Feasibility and applicability analysis of CO2-ECBM based on CO2

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Feasibility and applicability analysis of CO2ECBM based on CO2-H2O-coal interactions Xiaolei Liu, Caifang Wu, and Kai Zhao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01663 • Publication Date (Web): 16 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017

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Title Page Title: Feasibility and applicability analysis of CO2-ECBM based on CO2-H2O-coal interactions Author names and affiliations: Xiaolei Liua, b, Caifang Wua, b *, Kai Zhaoc a. Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process of the Ministry of Education, China University of Mining and Technology, Xuzhou, Jiangsu Province 221008, China b. School of Resources and Geosciences, China University of Mining & Technology, Xuzhou, Jiangsu Province 221008, China c. Eighth Oil Production Plant of Daqing Oilfield Co., Ltd., PetroChina, Daqing Heilongjiang Province 163514, China * Corresponding author: Caifang Wu Phone number: +86 13813291526 Fax: +86 051683885387 Email: [email protected] Address: Room 338, Key Laboratory of Coalbed Methane Resource and Reservoir Formation Process, Ministry of Education, South Jiefang Road, Xuzhou, Jiangsu Province 221008, China

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Abstract In view of the failed field test of CO2-ECBM, the feasibility and applicability of CO2-ECBM were analyzed based on interactions among CO2, H2O and minerals in the coal reservoir by conducting flooding experiments of coal samples with nuclear magnetic resonance (NMR) technology. CO2 can change the wettability of the coal surface, and adsorbed water could be changed into free water. CO2 is adsorbed constantly on the coal surface, leading to the swelling of coal matrix. The effect of the dissolution of minerals and flooding pressure, to some extent, can improve the permeability of reservoir, but the improvement of permeability is not obvious with increasing effective stress. Due to severe stress sensitivity, fluid in the ultra-low permeability and deep coal reservoir is difficult to flow. Furthermore, the swelling of coal matrix leads to the increase of effective stress, which in turn reduces the permeability quickly. The positive effects of CO2-ECBM are tough to compensate negative effects of stress sensitivity and swelling of coal matrix. Therefore, the application of CO2-ECBM should be taken into consideration in the exploitation of ultra-low permeability and deep coal reservoir. Keywords: CO2-ECBM; flooding; matrix swelling; stress sensitivity; wettability

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1. INTRODUCTION CO2 is a greenhouse gas. At present, how to effectively deal with CO2 has received much attention. It can achieve dual benefit to inject CO2 into unminable coal seams in the process of coalbed methane (CBM) development. It can not only achieve CO2 sequestration, but also enhance the production of CBM (CO2-ECBM).1-3 Since 1980s, CO2-ECBM has been widely studied in the world.1-7 Numerical simulation and physical simulation have been used to analyze the interaction of CH4 and CO2 in terms of coal rank, adsorption capacity and energy.2, 6-10 The results show that the adsorption capacity of CO2 is higher than that of CH4 under the same conditions. Injection pressure, injection rate and salinity of formation water could influence the effect of CO2 on CBM production.11-14 However, the results are not identical. Reznik et al.11 pointed out that the higher the CO2 displacement pressure, the more the methane is replaced. Mazumder et al.12 suggested that the injection pressure has little effect on CBM production. Liu and Wu13 presented that the injection pressure plays a dual role in the process of CO2 displacement, which is related to the permeability of the reservoir and the desorption pressure of CBM. In brief, substantial studies conclude that CO2-ECBM has a better effect. Unfortunately, the results of field tests and laboratory experiments are inconsistent. The final results are unsatisfactory in the pilot test.6 CBM cannot be effectively extracted, and even a significant reduction in reservoir permeability has been caused.15 In the CO2-ECBM pilot test of San Juan basin, the permeability decreased from 100 mD to 1 mD after injection.15 Facing the inconsistency and unfavorable results, the further research is needed. The interactions among CO2, CH4, H2O and coal matrix play a key role when CO2 is injected into coal seams. The pH of formation water decreases when CO2 dissolves into the formation 3


