Implications for coal seam CO2 sequestration

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Analysis of effects of CO2 injection on coalbed permeability: Implications for coal seam CO2 sequestration Erlei Su, Yunpei Liang, Quanle Zou, Fanfan Niu, and Lei Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01190 • Publication Date (Web): 31 May 2019 Downloaded from http://pubs.acs.org on June 1, 2019

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Analysis of effects of CO2 injection on coalbed permeability: Implications for coal seam CO2 sequestration Erlei Su 1, Yunpei Liang 1,*, Quanle Zou 1,*, Fanfan Niu 2, and Lei Li 1,3,4 1

State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China; Zhengzhou Engineering Co., Ltd. of China Railway Seventh Group, Zhengzhou 450052, China; 3 State Key Laboratory of Gas Disaster Monitoring and Emergency Technology, Chongqing 400037, China 4 China Coal Technology and Engineering Group Chongqing Research Institute, Chongqing 400037, China 2

ABSTRACT A proper understanding of permeability reductions of CH4-containing coal seams after CO2 injection is essential, as coal permeability is the key parameter influencing the efficiency of enhanced coalbed methane recovery with CO2 sequestration and theoretical research on it is lacking. The main objective of this study was to accurately quantify the effects of CO2 injection on coalbed permeability. Therefore, permeability decrease coefficients and permeability rebound and recovery pressures of a binary gas (CH4 + CO2) are proposed based on the Shi–Durucan and extended Langmuir models. Then, the trends of these parameters under the influence of the main influencing factors are detailed. Specifically, the permeability decrease coefficient increased with an increase in CO2 proportion and increased rapidly when the reservoir gas pressure was low. Permeability recovery pressure decreased with an increase in CO2 proportion; the range of decrease was larger at low CO2 proportions. CO2 proportion had little effect on the permeability rebound pressure. Besides, the larger the Langmuir volume constant of CO2, the larger was the permeability decrease coefficient and permeability rebound pressure, and the smaller the permeability recovery pressure. However, the effect of the Langmuir pressure constant on these parameters was relatively weak. Finally, in light of these results, the implications of different characteristics of permeability evolution for CO2 injection pressure adjustment in the process of enhanced coalbed methane recovery with CO2 sequestration are discussed from a macroscopic perspective. The results of this study may provide a reference to select appropriate coal seams and injection pressures for CO2 sequestration. 1. INTRODUCTION The progress of human society and the development of the industrial economy has led to substantial increases in energy consumption.1-3 It is predicted that global energy consumption will double from its current value by 2050, and that fossil fuels are likely to continue to be a major source of energy for decades to come.4-6 The increasing concentration of CO2 in the atmosphere, due to burning of fossil fuels, is believed to be the main cause of global warming.7-9 To overcome this challenge, geological sequestration, as a critical component of Carbon Capture and Storage, is considered to be the most ACS Paragon Plus Environment

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effective way to prevent CO2 emissions into the atmosphere.10-12 Storage options for geological sequestration include exhausted petroleum and gas reservoirs, deep brine aquifers, and unmineable coal seams.13, 14 Considering the latter, storage in coal seams can not only provide CO2 storage, but also enhance coalbed methane recovery. This process is known as enhanced coalbed methane recovery with CO2 sequestration (CO2–ECBM) and has become a topic attracting considerable research.15-18 Some countries have implemented CO2 coal seam sequestration projects, including the United States, Canada, Poland, Australia, Japan, and China;19, 20 however, none have so far successfully achieved their predetermined CO2 injection capacity. For example, at the Allison Unit in the United States, the rate of CO2 injection was reduced by 40% in the initial injection stages.21 The Ishikari Basin in Japan experienced a 70% reduction in its injection capability during the first year of operation.22 Loss of injection capacity was also observed in a micro-pilot field test in Qinshui Basin, China.23 These problems were caused by loss of permeability of the coal seams after CO2 injection.24-26 As the permeability of a coal seam decreases, the migration of CO2 is restricted, leading to a reduction of the amount that can be injected and so the predetermined CO2 storage capacity cannot be achieved. At the same time, because limited CO2 reduces the displacement of CH4, coalbed methane (CBM) recovery is seriously affected. The permeability of a coal seam is the key parameter affecting its CO2 storage capacity and CBM recovery;27-29 therefore, a proper understanding of permeability changes of CH4-containing coal seams after CO2 injection is essential to improving CO2–ECBM technology. The evolution of coal seam permeability is an active research area and numerous studies have been presented. Based on the assumption of uniaxial strain, Gray30 first considered matrix shrinkage in coal permeability changes. Palmer and Mansoori31 established the widely used PM model, a permeability model that takes into account the effects of matrix shrinkage and effective stress. Assuming that the change in cleat permeability was dominated by the effective horizontal stress, Shi and Durucan32 developed the SD model for pore pressure-dependent permeability. Cui and Bustin33 deduced a stressdependent permeability model (CB model) by quantifying the influence of volume strain caused by reservoir pressure and gas adsorption on the permeability of a coal seam. In summary, coal permeability is mainly affected by two factors: effective stress and gas adsorption/desorption-induced deformation. The above studies all focused on permeability evolution when the coal seam contained only CH4. During on-site implementation of CO2–ECBM, CO2 is injected into the coal seam through a surface well, at which time both CO2 and CH4 are contained in the coal seam. The injected CO2 will change the effective stress of the coal seam and generate competitive adsorption with CH4, so analysis of the permeability evolution must consider this binary gas during CO2–ECBM.

