the background T2 relaxation. (4) Place the samples into the water saturation device. The samples were saturated with ionized water for 72 h at 20 MPa. (5) After saturation and weighing, the samples are placed into the NMR apparatus to test the T2 relaxation. (6) Conduct scCO2-H2O geochemical reaction experiment on the coal core samples. Repeat the above steps (4) and (5).

2.5. X-ray CT scanning CT scanning of sample XJ15# before and after reaction used an Xradia 510 Versa high resolution 3D X-ray microscopy imaging system manufactured by Zeiss. Non-destructive high resolution 2D CT slice image was obtained through X-ray microscopy imaging of optical lens, which was then used to construct 3D image. The scanning voxel resolution before the reaction was 4.05 μm, while that after the reaction was 4.15 μm. The samples before and after reaction were scanned at the same parameter setting, i.e., the temperature was 20 °C; the exposure time of single scan was 120 s; and the total number of 2D images was 1,016.

2.3.2. NMR fractal dimension calculation Fluid content and pore structure can be reflected by proton nuclear magnetic resonance under radio-frequency field. The relaxation characteristics can be expressed by the following equation [31]:

ρ 1 S ≈ ρ2 = FS 2 T2 V r

∑ VPi

(2)

2.5.1. Model specification of pores and fractures network structure 3D visualization software (AVIZO) was used for data analysis and reconstruction of coal structure of the scanned sampled. First, preprocesses of the slice: noise reduction, contrast enhancement and edge sharpening were conducted on the slices with Gaussian filter algorithm; second, 3D visualization reconstruction of coal structure: image segmentation and visualization reconstruction were conducted on the seam structure with binarization algorithm; third, data extraction: data extraction (equivalent diameter, pore surface area, pore volume, coordination number, shape factor, etc.) on the coal structure was done with pore-scale network modeling based on maximum sphere algorithm, which was also convenient for observing pore-channel distribution and connection; last, AVIZO distribution analysis module was used to plot the structural distribution of the extracted coal section.

(1)

where, T2 represents transverse relaxation time, ms; S represents the pore surface area, nm2; V is the pore volume, nm3; ρ2 denotes the transverse surface relaxivity coefficient, nm/ms; r is the pore radius, nm; and Fs represents the geometry factor. Therefore, there is a consistent one-to-one match between T2 value and the pore radius; the greater T2 value, the greater corresponding pore radius [32]. NMR fractal theory is proposed to confirm the geometric characteristics of the pore-fracture system [31]. Generally, NMR experiments are performed at two conditions: saturated water condition and irreducible water condition. The NMR fractal dimensions are acquired 371

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Fig. 1. High resolution 3d X-ray microscopic imaging system.

2.5.2. 3D reconstruction location The sample was glued on the sample table before scanning, however, CT scanning was performed again after the scCO2-H2O-coal geochemical reaction, so, the sample was required to be glued again, which could cause certain angle of inclination between the two CT scanning. Though it is hard to be observed visually, it differs significantly in CT scanning. In Fig. 1, the black is the pore, the grey is organic matter and the white is inorganic mineral. In case the minerals in the rectangle are located, minerals at the bottom would have a large deviation. Therefore, the images were aligned according to the characteristic mineral until the characteristic minerals coincided completely and input the required rotation angle of the sample in AVIZO for further comparison. To reduce the error, the model with larger dimensions of the sample was reconstructed, the side length of which was set as 300 μm.

and pore size can be found by means of fractal dimension and peak and trough points [31,32]. The sample BF3#-1000 is taken as an example, as shown in Fig. 2. The lg(T2) and lg(VP) can be divided into four segments by the lg(T2) value (< −0.22, −0.22 to 0.78, 0.78 to 2, and > 2) that correspond to the T2 value (< 0.6, 0.6 to 6, 6 to 100, and > 100 ms) and represent the different pore-fractures: adsorption pore (< 10 nm), transition pore (10–100 nm), seepage pore (100–1000 nm), and fracture (> 1000 nm) respectively. For log(T2) < −0.22, the T2 spectrum corresponds to micropore or adsorption pores, where pore size and pore volume are extremely small, causing saturated water to be more sensitive to external magnetic fields, and T2 time to be short and rapidly changing [32]. The curve slope in this case is very steep. For log(T2) > −0.22, the T2 spectrum corresponds to mesopore-fracture or seepage pores. Pore size and pore volume are large, causing saturated water to be less sensitive to external magnetic fields [32], and T2 time to be large and slowly changing. Therefore, the curve slope is very flat.

