Academic Journal of Science and Technology ISSN: 2771-3032 | Vol. 11, No. 1, 2024 174 The Response Characteristics of Pore Volume and Specific Surface Area of Different Phases of Carbon Dioxide Injection into Anthracite Coal Pores Jiale Yi School of Resources & Environment, Henan Polytechnic University, Jiaozuo 454000, China Abstract: After the coal sample reacts with CO2, the porosity and pore volume change. Compared with dry raw coal sample, the porosity of coal after CO2 adsorption is increased. With the increase of CO2 injection pressure, the porosity of coal will gradually increase, especially when CO2 enters the supercritical phase. In terms of pore capacity of coal samples, CO2 adsorption time, CO2 adsorption pressure and CO2-H2O treatment do not show obvious regularity in the changes of pore capacity of coal samples, but in general, the increase of CO2 adsorption pressure and CO2-H2O treatment have a positive effect on the increase of pore capacity of coal samples, especially ScCO2-H2O. The mesoporous pore capacity increased by 137.1%. In terms of macro pore volume of coal sample, the change of macro pore volume after CO2 adsorption is more regular. Extending CO2 adsorption time, increasing CO2 adsorption pressure and CO2-H2O treatment all have positive effects on the increase of macro pore volume of coal sample. Under 8MPaScCO2-H2O, macro pore volume reaches a maximum of 0.016462ml/g. The increase also reached 130.4 percent. On the mesoporous specific surface area of coal sample, increasing CO2 adsorption pressure, prolongating CO2 adsorption time and CO2-H2O treatment can increase the mesoporous specific surface area of the sample. The mesoporous specific surface area of the sample at 8 MPa ScCO2-H2O is 0.631 m2/g, which reaches the maximum in this experiment, with an increase of 334.27%. Keywords: Porosity, Pore volume, Pore specific surface area, ScCO2. 1. Introduction Coal reservoirs, as a type of porous medium, are composed of interconnected pores and fractures. The development characteristics and connectivity of these pores and fractures directly impact the desorption, diffusion, permeation processes of methane, as well as the production characteristics, thereby determining the effectiveness of CO2 displacement of CH4 after injection into coal seams. Currently, research on coal pore and fracture characteristics mainly focuses on: investigating the development characteristics of pore and fracture networks in coal reservoirs [1, 2]; studying the effects of pores and fractures on gas adsorption/desorption, fluid migration, gas content, permeability, coalbed methane production, and effective implementation of CO2-ECBM based on various models. The genesis types and complex morphologies of coal pores vary widely in size. Scholars both domestically and abroad have conducted extensive research on the classification of coal pore structures for different research purposes. Among them, the most widely used in China's coalbed methane industry is the IUPAC classification system based on coal adsorption properties: micropores (<2 nm); mesopores or transitional pores (2~50 nm); macropores (>50 nm), and the decimal classification system proposed based on industrial adsorbents: micropores (<10 nm); transitional pores (10~100 nm); mesopores (100~1000 nm); macropores (>1000 nm). Researchers believe that the genesis, morphology, and development characteristics of coal pores are closely related to coal maceral composition, metamorphism, and deformation [3]. 2. Sample and Methods 2.1. Sample collection and preparation The samples used in the experimental research of this paper were taken from the Datong Mine No. 3 coal mine in Gaoping City, Shanxi Province, China. These coal samples were all taken from adjacent positions of the same large block. To ensure the accuracy and reliability of the experiments, three small pieces of original samples were randomly placed in sealed bags. The reflectance of vitrinite and microcomponents were tested using the Axio Scope.A1 multifunctional microscope produced by ZEISS company. The experimental conditions were room temperature at 23°C and humidity at 