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It is believed that coal preferentially absorbs CO2 and increases adsorption-induced swelling, which leads to lower permeability. Ottiger et al.34 measured adsorption of a mixed gas of CO2 and CH4 by coal from the Sulcis Coal Province in Italy. The experimental results showed preferential adsorption of CO2 over CH4; the higher the concentration of CO2, the higher was the adsorption. Brochard et al.35 implemented molecular simulations of competitive adsorption, and found that swelling of the coal matrix linearly increased with the mole fraction of CO2. Lin et al.36 conducted laboratory experiments to study permeability changes during the ECBM process. They found that as the concentration of CO2 in the injection gas increased, the permeability of the coal decreased: use of pure CO2 led to the greatest permeability reduction. As seen from the above results, CO2 undoubtedly affects permeability evolution of coal, but no scholars have yet quantitatively analyzed the reduction of permeability by CO2 injection. Besides, many studies have shown that the permeability can rebound and recover during CBM production, which will also affect the efficiency of CBM recovery.31, 37, 38 If the ability of coal seam permeability to rebound and recover after CO2 injection is understood, its ability to store CO2 can be better evaluated. It is therefore meaningful to study the influence of CO2 on coal permeability reduction and subsequent rebound and recovery. In this study, the effect of CO2 injection on coalbed permeability was accurately quantified by proposing the concepts of permeability decrease coefficients, permeability rebound, and recovery pressure of a binary gas, based on the SD and extended Langmuir models. The main factors affecting these three parameters were determined to analyze the influences of different factors on permeability changes during CO2–ECBM. Based on these results, the selection of appropriate coal seams and injection pressures for CO2 sequestration, under different conditions of permeability evolution, is macroscopically discussed. 2. THEORY 2.1. Analysis of Permeability Changes During Enhanced Coalbed Methane Recovery with CO2 Sequestration Effective stress changes and adsorption/desorption-induced deformation caused by gas pressure changes play a competitive role in the evolution of coal permeability. In the process of CBM, as the coal reservoir gas pressure decreases, the effective stress increases, causing the coal skeleton to compress and leading to a decrease of permeability. In contrast, gas desorption causes the coal matrix to shrink, which increases the permeability.39, 40 In the process of CO2–ECBM, CO2 is injected into the coal seam through an injection well, and can then occupy the original adsorption sites of CH4 because of its better adsorptive


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capacity on the coal seam; thereby increasing the production of CBM, as shown in Fig. 1a. However, the reservoir gas pressure increases due to CO2 injection, which leads to reduction of the effective stress. At the same time, the injected CO2 can cause competitive adsorption with CH4. The coal matrix will shrink with the desorption of CH4, and swell with adsorption of CO2. Because adsorption of CO2 is stronger, CO2 adsorption-induced swelling is greater than CH4 desorption-induced shrinkage, and the matrix could eventually swell. It is interesting to note that the reservoir gas pressure decreases gradually with distance from the injection well after CO2 injection (Fig. 1b), so the reduction of effective stress correspondingly decreases with increase in distance. The proportion of CO2 also gradually decreases with distance from the injection well (Fig. 1c), and therefore CO2–CH4 adsorption-induced swelling decreases.41, 42 Owing to the variations of gas pressure and CO2 proportion with distance from the injection well, matrix deformation and effective stress effects differ remarkably at different positions, as do the permeability values.

Figure 1. Schematic of enhanced coalbed methane recovery with CO2 sequestration.