3. Results 3.1. NMR

3.1.2. Porosity change The porosity of the sample can be obtained by correcting the NMR signals of the sample according to the standard sample with known porosity and NMR signal. It is suggested that the porosity increased significantly after the reaction. The porosity before the reaction is 0.64%–4.04%, and that after reaction is 2.35%–6.04%, i.e., the porosity after reaction is 1.02–4.01 times of that before reaction (Table 2); moreover, the deeper the simulated burial depth is, the larger the porosity increases. Through the comparison between NMR T2 mappings before and

3.1.1. Pore-fractures identified by T2 spectra Previous research found that there is a positive correlation between T2 and pore size. The longer T2 is, the larger the corresponding pore size. The T2 distribution characteristics, including the number, area and peak position, can be used to analyze coal pore-fracture types. From the comparison of NMR T2 mapping before and after the reaction (Figs. 2 and 3), it could be seen that all the samples have three peaks. The starting point is 0.03 ms, the first wave trough is at 6 ms and the second is at100 ms. Previous studies have shown that the relation between T2

Fig. 2. Pore size division and the calculation of multiscale NMR fractal dimensions in coals. Note: The slopes of lg(Vp) vs. lg(T2) represent the value of 3-D. D1, D2, D3, and D4 represent the fractal dimensions of adsorption pore, transition pore seepage pore, and fracture, respectively. D is the fractal dimension for NMR. 372

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Fig. 3. NMR T2 mapping before and after the reaction.

Porosity contributed by the seepage pores and fractures of the samples parallel to bed stratification increased obviously, but that of the samples vertical to the stratification increased less. Meanwhile, the value on X-axis corresponding to the end of the fracture peak of vertical to stratification samples became smaller (Fig. 3), but that of the parallel to stratification sample did not change, which resulted in different changes of the calculated porosities contributed by the fractures; even there was the phenomenon of decreasing (BF3#-1000 m and XJ3#-2000 m). This reason for this due to the

after reaction (Fig. 3), it could be seen that high-rank coal develops mainly adsorption pores and transitional pores and even so after the reaction. According to the previous fractal characteristics, the porosity contributed by the adsorption pore, the transitional pore, the seepage pore and the fracture were calculated respectively (Table 3). It is found that the adsorption pores and the transitional pores contributed more increase in porosity, 0.631%–1.783% and 0.190%–1.353%, respectively; while seepage pores and fracture contributed less increase in porosity, 0.0019%–0.430% and −0.440%–0.316%, respectively. 373

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Table 2 Changes of sample parameters after the geochemical reaction. Sample

BF3#-1000⊥ BF3#-2000⊥ XJ3#-1000⊥ XJ3#-2000⊥ YW3#-1000// YW3#-2000// XJ15#-1000// XJ15#-2000//

Length (mm)

△m(After-Before) (g)

−0.50 −0.15 −0.54 −0.11 −0.74 −0.42 −0.56 −0.43

41.44 41.34 36.01 35.46 44.02 56.28 38.78 50.90

Nuclear magnetic porosity

Permeability

Before(%)

After(%)

After/Before

Before(mD)

After(mD)

After/Before

4.04 2.20 2.98 3.76 0.64 0.76 2.55 2.61

4.54 5.62 3.57 3.84 2.35 3.04 5.48 6.04

1.12 2.55 1.20 1.02 3.68 4.01 2.15 2.32

0.037 0.002 0.015 0.013 0.004 0.005 0.048 0.074

0.197 0.085 0.039 0.146 0.125 0.529 0.198 0.567

5.30 36.68 2.51 10.98 32.45 115.10 4.12 7.67

Note: Length is the length of the sample, mm; △m(After-Before) is the quality difference of post-reaction and pre-reaction samples, g. Table 3 The changes of porosities contributed by adsorption pores, transition pores, seepage pores, and fractures. Changes of porosities

Adsorption pore (%) Transition pore (%) Seepage pore (%) Fracture (%)

BF3#

BF3#

XJ3#

XJ3#

YW3#

YW3#

XJ15#

XJ15#

−1000⊥

−2000⊥

−1000⊥

−2000⊥

−1000//

−2000//

−1000//

−2000//

0.631 0.144 0.166 −0.440

1.583 1.353 0.308 0.177

0.265 0.190 0.019 0.115

0.049 0.135 0.037 −0.138

0.717 0.578 0.193 0.227

0.900 0.721 0.351 0.313

1.234 1.285 0.268 0.147

1.783 0.905 0.430 0.316

at the seepage stage, YW3#-2000// decreased 0.01 at the fracture stage, while BF3#-2000⊥ increased 0.03. It was possible that slight difference in YW3#-2000// and BF3#-2000⊥ samples were related with different minerals filled in the fracture. However, generally, scCO2-H2O had little influences on the complexity of pore of the 25 mm coal core samples.

existence of large pore-fractures in the samples, pores enlarged after scCO2-H2O reaction, making them insufficient for restraint free water, i.e., exceeding the measurable range of NMR. Therefore, the measured porosities contributed by the fractures decreased. It could be seen from above the increase in large pores and fractures of vertical to stratification samples is greater than that of the parallel to stratification samples.

