desorption Behavior of CH4 on Shale during CO2 Huff-and

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Adsorption/desorption Behavior of CH4 on Shale during CO2 Huff-and-Puff Process Ligong Li, Chao Li, and Tianhe Kang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00920 • Publication Date (Web): 12 May 2019 Downloaded from http://pubs.acs.org on May 12, 2019

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Adsorption/desorption Behavior of CH4 on Shale during CO2 Huff-and-Puff Process

Ligong Lia, Chao Lia, Tianhe Kanga* aKey

Laboratory of In-situ Property-improving Mining of Ministry of Education, Taiyuan University of Technology, Taiyuan, Shanxi, China, 030024

*Corresponding author: Tianhe Kang Professor College of Mining Engineering Taiyuan University of Technology Tel: 0351-6014627 E-mail: [email protected] 1 ACS Paragon Plus Environment

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Abstract CO2 huff-and-puff method has been widely adopted for enhancing CH4 recovery in shale reservoirs. Revealing the behavior of CH4 adsorption/desorption in shale during CO2 huff-andpuff process clarifies the recovery mechanisms of CH4 from shale reservoirs. In our work, the question of how CO2 plays a role in affecting the adsorption/desorption of CH4 is investigated using low-field nuclear magnetic resonance (NMR) technique. In addition, phase-transition of CH4 is also analyzed to investigate how the existing states of CH4 transfer in shale during this process. Specifically, the states that CH4 resides in shale are firstly recognized by analyzing the measured T2 spectrum of shale after injecting CH4. CO2 huff-and-puff tests are then conducted to investigate how CO2 impacts the adsorption/desorption behavior of CH4 on shale samples. Furthermore, the T2 signals of shale during depressurization is measured to investigate the state transform of CH4 in shale during CO2 huff-and-puff process. Test results show that three states are observed for CH4 storing in shale samples, i.e., bulk CH4, free CH4 at pore center, and adsorbed CH4 on pore surface. After injecting CO2, the adsorbed CH4 will be desorbed from the shale surface, which thus increases the free CH4 at pore center. During depressurization, the free CH4 is more readily produced from the shale samples, while the adsorbed CH4 is hard to be recovered; more advance technology should thereby be proposed for enhancing the adsorbed CH4 from shale reservoirs. This work is expected to inspire new understanding of the mechanisms of CH4 recovery using CO2 huff-and-puff methods. Keywords: adsorption/desorption behavior; CO2 huff-and-puff; adsorbed CH4; low-field NMR technique

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1. Introduction Shale-gas production has been increasing as an important portion of the energy supply in North America. Shale gas in such shale reservoirs is generally preserved in two states, i.e., free-gas state, and adsorbed-gas state [1-2]. Shale rocks in shale reservoirs usually have higher content of organic matter, such as kerogen, than the conventional ones, rending hydrocarbons in shale reservoirs tending to adsorb on shale surface [3]. In other words, the amount of shale gas in the adsorbed-state is highly affected by the organic matter in shale reservoirs. A big proportion of reserves in organic-rich shale is generally kept as the adsorbed gas. Previous study has proposed that the adsorbed reserves can take account for as high as 85 vol% of the total amount in shale [4]. Therefore, a recovering approach that can release the adsorbed CH4 is significant for shale gas production. It has been found that CO2 is more readily adsorbed on shale surface, which enables CO2 to efficiently recover CH4 from shale reservoirs [5-6]. Previous works were conducted extensively to reveal the efficiency of CO2 for enhancing CH4 recovery. Molecular simulation methods specifically take into consideration the surface-fluid interactions, which enables it to reveal the recovering mechanisms of CO2 to shale CH4 from pore-scale perspective. By applying the grand canonical Monte Carlo simulations, it is found that CO2 generally forms adsorption layers on organic pore surface, which present significantly higher adsorbed density in the adsorption layer than that of CH4 [7]. It suggests that CO2 exhibits a stronger adsorption capacity than that of CH4. Numerical simulation works also observed similar founding to the study above mentioned [8-13]. Recently, extensive experimental studies were performed to measure CH4, CO2, and their mixtures on typical shale cores [14-23]. Pino and Bessieres [24] conducted the measurements on the adsorption of the mixtures of CH4-CO2 3 ACS Paragon Plus Environment