2.2. Dynamic Model for Coal Permeability after CO2 Injection

At present, the SD model (Eq. 1) is one of the most widely used permeability models for CBM. It was developed to describe permeability changes under the uniaxial strain conditions prevailing in coalbed reservoirs.32 It assumes that the total vertical stress remains unchanged during CBM production.

k  k0 e

 v  pb E pb 3C f    p  p0   max,CH 4  CH 4  0 CH 4 31 v   1 pbCH 4 1 p0bCH 4  1 v

   

,

(1)

where k is coalbed permeability, mD; k0 is coalbed permeability before CO2 injection, mD; Cf is cleat volume compressibility, MPa−1; v is Poisson's ratio of the coal seam, fraction; p is coal reservoir gas


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pressure, MPa; p0 is initial reservoir gas pressure, MPa; E is Young's modulus of the coal seam, MPa;

bCH

4

is the Langmuir pressure constant of CH4, MPa−1.  max,CH 4 is the Langmuir volumetric strain constant of CH4, and can be calculated using the formula  max,CH 4 = CH 4 aCH 4 . CH 4 is the shrinkage/swelling coefficient of CH4 and aCH 4 is the Langmuir volume constant of CH4, m3/t. In the CO2–ECBM process, the effect of CO2 injection on effective stress and adsorption/desorption-induced deformation should be considered. When discussing permeability changes during CO2–ECBM, k0 is defined in this article as the permeability of coal seams containing only CH4 and k is the absolute permeability for the binary gas (CH4 and CO2). Assuming that all in situ methane is fully displaced by injected CO2, the permeability model during the CO2–ECBM process is obtained, based on the extended Langmuir equation and Eq. 1, as shown in Eq. 2:43

k  k0 e

 v E   max,CH 4 pCH 4 bCH 4  max,CO2 pCO2 bCO2  max,CH 4 p0bCH 4  3C f     p  p0   31 v   1 pCH 4 bCH 4  pCO2 bCO2 1 p0bCH 4  1 v

   

,

(2)

where  max,CO2 is the Langmuir volumetric strain constant of CO2 and can be calculated using the formula

 max,CO = CO aCO . CO is the shrinkage/swelling coefficient of CO2 and aCO is the Langmuir volume 2

2

2

2

constant of CO2, m3/t.

bCH

2

4

is the Langmuir pressure constant of CO2, MPa−1. It is important to note that

p  pCO2  pCH 4 , where pCO2 and pCH 4 are the partial pressures of CO2 and CH4 (MPa), respectively.

From Dalton's law of partial pressure:44  pCH 4  p 1    ;   pCO2  p ,

(3)

where α is the CO2 proportion: 0    1 . Equation 3 can be substituted into Eq. 2 to obtain coal permeability after CO2 injection:

k  k0 e

 v E   max,CH 4 p 1 bCH 4  max,CO2 p bCO2  max,CH 4 pCH 4 ,0bCH 4  3C f     p  p0   31 v   1 p 1 bCH 4  p bCO2 1 pCH 4 ,0bCH 4  1 v

   

.

(4)

Based on this permeability model, Shi and Durucan successfully matched CO2–ECBM field data using METSIM2 software, including those of the Allison Unit, Alberta Fenn Big Valley, and Yubari pilot projects.43,

45, 46

However, Shi and Durucan did not analyze the effect of CO2 injection on

permeability reduction, rebound, and recovery, because these papers were not targeted to these phenomena.


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2.3. Permeability Decrease Coefficients and Rebound, and Recovery Pressures When a coal reservoir gas only contains CH4, its permeability is kCH . When the coal reservoir gas 4

contains (1 – α) proportion of CH4 and α proportion of CO2, its permeability is k CO2 . Assuming that other parameters remain unchanged, the reduction in permeability caused by α proportion of CO2 can be obtained: k  kCH 4  k CO2 .

(5)

To compare k under different conditions, the parameter η can be defined:



Vk . kCH 4

(6)

Equation 4 is substituted into Eq. 6 and simplified to obtain:

 e



EC f   max,CH 4 pbCH 4  max,CH 4 p 1 bCH 4  max,CO2 p bCO2   1 v  1 pbCH 4 1 p 1 bCH 4  p bCO2

   

1.