3.2. MIP 3.1.3. NMR fractal characteristics According to the NMR T2 distribution and fractal theory (Eq. (2)), the fractal dimensions of the NMR were calculated. D1, D2, D3, and D4 represent the fractal dimensions of adsorption pore, transition pore seepage pore, and fracture, respectively. D is the fractal dimension for NMR. D1, D2, D3, D4, and D are demonstrated in Fig. 2 and listed in Table 4. Table 4 shows the ranges of D1, D2, D3, D4, and D from −0.03 to −0.12, from 2.70 to 2.78, from 2.94 to 2.96, from 2.94 to 2.99, and from 2.60 to 2.64, respectively. Generally, surface fractal dimension is between 2 and 3. D1 of micropore does not conform to the definition of surface fractal dimension. Therefore, D1 is actually meaningless for describing the inner pore characteristics. This is the same as previous studies [29,31,32]. Two relaxation mechanisms, T2B and T2D, have a great influence on coal. D2, D3, D4, and D meet the definition of fractal dimension. It could be seen from the calculation results that there was no obvious change in fractal dimensions of each stage and the total fractal dimension after reaction. There were slight decreases of about 0.01 for 2000 m samples at the transitional stage, YW3#-2000// decreased 0.02

The coal structure after scCO2-H2O reaction changed greatly. From the changes of incremental pore volumes of the MIP with the pore diameters (Fig. 4), the adsorption pores of the samples developed in a large amount due to the high degree of metamorphism; after the reaction, the pores of the coal samples showed bipolar distribution, the pore size distribution was with obvious regularity; moreover, the peak of coal pore volume distributed mainly within the range larger than 30 μm and smaller than 100 nm, while pore volume between 100 nm–30 μm was small. Besides, after the reactions at the 2000 m simulated burial depth condition, pores over 30 μm have the larger pore volume than that after the reactions at the 1000 m simulated condition; while for pores between 100 nm–30 μm, those pore volumes after the reactions at the 1000 m simulated burial depth condition had more amount than those after the reactions at the 2000 m condition. Therefore, it could be concluded that the deeper the simulated burial depth is, the more favorable it is for pore enlarging. It could be seen from Table 5, the volumes of the adsorption pores and fractures after reactions increased most. The higher the reaction

Table 4 Fractal dimension of D1, D2, D3, D4, and D before and after the reaction. Sample

D1-Before

D1-After

D2-Before

D2-After

D3-Before

D3-After

D4-Before

D4-After

DBefore

DAfter

BF3#-1000⊥ BF3#-2000⊥ XJ3#-1000⊥ XJ3#-2000⊥ YW3#-1000// YW3#-2000// XJ15#-1000// XJ15#-2000//

−0.08 −0.03 −0.06 −0.06 −0.20 −0.06 −0.12 −0.05

−0.09 −0.09 −0.06 −0.07 −0.06 −0.06 −0.10 −0.10

2.71 2.72 2.77 2.78 2.77 2.74 2.70 2.72

2.71 2.71 2.77 2.77 2.77 2.74 2.70 2.70

2.94 2.97 2.94 2.94 2.94 2.96 2.94 2.96

2.94 2.96 2.94 2.94 2.94 2.94 2.94 2.96

2.98 2.94 2.97 2.96 2.99 2.99 2.98 2.98

2.99 2.97 2.97 2.97 2.98 2.98 2.98 2.98

2.61 2.63 2.63 2.62 2.63 2.64 2.60 2.62

2.61 2.62 2.63 2.62 2.63 2.63 2.60 2.61

374

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Fig. 4. Change of incremental pore volumes from the MIP of the coal samples before and after the reaction.

and the porosity increased from 1.49% to 6.76%.

temperature and pressure are, the more the pore volume of fractures increases; while there are no such rules for other pores. The pore volumes of transitional pores and seepage pores of carbonate-rich samples after the reaction (YW3# and XJ3#) are even lower than that of the original samples. The total pore volume of carbonate-rich samples does not increase with the increase of the reaction temperature and pressure. However, it could be seen from Fig. 4 that the stages of larger pore diameters, the pore volume increases with the increase of the temperature and the pressure. Therefore, it could be speculated that it generated larger fractures, exceeding the range of MIP or causing the samples to shatter.

3.3.1. Change characteristics of minerals Mineral data of XJ15#-2000 sample were extracted and it could be found that the minerals in the coal before reaction were mainly particles < 10 μm, covering 94% of the total minerals. With the increase of the radius, the number of mineral particles decreased sharply. After the geochemical reaction, minerals in the coal decreased significantly (Table 7). It could be seen that the quantity of the total minerals reduced by 4.08%, the provided superficial area decreased by 35.95% and the volume decreased by 63.8%, among which mineral quantities with the radius of 2–5 μm and 25–50 μm increased instead and the surface area and volume it provided also increased, but particles with other dimensions decreased to varying degree, among which particles with the radius of 20–25 μm decreased the most. Shape factor, the ratio of the minimum radius to the maximum radius of mineral particle, was selected to compare the changes in the shape of all mineral particles before and after the reaction. It could be seen from Fig. 6, before the reaction, mineral particles with 0.2–0.3 shape factor are the most. After the reaction, mineral particles with polygon shapes which were close to circular (approach 1) had a large reduction, but minerals within 0.2–0.3 increased instead. It can be seen that the dissolution of minerals leads to the complicated shapes of mineral particles.

