The effect of temperature on methane adsorption in shale gas

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The effect of temperature on methane adsorption in shale gas reservoirs Jie Zou, Reza Rezaee, and Kouqi Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02639 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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The effect of temperature on methane adsorption in shale gas reservoirs Jie Zou a*, Reza Rezaee a and Kouqi Liu b a

Department of Petroleum Engineering, Curtin University, WA 6151, Australia Department of Petroleum Engineering, University of North Dakota, Grand Forks, ND 58202, USA * Corresponding author, email address: [email protected] b

Abstract Methane adsorption isotherms on shale were investigated at 25℃, 45℃, 60℃, 80℃ with pressure up to 7MPa (1015psi). A total of 6 shale samples with low total organic carbon (TOC) from Perth Basin and Canning Basin (Western Australia) were studied to quantify the effect of temperature on methane adsorption in shale gas reservoirs. Pore structure of shale samples was measured using low-pressure nitrogen and carbon dioxide adsorption. At the low temperature (25℃), the methane adsorption isotherms show a general increase of methane adsorption with TOC. However, the trend is not in line with methane adsorption at high temperature (80℃). At 80℃, Sample AC-2 with 0.64%TOC has larger maximum methane adsorption capacity than sample AC-4 with 1.03%TOC, indicating that the effect of temperature on methane adsorption for different shale samples is different. As the temperature increases, the decrease rate of methane adsorption on low TOC samples is smaller than that of the samples with high TOC content. All the experimental methane adsorption isotherms fit well with the Langmuir equation. The Langmuir volume of sample AC-2 (0.64%TOC), AC-4 (1.03%TOC) and AC-5 (0.23%TOC) is very close to each other at high temperature (80℃). The thermodynamic parameters of methane adsorption on shale samples were determined. For the studied shale samples, the heat of adsorption and the standard entropy range from 4.5 to 14.5 kJ/mol and from 42.0 to 74.7J/mol/K, respectively. Keywords: temperature, kerogen, clay minerals, methane adsorption, shale gas

1. Introduction Methane adsorption in shale gas system has drawn much attention because of the significance of shale gas to mitigate the energy crisis of the world. With the development of oil and gas industry and the increasing demand for energy, shale gas has been widely explored and developed all over the world. However, there are many uncertainties in evaluation and development of shale gas due to the complicated adsorption properties of methane in the 1 ACS Paragon Plus Environment

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shale gas system. As reported, methane is stored as free gas and adsorbed gas in shale gas reservoirs. The percentage of adsorbed gas could be up to 85% in shale gas reservoir.1 The adsorbed gas has also been reported to influence the gas flow and permeability, which are crucial to the field development plan.2 Therefore, understanding the theory of methane adsorption characteristics is required to reduce uncertainties of reservoir evaluation and design economical production strategies. Methane adsorption characteristics of shale gas reservoir are controlled by various factors.3-7 Organic matter and inorganic matter in shale both can adsorb methane and control the pore structure.8-10 Total organic carbon (TOC) is one main controlling factor of methane adsorption and has a positive linear correlation with methane adsorption capacity in shale gas system. 6, 7, 11 The maturity of organic matter was also regarded as another controlling factor of methane adsorption.6,

7, 12

TOC-normalized

adsorption capacity was observed to increase with the increasing thermal maturity up to some critical value and followed by a reversed trend at high maturity.7 Two reasons based on the previous reports can be used to explain this trend. One reason can be explained as that micropores (less than 2 nm) are generated with thermal maturation, which can offer more volume for methane adsorption.11 Another reason is due to the changes of surface chemistry of kerogen with thermal maturation: the affinity between methane and kerogen increases with maturity because of increasing aromaticity.6 In addition of the maturity and the TOC, the kerogen type was also reported to have an effect on methane adsorption capacity. Methane adsorption capacity of individual kerogen types follows the order: type III > type II > type 1.6, 13

Apart from the organic matter, for the clay-rich shales, the inorganic matter can also

influence the total methane adsorption capacity.6, 7, 14, 15 For the pure clay minerals, the methane adsorption capacity was reported to decrease in the order: semectite > mixture of illite and semectite > kaolinite>chlorite > illite.8 The methane adsorption capacity of clays is reduced with existing of moisture in shales. Due to the hydrophilicity of clay, moisture occupies the methane adsorption sites on clay.7, 16 In addition, external parameters such as pressure and temperature can affect the methane adsorption as well. Methane adsorption is an exothermic process and, methane adsorption capacity reduces at the high temperature.

