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Experimental study on selective adsorption/desorption of CO and CH behavior on shale under high pressure condition 2

4

Zihao Yang, Zhaoxia Dong, Liu Wang, Taiheng Yin, Xinyu Fan, Meiqin Lin, and Juan Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02068 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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Energy & Fuels

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Experimental study on selective adsorption/desorption of CO2 and CH4 behavior on shale under high pressure condition

5

Zihao Yang,* Zhaoxia Dong, Liu Wang, Taiheng Yin, Xinyu Fan, Meiqin Lin and

6

Juan Zhang

1 2 3

7

Institute of Enhanced Oil Recovery, China University of Petroleum (Beijing), Fuxue

8

Road 18, Changping, Beijing, 102249, China.

9

Abstract: The mechanisms of CO2 enhancing coal bed methane (ECBM) are

10

investigated by many works. The competitive adsorption of CO2 and methane on coal

11

is believed to be the dominant effect. However, whether CO2 injection can be used for

12

improving shale gas recovery is not well answered yet. The adsorption and desorption

13

behaviour of CO2, methane or their mixture on shale is seldom reported. To evaluate

14

the feasibility of the utilization of CO2 for enhancing shale gas recovery, adsorption

15

and desorption experiments of methane, carbon dioxide, and mixtures of the two

16

gases on shales from Qaidam basin have been operated by manometric method at

17

50 °C and pressures up to 5.0 MPa. The results show that for single component of

18

CO2 or methane, preferential adsorption of CO2 on shale is observed. Meanwhile, the

19

adsorption behavior of mixtures of the two gases manifests that CH4 is preferentially

20

adsorbed on shale instead of CO2, which means that during the adsorption process, the

21

existence of CO2 promoted the sorption of CH4. For the process of desorption, an

1

ACS Paragon Plus Environment

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opposite phenomenon is obtained, it is presented that with the pressure decreasing,

2

CO2 shows a higher adsorption ability on shale. During this period, the existence of

3

CO2 facilitates desorption of CH4, which may result in a higher recovery of shale gas.

4

Based on the competitive adsorption experimental results, the utilization of CO2 to

5

improve shale gas recovery is practicable theoretically.

6 7

KEYWORDS: Shale Gas; Carbon Dioxide; Competitive Adsorption; Desorption

8 9

1. Introduction

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As an unconventional natural gas reservoir, shale gas has achieved huge success

11

in the exploration and development in the United States in recent years, which is

12

greatly benefited from the advancements of horizontal drilling and hydrofracture

13

technologies.1 However, some issues caused by these technologies, such as excessive

14

consumption of water, leakage of shale gas, environmental pollution and treatment of

15

waste water, are emerging. Meanwhile, for shale reservoirs with strong water

16

sensitivity, the efficiency of such technologies will be reduced. Therefore,

17

environmental friendly alternative technologies for shale gas recovery attract people's

18

attention.

19

Under the double pressure of energy shortages and increasingly severe natural

20

disasters and social harm caused by the emission of greenhouse gases, the utilization

21

of CO2 for exploring oil and gas can bring both environmental and economic

22

benefit.2,3 Many works has been done to investigate the feasibility and mechanisms of 2


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CO2 enhancing coal bed methane (ECBM).4-9 The preferential adsorption of CO2 on

2

coal

3

displacing the coalbed methane by CO2 injection; on the other hand，the reduction of

4

the partial pressure of CH4 and the effect of repressurizing the coal reservoir by gas

5

can both accelerate desorption of methane from the coalbed.10-12

is

demonstrated

to

be

the

most

important

mechanism

of

6

As reported, shale gas has the similar occurrence mechanism to that of coal bed

7

methane,13 meanwhile researchers have already studied the adsorption behavior of

8

single component of CO2 and CH4 on shale. During the process of adsorption, all the

9

studies have come to the unanimous conclusion that, adsorption capacity of CO2 on

10

shale is significantly higher than CH4, which is to say that CO2 has the ability to

11

displace the adsorbed CH4 from the shale surface in porous media.14-22 This result is

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also considered to be one of the important theoretical premises for the application of

13

CO2 to enhance gas recovery (CO2-EGR). Consequently, the utilization of injecting

14

CO2 is hopefully an alternative technology of enhancing shale gas recovery instead of

15

horizontal drilling or hydrofracture technology. Moreover, the previous work have

16

also provided the details of sample preparation,22 pre-treatment and the method of

17

adsorption experiment, which has referential value for further study.