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water. Then CO2 solution can react with minerals in coal. The dissolution of carbonate minerals can improve reservoir permeability. From the view of adsorption capacity, the adsorption capacity of CO2 is stronger than that of CH4,4, 8, 16-19 which can reduce the amount of adsorbed methane, and increase free methane. At the same time, the wettability of coal surface can be changed when CO2 is in contact with the coal surface.20-23 During numerical simulations, some scholars assumed that the porosity and permeability of coal are constant without considering the effect of stress and matrix swelling.6 Some scholars only studied the effect of matrix shrinkage on permeability of coal reservoir.18 However, some negative effects can reduce reservoir permeability. Previous studies show that the swelling of coal matrix caused by adsorption has a direct effect on the permeability. The swelling of coal matrix is proportional to the content of adsorbed CO2.2, 24 With increasing CO2 content and confining pressure, the permeability decreases obviously.24, 25 Besides, the swelling of clay minerals can block the migration channel and reduce the permeability.26 Anggara et al.24 suggested that the dissolved carbonate minerals can precipitate as the pressure decreases and the injection stops by numerical simulation. Different research perspectives present different conclusions. Previous studies paid more attention to reactions occurring after CO2 injection into the coal seam, such as adsorption capacity, wettability or the reaction of CO2 aqueous solution and minerals. Generally speaking, most studies show that CO2 can be utilized to achieve dual benefit, and few scholars have put forward the conditions of CO2-ECBM.15, 26 It is well known that the degree of difficulty of fluid migration in reservoir plays an important role in the process of CBM production. In this case, in order to verify the applicability of CO2-ECBM technology in CBM development, the feasibility of CO2-ECBM 4


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technology for ultra-low permeability reservoir and deep reservoir is explored. In this study, the flooding experiments with different permeability samples were carried out under different conditions. According to reactions in the process of CO2 injection into coal samples and based on the previous research, the influences of main reactions and their positive and negative effects on the permeability after CO2 injection into coal samples were comprehensively analyzed. The feasibility and applicability of CO2-ECBM technique were analyzed emphatically according to the change of permeability and the difficulty of migration of the fluid in coal samples.

2. MATERIALS AND METHODS 2.1. Coal samples The samples were collected from No. 3 coal seam of the Yameidaning mine (DN), No. 15 coal seam of the Lean mine (LA) in Qinshui Basin, and No. 16 coal seam of the Runfeng mine (RF), west of Guizhou Province. The sample size was prepared according to the core holder. The diameter of coal pillar is 25 mm with the length of 27-57 mm. In order to compare the displacement effects of coal samples with different permeability grades, DN coal sample and LA coal sample with basically identical parameters but different permeability were selected. In order to study the dissolution of minerals in CO2 water solution, RF coal sample with relatively serious mineral filling was selected. The basic parameters of coal samples and semi-quantitative test results of mineral content of RF are shown in Table 1 wherein Vdaf means yield of volatiles in the dry ash free state, and the Ro,max represents that the mean maximum vitrinite reflectance in oil immersion. 2.2. Experimental setup The experimental device of NMR is consistent with the experimental device of Liu and Wu13 5


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(Figure 1). The displacement device is mainly composed of non-magnetic core holder and NMR detector. The confining pressure liquid is non-magnetic fluorine oil. The instrument used in the flooding experiment of CO2 water solution is a simulated displacement device with high temperature and pressure. The displacement system includes ISCO pump, intermediate container, hand pump and gas and water collecting device (Figure 2). The experiment was carried out at the State Key Laboratory of Heavy Oil Processing, China University of Petroleum. 2.3. Principle of T2 spectrum The fluid states are mainly distinguished according to the range of T2. The principle of T2 is the same as previously reported.13, 27, 28 The principle is as follows.