(7)

In this paper, this parameter is defined as the permeability decrease coefficient. It reflects the contribution of CO2 to permeability reduction, which obviously lies between 0 and 1: the larger the permeability decrease coefficient, the greater is the reduction in permeability caused by CO2. It can be observed from Eq. 7 that the permeability decrease coefficient is related to Young's modulus and Poisson's ratio of the coal seam, the Langmuir pressure constants of CH4 and CO2, Langmuir volumetric strain constants of CH4 and CO2, and the CO2 proportion. Equation 4 describes the relationship between permeability and gas pressure during CO2–ECBM. The right-hand side of the equation indicates that the permeability is influenced by two factors: effective stress and its competitor, adsorption-induced swelling. Generally, with a decrease of gas pressure, reservoir permeability will rebound and recover. Therefore, the permeability rebound and recovery pressures of binary gases reflect the ability for permeability improvement during CO2–ECBM. Firstly, to analyze the permeability rebound of a binary gas, a function of gas pressure is defined, based on Eq. 4: f  p   eg p ,

(8)

where g(p) is given by:

 v E   max,CH 4 p 1    bCH 4   max,CO2 p bCO2  max,CH 4 p0bCH 4 g  p   3C f      p  p0   3 1  v   1  p 1    bCH 4  p bCO2 1  p0bCH 4  1  v The first and second derivatives of g(p) are, respectively:


    

(9)

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  3C f  E   max,CH 4 1    bCH 4   max,CO2  bCO2 g  p  v  2 1 v  3  1  p 1    b   b  CH 4 CO2  '

g ''  p  









  1    b   b 

2 EC f  max,CH 4 1    bCH 4   max,CO2  bCO2

1  v  1  p  1    bCH

CH 4

  ;   

  bCO2

3

4

(10)

.

(11)

CO2

g"(p) is always greater than 0, so f(p) has a minimum value of prb, which is obtained by solving for g′(p) = 0:

prb 

E  max,CH 4 1    bCH 4   max,CO2  bCO2  1 3v . 1    bCH 4   bCO2





(12)

Equation 12 shows that the permeability rebound pressure of a binary gas is related to Young's modulus and Poisson's ratio of the coal seam, the Langmuir pressure constants of CH4 and CO2, Langmuir volumetric strain constants of CH4 and CO2, and the CO2 proportion. If f(p) = 1, this means that the permeability at this time is equal to the permeability before CO2 injection. If p  p0 , the corresponding reservoir gas pressure for permeability recovery can be defined as the recovery pressure prc, which can be obtained by solving Eq. 13:

 v E   max,CH 4 prc 1    bCH 4   max,CO2 prc bCO2  max,CH 4 p0bCH 4 0  3C f      prc  p0   3 1  v   1  prc 1    bCH 4  prc bCO2 1  p0bCH 4  1  v

   . (13)  

After simplification, Eq. 13 becomes:

v  prc  p0  

E   max,CH 4 prc 1    bCH 4   max,CO2 prc bCO2  max,CH 4 p0bCH 4   3  1  prc 1    bCH 4  prc bCO2 1  p0bCH 4

  =0 . 

(14)

The analytical solution of Eq. 14 is the recovery pressure of the binary gas. Unfortunately, there is no specific analytical solution because solution of the polynomial is complicated. It is, nevertheless, clear that the permeability recovery pressure of a binary gas is related to Young's modulus and Poisson's ratio of the coal seam, Langmuir pressure constants of CH4 and CO2, Langmuir volumetric strain constants of CH4 and CO2, the CO2 proportion, and the initial reservoir gas pressure. 3. RESULTS AND DISCUSSION 3.1. Factors Influencing Permeability Decrease, Rebound, and Recovery 3.1.1. CO2 proportion According to the theoretical analysis presented in Section 2.3, CO2 proportion has an impact on the


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permeability decrease, rebound, and recovery. To study the evolution of permeability with reservoir gas pressure under different CO2 proportions, the other parameters of coal were set as fixed values, as shown in Table 1. The permeability ratio was calculated as a function of reservoir gas pressure for different CO2 proportions, as shown in Fig. 2. According to these data, at the same reservoir gas pressure, the higher the proportion of CO2 in the mixed gas, the smaller is the permeability ratio. Specifically, when the proportions of CO2 were 20%, 40%, 60%, 80%, and 100%, the permeability ratios corresponding to a gas pressure of 6 MPa in the reservoir were 0.3269, 0.1344, 0.0652, 0.0357, and 0.0215, which caused reductions in permeability of 67.31%, 86.56%, 93.48%, 96.43%, and 97.85%, respectively. The maximum permeability decreased by nearly two orders of magnitude. Similar results were observed in field tests.26 Table 1 Basic parameters used for analyzing the effect of CO2 proportion. Parameter