3.3. X-ray CT Minerals, pores, coal matrix and other components of the coal differ greatly in density and attenuation coefficient, which could result in differences in CT number distribution. Generally speaking, CT number of minerals is about 3000HU, CT number of coal matrix is 1000–1600HU and CT of the pores is less than 600HU, therefore, quantitative distribution could be conducted according to the value of CT number. Reconstruction of the sample is as shown in Fig. 5, in which black is organic matters, green is mineral, and purple is pores and fractures. From Fig. 5 and Table 6, it could be seen that the mineral volume after reaction decreased by about 0.075 mm3, while the pore volume increased by about 0.104 mm3; the maximum radius of the pores and fractures increased from 124.71 μm to 234.55 μm, while the maximum radius of the mineral particles decreased from 176.18 μm to 93.13 μm;

3.3.2. Change characteristics of pores and fractures Pore and fracture data of the samples were extracted and it could be found that pores and fractures in the coal before reaction were

Table 5 Pore volume variation after scCO2-H2O reaction (mg/L). Type

BF3#-1000 m

BF3#-2000 m

XJ3#-1000 m

XJ3#-2000 m

YW3#-1000 m

YW3#-2000 m

XJ15#-1000 m

XJ15#-2000 m

Adsorption pore (mg/L) Transition pore (mg/L) Seepage pore (mg/L) Fracture (mg/L) Total

0.0016 0.0009 0.0017 0.0086 0.0130

0.0012 0.0013 0.0016 0.0134 0.0176

0.0010 0.0006 0.0018 0.0048 0.0082

0.0010 0.0006 0.0017 0.0102 0.0136

0.0016 −0.0014 0.0000 0.0197 0.0199

0.0012 −0.0021 −0.0001 0.0192 0.0182

−0.0001 0.0017 0.0022 0.0068 0.0106

0.0004 −0.0003 −0.0001 0.0070 0.0070

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Fig. 5. 3D visualization reconstruction of coal seam.

mainly < 10 μm, covering about 99% of the total pores and fractures. With the increase of the radius, the numbers of the pores and fractures decreased sharply. After the geochemical reaction, pores and fractures in the coal increased significantly (Table 8). The total number of the pores and fractures increased to 2.47 times of the original, the provided area increased to 4.67 times and the volume increased to 4.89 times, among which pores and fractures with the dimension of 10–20 μm increased the most. Shape factor, the ratio of the minimum radius to the maximum radius of pores and or fractures, was selected to compare the changes in the shape of all pores and or fractures before and after the reaction. It could be seen from Fig. 7, before the reaction, pores and fractures with 0.2–0.3 shape factor are the most, and they are mostly in strips. After the reaction, pores and fractures within 0.2–0.3 increased the most. It can be seen that with the dissolution of minerals, all kinds of pores are improved, but the strip one is the main type.

φ k = ( )3 k0 φ0

(5)

where k is the post-reaction permeability; k0 is the pre-reaction permeability; φ is the post-reaction porosity; φ0 is the prereaction porosity. φ Fig. 8 shows the relationship between k and ( φ )3 of the samples. It k0 0 can see a good linear correlation can be found except for BF3#-1000⊥ and XJ3#-2000⊥.Therefore, the measured porosity by NMR and the measured permeability by permeameter are consistent with the change mechanism of sample permeability. As shown in Fig. 3 and Table 3, the fracture peak of parallel to stratification samples increased obviously, but that of the vertical to stratification samples had less increase and some even decreased. Meanwhile, the value on X-axis corresponded to zero point of the third peak of the vertical to stratification samples became smaller, but the value of the parallel to stratification sample did not change. It may be due to the existence of large pore-fractures in the samples, which became larger after scCO2-H2O reaction, so that it was insufficient to restraint free water, i.e., exceeding the measured scope of NMR, which resulted in decrease of the measured pores and fractures [31,32]. However, the T2 value of the crack peak of the BF3#-1000⊥ and XJ3#2000⊥samples attenuate most, resulting in the maximum porosity deviation (less than the actual value) after the reaction. That is also the reason for the deviation of the two differential points from the linearity in Fig. 8. Thus, the increase of the pore volume of vertical stratification sample was larger than that of the parallel stratification sample. Coal is an anisotropic medium, so, with the injection of scCO2,

4. Discussion 4.1. Influences of scCO2-H2O on the connectivity of pores and fractures 4.1.1. Overall permeability changes In order to verify whether the measured values of porosity and permeability follow the permeability porosity relationship as described in Eq. (5) [35].