1

T2

= ρ

S V

(1)

where ρ is the surface relaxation rate and S/V is the specific area of pores. As the specific area increases, T2 becomes shorter. Meanwhile, the specific area of coal is mainly controlled by micropores. Therefore, the smaller the pore is, the shorter the transverse relaxation time will be. 2.4. Experimental methodology 2.4.1. CO2 flooding of DN The coal sample had been saturated with distilled water for 5 days under vacuum condition before the experiment. The experimental device of gas flooding water is shown in Figure 1. The displacement pressure (injection pressure) was adjusted to 2.5 MPa at the beginning according to the preliminary experiments. In case the fluid pierces the sample radially and in order to ensure the integrity of the sample, the confining pressure was set to 6 MPa. The confining pressure is the radial pressure, and it corresponds to the overburden pressure. After a period of stability, there was no gas at the outlet, and the displacement pressure was slowly adjusted to 4 MPa. The bubbles 6


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came out slowly after 3 h, indicating that CO2 passed through the sample. In order to slow down the migration rate of CO2 in coal samples and prolong the contact time between CO2 and coal sample, the displacement pressure was adjusted to 3 MPa for 9 h. The process was recorded by NMR signals. 2.4.2. Water flooding and CO2 flooding of LA The coal sample was not saturated before the experiment. The water flooding was carried out before CO2 flooding. Distilled water was used in this experiment. The device of water flooding is assembled according to Liu and Wu,13 and it had been evacuated for 13 h before water flooding. The purpose of the water flooding was to make the sample contain water in saturated state. According to the displacement test of DN, the confining pressure and displacement pressure were set to 6 MPa and 3.5 MPa respectively considering the lower permeability of the sample. The flow rate of water flooding was basically zero after 1 h. In order to make the coal sample contain water in saturated state as soon as possible, the displacement pressure was adjusted to 4 MPa, and the confining pressure was adjusted to 7 MPa at the same time. The flow rate of water flooding was basically zero after 2 h. The displacement pressure slowly regulated to 5 MPa. There was no water at the outlet after 8.5 h at the constant displacement pressure of 5 MPa. Eventually, the confining pressure was adjusted to 10 MPa. The displacement mode changed to constant flow with flow rate of 0.01 mL/min. The experiment stopped after about 6 h and the displacement pressure rose to 9 MPa. The total time of water flooding was 18.5 h. Water was not observed at the outlet throughout the experiment. After that, CO2 flooding was started with the confining pressure of 10 MPa according to Figure 1. In view of the water flooding test, the displacement pressure was adjusted to about 4 MPa at the beginning. However, there was no gas coming out from the outlet after a 7


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period of time. Then the displacement pressure was gradually adjusted to 5.86 MPa. CO2 flooding lasted for 11.5 h. The process was recorded by NMR signals. It should be explained that the purpose of this experiment is to study the effect of CO2 on the water in coal sample and explain the degree of difficulty of fluid migration in coal sample. The saturated condition of water is not an essential condition. At the same time, in order to ensure the progress of the experiment, although the actual experimental condition is different from the coal sample of DN, it does not affect the final experimental results. This is explained in the following text. 2.4.3. Effect of CO2 on water in coal sample In order to further verify that CO2 causes the change of adsorbed water to free water, the coal sample of DN with saturated water was put into a sealed device. CO2 was injected into the device at 4 MPa. Afterwards, the signals were collected. The experiment lasted for 30.5 h. 2.4.4. Flooding experiment of CO2 aqueous solution According to the displacement test of DN, the displacement pressure was designed to be 2 MPa considering the higher permeability of the sample. Before the experiment, the sample (RF) was dried at 80 °C for 8 h under vacuum condition and was saturated in formation water for 16 h with a pressure of 2 MPa under vacuum condition. The ion content of the original formation water is shown in Table 2. The ratio of the absolute value of the volume difference to the time is the flow rate at the intermediate moment. The flow rate is positive when the liquid comes out. The experimental process is as follows. 1) The preparation of the experiment Firstly, the vessel for CO2 aqueous solution in Figure 2 was rotated 180° and the confining pressure was adjusted to 2.5 MPa. Then the six-way valve I-1, I-3, II-1, II-3 and II-4 and the valve 8


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1, 2, 3 and 5 were opened. Afterwards, in order to exhaust the air of the pipeline, the flow rate of ISCO pump was set to 20 mL/min. It was closed when the liquid flowed into cylinder 3. After that, the valve 5, six-way valve I-1 and II-3 were closed and six-way valve I-2 and I-5 were opened. Finally, CO2 was injected to the vessel at 2 MPa. Meanwhile, the amount of distilled water was measured by cylinder 1. 2) The preparation of CO2 aqueous solution and flooding experiment According to Figure 2, six-way valve I-1, I-3, II-1, II-3 and II-4 and valve 1 and 2 were opened. The pressure of ISCO pump was adjusted to 2 MPa and maintained for a period of time. After the CO2 in the displacement vessel was in equilibrium with the aqueous solution, the valve 3 and 4 were opened. Meanwhile, the water and gas of the downstream were recorded. The flooding experiment lasted for about 136 h.