Many methane adsorption studies at ranged

temperature have shown that methane adsorption isotherms varies significantly at different temperatures.7,

17, 18

Adsorbed methane content decreases with rising temperature and the

change of adsorbed gas content with increasing temperature is greater at higher pressure.19, 20

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However, up to date, the study of the effect of temperature on methane adsorption of low TOC shale samples has rarely been mentioned and is still limited. This study attempted to quantify the ifacct of the temperature on themethane adsorption characteristics of shale samples with TOC range (0.23% to 3.07%). The comparison of methane adsorption at ranged temperature between low and relatively high TOC shale samples can contribute to estimating the methane adsorption capacity in shale gas reservoir. 2. Samples and experimental methods 2.1. Samples Shale samples used in this study are from Carynginia Formation, Western Australia. Six samples were collected: one from Canning Basin and the other 5 samples from Perth Basin. The Perth Basin has been estimated to store up to 32.7 Tcf of technically recoverable shale gas.21 The samples were chosen to represent low TOC samples. Table 1 and Table 2 show the mineralogical composition and geochemical analysis, respectively. Total organic carbon (TOC) of 6 samples range from 0.23 to 3.07%. Virtrinite reflectance (R o ) of the samples is calculated from T max and varies between 0.98% and 1.21% except for the Sample 5.21 For Sample AC-5, the TOC is too low to measure the virtrinite reflectance. The HI data shows that the kerogen in AC samples belong to Type III while the kerogen in sample T-1 belong to the Type II. AC-1and T-1 have more clay minerals than others. All samples were crushed into powder and passed through a 60 mesh sieve ( Sample AC-1 > sample AC-3 > Sample AC-4 > Sample AC-5, which is consistent with the order of TOC (Figure 7). In order to quantify the effects of the pressure and temperature on the methane adsorption, we derived the adsorption quantity of the samples under various temperature and pressure and the results can be seen in Figure 8. Adsorbed methane decreases with increasing temperature at each pressure. The curve slope, regarded as the decrease rate of methane adsorption with increasing temperature, increases with increasing pressure from 5 to 50bars(725.2psi) but remains stable when pressure keeps increasing from 50 to 70bars(1015.3psi).


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Methane adsorption isotherms can be well fitted by Langmuir equation and the fitting results are presented in Table 4. Langmuir volume decreases and Langmuir pressure increases with increasing temperature. Langmuir volume of shale samples shows a different degree of change from 25℃ to 80℃. For instance, Samples AC-1 and AC-4 decrease from 51 to 37 (scf/ton) (27.5% change) and 34 to 20 (scf/ton) (41.1% change), respectively. However, Samples AC-2 and AC-5 both decrease from 26 to around 20 (scf/ton) (23.1% change).

AC-1

40 35

25℃

30

45℃

25 20

60℃

15

80℃

10 5

20

25℃

15

45℃ 60℃

10

80℃

5 0

0 0

200

400

600

0

800 1000 1200

25℃

25

45℃

20

60℃

15 10 5

Adsorbed gas(scf/ton)

30

0

45℃ 10

60℃ 80℃

5


25℃

0 600

25℃ 45℃

15

60℃ 10

80℃

5

200

400

800

Pressure(psi)

600

800 1000 1200

Pressure(psi)

T-1

70

15

400

1000 1200

20

0

AC-5

200

800

0

200 400 600 800 1000 1200 Pressure(psi)

20

600

AC-4

25

35

0

400

Pressure(psi)

AC-3

0

200

Pressure(psi)

40 Adsorbed gas(scf/ton)

AC-2

25



45


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60 50

25℃

40

45℃

30

60℃

20 10 0

1000 1200

0

200

400 600 800 1000 1200 Pressure(psi)

Figure 6.Methane adsorption isotherms for the shale samples at different temperatures.


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70 60

Adsorbed gas (scf/ton)

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50 AC-1 40

AC-2 AC-3

30

AC-4

20

AC-5 T-1

10 0 0

200

400

600

800

1000

1200

Pressure (psi) Figure 7.Comparison of methane adsorption isotherms for the shale samples at 25℃. Note that sample AC-2 has more adsorbed gas content than sample AC-4 at the first 3 pressure points.