60%. The tests were conducted according to the recommended standards of the petroleum industry SY/T 5124-2012 and SY/T 5124-1995. Table 1 shows the test results of this experiment. The results indicate that the random average reflectance of vitrinite is 2.34%, 2.29%, and 2.16% respectively, indicating high-rank coal. Microcomponent analysis shows that these samples are predominantly vitrinite, accounting for 97.5%, 96.0%, and 97.3% respectively, with some inertinite, accounting for 2.5%, 4.0%, and 2.7% respectively. The total organic matter accounts for 93.1%, 95.4%, and 96.3% respectively, pyrite accounts for 0.1% in all cases, and other minerals account for 6.8%, 4.5%, and 3.6% respectively. 175 Table 1. Basic information of the coal samples. Sample source Total rock (%) maceral (%) Vitrinite reflectance (%) Total organic matter Pyrite Other minerals Liptinite Vitrinite Inertinite Min Max Average value The 3# coal of Datong Mine 93.1 0.1 6.8 / 97.5 2.5 2.11 2.51 2.34 95.4 0.1 4.5 / 96.0 4.0 2.21 2.45 2.29 96.3 0.1 3.6 / 97.3 2.7 1.99 2.35 2.16 2.2. Water injection and gas injection experiments All samples in the experimental procedure were subjected to 24 hours of drying at 105°C in an oven to remove all moisture before conducting the triaxial experiments. Then, according to the experimental plan, the samples were prepared under the corresponding CO2 pressure. After the pressure gauge readings stabilized, continuous CO2 injection was carried out for 3 days and 6 days at the given pressure and temperature [5, 6]. Studies have shown that the structure of coal still undergoes slow changes even after one year of CO2 injection, but the main interaction between coal and CO2 is completed within the first few days. Therefore, considering the timeliness of CO2 adsorption, this study adopted continuous injection times of 3 days and 6 days. Table 2. Experimental conditions for coal powder. Coal sample identification number Experimental temperature (℃) Moisture condition Gas injection type Gas injection time (h) Gas injection pressure (MPa) 1 40 Dry / / / 2 Dry CO2 72h 4MPa 3 Dry CO2 72h 6MPa 4 Dry CO2 72h 8MPa 5 Dry CO2 144h 4MPa 6 Dry CO2 144h 6MPa 7 Dry CO2 144h 8MPa 8 Immersion water CO2 144h 4MPa 9 Immersion water CO2 144h 6MPa 10 Immersion water CO2 144h 8MPa 2.3. Low-temperature N2 adsorption experiment The low-temperature nitrogen adsorption experiment was conducted using the ASAP 2460 multi-station automatic surface area and pore size analyzer manufactured by Micromeritics Instrument Corporation in the United States, following the international standards ISO 15901-2:2006 and ISO 15901-3:2007. The sample used in the experiment was coal powder with a particle size of 0.18-0.25 mm. Nitrogen was used as the adsorption medium, with the analysis bath temperature set at -195.85°C and the relative pressure range from 0.005 to 0.996. In the analysis of nitrogen adsorption data, the multi-layer adsorption model proposed by Brumauer-Emmett-Teller (BET) was used to calculate the specific surface area, and the Barrett-Joyner-Halenda (BJH) equation was used to determine the pore size. The BET model is a widely used theoretical tool for calculating adsorption isotherms, suitable for studying the adsorption behavior of adsorbent surfaces in gases or solutions. Figure 1. Physical diagram of the low-temperature N2 adsorption experimental instrument. 2.4. High-pressure mercury experiment. The mercury intrusion porosimetry test gradually injects liquid mercury into the pores under external pressure. Smaller pore sizes require higher pressure to be filled. The experiment utilized the AutoPore IV 9520 fully automatic mercury porosimeter from Micromeritics Instrument Company, USA, following the international standard ISO 15901-1:2005. The coal sample used in the experiment was small coal blocks with a volume of about 3-4 cm³. During the experiment, the injection pressure of mercury ranged from 0.0099 MPa to 176 413.46 MPa. The measurement lower limit was 5 nm, employing a computer-program-controlled point-by-point measurement method, and the samples were dried for 24 hours before the experiment. The pore size calculation employed the Washburn equation, with a surface tension of mercury set at 0.48 N/m and a contact angle between mercury and coal set at 140°. Fig. 2 High-pressure mercury intrusion porosimetry instrument physical diagram. 