Table 6 Scanned coal minerals and pore parameters. Sample

Porosity (%)

Pore volume (mm3)

Matrix volume (mm3)

Mineral volume (mm3)

Pore radius (μm)

Mineral radius (μm)

Before After Difference

1.49 6.76 5.28

0.027 0.130 0.104

1.650 1.757 0.107

0.117 0.042 −0.075

2.57–124.71 2.57–234.55

2.51–176.18 2.57–93.13

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Table 7 Parameter changes of mineral particles. Superficial area (mm2)

Volume (mm3)

Radius

Count

(μm)

Before

After

After/Before (%)

Before

After

After/Before (%)

Before

After

After/Before (%)

2–5 5–10 10–15 15–20 20–25 25–50 50–100 > 100 Total

14,118 6630 914 216 75 52 9 5 22,019

14,574 5471 767 201 44 55 9 0 21,121

103.23 82.52 83.92 93.06 58.67 105.77 100.00 0 95.92

1.60 4.15 2.36 1.45 1.08 2.20 2.55 10.09 25.49

1.72 3.53 2.05 1.39 0.65 4.28 2.70 0 16.32

107.27 85.01 86.74 96.07 60.26 194.49 106.09 0 64.05

0.002 0.009 0.007 0.004 0.004 0.008 0.013 0.070 0.117

0.003 0.008 0.006 0.004 0.002 0.008 0.012 0 0.042

109.41 84.16 84.60 93.76 56.15 100.45 93.72 0 36.20

Fig. 7. Changes of shape factors of pores and fractures Note: The shape factor is the ratio of the minimum radius to the maximum radius of pore or fracture. Count is the count of pores and fractures.

Fig. 6. Changes of shape factors of mineral particles Note: The shape factor is the ratio of the minimum radius to the maximum radius of mineral particle. Count is the count of mineral particles.

expansion of coal was directional. Through studies in the literature that the expansion represented on vertical stratification was indeed larger than that generated in parallel stratification [35–37]. Permeability results showed that the permeability of the samples after scCO2-H2O reaction increased (Table 2), and the deeper the experimental condition was, the higher the increasing ratio of the permeability was. However, the increasing ratios of the permeability of parallel to stratification samples were far higher than those of the vertical to stratification samples, among which the permeability of YW3#-2000// after reaction was 115.1 times of that before the reaction. It could be seen that though the expansion degree of the parallel to stratification is smaller than that of the vertical to stratification, its connectivity is higher than cleat, the water-rock interaction becomes more significant and the pores and fractures changed more. Fig. 8. Relationship between porosity and permeability.

Table 8 Parameter changes of pores and fractures. Superficial area (mm2)

Volume (mm3)

Radius

Count

(μm)

Before

After

After/Before (%)

Before

After

After/Before (%)

Before

After

After/Before (%)

2–5 5–10 10–15 15–20 20–25 25–50 > 100 Total

57,289 6979 161 23 10 7 0 64,469

131,826 25,583 1652 223 48 29 1 159,362

2.30 3.67 10.26 9.70 4.80 4.14 / 2.47

5.78 3.73 0.52 0.23 0.19 3.04 0.00 13.48

14.20 17.53 5.76 2.32 1.03 2.45 19.71 63.00

2.46 4.70 11.10 10.25 5.34 0.80 / 4.67

0.009 0.007 0.001 0.000 0.000 0.009 0.000 0.027

0.022 0.032 0.011 0.005 0.002 0.005 0.054 0.131

2.50 4.62 10.08 9.38 4.50 0.57 / 4.89

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VClo = VEje,

4.1.2. Effectively interconnected pores and non-effectively interconnected pores The pore morphology of coal is complex, the width of the pores and the connected pore throats are variable, and some of the pores have smaller entry pore throats. After the mercury is injected into the pores, the mercury cannot be completely exited due to the small pore throat and the contact angle between the mercury and the coal surface. As a result, part of the mercury stays in the pores, forming the intrusion and extrusion mercury hysteresis loop [12]. Therefore, the hysteresis loop can characterize the connectivity characteristics of the pore fracture network. It was believed that the narrower the hysteresis loop of mercury intrusion and extrusion is, the better the connectivity of the pores is. That is, the closer the amount of mercury extrusion to the amount of mercury intrusion is, the higher the proportion of effectively connected pores and cracks in coal is [38]. Therefore, the pore in coal measured by mercury injection experiment can be divided into two categories: noneffectively connected pore and effectively connected pore. The noneffectively connected pores include the semi-closed pores (ink-bottle pores), which throats are smaller than diameter the aperture of the howler, and the open pores with the small throats. According to the intrusion and extrusion mercury curve, the change of effectively and non-effectively connected pores amount after the action of scCO2 can be analyzed qualitatively [22]. It can be seen from the mercury curve morphology of the tested coal samples that the coal samples showed certain mercury extrusion hysteresis before and after the action of scCO2-H2O system (Fig. 9), which indicated that there were both effectively connected pores and a certain amount of non-effectively connected pores, that can be calculated quantitatively by Eqs. (5) to (7) (Fig. 10).