Sample AC-1 AC-2 AC-3 AC-4 AC-5 T-1

Table 4 Langmuir parameters for the shale samples at different temperatures

Langmuir parameters

25℃

45℃

60℃

80℃

P L (psi) V L (scf/ton) P L (psi) V L (scf/ton) P L (psi) V L (scf/ton) P L (psi) V L (scf/ton) P L (psi) V L (scf/ton) P L (psi) V L (scf/ton)

218.9 51.3 256.1 26 263.8 46.5 422.1 34.2 383.5 26.5 338.2 80.6

277.8 46.3 306.7 23.1 352.3 35.2 544.6 29.5 398.3 24 371.7 63.7

346.9 40.2 322.5 20.6 492.2 26 562 22.2 445.3 22.1 481.4 50.3

433.8 37.6 429.2 19.8 N/A N/A 729.7 20.5 505.4 20.7 N/A N/A


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AC-2

45

45

40

40

35

35

30

70bars 60bars 50bars 40bars 30bars 20bars 10bars 5bars

25 20 15 10 5



AC-1

30


25 20 15 10 5 0

0 20

30

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70

20

80

30

Temperature(℃)

40

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30 25


20 15 10 5 0 40

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30 70bars 60bars 50bars 40bars 30bars 20bars 10bars 5bars

25 20 15 10 5 0 20

80

30

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50

60

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80

Temperature (℃)

Temperature (℃) AC-5

45

60

AC-4

45

20

50

Temperature(℃)

AC-3

T-1 60

40 35

50

30 70bars 60bars 50bars 40bars 30bars 20bars 10bars 5bars

25 20 15 10 5



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0 20

30

40

50

60

70

Temperature(℃)

40


30 20 10 0

80

20

30

40

50

60

Temperature (℃)

70

80

Figure 8. Comparison of correlations between temperature and adsorbed methane under isobaric conditions. The points are the experimental results and the dotted curves are the linear regression fitting results.


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4. Discussion 4.1. Methane adsorption characteristics at low temperature Methane adsorption characteristics at low temperature are controlled by both composition and pore structure. The organic fraction (TOC) is the main controlling factor of methane adsorption capacity for shale at low temperature (25℃). As shown in Figure 9, Langmuir volume at 25℃ has a positive linear correlation with total organic carbon, which is described using Eq. 11 and 12.

V L = 9.995TOC + 23.326 V L = 15.178TOC + 19.232

R2=0.916

(11)

R2=0.794

(12)

Where VL is the Langmuir volume in scf/ton, and TOC is the total organic carbon in wt%. The Eq. 11 was determined by using the 5 shale samples from the same borehole AC. The Eq. 12 was determined using all the 6 shale samples, with a smaller correlation coefficient. The smaller correlation coefficient (0.79) results from the higher Langmuir volume of sample T-1 comparing to sample AC-1, even though the two shale samples have close TOC. Apart from TOC, more parameters are required to be considered in order to get the Langmuir volume. Sample AC-2 with lower TOC than sample AC-4 has higher adsorbed methane at a pressure less than 30bars (435.1psi) at 25℃. This notable phenomenon possibly results from the larger micropore volume of sample AC-2, which has been explained using molecular simulation.37 Smaller pores from micropore volume have higher adsorption capacity than larger pores at low pressure.

90 80

Langmuir volume at 25℃ （scf/ton)

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y = 15.178x + 19.232 R² = 0.7938

70 60 50

Correlation 1

40 y = 9.9952x + 23.326 R² = 0.916

30

Correlation 2

20 10 0 0

1

2

TOC(wt%)

3

4

Figure 9. Correlation between Langmuir volume at 25℃ and TOC: correlation 1 is fitted from the 5 AC shale samples using linear regression; correlation 2 is fitted from all the shale samples using linear regression.