18

However, for CO2-EGR, the practical situation is that CO2 and CH4 are mixed in

19

the reservoirs. Thus the adsorption behavior of the mixture is of significant

20

importance. The result will provide assertive evidence for whether CO2-EGR is

21

practicable for shale gas. Unfortunately, most previous researches focused on the

22

pure component adsorption on shale, there is almost no literature reporting the result 3


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of sorption behavior of the mixture on shale. The basic data of this field is devoid. To

2

enrich the research in this field, study on the adsorption behavior of CH4 + CO2

3

system is of great importance. According to the literatures,23-26 the experimental

4

method of mixture gas sorption behaviors on coal is well established and can be used

5

for this study. Meanwhile, based on the experimental data, the adsorption rule of CH4

6

+ CO2 binary system on coal presents complex characteristic which is different with

7

single component adsorption. For the adsorption process on different coal, which

8

kind of gases was preferential adsorbed was not assured, and the adsorption

9

mechanism of binary system was not well investigated yet. Therefore, for the

10

adsorption and desorption process, only studying the single gas sorption is

11

insufficient, the research on the binary mixture system of CH4 and CO2 is imperative.

12

In this work, isothermal adsorption experiment was operated to investigate the

13

adsorption and desorption behavior of CO2, CH4 and their mixtures on shale. The law

14

of adsorption of these systems on shale was illuminated. According to the

15

competitive adsorption experimental results, the utilization of CO2 to improve shale

16

gas recovery is practicable theoretically.

17

2. Experimental section

18

2.1. Physical properties of shale

19

The shale used in this work was obtained from Qaidam basin. The pyrolysis

20

parameter was analyzed by Rock-Eval technology developed by France Petroleum

21

Research Institute. For conducting the pyrolysis analysis, the shale samples were

22

pulverized to 100-200 mesh powder. The details introductions of Rock-Eval can be 4


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27,28,29

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found in the published literatures

. The pyrolysis analysis process was briefly

2

described as follows. The shale sample was first heated at 300 oC for 3 minutes in an

3

inert atmosphere. Then, a ramped heating stage of 25 oC/min was conducted until the

4

temperature reached 650 oC and kept the temperature constant for 1 min. Followed,

5

the sample was transferred into an oxic chamber where it is heated to 850 ºC, burning

6

off all the remaining organic-matter. The shale sample pyrolysis analysis result was

7

shown in Table 1. The result shows that the total organic carbon (TOC) of the shale

8

sample is 4.86%.

9

Table 1. Results of Rock pyrolysis analysis

10 TOC（%）

Tmax（℃）

S1（mg/g）

S2（mg/g）

S3（mg/g）

S4（mg/g）

4.86

433

2.13

28.47

0.89

23.25

11 12

The mineral composition of the shale was analyzed by X-Ray Diffraction (XRD)

13

and the result is listed in Table 2. It can be seen that the content of

14

brittle minerals such as quartz and feldspar is up to 40.1%, which is a relative higher

15

level. This data suggests that the shale sample is easily to be fractured. Meanwhile,

16

the content of clay is over 55%. As reported,30 clay minerals are an important

17

adsorptive support for gases as well as organic matter, therefore the relative higher

18

content of clay bring benefits for shale gas adsorption.

19 20

Table 2. Analysis results of minerals in total shale rock 5


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Potash Composition

Quartz

Plagioclase

Calcite

Siderite

Clay

5.6

1.0

3.0

55.9

feldspar content 32.2

2.3

（wt%） 1 2

According to Kang’s reasearch,14 the pores are mainly existing in the organic

3

matter and the most of them are nanoscale micropore or mesopore. The method of

4

low-pressure nitrogen adsorption is adopted to analyze the pore structure of shale.31-33

5

The apparatus is ASAP 2460 surface area and porosity analyzer provided by

6

Micromeritics Instrument CORP, America.