VInter = VEje, = VInj,

sta des

− VEje, − VEje,

VTotal= VInter + VClo = VInj, des = VEje, sta

(7)

where VInter is the volume of the effectively interconnected pores, cm3·g−1; VEje,sta is the pore volume corresponding to the start of the mercury ejection, cm3·g−1; VEje,des is the pore volume corresponding to the end of the mercury ejection, cm3·g−1; VInj,des is the pore volume corresponding to the end of the mercury intrusion, cm3·g−1; VTotal is the total pore volume from the MIP, cm3·g−1; and VClo is the volume of the non-effectively interconnected pores, cm3·g−1. The interconnected pores and fractures of the tested coal samples after the reactions increased to some degree compared with the original samples. Effectively interconnected pores increased by at least 0.3%, and as high as 20%. Among them, the effectively pores of BF3#-1000 m increased by 0.0033 ml/g, and that of BF3#-2000 m increased by 0.0037 ml/g, which had the largest increase of effectively interconnected pores. Moreover, compared with interconnected pores, the content of non-effectively interconnected pores changed obviously, the increasing ratio was 92% more than that before the reaction, and maximally more than 102%. Non-effectively pores of YW3#-1000 m coal sample increased by 0.0183 ml/g, while that of BF3#-2000 m increased by 0.0139 ml/g which had the largest increasing amount of non-effectively interconnected pores. The effectively interconnected pores of the original samples averaged at 0.0193 ml/g (accounting for 79%), the volume of non-effectively interconnected pores averaged at 0.0052 ml/g (accounting for 21%), and the effectively interconnected pores covered 79%, from which it could be seen that the effectively interconnected pores dominated. The effectively pores and fractures of the coal seam after reaction averaged at 0.0209 ml/g (accounting for 55%), the volume of non-effectively interconnected pores and fractures averaged at 0.0171 ml/g (accounting for 45%). The amount of both was basically the same and the amount of non-effectively interconnected pores and fractures was slightly higher than that of effectively interconnected pores and fractures. It could be seen thus scCO2 reaction could make the proportion of non-effectively pores and fractures in coal samples increase. In conclusion, the effect of scCO2-H2O on effectively connected pores is relatively week, while a large number of non-effectively connected pores are generated. This not only confirms the important effect of scCO2 on pore volume increase, but also indicates that scCO2

des des

des

(5)

(6)

Then,

Fig. 9. Cumulative pore volumes of coal samples with and without scCO2-H2O treatment. 378

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Fig. 10. Effectively interconnection pore volume distribution (left) and non-effectively connected pore volume distribution (right).

improves the local connectivity of coal samples.

Table 9 Parameter changes of pore throat before and after the reaction.

4.1.3. 3D pore interconnection model Pore throat is the interconnection channel of the pores and fractures, which is important for fracture connectivity [39]. Based on maximum sphere algorithm with pore network model, data of coal structure could be extracted easily (including equivalent diameter, pore surface area, pore volume, coordination number, shape factor, etc.) and the distribution and connection of pore throat could be observed. 3D distribution of coal pore throat and throat volume distribution are shown in Fig. 11. Before the reaction, the connectivity of the samples was poor, most of the pores were isolated and both the pores and throats developed unevenly; while after the reaction, the connectivity of the sample pores was good, the pores were interconnected with one another through the throat and developed evenly. According to statistics, the number of throats increased to 5.91 times of the original, and pore throats with radius of 1.0–1.5 μm are dominant (Table 9). Fig. 12 shows that before the reaction, the coordination numbers focused on about 1 to 3, are about 88% of the total. The maximum

Sample

EqRadius (μm)

Count

Length (μm)

Area (μm2)

Volume (μm3)

Before

<1 1.0–1.5 1.5–2.0 2.0–2.5 Total

69 65 34 6 174

2101.15 2862.32 877.66 157.19 5998.31

207.24 276.21 318.51 98.85 900.82

2101.15 2862.32 877.66 157.19 5998.31

After

1.0–1.5 1.5–2.0 2.0–2.5 2.5–3.0 >4 Total

806 140 52 18 13 1029

311308.69 58595.39 20690.82 5823.42 4218.90 400637.21

3285.61 1374.22 792.84 395.69 1954.46 7802.83

1265554.82 573657.38 312487.00 127613.48 547359.31 2826671.99

coordination number is 17, suggesting the pore interconnecting path or the gas flow path was not developed, which limits pore connectivity. After the reaction, the coordination numbers with the maximum increase are still 1, 3 (the increasing times are respectively 6.74 and