Value

Reference

Initial reservoir gas pressure (MPa) Cleat volume compressibility (MPa-1) Young’s modulus (MPa) Poisson’s ratio Langmuir volume constant of CH4 (m3/t) Langmuir volume constant of CO2 (m3/t) Langmuir pressure constant of CH4 (MPa-1) Langmuir pressure constant of CO2 (MPa-1) Shrinkage/swelling coefficient of CH4

6 0.139 2900 0.35 28 44 1.785 0.972 3.8×10-4

Mavor et al.47 Shi et al.46 Shi et al.46 Shi et al.46 Yamaguchi et al.48 Yamaguchi et al.48 Yamaguchi et al.48 Yamaguchi et al.48 Shi et al.46

Shrinkage/swelling coefficient of CO2

3.8×10-4

Shi et al.46

Figure 2. Evolution of permeability with different CO2 proportions.

To quantitatively analyze the influence of CO2 proportion on permeability, the permeability decrease coefficient was calculated, as given in Fig. 3. The higher the CO2 proportion, the greater were the permeability decrease coefficients, indicating that CO2 has a strong impact on permeability. With a


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decrease of reservoir gas pressure, the permeability decrease coefficient first decreased slowly, and then rapidly: for example, when the CO2 proportions in the mixed gas were 20%, 40%, 60%, 80%, and 100% and the gas pressure in the reservoir decreased from 6 MPa to 1 MPa, the permeability decrease coefficients decreased by 0.2954, 0.2656, 0.1995, 0.1441, and 0.1036, respectively. Correspondingly, when the reservoir gas pressure decreased from 12 MPa to 6 MPa, the permeability decrease coefficients decreased by 0.0137, 0.0069, 0.0029, 0.0010, and 0.0002, respectively. This indicated that the permeability was less affected by CO2 when the reservoir gas pressure was relatively low or the CO2 proportion was relatively small.

Figure 3. Evolution of permeability decrease coefficients with different CO2 proportions.

With different CO2 proportions, as shown in Fig. 2, the permeability ratios all decreased at first and then increased as the reservoir gas pressure dropped, eventually exceeding 1; however, it can be seen from the permeability ratio curve in Fig. 2 that the rebound and recovery pressures differed for different CO2 proportions. The reservoir gas pressures corresponding to permeability rebound and recovery at different CO2 proportions were extracted from Fig. 2 and are presented in Fig. 4. The larger the CO2 proportion, the smaller was the permeability recovery pressure. Interestingly, the lower the CO2 proportion, the greater was the influence on permeability recovery pressure. More precisely, when the CO2 proportions increased at intervals of 20%, the recovery pressures decreased by 3.23 MPa, 0.58 MPa, 0.28 MPa, 0.17 MPa, and 0.11 MPa, respectively. However, as the CO2 proportion increased, the permeability rebound pressure first increased and then decreased. When the CO2 proportion increased from 0 to 100%, the rebound pressure only increased by 0.27 MPa, which indicated that the effect of CO2 proportion on the rebound pressure is not as great as that on the recovery pressure.


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Figure 4. Evolution of rebound and recovery pressures under different CO2 proportions.

3.1.2. Langmuir volume and pressure constant ratios

Langmuir volume and pressure constant ratios of CH4 and CO2 determine the adsorption/desorptioninduced deformation of a coal matrix under different pressures, which then affects permeability. To study the influence of different Langmuir volume (Langmuir volume constant of CH4 : Langmuir volume constant of CO2) and pressure (Langmuir pressure constant of CH4 : Langmuir pressure constant of CO2) constant ratios on permeability change, other parameters of the coal seam were fixed, as given in Table 2. The Langmuir volume constant of CH4, as the benchmark, was 28 m3/t; the benchmark Langmuir pressure constant of CO2 was 0.972 MPa−1. Permeability ratios under different Langmuir volume and pressure constant ratios varied with reservoir pressure, as shown in Figs. 5 and 6, respectively. At the same reservoir gas pressure, the larger the Langmuir volume constant of CO2, the smaller was the permeability ratio. Specifically, when the Langmuir volume constant of CO2 was 1, 1.5, 2, and 2.5 times that of CH4 (the reasons for these ratios are explained in Section 3.2), the permeability ratios corresponding to 6 MPa in the reservoir were 0.6558, 0.0870, 0.0115, and 0.0015, decreasing by 34.42%, 91.30%, 93.48%, 98.85%, and 99.85%, respectively. This is because the larger the Langmuir volume constant of CO2, the larger was the adsorption swelling after CO2 injection. Table 2 Basic parameters used for analyzing the effect of Langmuir volume constant ratio. Parameter