3. RESULTS AND DISCUSSION 3.1. Interaction of CO2, H2O and coal surface 3.1.1. CO2 flooding of DN According to previous studies,20,21 CO2 can change the contact angle of water on the coal surface, and then change wettability. When CO2 was injected into the sample, the contact angle of water on the coal surface increased. Capillary pressure decreased with increasing contact angle on the basis of capillary pressure equation, which made the adsorbed water come out easily from the micropores of coal. The variation of adsorbed water was in fluctuation before CO2 passed through the sample. After CO2 passed through the sample, it showed a decreasing trend and a better linear relationship (Figure 3), indicating that the wettability of coal surface was changed in the presence of CO2. However, some of the desorbed water was adsorbed again because of flooding pressure 9


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before CO2 passed through the sample.13 The CO2 and flooding pressure led to the fluctuation in the initial stage. After CO2 passed through the sample, both the original free water and transformation of water were discharged. Therefore, the signal of adsorbed water was reduced. 3.1.2. Water flooding and gas flooding of LA 1) Water flooding As shown in Figure 4, the change of adsorbed water was very small, showing that the sample with low permeability was difficult to displace. The adsorbed water stopped increasing when the confining pressure and flooding pressure increased to 10 MPa and 9 MPa, respectively. It could be concluded that stress sensitivity occurred when the confining pressure increased to 10 MPa. Firstly, the rate of saturated water in coal sample was not so fast. Moreover, water did not come out during the flooding process. Besides, the signal of adsorption water changed very sharply when the confining pressure increased to 10 MPa. Based on the above reasons, the stress sensitivity was determined in this study. It resulted in the closure of some fractures in the coal sample, which led to a further reduction of the permeability. These reasons made it difficult for water to flow. 2) CO2 flooding The permeability of the coal sample with the confining pressure of 10 MPa was lower than that of the original coal sample, which represents a lower permeability sample. As observed from Figure 4c and d, the adsorbed water was decreased, but the decrease was not very obvious after about 12 h. It inferred that the wettability of CO2 in the coal sample increases under high pressure, resulting in a decrease in the adsorbed water signal.20, 21 In addition, gas did not come out in the whole process, demonstrating that gas was difficult to transport in the coal sample. It is noteworthy that although the displacement pressure was higher in sample LA than that in sample 10


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DN in the end, the displacement pressure was basically the same at the beginning, and the confining pressure of LA was greater than that of DN, leading to further lower permeability of LA. This corresponded to a comparison with a lower permeability sample than LA. However, there was no gas coming out from the outlet after a period of time. Then the displacement pressure was adjusted to 5.86 MPa gradually. The difficulty of fluid migration in ultra-low permeability coal sample was highlighted. In addition, water could hinder the migration of gas. Meanwhile, the water content in sample LA was lower than that of sample DN. In theory, the migration of gas should be faster in LA. However, the migration of gas in LA was very slow, which also showed that the fluid migration was very difficult for the coal sample with an extremely low permeability. 3.1.3. The effect of CO2 on adsorbed water of coal surface Because of the device being interlinked with the outside in the above, the content of water varied irregularly. Therefore, it was not confirmed that the adsorbed water was converted to free water. Given that, the experiment in a closed system was designed. As presented in Figure 5a, according to the principle of T2 spectrum, S1, S2 and S3 represent the adsorption water, capillary bound water and free water, respectively. The results are consistent with Guo et al.27 The adsorbed water mainly existed in the sample, and the capillary bound water and free water were seldom. After a period of time, the state of water in the sample changed obviously (Figure 5). The adsorbed water decreased, and the free water increased significantly. Simultaneously, the capillary bound water and the total of water (St) for three states remained basically unchanged. This is because the compressibility of water is very small, and the capillary bound water only acted as a transient. The result confirmed that the adsorbed water was gradually transformed into free water. Therefore, when CO2 is in contact with the coal surface, it will change the wettability of coal 11