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on shale; and they observed a preferential adsorption of CO2 over CH4. In addition, Rios et al. [22] measured the CH4 and CO2 adsorption on activated carbon samples. The adsorption capacity of CO2 is found to be significantly higher than the capacity of CH4 under the same conditions. Most of these works compared the relative capacity of CO2 adsorption over CH4. Limited studies were conducted to explore the influence of CO2 on the adsorption of CH4 in shale. Recently, Zhao and Wang (2019) performed NMR test to explore how CO2 impact on the CH4 adsorption in shale; they observed that a big proportion of the adsorbed CH4 can be desorbed due to the injected CO2 from shale surface, increasing the total amount of free CH4 in shale. After CO2 injection, depressurization approach is generally adopted to recover CH4 from shale reservoirs. However, this study failed to perform the subsequent works. Although CO2 has been widely proposed as an efficient agent, the questions of how the injected CO2 affect the adsorbed CH4 and how the desorbed CH4 is recovered from shale with the depressurization method from a pore-scale perspective are still scarcely touched. Low-field NMR technique is generally applied to analyze hydrogen-containing fluids existing in porous media, e.g., water and methane [26-28]. This technique has been applied for measuring porosity, permeability, and pore size distribution of rocks or determining the wettability of rock surface etc. [29] Studies have been conducted to explore the NMR relaxation of methane in big space, the so-called bulk space [30-31]. Recently, researchers were dedicating to revealing the adsorbed methane in porous media using the NMR technique [32-34]. Using NMR technique, Alexeev et al. (2004) [32] investigated the existing states of methane in coal samples; it was found that methane can adsorb in solid solutions in addition to the free methane in bulk. Similarly, Guo et al. (2007) [33] observed that methane can exhibit three kinds of relaxation 4 ACS Paragon Plus Environment

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mechanisms in coals using the low-field NMR setup, among which the free and adsorbed methane are represented. In recent years, Yao et al. (2014) [34] also observed three different T2 types of methane on coals, which are proposed to represent bulk methane among coal particles, free methane in pore center, and adsorbed methane on shale surface. Shale is extremely tight, resulting that the fluid properties in shale is quite different from that in bulk, leading to the difficulty to detect shale fluid [35]. In this study, we investigate the adsorption/desorption behavior of CH4 on shale during CO2 huff-and-puff test using low-field NMR technique. The objective of this study is to reveal the fundamental mechanisms of CO2 huff-and-puff method for enhancing shale gas recovery and then best utilize the CO2 for displacing CH4 in shale formation. It may be known for the first time that the NMR technique is applied to reveal effect of CO2 on the adsorption/desorption behavior of CH4 during CO2 huffand-puff process. 2. Experimental Section 2.1 Experimental Materials The purities of CO2 and CH4 used are 99.95% and 99.98%, respectively. Shale samples are extracted from 865 m in Sichuan Basin of China. Both shale samples are squeezed into small particles with an averaged diameter of 1.50 mm. To extract the in-situ moisture residing in shale particles, the crushed shale particles are stored in an oven at 423.15 K and vacuumed for 48 hours. 2.2 Shale Sample Characterization The Hitachi SEM setup is employed to characterize the surface morphology of both shale samples. Before surface scanning, a golden film with an even thickness of about 10 nm is first 5 ACS Paragon Plus Environment

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coated on the surface of shale, which is used to enhance the conductivity of shale surface. In addition, the combustion method is applied to characterize the total organic carbon (TOC) content in both shale cores. the details for measuring the TOC content can be found anywhere [1]. Based on the measurement, the TOC content of shale #1 and #2 are 3.15% and 2.67%, respectively. Figure 1 shows the FE-SEM images of both shale samples. The X-ray spectroscopy analysis is then conduced on both the selected points “a” and “b” in the two shale cores. A high carbon content, indicating organic matter, is observed in both selected points. As shown in Figure 1, bunches of pores coexist around the selected points in the shale samples, suggesting that nanopores in shale samples can be resulted from organic matter, such as kerogen.

Figure 1. The SEM images of the core samples (a) #1 and (b) #2. 2.3 Experimental Setup The CO2 huff-and-puff setup mainly comprises of an NMR apparatus (Mini-MR, China), a syringe pump (ISCO, USA), a manual pump (Huake, China), and a nonmagnetic core holder (Niuman, China). The core holder is fabricated using PEEK material, which is nonmagnetic and can hold pressures as high as 25.0 MPa and temperatures as high as 373.15 K. The manual pump 6 ACS Paragon Plus Environment