3. Results and Discussion 3.1. Changes in porosity and pore volume of coal after CO2 injection. Based on previous research findings, low-temperature N2 adsorption experiments based on the BJH model are more accurate for characterizing pores smaller than 50nm, while there are significant errors in the high-pressure mercury intrusion method within the range of less than 30nm. Therefore, in order to obtain maximum experimental accuracy, 50nm is selected as the dividing point for the two aforementioned experimental methods for joint analysis (pores with diameters larger than 50nm are characterized using high-pressure mercury intrusion experimental data, while pores smaller than 50nm are characterized using N2 adsorption experimental data). The division of pore size is very important for studying the pore distribution characteristics of coal samples, and a targeted pore size division scheme can clearly reflect the distribution characteristics of different-sized pores. To ensure the accuracy of this experiment, the multi-scale pore structure characteristics of coal samples were studied based on low- temperature N2 adsorption experiments and high-pressure mercury intrusion experiments, and the pores were divided into mesopores (2~50nm) and macropores (larger than 50nm). Table 3. The porosity and pore volume of coal samples before and after reaction. Number Mercury Intrusion Porosity (%) Porosity Growth (%) Mesopores (×10-4ml/g) Mesopore Growth (%) Macropores (×10-3ml/g) Macropore Growth (%) Total Pore Volume (×10- 3ml/g) Total Pore Volume Growth (%) 1 3.50 - 2.48 - 6.897 - 7.145 - 2 3.69 5.43 3.17 27.82 7.903 14.59 8.22 15.05 3 4.09 16.87 1.43 -42.34 10.588 53.52 10.731 50.19 4 4.13 17.98 6.4 158.06 10.981 59.21 11.621 62.65 5 4.21 20.31 4.51 81.85 10.969 59.04 11.42 59.83 6 4.24 21.06 1.74 -29.84 11.28 63.55 11.454 60.31 7 4.38 25.13 4.23 70.56 98.43 42.17 10.266 43.68 8 4.52 29.02 3.57 43.95 13.299 92.82 13.656 91.13 9 4.64 32.58 1.4 -43.55 13.538 96.29 13.678 91.43 10 4.97 41.95 5.88 137.10 15.874 130.16 16.462 130.40 Figure 3. Change in Mercury Intrusion Porosity of Coal Samples Before and After Reaction. The porosity of coal seams is an important parameter for evaluating permeability. In this experiment, the porosity was measured using the mercury intrusion method. With the CO2 adsorption time remaining constant (72 hours), as the CO2 adsorption pressure increases, the porosity of the coal sample gradually increases. When the adsorption pressure increases from 4 MPa to 6 MPa and then to 8 MPa, the porosity increases from the initial 3.5% to 4.13%, showing an increase of 17.98%. Similarly, with a CO2 adsorption time of 144 hours, as the CO2 adsorption pressure increases from 4 MPa to 6 MPa and then to 8 MPa, the porosity of the coal sample gradually increases from the initial 3.5% to 4.38%, showing an increase of 25.13%. It can be seen that the mercury intrusion porosity of the coal sample increases with the increase in CO2 adsorption pressure. When the CO2 adsorption pressure is held constant (4 MPa) and the adsorption time is extended from 72 hours to 144 hours, the porosity of the sample increases from 3.69% to 4.21%. Similarly, when the CO2 adsorption pressure is 6 MPa, extending the CO2 adsorption time increases the porosity of 177 the sample from 4.09% to 4.24%. When the CO2 adsorption pressure is 8 MPa, extending the CO2 adsorption time increases the porosity of the sample from 4.13% to 4.38%. Therefore, it can be observed that extending the adsorption time under constant CO2 adsorption pressure also increases the mercury intrusion porosity of the coal sample. This experiment also considered the influence of the coupling factors of water and CO2. After treatment with 4 MPa CO2- H2O, the porosity of the sample was 4.52%, after treatment with 6 MPa CO2-H2O, the porosity was 4.64%, and after treatment with 8 MPa CO2-H2O, the porosity was 4.97%. It