Fig. 11. Changes of pore throat and interconnected pores. 379

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water-rock reaction contact surface is larger and the geochemical reaction is more intense, which results in the increase of interconnected pores and fractures. 4.2.2. Dissolution of carbonate minerals Previous studies have shown that scCO2-H2O dissolves calcite significantly. The calcite on the surface is dissolved completely and calcite inside is exposed accompanied by the appearance of corroded crystal cone and new crystal surface; there are plenty of diamond-shaped corroded pits on the surface of dolomite; while changes of other minerals have fewer influences on the pores and fractures [23]. It could be seen that after the reaction, changes of pores and fractures are influenced mainly by the dissolution of carbonate minerals. CT results (Tables 6–8; Figs. 5–7) also verified that the number and volume of the minerals after the reaction decreased, quantity of small particle minerals increased and numbers of large particle minerals decreased; correspondingly, the numbers and volumes of the pores increased and pores with large diameters also increased. Because minerals are easily dissolved towards the mineral center along the differential deformation joint, the mineral shape tends to be a long strip from round (shape factor 0.2–0.3); the number of strip pores increases correspondingly (shape factor 0.2–0.3). Besides, it could be seen the quality of NMR samples decreased (Table 2), which might be caused by mineral dissolution, or by peeling off of fine slack coal. Carbonate minerals contained in the coal fill mainly in plant crystal cells or in fractures [23,43], while after the carbonate filled in crystal cells is dissolved, isolated pores of different sizes or pore groups distributed in blocks would be formed in the coal, resulting in honeycomb in the coal, which is poorly or partially connected [22,23]. Therefore, the mercury intrusion data showed that the increased amount of effectively interconnected pores and fractures was less than that of noneffectively interconnected pores and fractures (Fig. 9). CT results also showed that after the reaction, the pore volume increased from 0.027 mm3 to 0.131 mm3, and the interconnected pores increased from 0.008 mm3 to 0.054 mm3, i.e., the increase of interconnected pores was less than that of non-interconnected pores; however, its increasing ratios were higher and its permeability was improved obviously. Permeability of YW3# sample increased the most (Table 2). The permeability after 1000 m simulated experiment was 32.45 times of that before the reaction; permeability of 2000 m simulated experiment even reached 115.10 times. It was speculated that calcite thin film might have been developed between the layers, which dissolved under scCO2-H2O reaction, resulting in substantial increase of its permeability.

Fig. 12. Changes of coordination number.

7.19). But the pores and fractures of larger coordination numbers increase in large amount, especially those whose coordination numbers are larger than 10. Therefore, gas flow paths increase and the connectivity improves. This is because plenty of pores and fractures of the samples are interconnected and larger pores are formed. It could be seen from Fig. 11 that the volume of the interconnected pores increased and the interconnected pores expanded in the vertical direction along the original fracture surface. Before reaction, the interconnected pore volume was 0.008 mm3 and after the reaction, the volume increased to 0.054 mm3. The pore tortuosity degree dropped from 71.83 to 15.23, which suggests the resistance of capillary tubes of the throat reduced and the gas flow path in the matrix was shortened, which was favorable for gas flow and production.

4.2. Main reasons for changes in pore interconnected characteristics 4.2.1. Coal expansion Gas adsorption in micropore of the coal could cause a decrease of coal surface free energy, which could cause expansion of coal volume [40]. When the pressure exerted on the coal is withdrawn, the coal would restore to the original shape [41]. However, with the existence of stratum water, changes of coal structure by CO2 is not completely reversible. According to Liu et al [12], in addition to coal swelling caused by gas adsorption, dissolution of layered distributed or fracture filled carbonate could make the coal undertake different pressures. Under the function of the pressure, the enlargement of the original fractures and the addition of new fractures could also cause expansion of the coal. Moreover, it has been verified by experiment that broadening of the original fractures occurs in the late stage of the experiment (about 120 h later), while in the initial stage of the reaction, larger fractures in the coal compact under the pressure [41]. The reaction in this experiment lasted for 240 h. The reaction of CO2 on coal was much longer and the fracture widths increased more. Connectivity of NMR seepage pores and fractures becomes better (Fig. 2) and the permeability increased greatly (Table 2), all of which showed the occurrence of irreversible coal expansion; however, CT image (Fig. 1) verified that new fractures generated while the original fractures enlarged after the reaction. However, adsorption induced swelling of coal is anisotropic and the expansion in vertical to stratification direction is higher than that in the parallel direction [35–37,41–42]. It is showed by NMR T2 mapping that there is intense attenuation at the end of fracture peak in vertical to stratification direction, and the T2 ending value is smaller (Fig. 3), which is possible because the fracture opening could not hold the free water due to a large expansion in vertical stratification direction. The permeability results show that the improvement of permeability is higher than that in the vertical direction (Table 2). This is because the connectivity of the stratification is more powerful than the cleat, the