7

The shale samples with a particle size of < 0.113 mm are most suitable for

8

low-pressure nitrogen adsorption experiments.34 Therefore, for conducting the

9

low-pressure nitrogen adsorption experiments the samples were crushed to 120-230

10

mesh size grains. Before analysis with either N2, the shale samples were automatically

11

degassed by heating at ∼110 °C in a vacuum for about 24 h. The low-pressure N2

12

adsorption isotherms were measured over the relative equilibrium adsorption pressure

13

(P/Po) ranges of 0.005–0.995 at 77.4 K. The pore volume, surface area, and pore size

14

distribution (PSD) could be calculated based on the N2 adsorption data using the

15

density functional theory method (DFT).34,35,36 The low-pressure nitrogen adsorption

16

experiments result is shown in Figure 1 and Table 3. In terms of the adsorption

17

isotherm category by International Union of Pure and Applied Chemistry (IUPAC), it

18

can be seen from Figure 1, the N2 adsorption curve on shale is similar with type Ⅳ, 6


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1

which indicates that mesopore is dominant. Simultaneously, single peak appears

2

obviously at 3nm, which suggests that pore size is mainly nanoscale. As shown in

3

Table 3, the specific surface area of the sample is 23.588 m2/g, which is much higher

4

than that of tight sandstone.37 This relative higher surface area can bring advantages

5

for gas sorption. It also can be seen that the mesopore provides main contribution to

6

specific surface area. Pore Width (nm) 0

20

40

60

80

3.5

3

Quantity Adsorbed (cm /g)

4.0

0.0010

3

dV/dw (cm /g.nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.0005

3.0 2.5

0.0000

2.0 1.5 1.0

Adsorption Desorption

0.5 0.0

0.0

0.2

0.4

0.8

1.0

Relative Pressure(P/P0)

7 8

0.6

Figure 1. Nitrogen adsorption and desorption isotherms and Pore size distributions

9

Table 3. Nitrogen surface areas and pore volume

10

Parameter

Pore types

Percentage（%）

Micropore Mesopore Macropore

Total

Micropore Mesopore Macropore

Specific surface 3.990

19.572

0.019

23.588

2

area（m /g）

7


16.9

83.0

0.1

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Pore volume 0.005167

0.047828

0.00037

0.053365

9.7

89.6

（cm3 /g） 1

2.2. Experimental set-up

2

Shale samples were pulverized and sieved to obtain 120-230 mesh powder, and

3

dried in the oven for over 48 h at 105 °C to remove moisture maintained in rock

4

samples, and then sealed for future experiments. CO2 and CH4 with a purity of 99.9 %

5

are purchased from Beijing AP Beifen Gases Industry Company.

6

Based on the references which reported shale gas adsorption and desorption

7

experiments,22,24,38,39 we built up a device to study the single and multi-component

8

gases adsorption-desorption experiment adopting the experiment method which was

9

well introduced in the literatures. The schematic diagram is shown in Figure 2. For

10

adsorption and desorption behavior measurement, the main part apparatus mainly

11

consists of a sorption cell, a reference cell, and a series of valves and pipelines. The

12

main part is placed in the air bath oven under 50 °C. The temperature can be

13

controlled constant within ±0.15 °C of the set point. A pressure sensor with precision

14

of 0.05% is equipped (Max 25 MPa) and connected to the computer to record the

15

pressure variation of the system. For obtaining the accurate data of adsorption and

16

desorption, besides recording the pressure and temperature, the composition of the

17

equilibrium gas is also required to be analyzed by gas chromatography. To avoid the

18

error caused by the pressure variation during gas sampling process, a sampling cell is

19

designed as void volume. During the adsorption experiment, the sampling cell is

20

connected with adsorption cell, under the action of diffusion, the gas composition of 8


0.7

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the both cells are coincident with each other. When the adsorption equilibrium was

2

established, the valve between adsorption cell and sampling cell was closed and then

3

the equilibrium gas was collected for further composition analysis. Leak test was

4

operated before each experiment. Helium was injected until the pressure reaching 12

5

MPa. The system was kept in the oven under 50 °C for 24 hours. If the leakage rate of