Value

Reference

Initial reservoir gas pressure (MPa) Cleat volume compressibility (MPa-1) Young’s modulus (MPa) Poisson’s ratio CO2 proportion factor

6 0.139 2900 0.35 0.6

Mavor et al.47 Shi et al.46 Shi et al.46 Shi et al.46 This study

Shrinkage/swelling coefficient of CH4

3.8×10-4

Shi et al.46

Shrinkage/swelling coefficient of CO2

3.8×10-4

Shi et al.46


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Figure 5. Evolution of permeability with different Langmuir volume constant ratios.

As the Langmuir pressure constant of CH4 increased, the permeability ratio decreased (Fig. 6). For example, when the Langmuir pressure constant of CH4 was 1, 1.5, 2, and 2.5 times that of CO2, the permeability ratios corresponding to a gas pressure of 6 MPa were 0.1427, 0.0863, 0.0575, and 0.0409. However, these changes were relatively small compared with those of the Langmuir volume constant, indicating the Langmuir pressure constant ratio has less effect on the permeability than the Langmuir volume constant ratio.

Figure 6. Evolution of permeability with different Langmuir pressure constant ratios.

Permeability decrease coefficients were similarly used to quantitatively analyze the effect of Langmuir volume and pressure constant ratios, as given in Figs. 7 and 8, respectively. According to Fig. 7, the closer the Langmuir volume or pressure constant ratio was to 1, the smaller was the permeability decrease coefficient. Specifically, when the Langmuir volume constants of CO2 were 1, 1.5, 2, and 2.5 times of that of CH4, the permeability decrease coefficients corresponding to a gas pressure of 6 MPa in the reservoir were 0.3443, 0.9130, 0.9884, and 0.9985, respectively. The effect of Langmuir pressure constant ratio on permeability decrease coefficients was relatively weak in contrast. When the Langmuir ACS Paragon Plus Environment

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volume constant of CH4 was 1, 1.5, 2, and 2.5 times of that of CO2, the permeability decrease coefficients corresponding to a gas pressure of 6 MPa in the reservoir were 0.8573, 0.9137, 0.9425, and 0.9591, respectively.

Figure 7. Evolution of permeability decrease coefficients with different Langmuir volume constant ratios.

Figure 8. Evolution of permeability decrease coefficients with different Langmuir pressure constant ratios.

The values of the reservoir gas pressures corresponding to permeability rebound and recovery with different Langmuir volume and pressure constant ratios, taken from Figs. 5 and 6, are presented in Figs. 9 and 10, respectively. With an increase of the Langmuir volume constant of CO2 or Langmuir pressure constant of CH4, the permeability rebound pressure increased, but the recovery pressure decreased. The data in Fig. 9 reveal that the permeability recovery pressures changed by 1.17 MPa, 0.32 MPa, and 0.15 MPa with the change of Langmuir volume constant ratio; however, with the change of Langmuir pressure constant ratio, the recovery pressure changed by 0.10 MPa, 0.10 MPa and 0.09 MPa. The effect of Langmuir pressure constant ratio on the permeability rebound pressure was similar. The above results confirmed that the Langmuir pressure constant ratio has less effect on permeability.


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Figure 9. Evolution of rebound and recovery pressures under different Langmuir volume constant ratios.

Figure 10. Evolution of rebound and recovery pressures under different Langmuir pressure constant ratios.

3.2. Implications for Coal Seam CO2 Sequestration Figure 11 summarizes the Langmuir volume and pressure constants of some typical coal seam CO2 sequestration projects.20 The Langmuir volume constants of different projects vary greatly, while those of the Langmuir pressure constant are relatively similar. For anthracite, the Langmuir volume constant of CO2 is 1.03–1.08 times that of CH4 and the Langmuir pressure constant of CH4 is 2.80–2.84 times that of CO2. For bituminous coal, the Langmuir volume constant of CO2 is 1.43–2.94 times that of CH4 and the Langmuir pressure constant of CH4 is 0.85–3.50 times that of CO2. For lignite, the Langmuir volume constant of CO2 is 9.66 times that of CH4. According to the analysis in Section 3.1.2, the Langmuir volume constant ratio has an obvious influence on permeability, while the effect of the Langmuir pressure constant ratio is relatively weak. Therefore, this section mainly discusses the influence of Langmuir volume constant on coal seam CO2 sequestration. When there is a small difference between the CO2 and CH4 Langmuir volume constants, the