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surface and convert the adsorbed water into free water as well as replace CH4. Accordingly, CH4 is also gradually desorbed with the production of water. Based on the comparison of the two flooding experiments, it is found that the permeability has important influence on the flooding effect. CO2 could quickly pass through the sample DN at 4 MPa. However, CO2 did not drive through the sample LA at about 6 MPa after 12 h. Therefore, it is very difficult for gas and water to transport in ultra-low permeability reservoirs. After the flooding, the permeability of sample LA was tested under the same conditions. It is 0.00665 mD which is lower than that before the flooding. According to the change of the adsorbed water in the late period of water flooding, the stress sensitivity was caused with increasing confining pressure, which resulted in the decline of permeability. The results show that the stress sensitivity in the ultra-low permeability reservoir is more obvious, and the migration of CO2 in the reservoir is more difficult. Correspondingly, the coal sample became more CO2-wetting with increasing pressure,21 and the coal matrix adsorbed more CO2 due to the reduction of adsorbed water, which caused the swelling of coal matrix in turn and decreased the permeability. 3.2. The effect of CO2 aqueous solution and minerals in coal The purpose of this experiment is to show that CO2 aqueous solution can react with minerals in coal to improve the permeability of the reservoir in a certain range of permeability, and its effect is not obvious for the ultra-low permeability reservoir. CO2 aqueous solution was prepared according to Figure 2. The flow rate of ISCO pump could reach 21.18 mL/h before the displacement, which indicated that CO2 was quickly dissolved in formation water. The average flow rate of ISCO pump was 0.24 mL/h after 14 h. Given that, this study concluded that CO2 and formation water tended to be a balance. Then the flooding of CO2 aqueous solution was begun. 12


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The average flow rate of the pump was in fluctuation during the displacement, revealing that the flooding fluid constantly broke through resistance. The flooding fluid suffered great resistance when it reached the core due to low permeability of the coal sample. Therefore, the pressure of flooding fluid began to drop and broke the balance between CO2 and formation water, and some CO2 escaped from the formation water. Some CO2 was adsorbed by the coal surface, and the other part of CO2 flowed to the downstream through the fracture of the sample. According to formula (2) and (3), the effective permeability of water and gas could be obtained.

Kw =

Kg =

qw ⋅ µ w ⋅ L ⋅100 A ⋅ ( p1 − p2 ) 2 ⋅ p2 ⋅ q g ⋅ µ g ⋅ L 2

2

A ⋅ ( p1 − p2 )

(2)

⋅ 100

(3)

where Kw and Kg are the effective permeability of water and gas, respectively (×10-3 µm2); qw and qg are the flow rate of water and gas, respectively (mL/s); µw and µg are the viscosity of water and gas respectively (mPa•s); L is the length of the sample (cm). A is the sectional area of the sample (cm2); p1 is the inlet pressure of the sample (MPa); p2 is the outlet pressure of the sample (this text takes 0.1 MPa). The water flow rate and gas flow rate at the outlet during the displacement process are shown in Figure 6. Considering the injection pressure and outlet pressure, respectively, 2MPa and 0.1MPa, the CO2 viscosity was taken as the average value. The viscosities of CO2 and formation water are 15.35 × 10-6 Pa•s and 0.8007 × 10-3 Pa•s respectively at 30 °C.29, 30 The size of the sample can be regarded as constant. Therefore, water flow rate and gas flow rate are proportional to their respective effective permeability under the condition of constant flooding pressure. In this context, the dynamic change of the effective permeability can be reflected by the change of the 13


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liquid flow rate. As observed from Figure 6, the flow rate of water could reach 1.44 mL/h in the initial stage of flooding, and then declined rapidly. Finally, the average flow rate of water was 0.31 mL/h. The effective permeability of water decreased exponentially with time (Figure 7). The correlation coefficient of formula (3) is up to 0.915. The variation trend of formula (4) is consistent with the change of the permeability with effective stress. The reason is that the internal stress generated by the swelling is superimposed on the original stress, making the effective permeability of water decline exponentially with time.