can be seen that their porosity is greater than that of samples adsorbed with CO2 alone, and the porosity also increases with the increase in CO2 adsorption pressure. After treatment with supercritical CO2 and water, sample 10 reached the maximum porosity (4.97%) in this experiment, with the maximum increase of 41.95%. Therefore, extending the CO2 adsorption time, increasing the CO2 adsorption pressure, and performing CO2 adsorption after water immersion will increase the mercury intrusion porosity of the coal sample. Figure 4. Changes in Porosity of Coal Samples Before and After Reaction. The porosity in coal is mainly contributed by macropores, while mesopores have smaller porosity, and the porosity of some samples decreases after CO2 adsorption. Mesopore porosity shows varying degrees of increase and decrease after CO2 adsorption, while macropores only show different degrees of enlargement after CO2 adsorption. From the perspective of CO2 adsorption time, the mesopore porosity of coal samples adsorbed with 4 MPa CO2 shows an increasing trend, specifically, the mesopore porosity increases from 0.000317 ml/g to 0.000451 ml/g after extending the adsorption time. The mesopore porosity of coal samples adsorbed with 6 MPa CO2 shows a decreasing trend. The mesopore porosity of coal samples adsorbed with 8 MPa CO2 increases, but with extended adsorption time, the increase in mesopore porosity decreases, from 158.06% to 70.56%. This data indicates that the change in mesopore porosity due to CO2 adsorption time is not regular. From the perspective of CO2 adsorption pressure, when adsorbed for 72 hours, 4 MPa CO2 adsorption increases the mesopore porosity of the sample to 0.000317 ml/g, 6 MPa CO2 adsorption decreases it to 0.000143 ml/g, and 8 MPa CO2 adsorption increases it to a maximum of 0.00064 ml/g. When adsorbed for 144 hours, 4 MPa CO2 adsorption increases the mesopore porosity of the sample to a maximum of 0.00451 ml/g, 6 MPa CO2 adsorption decreases it to 0.000174 ml/g, and 8 MPa CO2 adsorption increases it to 0.000423 ml/g. Overall, increasing CO2 adsorption pressure promotes an increase in mesopore porosity in coal samples.Additionally, this experiment considered the effect of CO2 adsorption on mesopore porosity of coal samples after water immersion. Overall, CO2-H2O promotes an increase in mesopore porosity of coal samples, especially ScCO2-H2O, which increases mesopore porosity by 137.1%, reaching 0.000588 ml/g. After CO2 adsorption, the pattern of macropores in the samples becomes more pronounced. From the perspective of CO2 adsorption time, the macropore porosity of coal samples adsorbed with 4 MPa CO2 shows an increasing trend, with the macropore porosity increasing from 0.00822 ml/g to 0.01142 ml/g after extending the adsorption time. The macropore porosity of coal samples adsorbed with 6 MPa CO2 also shows an increasing trend, with the macropore porosity increasing from 0.010731 ml/g to 0.011454 ml/g after extending the adsorption time. The macropore porosity of coal samples adsorbed with 8 MPa CO2 also increases, but after extending the adsorption time, the increase in macropore porosity decreases slightly, from 62.65% to 43.68%. Therefore, extending CO2 adsorption time under constant pressure has a positive effect on increasing macropore porosity in samples.From the perspective of CO2 adsorption pressure, when adsorbed for 72 hours, the macropore porosity increases with increasing CO2 adsorption pressure, specifically, the macropore porosity is 0.00822 ml/g at 4 MPa, 0.010731 ml/g at 6 MPa, and 0.011621 ml/g at 8 MPa. Similar trends are observed when adsorbed for 144 hours, with the exception of a smaller increase in macropore porosity at 8 MPa. This data indicates that increasing CO2 adsorption pressure also has a positive effect on increasing macropore porosity in samples.After water immersion, followed by CO2 adsorption, the data indicates that higher adsorption pressure results in larger macropore porosity, with the macropore porosity being 0.013656 ml/g at 4 MPa CO2-H2O, 0.013678 ml/g at 6 MPa CO2-H2O, and reaching a maximum of 0.016462 ml/g at 8 MPa ScCO2-H2O, with an increase of 130.4%. Additionally, the total porosity of coal samples is mainly contributed by macropores, so the variation in total porosity is similar to that of macropores. Therefore, extending CO2 adsorption time, increasing CO2 adsorption pressure, and performing CO2 adsorption after water immersion all have a positive effect on increasing the total porosity of coal samples. 