6

each cell is lower than 6.89×10-4 MPa/h, the system is considered with

7

excellent air-tightness.38

8

To obtain accurate data, it is necessary to calibrate the volume of the reference

9

cell (VRC), adsorption cell (VAC) and sampling cell (VSC). The void volume of the pines,

10

pressure sensor and six-way valve was included as a part of the volume of reference

11

cell. The blank volumes of the cells are measured by helium expansion. The mass

12

conservation equations of helium were established to calculate the volume. The

13

densities of helium under different pressures were calculated by Aspen Plus.（VRC

14

=54.077 cm3、VAC =170.010 cm3、VSC =9.193 cm3）. After calibrating, a certain amount

15

of shale particles were packed in the adsorption cell and then the residual volume (Vd)

16

was measured by the method similar with the calibrating method.

17

2.3. Adsorption and desorption experiments

18

For single gas adsorption and desorption measurement, after the process of

19

set-up, the system was vacuumed and adsorption experiment was operated. Based on

20

the previous result, adsorption equilibrium is considered to be achieved when the

21

pressure variation was less than 6.89×10-4 MPa/h.38 The pressure was increased step

22

by step up to 5MPa after each equilibrium being established. When the adsorption 9


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1

experiment was accomplished, the valve between adsorption cell and six-way was

2

closed. A certain amount of gas was released from reference cell and sampling cell

3

through needle valve, then the needle valve was closed and the pressure of a new

4

equilibrium was recorded. Next, the valve between adsorption cell and six-way was

5

opened and the pressure was recorded after adsorption equilibrium was obtained. To

6

avoid air diffusing into the system, a one-way flow valve was connected at the outlet

7

end of the needle valve. The procedure restarts until the pressure minimum is reached.

8

The amount of gas during adsorption or desorption period was calculated by mass

9

conservation. For the adsorption and desorption of mixture of CO2 and CH4, the

10

composition of equilibrium gases was analyzed by gas chromatograph. Different with

11

pure component adsorption process, the gas composition maybe changed after

12

adsorption equilibrium being achieved, thus the whole system needs to be vacuumed

13

after each experiment.

14 15

Figure 2. Schematic diagram of isothermal adsorption and desorption apparatus

16

3. Results and discussion

17

3.1. Isothermal adsorption and desorption curve of CH4 and CO2

18

The isothermal adsorption and desorption curves of CH4 and CO2 on shale 10


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1

samples under different pressures up to 5 MPa at 50 °C are shown in Figure 3. 0.30

CH4 Adsorption CH4 Desorption CO2 Adsorption CO2 Desorption

0.25

Excess Adsorption (mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.20

0.15

0.10

0.05

0.00 0

2

1000

2000

3000

4000

5000

Pressure (kPa)

3

Figure 3. CH4 and CO2 adsorption and desorption isotherms

4

It can be seen from Figure 3 that with the pressure being increased, the

5

adsorption amount of both CO2 and CH4 is aggrandized. Under the same condition of

6

pressure, the adsorption amount of CO2 on shale is significantly higher than that of

7

CH4. The hysteresis phenomenon is presented during the desorption procedure and for

8

CO2 gas the hysteresis phenomenon is more obvious than that of CH4 which implies

9

that the interaction between CO2 and shale is more stronger than that between CH4

10

and shale. Based on this result, it can be concluded that the adsorption ability of CO2

11

on shale is much higher than that of CH4, which is identical with the mechanisms of

12

CO2 enhancing coal bed methane (ECBM) and it can be tentatively considered that

13

the utilization of CO2 for enhancing shale gas is feasible. However, the subsequent

14

experiments are needed to furtherly prove this conclusion.

15

3.2. Isothermal adsorption and desorption of premixed gases

16

For the practical situation, there is already free shale gas existing in the reservoir, 11


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1

the actual selective adsorption behavior involves mixtures of CO2 and CH4.

2

Therefore, it is necessary to study the sorption property of mixture gas to investigate

3

whether CO2 can be utilized for shale gas recovery. In this work, 41.9%CO2 +

4

58.1%CH4 was prepared as mixture gas to study competitive adsorption.