4.3. Reaction degree and pore volume change characteristics of each scale It is noteworthy that, from Tables 3 and 5 and Figs. 3 and 4, the NMR results showed that transformation of adsorption pore volume after the reaction was the greatest, while the MIP results showed that transformation of fracture pore volume was the greatest. Besides, the volumes of adsorption pores, transitional pores, seepage pores and fractures in the NMR results basically increased with the increase of the simulated experimental temperature and pressure conditions, but the MIP results showed that volumes of adsorption pores, transitional pores, seepage pores had poor relations with the changes of the simulated temperature and pressure, even on the whole, the volume of stage pores < 100 nm under 1000 m reaction condition increased more than that under 2000 m reaction. This was actually caused by different dimensions of the samples. The MIP samples were of smaller size, which was about 4–8 mm, in which water-rock reaction was more sufficient; while NMR samples were coal cores with the diameter of 25 mm, so the samples were large in size and compact in texture, therefore, their initial permeability was poor. Therefore, the NMR samples can be viewed as incomplete samples, but the MIP samples are relatively complete, or the NMR samples could be regarded as the MIP samples at an early stage of the reaction. Similar samples after 2000 m simulated reaction 380

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To epitomize, scCO2-H2O plays a positive role in reforming the pore and fracture system and can improve the permeability of the coal reservoir in a short time, which is also one of the main reasons for the gas production increase in the engineering test. The influence of long-term scCO2-H2O on reservoir pore and fracture system can be further explored, because carbonate mineral precipitation may occur in the later stage. It will be helpful to understand the stability of long-term methane production and the ability of reservoir to store CO2.

had faster reaction rate due to higher temperature and pressure, so the complete degree of reaction was higher than 1000 m samples. The higher reaction temperature and pressure and smaller reaction particles could accelerate the rate of reaction. It could be seen from the differences that abundant small-sized pores could be corroded on the surface of carbonate minerals first and with the progress of the reaction as well as the strengthening of the pore enlargement, small-sized pores connect gradually to form large-sized pores, while small-sized pores are formed gradually within. Therefore, the more complete the carbonating reaction is, the greater the increase amount of large-sized pore volumes is, while the increasing of smallsized pores becomes slower and slower (Fig. 4). With mass corrosion of carbonate samples, the skeleton density decreases and the sample plasticity increases, so the coal seam undertakes different pressures. New fractures are easily formed under pressure difference. Since the mercury samples are smaller, with the increase of the pressure, the samples are easily crack. This is also why the pore volume of carbonaterich samples (YW3# and XJ15#) at fracture stage under 2000 m reaction conditions is smaller than that under 1000 m reaction condition (Table 4).

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5. Conclusions Using high metamorphic coal in the south of Qinshui Basin as the object, this study simulated geochemical reaction between scCO2-H2O system and coal rock under 1000 m (45 °C, 10 MPa) and 2000 m (80 °C, 20 MPa) conditions. Moreover, with NMR, MIP, X-ray CT and overburden pressure pore permeability experiments, the characteristics of scCO2-H2O transforming pore and fracture structures were analyzed to further study the characteristics of and reasons for permeability change of the samples. The following conclusions were drawn: (1) ScCO2-H2O reaction with coal could add/expand pores and fractures, which could cause the increase of total pore volume, connected pore volume, pore throat quantity. The radius of pore throat was mainly < 1.5 m, with larger coordination number. As a result, the permeability of the sample was greatly improved, and it increased with the increase of the simulated temperature and pressure. The maximum amplifications of 1000 m and 2000 m were 35.68 times and 114.10 times respectively. Since the connectivity of the stratification is stronger than the cleat, the contact area of scCO2-H2O-coal is larger and the increase of porosities and permeabilities of parallel stratification samples are larger than that of the vertical stratification samples. (2) Under the expansion of coal, micro-fracture grows/expands and the connectivity of the seepage pore and fracture become better. Under water-rock interaction, the dissolution of minerals leads to an increase in the number, specific surface area and volume of pores, with a shape factor of 0.2–0.3. The effectively connective pores and fractures are easily developing along the original micro-fracture direction; however, the increase of effectively interconnected pores and fractures content is far less than non-effectively interconnected pores and fractures, which is in positive correlation with the simulated temperature and pressure. (3) Reaction extent of the samples differs and the changing characteristics of pore volumes at different stages are different. The larger the sample size is, the more intensive the transformation of scCO2-H2O on the volumes of the adsorption pore and the transitional pore is; moreover, with the rise of the simulated temperature and pressure, the greater the pore volume of each stage increases. The smaller the sample size is, the higher the complete degree of reaction is; and the more powerful the transformation of pore volume at the fracture stage is. It could also be seen that with the rise of the simulated temperature and pressure, the higher the increase of the pore volume is at the fracture stage (especially > 30 μm), the slower the increase of the adsorption pore and the transitional pore is. 381

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