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permeability decrease coefficient is relatively small, which is conducive to the migration of CO2 to reach the predetermined CO2 storage capacity. However, under these conditions, very little CO2 can displace each adsorbed CH4 molecule, which means that the CO2 storage capacity of coal seam is limited. With an increase of CO2 Langmuir volume constant, the CO2 needed to displace the same CH4 increases, indicating that more CO2 can be stored, but the reduced permeability caused by CO2 is more serious. For example, according to Section 3.1.2, when the Langmuir volume constant of CO2 was 2.5 times that of CH4, the permeability decreased by nearly three orders of magnitude, which will seriously affect the migration of CO2. Therefore, when selecting a site for CO2 storage, both the CO2 storage capacity of the coal seam and the reduction of permeability should be considered.

Figure 11. Summary of Langmuir volume and pressure constants in coal seam CO2 sequestration projects using the data presented in Pan et al.20. (The minimum and maximum values are calculated from the range of the Langmuir constants.)

Based on the above analysis and the data in Section 3.1.1, the evolution of coal permeability with reservoir gas pressure during coal seam CO2 sequestration for different Langmuir volume constants and initial permeabilities is shown in Fig. 12. For economical CO2–ECBM projects, previous studies showed that permeability must exceed 1 mD.49, 50 Therefore, there may be three situations for the permeability evolution of a coal seam: 1) when the pure CO2 permeability is higher than 1 mD at any reservoir gas pressure (Fig. 12a, Permeability curve 1); 2) when the pure CO2 permeability is less than 1 mD at almost all pressures (Fig. 12a, Permeability curve 2); and 3) when the pure CO2 permeability is partly lower than 1 mD and the permeability gradually increases with a decrease of CO2 proportion (Fig. 12b). ACS Paragon Plus Environment

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Figure 12. Evolution of permeability during coal seam CO2 sequestration.

If the pure CO2 permeability is higher than 1 mD at any reservoir gas pressure (Fig. 12a, Permeability curve 1), the coal seam is relatively suitable for CO2 sequestration projects because CO2 injection will not be severely affected by the falling permeability. If the pure CO2 permeability is less than 1 mD at almost all pressures (Fig. 12a, Permeability curve 2), then permeability drops significantly when CO2 is injected into such a coal seam and CO2 can only migrate around the injection well. In the Ishikari Basin, the Langmuir volume constant of CO2 is 1.57 times that of CH4 and the initial permeability of coal seam is only 1 mD.13 This may be why the Ishikari Basin witnessed a 70% reduction in its injection capability during the first year of operation. Under the third condition, if the permeability of pure CO2 is partly lower than 1 mD and the permeability gradually increases with a decrease of CO2 proportion (Fig. 12b), then, according to the analysis presented in Section 2.1, the reservoir gas pressure and CO2 proportion gradually decrease with distance from the injection well after CO2 injection. Therefore, when the permeability at the injection well is at point A in Fig. 12b, the evolution of permeability must follow the direction of the blue arrow as the distance from the injection well increases; however, because the permeability in a certain area around the injection well is lower than 1 mD, CO2 cannot migrate far away. If the injection pressure is increased, then, when a certain pressure value is reached, the permeability at the injection well will change to point C (1 mD). As the distance from the injection well increases, the permeability evolves along the direction of the blue arrow and exceeds 1 mD. Therefore, under the injection pressure corresponding to point C, the coal reservoir is suitable for CO2 sequestration. If the injection pressure is reduced, then when a certain pressure value is reached, the permeability at the injection well will change to point B (1 mD). Similarly, under an injection pressure corresponding to point B, the coal reservoir is suitable for CO2 sequestration. Comparing points B and C, point B is a more suitable injection pressure for CO2 coal seam storage. There are three reasons: the first is that when the reservoir gas pressure is