3.2. Change characteristics of pore specific surface area of coal after CO2 injection The gas adsorption capacity of coal is closely related to the pore volume and specific surface area of coal, especially the pore volume and specific surface area of nanopore. In this experiment, changes in specific surface area of different apertures were analyzed based on low-temperature N2 adsorption experiment and high-pressure mercury injection experiment. The specific surface area of mesoporous pores was the BET specific surface area tested by liquid nitrogen method, and that of macroporous pores was the specific surface area tested by mercury injection method. The distribution of specific surface area of different apertures before and after CO2 adsorption was shown in Table 4. 178 Table 4. Pore specific surface area before and after coal sample reaction. Number mesoporous (m2/g) Mesoporous increase (%) macrohole (m2/g) Macrohole increment (%) Total pore specific surface area (m2/g) Total pore specific surface area increase (%) 1 0.1453 - 0.137465 - 0.282765 - 2 0.1513 4.13 0.141311 2.8 0.292611 3.48 3 0.1598 9.98 0.129163 -6.04 0.288963 2.19 4 0.1669 14.87 0.146002 6.21 0.312902 10.66 5 0.1857 27.8 0.12622 -8.18 0.31192 10.31 6 0.1988 36.82 0.150999 9.85 0.349799 23.71 7 0.2539 74.74 0.132318 -3.74 0.386218 36.59 8 0.298 105.09 0.116302 -15.4 0.414302 46.52 9 0.4964 241.64 0.11801 -14.15 0.61441 117.29 10 0.631 334.27 0.13198 -3.99 0.76298 169.83 Figure 5. Changes of specific surface area before and after coal sample The experimental data show that the total pore specific surface area of coal samples in this experiment is mainly contributed by mesoporous pores. The mesoporous specific surface area of the raw coal sample is 0.1453 m2/g, and the mesoporous specific surface area increases to 0.1513 m2/g under the adsorption condition of 4 MPa CO2 for 72 hours, and 0.1598 m2/g under the adsorption condition of 6 MPa CO2 for 72 hours. Under the condition of 8 MPa CO2 adsorption for 72 hours, the specific surface area of the mesopore increased to 0.1669 m2/g. It can be seen that under the same CO2 adsorption time, the specific surface area of the coal sample mesoporous will also increase with the increase of CO2 adsorption pressure, which can also be clearly seen in Fig.3-3. This is also reflected in the case of 144 hours of CO2 adsorption. The specific surface area of the mesopore increases to 0.1857 m2/g at 4 MPa CO2 adsorption for 144 hours, and 0.1988 m2/g at 6 MPa CO2 adsorption for 144 hours. Under the condition of 8 MPa CO2 adsorption for 144 hours, the specific surface area of the mesopore increased to 0.2539 m2/g, and the increase rate reached 74.74%. When the CO2 adsorption pressure is 4 MPa, the adsorption time is extended from 72 hours to 144 hours, and the specific surface area of the mesopore is also increased to 0.1857 m2/g. It can be seen that under the condition that the CO2 adsorption pressure remains unchanged, the specific surface area of mesoporous pores will increase with the extension of CO2 adsorption time. This is also reflected in the case of 6 MPa and 8 MPa, when the CO2 adsorption pressure is both 6 MPa, the adsorption time is extended from 72 hours to 144 hours, and the specific surface area of the mesopore is increased from 0.1598 m2/g to 0.1988 m2/g. When the CO2 adsorption pressure is 8 MPa, the adsorption time is extended from 72 hours to 144 hours, and the specific surface area of the mesopore is increased from 0.1669 m2/g to 0.2539 m2/g. The increase of mesoporous specific surface area of the samples treated with CO2-H2O was significantly higher than that of the samples adsorbed CO2 alone. In addition, when the samples were treated with CO2-H2O, the mesoporous specific surface area of