K w = 0.0141 ⋅ exp(−0.0511 ⋅ t ) + 0.044

(4)

where t is time (h). The flow rate of gas was unstable, and the change of the flow rate was undulate with time, which reflected the complexity of seepage and diffusion of gas in the coal sample. From 38 h to the end of the flooding, the cumulative volumes of ISCO pump fluid, collected water and collected gas were 37.06 mL, 35.66 mL and 36.15 cm3, respectively. Taking into account the solubility of CO2,31 if there is no adsorption, more gas should be collected. Given that, some CO2 was adsorbed on the pore surface of coal matrix during this period, resulting in the swelling of coal matrix. In order to prove the reactions between CO2 aqueous solution and minerals in coal, it is necessary to analyze the collected water. Cation concentration of the original solution and the collected solution was tested by ICP-700 inductively coupled plasma spectrometer (Figure 8). As shown in Figure 8, S and RF represent the formation water before and after reaction, respectively. The concentration of iron ion (Fe2+, Fe3+) was very little, and Al3+, Ca2+, K+, Mg2+ 14


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and Na+ were significantly changed, demonstrating that the reactions between CO2 aqueous solution and minerals reacted strongly. Compared with the original dry coal sample, the permeability is increased by 2.12 times at 2 MPa of confining pressure after the reactions (Figure 9). However, the change of permeability is very small with increasing effective stress. The positive effects of mineral dissolution and flooding pressure on the permeability of the coal sample can be revealed when the permeability is good. The swelling and stress sensitivity of the matrix are more significant in the ultra-low permeability and deep reservoir, which can result in the decline of permeability. Although high temperature in the deep is conducive to the injection of CO2, detailed argument is also necessary. CO2 can change the wettability of coal surface, which can transform the adsorbed water into free water. In addition, according to previous studies, CO2 can displace CH4 and change it into free methane.8 Besides, CO2 aqueous solution can react with minerals in the coal, which is beneficial to the improvement of the reservoir permeability in a certain extent. Furthermore, the displacement pressure can cause some fractures, which can increase the permeability of the reservoir. However, CO2 has been adsorbed on the coal surface in the whole process. The amount of adsorbed CO2 is much higher than the original amount of adsorbed water and adsorbed methane, which can cause the swelling of the coal matrix and is very unfavorable for reservoir. Although there are some positive effects for ultra-low permeability reservoirs, the negative effects caused by matrix swelling cannot be offset. Besides, the migration of fluid was very difficult when the adsorption expansion of CO2 replacement CH4 was not considered. Therefore, the permeability will decrease faster after considering the swelling effect of this part. This is one of the reasons for the failure of many field trials. Therefore, the technology of CO2-ECBM should be taken into 15


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consideration in the exploitation of ultra-low permeability reservoirs.

4. CONCLUSIONS According to the flooding experiment, the reactions of CO2 injection into coal reservoir were discussed. The wettability of coal surface is changed, and the adsorbed water is converted into free water. On the other hand, because of the stronger adsorption capacity of CO2, the adsorbed CH4 is converted into free methane. CO2 is adsorbed on the coal surface during these two processes, resulting in the swelling of coal matrix. In addition, the reactions between CO2 aqueous solution and minerals, and the effect of displacement pressure can improve the permeability of the reservoir. In contrast, the effect of adsorption swelling is more significant for the ultra-low permeability reservoir. Moreover, the stress sensitivity in the deep coal reservoir is very strong. Coupled with the adsorption swelling, permeability will be very low and the migration of fluid will become more difficult. Therefore, it is not proper to apply the technology of CO2-ECBM. Additionally, the comprehensive test of various reactions during the injection could be achieved through continuous improvement of experimental equipment. NOTES The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (41572140), the National Major Special Project of Science and Technology of China (2016ZX05044), the Fundamental Research Funds for the Central Universities (2015XKZD07), and the Qing Lan Project.

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64-75.

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Page 19 of 23

Figures

Figure 1. Schematic diagram of gas flooding process.

Figure 2. Schematic diagram of CO2-saturated water flooding.

(a) 3000

(b) 25500 0h 6h 12 h

2500

25000

2000

Intensity (p.u.)