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is employed to provide confining pressures to shale cores through fluorocarbon oil. The fluorocarbon oil has zero hydrogen signals, which has no influence on the NMR response. The injected CO2 and CH4, are first contained in the transfer cylinder. It is noted that the NMR apparatus possesses a gradient of 0.025 T/m in the X, Y, and Z directions within a frequency range of 1~30 MHz. The NMR setup is used to measure the T2 signals of the two shale cores. 2.4 Experimental Procedures Figure 2 presents the schematic for conducting the CO2 huff-and-puff method on the CH4saturated shale samples. In this work, NMR setup is applied to obtain T2 signals of the core samples at different stages of CO2 huff-and-puff process. CH4 contains hydrogen atoms, while CO2 does not; thus, the T2 spectrum from NMR measurements can reflect CH4 residing in shale samples. Figure 2 depicts the schematic for conducting CO2 huff-and-puff test on the CH4saturated core samples. To start the experiment, the crushed shale particles are first placed in the core holder. The loaded shale sample is then vacuumed for 24 h using a vacuum pump. Next, NMR scanning is applied on the dry shale particles; if remarkable T2 signal is observed, it suggests the shale particles were not dried adequately and another vacuum is required. CH4 is then injected into the shale samples at 0.5 ml/min by achieving different system pressures. Subsequently, NMR scanning is conducted to obtain the T2 signals of shale cores at given pressures. The measured T2 signals are then employed to analyze the initial CH4 distribution in shale samples. Next, shale samples are subjected to CO2 huff-and-puff tests. Specifically, CO2 is first pressured into both shale samples. It is noted that the original shale samples are initially saturated with CH4. T2 signals are then obtained at given time intervals. After 1440 min, shale samples are 7 ACS Paragon Plus Environment

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depressurized with stepwise manner. Multiple NMR tests are performed to measure the T2 spectra during the depressurization process. According to the measured T2 signals, the validation of CO2 huff-and-puff method is evaluated for enhanced shale CH4 recovery. Multiple cycles of CO2 huff-and-puff tests might be needed, and each NMR test is conducted for three times to confirm the repeatability.

Figure 2. Schematic for conducting the CO2 huff-and-puff method on the CH4-saturated shale samples. 3. Results and Discussion In this section, NMR technique is used to measure the T2 spectrum of shale samples during CO2 huff-and-puff process. Specifically, T2 spectrum after injecting CH4 is measured to investigate the states of CH4 residing in shale; T2 spectrum after injecting CO2 is obtained to explore how the introduced CO2 affect the CH4 adsorption/desorption behavior of CH4 on shale; meanwhile, T2 spectrum measured during depressurization is measured to evaluate the depleting exploration method for recovering shale CH4. 8 ACS Paragon Plus Environment

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3.1 Distribution of CH4 in Shale Samples Figures 3 and 4 show the measured T2 spectra of both core samples after saturating CH4 at the temperature of 353.15 K. Three different regions coexisting in the T2 spectrum are recognized, i.e., 1~200 ms, 200~2,000 ms, and 2,000~10,000 ms. Almost zero T2 signal is observed for the dry shale samples. It means that the shale core samples have been dried adequately. Additionally, we observe that the T2 signals increase as the increasing pressure. It is noted that the pressure indicates the pressure when CH4 reaches the adsorption/desorption equilibrium. Using the lowfield NMR technique, Yao et al. (2014) [34] conducted the measurements to evaluate the CH4 adsorption on coal samples; three different states are observed for the distribution of CH4 in coals: (a) adsorbed-state on shale surface, (b) free-state in pores, and (c) bulk CH4 between the coal particles. Similarly, it is reasonable to infer that the T2 peak in the range of 1~200 ms represents CH4 in the adsorbed-state on shale surface; T2 signals falling in the range of 200~2,000 ms represents the free CH4 locating in pore center, and T2 signals falling in the range of 2,000~10,000 ms refer to the bulk CH4. With the increase in the injection pressure, the magnitudes of the three T2 peaks tend to increase. This indicates that more CH4 adsorbs onto shale pore surface as the increasing pressure; meanwhile, the quantity of the free CH4 in pore center exhibits an increment with the increasing injection pressure. Different from shale sample #1, the magnitude of T2 spectrum in region b is higher than that in region a. It suggests that more free-state CH4 is observed than that of the adsorbed CH4 in the core sample #2. However, as for the core sample #1, the amount of adsorbed CH4 is remarkably more than that of the free CH4, emphasizing the importance of determining the adsorbed amount of CH4 in order to estimate the shale gas-in-place in shale gas reservoirs. Interestingly, we observe that the T2 spectrum is almost zero in region c at low pressure conditions. At low 9 ACS Paragon Plus Environment

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pressures, a small quantity of CH4 resides between shale particles, which is out of the detection limitation of the low-field NMR apparatus.