the samples increased significantly with the increase of adsorption pressure. The mesoporous specific surface area of the sample at 4 MPa CO2-H2O is 0.298 m2/g, that of the sample at 6 MPa CO2-H2O is 0.4964 m2/g, and that of the sample at 8 MPa ScCO2-H2O is 0.631 m2/g, reaching the maximum in this experiment. The increase reached 334.27%. Only the macropore specific surface area of 4 MPa CO2 adsorbed for 72 hours, 8 MPa CO2 adsorbed for 144 hours and 8 MPa CO2 adsorbed for 72 hours was larger than the original macropore specific surface area, and the increasing trend was not regular from the three variables. The macropore specific surface area of the other samples is smaller than that of the original, but the decreasing trend is not consistent. Therefore, whether the CO2 adsorption time is prolonged, the CO2 adsorption pressure is increased, or the specific surface area of the sample is treated with CO2-H2O, the specific surface area of the sample shows inconsistent rules. Therefore, the adsorption of CO2 has no regularity on the macro pore specific surface area of the coal sample. Since the total pore specific surface area of coal is contributed by mesoporous pores, its change trend is similar to that of mesoporous specific surface area, that is, increasing CO2 adsorption pressure, prolonging CO2 adsorption time and CO2-H2O treatment will increase the total pore specific surface area of coal samples. 4. Conclusions (1) After the coal sample reacts with CO2, the porosity and pore volume change. Compared with dry raw coal sample, the porosity of coal after CO2 adsorption is increased. With the increase of CO2 injection pressure, the porosity of coal will gradually increase, especially when CO2 enters the supercritical phase. For example, when CO2 is adsorbed for 4MPa72 hours, the porosity of the coal sample is 3.69%, and when CO2 is adsorbed for 8MPa72 hours, the porosity of the coal sample is 4.13%. With the increase of CO2 injection time, the porosity of coal will also increase. In addition, under the coupling effect of CO2 and water, the porosity of coal will further increase. After the treatment of supercritical CO2 and water, the porosity of sample 10 reaches the maximum in this 179 experiment (4.97%), and the increase rate also reaches the maximum (41.95%). (2) In terms of the pore capacity of coal samples, CO2 adsorption time, CO2 adsorption pressure and CO2-H2O treatment do not show obvious regularity in the changes of pore capacity of coal samples, but in general, the increase of CO2 adsorption pressure and CO2-H2O treatment have a positive effect on the increase of pore capacity of coal samples, especially ScCO2-H2O. The mesoporous pore capacity increased by 137.1%, reaching 0.000588ml/g. In terms of macro pore volume of coal sample, the change of macro pore volume after CO2 adsorption is more regular. Extending CO2 adsorption time, increasing CO2 adsorption pressure and CO2-H2O treatment all have positive effects on the increase of macro pore volume of coal sample. Under 8MPaScCO2-H2O, macro pore volume reaches a maximum of 0.016462ml/g. The increase also reached 130.4 percent. In addition, the total pore volume of coal samples is mainly contributed by macro pores, so the change of total pore volume is basically the same as that of macro pores. (3) For the mesoporous specific surface area of coal samples, increasing CO2 adsorption pressure, prolongating CO2 adsorption time and CO2-H2O treatment can increase the mesoporous specific surface area of the sample. The mesoporous specific surface area of the sample at 8 MPa ScCO2-H2O is 0.631 m2/g, reaching the maximum in this experiment, with an increase of 334.27%. On the macropore specific surface area of coal sample, the adsorption of CO2 has no regularity. Since only the specific surface areas of mesoporous and macro pores are studied in this experiment, the total pore specific surface area of coal in this experiment is contributed by mesoporous pores, and the change of total pore specific surface area is basically consistent with that of mesoporous specific surface area. 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