5

During the adsorption process, if the competitive adsorption phenomenon of CO2

6

and CH4 on shale occurs, the composition of mixture gases should be changed. In the

7

adsorbed phase, the content of more preferential gas will be higher than that of the

8

feed gas. This is to say that the content of more preferential gas in free phase (gas

9

phase of adsorption equilibrium) will be lower than that in feed gas. The gas

10

composition of feed gas and free phase was analyzed by gas chromatograph. The

11

result is shown in Figure 4. From Figure 4, it can be seen that under the experimental

12

pressures, the fraction of CO2 of both feed gas and free gas are almost coincident. This

13

phenomenon indicates that for mixture system, CO2 does not present more

14

preferential adsorption ability on shale than CH4, which is an opposite result with pure

15

gas adsorption.

16

In the classical theory, it is stated that the adsorption energy of CO2 is higher than

17

CH4. When the pressure decreases, CH4 is desorbed preferentially, consequently the

18

fraction of CH4 in free phase will increase which results in that the content of CO2

19

decreases. With the desorption processing, the desorption rate of CH4 decreases and

20

meanwhile the desorption rate of CO2 increases. As a result, from the desorption

21

curve it can be seen that the fraction of CO2 in gas phase firstly decreases and then

22

increases as pressure decreasing which matches the classical theory. But for all the 12


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Page 13 of 26

1

pressures conditions, CO2 concentration in free phase of desorption process is lower

2

than that in gas phase of adsorption process. In contrast, it can be inferred that the

3

fraction of CH4 in free phase during desorption is almost higher than that of

4

adsorption process. 0.46

Feed Adsorption Desorption

0.44

CO2 molar fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0.42 0.40 0.38 0.36 0.34 0

1000

2000

3000

4000

5000

Pressure (kPa)

5 6 7

Figure 4. CO2 molar fraction of the feed gas and equilibrium composition of adsorption and desorption gas as a function of pressure.

8

To further realize the adsorption law of each component in mixture gas, the total

9

adsorption quantity of mixture gas was calculated according to pressure variation and

10

the adsorption capacity of CO2 and CH4 of mixture system was also calculated. The

11

result is shown in Figure 5. For comparison, the adsorption quantity of pure gas was

12

presented as well. It can be seen from Figure 5 that the adsorption capacity of each

13

gas increases with pressure. Because the different adsorption ability of CO2 and CH4,

14

the phenomenon of competitive adsorption on specific adsorption site occurs, as a

15

result under the same pressure the adsorption capacity of mixture gas is between that

16

of pure CO2 and CH4.39 Meanwhile, it can be seen that for mixture gas system, the 13


Energy & Fuels

1

adsorption capacity of CH4 is higher than that of CO2. This result proves that during

2

mixture gas adsorption process, CH4 shows higher adsorption ability which is

3

opposite with pure gas adsorption. It is also noted that the CH4 adsorption capacity of

4

mixture gas is even a bit higher than that of pure CH4 under the same pressure, which

5

implies that in this case the existence of CO2 in the mixture gas promotes CH4

6

adsorption. 0.30

Pure CH4 Pure CO2 Mixture CH4 (Mixture) CO2 (Mixture)

0.25


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0.20

0.15

0.10

0.05

0.00 0

7

8

1000

2000

3000

4000

5000

Pressure (kPa)

Figure 5. Pure CH4 and CO2，and their mixture adsorption isotherms.

9

For a binary system, the adsorption capacity of a component in mixture gas is

10

depending on the partial pressure. If one of the component cannot be adsorbed or its

11

adsorption capacity can be neglected (for example binary system of CH4 and He), the

12

adsorption capacity of the other component should be coincided with that of its pure

13

gas under the same condition of pressure.10 For the condition of that both the

14

components can be adsorbed (for example binary system of CH4 and CO2), due to

15

competitive adsorption, the adsorption capacity of each component is different with

16

its pure component. To investigate the interaction of CH4 and CO2 during adsorption 14


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1

period, the adsorption capacity as a function of gas partial pressure is shown in Figure

2

6 and Figure 7. 0.09

Pure CH4 CH4 (Mixture) CH4 Patial Pressure

0.08


0.07 0.06 0.05 0.04 0.03 0.02 0.01 0

1000

2000

3 4

3000

4000

5000

Pressure (kPa)

Figure 6. CH4 adsorption isotherm as a function of CH4 partial pressure.