Amplitude (p.u.)

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

1500 1000 500 0 0.1

24500

y=-172.63x+24781.56 2 R =0.8724

24000 23500 23000

1

10 100 T2 (ms)

1000

10000

22500

0

2

4

6 8 Time (h)

10

12

14

Figure 3. Changes of adsorbed water in the process of CO2 flooding (DN). (a) T2 spectra; (b) intensity of absorbed 19


Energy & Fuels

water.

(a) 5000

(b) 24000 0.25 h 12 h 18.5 h

23700 Intensity (p.u.)

Amplitude (p.u.)

4000 3000 2000 1000

1

10 100 T2 (ms)

1000

Stress sensitivity

23100

22500

10000

(c) 5000

0

2

4

6

8 10 12 14 16 18 20 Time (h)

(d) 24500 0.5 h 6h 11.5 h

24400 24300 Intensity (p.u.)

4000 Amplitude (p.u.)

23400

22800

0 0.1

3000 2000 1000

24200 24100 24000 23900 23800

0 0.1

1

10

100

1000

10000

23700

0

2

4

T2 (ms)

6 8 Time (h)

10

12

Figure 4. Change of adsorbed water during flooding (LA). (a) T2 spectra of water flooding; (b) intensity of absorbed water during water flooding; (c) T2 spectra of CO2 flooding; (b) intensity of absorbed water during CO2 flooding.

S1

0h 3h 22.5 h 30.5 h

2000 1500 S3

1000 500 0 0.1

S2 1

10 100 T2 (ms)

1000

(b) 20000 St 16000 Intensity (p.u.)

(a) 2500

Amplitude (p.u.)

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 23

8000 S3

4000 0

10000

S1

12000

S2 0

5

10

15 20 Time (h)

25

30

35

Figure 5. T2 spectrograms of water at different times (DN in an enclosed space). (a) amplitude of water in different

20


Page 21 of 23

states; (b) intensity of water in different states.

1.6

Flow rate of water Flow rate of gas

1.4 Flow rate (mL/h)

1.2 1.0 0.8 0.6 0.4 0.2 0.0

0

20

40

60 80 Time (h)

100

120

140

Permeability (mD)

Figure 6. Change of liquid flow rate.

0.020 0.018 0.016 0.014 0.012 0.010 0.008 0.006 0.004 0.002

Kw=0.0141exp(-0.0511t)+0.044 2

R =0.915

0

20

40

60 80 100 120 140 Time (h)

Figure 7. Change of effective permeability.

Concentration (mmol/L)

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

27 24 21 18 15 12

S

RF

0.3 0.2 0.1 0.0

Al

Ca

Fe K Ions

Mg

Na

Figure 8. Change of ion concentration of collected liquid.

21


Energy & Fuels

0.30 Permeability (mD)

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

Original After reaction

0.25

y=0.07+2.32exp(-1.22x) 2 R =0.9901

0.20 0.15

y=0.06+0.2exp(-0.89x) 2 R =0.9971

0.10 0.05

2

3 4 5 6 Confining pressure (MPa)

7

Figure 9. Change of permeability for dry sample at different confining pressures.

22


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Page 23 of 23

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

Table Table 1 The parameters of coal sample. Ro,max

Mar

Ad

Vdaf

Porosity

Permeability

Chlorite

Pyrite

Calcite

Quartz

(%)

(%)

(%)

(%)

(%)

(mD)

(%)

(%)

(%)

(%)

DL

2.89

3.37

12.25

5.50

5.06

0.03590

-

-

-

-

LA

2.85

2.56

9.18

6.39

4.00

0.00679

-

-

-

-

RF

3.54

1.10

18.70

5.70

8.02

0.06140

15

10

2

3

Mar is the moisture content when received, Ad is the ash yield after drying, Vdaf means yield of volatiles in the dry ash free state, and Ro,max represents that the mean maximum vitrinite reflectance in oil immersion.

Table 2 The ion content of the original formation water (mmol/L). Al

Ca

Fe

K

Mg

Na

0.00032

0.086742

0.000205

0.093439

0.009634

11.20799

23


Feasibility and applicability analysis of CO2-ECBM based on CO2

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