Figure 3. T2 signals of CH4 in the core sample #1 at 353.15 K.

Figure 4. T2 signals of CH4 in the core sample #2 at 353.15 K.


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3.2 Adsorption/desorption of CH4 on Shale after Introducing CO2 CO2 huff-and-puff tests are then conducted on the two shale samples, which are pre-saturated with CH4. In this work, CO2 is injected into the CH4-saturated cores at higher pressure conditions; specifically, the highest pressure in shale sample #1 is 9.62 MPa after injecting CH4, while the pressure increases to 11.34 MPa after injecting CO2. Similarly, the highest pressure in the core sample #2 is 9.58 MPa after injecting CH4, while the pressure increases to 10.96 MPa after injecting CO2. After injecting CO2, both shale samples are then scanned to obtain T2 signals at every constant time intervals to explore the influence of CO2 on the adsorption/desorption of CH4. During the shutting-off period, which is the so called “soaking period”, the adsorbed CH4 desorbs from the shale surface, decreasing the relative amount of adsorbed while increasing the amount of free CH4 in shale samples. Figures 5 and 6 show the measured NMR signals of CH4 in both shale samples at different soaking time and 353.15 K. As for both shale samples, T2 signals in region a decrease and increase in region b as the soaking time increases b with the increasing soaking time. We have mentioned that the T2 spectrum in the three regions is proposed to represent the adsorbed CH4 on shale, free CH4 in the middle of pores, and bulk CH4 among shale particles, respectively. Thereby, it is reasonable to infer that CH4 adsorbed on shale pore surface is probably replaced by CO2, releasing CH4 from shale surface, which consequently enhances the relative amount of free CH4 in the middle of pores. In other words, CO2 enhances the desorption of CH4 from shale surface. Gas components generally have individual adsorbing capacities on shale rocks, which is the so-called selective adsorption. CO2 exhibits much higher adsorption on shale surface than CH4 [36], enabling CO2 being able to replace the adsorbed CH4 from organic pore surface. It


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suggests that CO2 could be an efficient agent used for recovering CH4 from shale reservoirs due to its higher adsorption than that of CH4.

Figure 5. T2 signals of CH4 in shale sample #1 in terms of soaking times.

Figure 6. T2 signals of CH4 in shale sample #2 in terms of soaking times. 12 ACS Paragon Plus Environment

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3.3 Adsorption/desorption of CH4 on Shale during Depressurization When soaking for 1,440 min after injecting CO2, both shale cores are depressurized with a stepwise manner; this process is designed to simulate the depleting exploration in shale reservoirs. NMR scanning is performed to measure the T2 signals during depressurization process to investigate the adsorption/desorption behaviors of CH4 in shale during depleting period. Figures 7 and 8 show the obtained T2 signals of both shale samples during depressurization. Generally, we observe that the T2 spectrum decreases in the three regions as system pressure decreases. Reduction in the system pressure enhances the desorption of CH4 from shale. In addition, both the adsorbed and free CH4 can be recovered from shale reservoirs with a depressurization method. Comparatively, a more significant reduction is observed in the magnitude of the T2 spectrum in regions b and c than that in region a. It signifies that the free CH4 in the middle of pores and the bulk CH4 in the free space is more readily to be produced from shale reservoirs. Conversely, T2 spectrum of CH4 in region a only has a slight decrement during the depletion process, suggesting that the adsorbed CH4 is harder to be produced by simply depressurization due to the strong attractions from the pore surface; thereby, more advanced method should be adopted.


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Figure 7. T2 signals of CH4 in shale sample #1 during depressurization.

Figure 8. T2 signals of CH4 in shale sample #2 during depressurization.