5 0.28

Pure CO2 Mixture (CO2) CO2 Patial Pressure

0.26 0.24


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.22 0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0

6 7

1000

2000

3000

4000

5000

Pressure (kPa)

Figure 7. CO2 adsorption isotherm as a function of CO2 partial pressure.

8

From Figure 6 it can be seen that the adsorption capacity of CH4 in mixture gas

9

is obviously higher than that of the same CH4 partial pressure. This result further

10

proves that the existence of CO2 does not block CH4 to be adsorbed but promotes the 15


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1

adsorption of CH4. This is to say, for binary gas system of CO2 and CH4, CH4 shows

2

advantage to be absorbed on shale. The same adsorption phenomenon of mixture gas

3

of CO2 and CH4 on low coal rank coal sample was also reported by Dai.40 Their

4

research also showed that the adsorption capacity of CH4 was affected by CO2 content

5

in the binary system.

6

Moreover, it can be seen from Figure 7 that the adsorption capacity of CO2 in

7

mixture gas is lower than that of the same CO2 partial pressure. When CO2 being

8

mixed with CH4, it can be concluded that CH4 molecules occupied adsorption site

9

preferentially which prevent subsequent CO2 adsorption. Therefore, according to

10

these result it seems that since there is already free CH4 in the shale gas reservoir,

11

injection of CO2 will facilitate CH4 adsorption and CO2 injection cannot be utilized

12

for shale gas recovery.

13

However, it should be noticed that the exploitation procedure of shale gas is

14

mainly depending on pressure deceasing process which means we should focus on the

15

desorption behavior rather than adsorption. Therefore, to evaluate whether CO2

16

injection can be used for shale gas recovery, the result of desorption should be

17

investigated. The adsorption and desorption isotherms of mixture, CH4 and CO2 are

18

shown in Figure 8. From Figure 8 it can be seen that under the same condition of

19

pressure, the excess adsorption capacities of CO2 during desorption process is

20

evidently higher than that of CO2 during adsorption period. Meanwhile, the condition

21

of CH4 is just the reverse. In contrast with adsorption process, during desorption

22

period, when being mixed with CH4, the adsorption of CO2 is promoted with pressure 16


Page 16 of 26

Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

decreasing, which is accordingly to say that during desorption process, CO2 present

2

advantage in competitive adsorption and more CH4 gas is replaced and desorbed. This

3

result indicates that CO2 has the potential to be used for enhancing CH4

4

Meanwhile, it also can be seen from Figure 8 that at the beginning of desorption

5

process, when the pressure just starts to decrease, the adsorption capacity of CO2

6

increases sharply to the maxim instead of decreasing. And then it decreases slowly as

7

pressure decreasing. For CH4 gas, the adsorption capacity decrease sharply almost as

8

soon as the pressure starts to decrease and then decreases slowly. During the pressure

9

decreasing period, CO2 shows preferential adsorption on shale samples, CH4 is

10

primarily to be desorbed due to preferential adsorption of CO2, thus when the pressure

11

begins to fall, the desorption rate of CH4 is relative higher and the excess adsorption

12

capacity decreases rapidly.

13

decreases gradually and the excess adsorption capacity decreases slowly.

As desorption process going on, desorption rate of CH4

14

When the pressure falling, because of that the adsorption energy of CH4 is lower,

15

CH4 is more easily to be desorbed from surface, which will provide vacant adsorption

16

sites. For CO2 gas, the decrease of pressure causes CO2 desorption, but at the

17

meantime, CO2 with higher adsorption ability will occupied part of the vacant

18

adsorption sites provided by CH4 desorption which results in the excess adsorption

19

capacity increasing. In terms of the result shown in Figure 8, it is investigated that the

20

adsorption amount of CO2 on vacant adsorption sites is larger than the desorption

21

amount due to pressure diminishing. At the beginning of desorption, due to the

22

relative higher desorption rate of CH4, lots of adsorption sites are available. Thus, at 17


Energy & Fuels

1

this period, the excess adsorption capacity of CO2 rises quickly. As the process going

2

on, desorption rate of CH4 decreases as a result the increment of adsorption capacity

3

are not sufficient to offset the desorption capacity caused by pressure decreasing; at

4

the same time with pressure falling further, the desorption rate of CO2 rises. Therefore

5

at the later stage of desorption, desorption plays a dominant role and the excess

6

adsorption capacity of CO2 declines.