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relatively low, permeability decrease coefficients reduce rapidly, indicating that the permeability is less affected by CO2. As the distance from the injection well increases, the CO2 proportion decreases and the permeability recovery pressure increases; therefore, the permeability is more easily recovered under lowpressure injection. The second reason is that the injection pressure required at point B is lower, which reduces the costs of compressing, storing, and transporting a high-pressure gas.51, 52 The third reason is that injecting high-pressure CO2 could obviously impair the mechanical properties of the coal;53-55 therefore, injecting high-pressure CO2 may not be conducive to long-term safe storage of CO2 in the coal seam, owing to the potential larger strength reduction. According to the above analysis, the basic parameters of coal seams should be known before CO2 sequestration projects, to understand the specific trends in permeability evolution. These characteristics of permeability evolution may provide a reference to select appropriate coal seams and injection pressures. Only using these guidelines can massive amounts of CO2 be injected and stored safely in coal seams. 4. CONCLUSIONS In this paper, permeability decrease coefficients and permeability rebound and recovery pressures of a binary gas are proposed to accurately quantify the effects of CO2 injection on coalbed permeability. The influences of the main factors on these three parameters are the predominant focus in this study. In addition, the selection of appropriate coal seams and injection pressures for CO2 sequestration are discussed. Based on the aforementioned results, main conclusions are drawn as follows: 1) Permeability decrease coefficients and permeability rebound and recovery pressures of a binary gas (CH4 and CO2) are proposed based on the SD and extended Langmuir models. The permeability decrease coefficient reflects the contribution of CO2 to permeability reduction. The permeability rebound and recovery pressures of the binary gas reflect the ability of permeability improvement during CO2– ECBM. Therefore, these three parameters can be used to compare the effect CO2 on coalbed permeability under different conditions. 2) With the increase of CO2 proportion in the binary gas, the permeability decrease coefficient increased and the permeability recovery pressure decreased, but the permeability rebound pressure remained relatively unchanged. When the reservoir gas pressure is low, the permeability decrease coefficient will decrease rapidly, indicating that the effect of CO2 on permeability is weak under low pressure. Besides, with the increase of Langmuir volume constant of CO2, the permeability decrease coefficient increased rapidly, indicating that CO2 will seriously reduce permeability. At the same time, the recovery pressure decreased and the rebound pressure increased. However, the effect of Langmuir


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pressure constant on permeability decrease coefficients, permeability rebound, and recovery pressure was relatively weak. 3) If the pure CO2 permeability is higher than the economic threshold value of 1 mD at any reservoir gas pressure, the coal seam may be suitable for CO2 sequestration. If the pure CO2 permeability is partly lower than 1 mD, the authors, enlightened by the trends shown in variations of permeability decrease coefficients and recovery pressure, suggest reducing the injection pressure to improve permeability. The results of this study may provide a reference for selection of appropriate coal seams and injection pressures for CO2 sequestration. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Tel: +86 13594608029 (Y.L.). *E-mail: [email protected] Tel: +86 17783372719 (Q.Z.). ORCID Erlei Su: 0000-0002-3165-6382 Quanle Zou: 0000-0002-6395-0455 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank the editor and anonymous reviewers for their valuable advice, and also thank Kathryn Sole, PhD, for editing the English text of a draft of this manuscript. This work is financially supported by the National Science and Technology Major Project of China (Grant No. 2016ZX05043005), the National Natural Science Foundation of China (51674050), and the State Key Research Development Program of China (Grant No. 2016YFC0801404), which are gratefully acknowledged. REFERENCES (1) Huang, Z.; Sednek, C.; Urynowicz, M. A.; Guo, H.; Wang, Q.; Fallgren, P.; Jin, S.; Jin, Y.; Igwe, U.; Li, S., Low carbon renewable natural gas production from coalbeds and implications for carbon capture and storage. Nat. Commun. 2017, 8, (1), 568. (2) Davis, S. J.; Caldeira, K.; Matthews, H. D., Future CO2 emissions and climate change from existing energy infrastructure. Science 2010, 329, (5997), 1330-1333. (3) Haszeldine, R. S., Carbon Capture and Storage: How green can black be? Science 2009, 325, (5948), 1647-1652. (4) Ma, T.; Fan, Q.; Li, X.; Qiu, J.; Wu, T.; Sun, Z., Graphene-based materials for electrochemical CO2 reduction. J. CO2 Util. 2019, 30, 168-182. (5) McCollum, D.; Bauer, N.; Calvin, K.; Kitous, A.; Riahi, K., Fossil resource and energy security dynamics in conventional and carbon-constrained worlds. Climatic Change 2014, 123, (3), 413-426. (6) Zou, Q.; Lin, B., Fluid–Solid coupling characteristics of gas-bearing coal subjected to hydraulic slotting: An experimental investigation. Energ Fuel 2018, 32, (2), 1047-1060. (7) Su, E.; Liang, Y.; Li, L.; Zou, Q.; Niu, F., Laboratory study on changes in the pore structures and gas desorption properties of intact and tectonic coals after supercritical CO2 treatment: Implications for coalbed methane recovery. Energies


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Implications for coal seam CO2 sequestration

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