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3.4 Phase-transition of CH4 in Shale In CO2 huff-and-puff scenarios, CH4 in shale will transform between the adsorbed and free states. With the knowledge of the volume of the injected CH4 and the proportion of T2 value at given regions, the specific gas moles of the adsorbed and free CH4 are calculated by knowing the gas density from NIST Chemistry WebBook [37]. The accuracy of the volume and pressure recorded are ±3.0%. Figure 9 presents variation of the moles of adsorbed CH4 and free CH4 in both shale cores in terms of soaking times. We observe that moles of the free CH4 in both shale samples first improves along with the soaking time, and then becomes stable. However, the amount of adsorbed CH4 decreases at the beginning of soaking and then levels off. After injecting CO2, CO2 can adsorb on shale tightly; as a result, the adsorbed CH4 is consequently replaced by CO2, leading to a increment in the moles of free CH4 and a decrement in the moles of adsorbed CH4 in shale samples. In addition, the moles of adsorbed CH4 as well as the free CH4 in core sample #1 is always more than that in the core sample #2. Gas storage in shale correlates with shale properties, such as BET surface area and TOC content; core sample #1 possesses a high TOC content than core sample #2, which may be the main reason causing the higher gas storage. Figure 10 exhibits variation of the moles of the adsorbed CH4 as well as the free CH4 in both shale samples during depressurization. We observe that the moles of free CH4 in both shale cores decreases when system pressure decreases; the final recovery of the free CH4 in shale cores #1 and #2 is 57.8% and 51.5%, respectively. On the contrary, the amount of adsorbed CH4 only shows a slight decrease in both shale samples, 7.8% and 20.6% for shale samples #1 and #2, respectively. It suggests than the depressurization method can efficiently recover the free CH4 in the middle of pores, while is not efficient in recovering the adsorbed CH4 form shale surface.


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Thereby, more advance technique should be adopted for enhancing shale CH4 recovery, especially the adsorbed CH4 on organic shale.

Figure 9. Variation of the moles of the adsorbed and free CH4 in both shale cores in terms of soaking times.

Figure 10. Variation of the moles of the adsorbed and free CH4 in both shale cores during depressurization. 16 ACS Paragon Plus Environment

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4. Conclusions and Suggested Future Works In this study, adsorption/desorption behavior of CH4 is investigated using the low-field NMR technique during CO2 huff-and-puff process by measuring the T2 spectrum of typical shale samples; specifically, both shale cores are first saturated with CH4 and then introduced with CO2. This study is expected to evaluate the feasibility of the recovering method, i.e., CO2 huff-andpuff method, in enhancing shale CH4 recovery from shale gas reservoirs. The following conclusions are obtained as,

• According to the T2 signals of CH4-saturated shale cores, three different states are observed for CH4 in shale, i.e., the free CH4 in the middle of pores, bulk CH4 among shale particles, and the adsorbed CH4 on shale surface. The total shale gas reserves can be dominated either by the adsorbed or the free CH4 depending on the physical properties of shale.

•

During the soaking periods, CO2 exhibits stronger adsorbing capacities on the organic shale surface than CH4, leading to the fact that a large proportion of adsorbed CH4 is displaced by CO2; it results in an increment in the moles of free CH4 in the middle of pores. The presence of CO2 enhances the desorption of CH4 from shale. Therefore, CO2 could be served as an efficient agent for the recovery of CH4 from organic shale due to its higher adsorption capacity than that of CH4;

• During depressurization, much more free-state CH4 is readily produced from shale samples; on the contrary, the adsorbed CH4 is hard to be recovered. It indicates that CO2 huff-andpuff approach may not be an efficient method for recovering the adsorbed CH4, and more advanced approaches should be adopted for shale gas reservoir development.


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The study is expected to provide fundamental understanding of the mechanisms of shale CH4 recovery using CO2 huff-and-puff method. More importantly, the efficiency of the CO2 huff-andpuff method is evaluated, and more fundamental understanding is obtained from this work; for instance, the CO2 huff-and-puff method may not be efficient for the recovery of the adsorbed CH4 from shale gas reservoirs. However, besides CH4, some heavier hydrocarbons may be also included in shale gas, such as C2H6, and C3H8 etc. It is thereby also significant to investigate how the injected CO2 affect the adsorption/desorption behavior of C2H6 and C3H8 on shale samples. In addition, molecular simulation methods can reveal the behavior of shale fluids from nano-scale perspective. The size of pores in shale is generally falling in the nano-scale. Thereby, in the future works, it is suggested in investigate how CO2 replace the adsorbed hydrocarbons from nanopores using molecular simulation methods. Acknowledgments The authors acknowledge the financial support from the National Science Foundation of China [Grant No. U1810102] to T. Kang. References [1] Liu, Y., Li H., Tian, Y., Jin, Z. 2018. Determination of Absolute Adsorption/Desorption Isotherms of CH4 and n-C4H10 on Shale from a Nanopore-Scale Perspective. Fuel. 218: 6777. [2] Wang, Y., Zhu, Y., and Liu, S. et al. 2016. Methane Adsorption Measurements and Modeling for Organic-Rich Marine Shale Samples. Fuel 172: 301-309. [3] Ross, D.J.K., and Bustin, R.M. 2009. The Importance of Shale Composition and Pore Structure upon Gas Storage Potential of Shale Gas Reservoirs. Mar. Pet. Geol. 26: 916-927. 18 ACS Paragon Plus Environment

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