7 0.16

Adsorption (Mixture) Adsorption (CH4 of Mixture) Adsorption (CO2 of Mixture) Desorption (Mixture) Desorption (CH4 of Mixture) Desorption (CO2 of Mixture)

0.14


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

0.12 0.10 0.08 0.06 0.04 0.02 0.00 0

8

1000

2000

3000

4000

5000

Pressure (kPa)

9

Figure 8. Adsorption and desorption isotherms of Mixture, CH4 and CO2.

10

By comparing the desorption ratio of CH4 in pure component and mixture gas,

11

the feasibility of exploiting shale gas by CO2 injection is evaluated. Desorption ratio

12

(Dr) is defined as the proportion of CH4 desorption capacity and the total adsorption

13

capacity of CH4. According to Figure 3 and Figure 8, desorption capacity is calculated.

14

Meanwhile, desorption ratio per pressure drop (Dr’) was also calculated to reflect the

15

difficult level of exploring CH4. The result is shown in Table 4. It can be seen that Dr 18


Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

and Dr’ of mixture are both higher than these of pure CH4, it suggested that

2

desorption ratio of CH4 can be enhanced by 13.5% by the existence of CO2. This

3

result demonstrates that the utilization of CO2 for enhancing shale gas recovery has

4

certain theoretical feasibility.

5 6

Table 4. Comparison of desorption rate at unit pressure drop between pure

7

CH4 and mixture desorption experiments

Experiment

Starting

End

point

point

/kPa

8

Adsorption

Adsorption

capacity of

capacity of

Dr’ Dr/%

stating point/

end point /

（mmol/g）

（mmol/g）

（%/kPa）

/kPa

Pure CH4

4954.5

470.0

0.076

0.016

78.9

0.0178

Mixture

5038.5

546.1

0.081

0.006

92.6

0.0207

4. Conclusion

9

To evaluate the feasibility of the utilization of CO2 for enhancing shale gas

10

recovery, adsorption and desorption experiments of methane, carbon dioxide, and

11

mixtures of the two gases on shale have been operated and the adsorption and

12

desorption behaviors have been discussed. Based on the above experimental results,

13

the following conclusions can be draw.

14

(1) For the shale sample, the mesopore is dominant and the pore size is mainly

15

nanoscale. Meanwhile relative higher surface area can bring advantages for gas

16

sorption. 19


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 26

1

(2) For pure CH4 and CO2, CO2 shows obvious advantage in adsorption.

2

However, shale has not exhibit preferential adsorption of CO2 for premixed gases.

3

Therefore the adsorption law of pure gas cannot be used to speculate the adsorption

4

behavior of premixed gas.

5

(3) For the mixture of CH4 and CO2, the different discipline of adsorption and

6

desorption process is presented. During adsorption process, CH4 is preferably

7

adsorbed; for desorption, CO2 shows higher adsorption ability on shale.

8

(4) The evaluation of shale gas recovery depends on desorption process rather

9

than adsorption process. In the desorption process, the existence of CO2 promotes

10

desorption of CH4, desorption ratio of CH4 was increased. Theoretically, CO2 can be

11

utilized for enhancing shale gas recovery.

12

AUTHOR INFORMATION

13

Corresponding Authors

14

*E-mail: [email protected] (Z.Y.)

15

Notes

16

The authors declare no competing financial interest.

17

Acknowledgements

18

This work was supported by the National Natural Science Foundation of China

19

(51774302), National Key Technologies R&D Program of China (2017ZX05009-004),

20

the

21

(2462015QZDX01) and Sinopec (P15027).

Science

foundation

of

the

China

University

20


of

Petroleum,

Beijing

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Desorption of CO2 and

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