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4.2. The effect of temperature on methane adsorption The effect of temperature on methane adsorption for shale samples is related to their composition. As shown in Figure 10, methane adsorption decreases with increasing temperature under isobaric condition. The decrease rate, used to describe the effect of temperature on methane adsorption, ranges from pressure to pressure and sample to sample. For each sample, the decrease rate increases with increasing pressure for pressure less than 50bars. Meanwhile, as a general rule of physisorption, adsorbed gas content increases with increasing pressure. The trend of adsorbed gas content with pressure is in line with the trend of the decrease rate with pressure. So it is reasonable to guess the decrease rate is related to adsorbed gas content. The more adsorbed gas content is, the more methane molecules would be influenced by the increased temperature. However, the correlation between the decrease rate and adsorbed gas content is not true for shale samples with different compositions. Figure 10 shows that the correlation between adsorbed gas content and the decrease rate follows this order: sample AC-3 > sample T-1 > sample AC-1 = sample AC-4 > sample AC-2 = sample AC-5. Sample AC-2 and AC-5 are less sensitive to the increased temperature on methane adsorption comparing to the other samples with relatively high TOC. Adsorbed gas content normalized by TOC at 80℃ presented in Figure 11 indicates samples AC-2 and AC-5 with low TOC have abnormally larger normalized adsorbed gas content than sample AC-1 and AC-4 with relatively high TOC. It has been reported that a certain amount of methane molecules are adsorbed on clay minerals in dry condition for low TOC shale.38 The proprotion of methane adsorbed on clay minerals is much higher for sample AC-2 and AC-5 than sample AC-1 and AC-4. The less sensitivity to the increased temperature for sample AC-2 and AC-5 could attribute to the higher propoertion of methane adsorbed on clay minerals, which implies the methane adsorbed on clay minerals is less sensitive to the increased temperature than that adsorbed on kerogen. Even when relatively high TOC and low TOC samples have the same adsorbed gas content at different pressures, the decrease rate of methane adsorption with increasing temperature is greater on relatively high TOC sample, which also suggests that the methane adsorbed on kerogen is more sensitive to the increased temperature on methane adsorption than on clay minerals. Furthermore, it has been shown that methane adsorbed on clay minerals has a positive correlation with BET surface area.9 Herein, since the large BET surface area, the maximum methane adsorption capacity described by the Langmuir volume of sample AC-5 (0.23%) at high temperature (80℃) is larger than sample AC-2 (0.64%) and no less than sample AC-4 (1.03%), which implies that with increasing temperature, a larger amount of methane adsorbed on kerogen is reduced than methane adsorbed on clay minerals. Note that the methane adsorption in this study was measured on dry samples. The presence of moisture has been reported to reduce the methane adsorption capacity greatly. The hypothesis is that


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moisture occupies the surface area of hydrophilic clay minerals.16, 39 However, the effect of moisture on methane adsorption would be different when taking the high temperature in the reservoir into consideration. Previous studies focused on the effect of moisture on methane adsorption at low temperature.13, 16, 39-42 The high temperature in shale reservoir condition may reduce the occupation of moisture on rock surface.43 The combined effect of moisture and temperature needs to be addressed in the future work.

Decrease rate of methane adsorption with temperature

0 -0.1 -0.2

AC-1

-0.3

AC-2

-0.4

AC-3 AC-4

-0.5

AC-5 T-1

-0.6 -0.7 0

10

20

30

40

50

60

Adsorbed gas content(scf/ton)

Figure 10. Comparison of correlations between adsorbed gas content and the decrease rate of methane adsorption with increasing temperature. The decrease rate is determined using the linear regression between adsorbed gas content and temperature in fig.8. 60

Adsrobed gas content normalized by TOC (scf/ton)

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50 AC-1

40

AC-2

30

AC-4

20

AC-5

10 0 0

200

400

600

800

1000

1200

Pressure(psi) Figure 11. Comparison of adsorbed gas content normalized by TOC at 80℃ for four shale samples. Samples AC-2 and AC-5 have larger adsorbed gas content normalized by TOC than samples AC-1 and AC-4.


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4.3. Thermodynamic parameters for methane adsorption Thermodynamic parameters were calculated from the temperature dependence of the Langmuir pressure according to Eq. 10, including the heat of adsorption and the standard entropy of adsorption listed in Table 5. Fig.12 shows that the heat of adsorption is positively related to the TOC. It has been reported that methane adsorbed on kerogen releases more heat than methane adsorbed on clay minerals.6, 8, 44 As for shale sample AC-2 and AC-5 with very low TOC in this study, a certain amount of methane adsorbed on clay minerals contributes to the low heat of adsorption. The heat of adsorption decreases with the proportion of methane adsorbed on clay minerals increasing. Meanwhile, the correlation coefficient is 0.83 in Figure 12, demonstrating that the heat of adsorption is not only related to the TOC. The heat of adsorption determined for shale samples in this study is ranging between 4.5 and 14.5 kJ/mol, which is smaller than the reported results on kerogen, organicrich shale and even Montmorillonite clay and I-S mixed clay.6-8, 44 The reason could be: 1. the studied shale samples have low TOC (0.23~3.07%); 2. the particle size used in this study (

The effect of temperature on methane adsorption in shale gas

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