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Experimental investigation on the effects of supercritical carbon dioxide on coal permeability#Implication for CO2 injection method Wei Li, Zhengdong Liu, Erlei Su, and Yuanping Cheng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03729 • Publication Date (Web): 21 Dec 2018 Downloaded from http://pubs.acs.org on December 25, 2018
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Experimental investigation on the effects of supercritical carbon dioxide on coal permeability：Implication for CO2 injection method Wei Lia,b,c,d, Zhengdong Liu a,b,c*, Erlei Sua,b,c, Yuanping Chenga,b,c Key Laboratory of Gas and Fire Control for Coal Mines (China University of Mining and Technology), Ministry of Education, Xuzhou 221116, China. b National Engineering Research Center for Coal and Gas Control, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China C School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China d Postdoctor Program, Wanbei Coal Electricity Group Limited Liability Company, Suzhou 234000,Anhui, China a
* Corresponding author at: National Engineering Research Center for Coal and Gas Control, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China. E-mail address:
[email protected].
Abstract CO2 storage in deep unrecoverable coal seams has become an effective method to curb greenhouse gas emission, which also contributes to an increase in coalbed methane (CBM) production. Due to high temperature and pressure in deep coal seams, the injected CO2 remains in a supercritical state. However, the influence of injecting supercritical CO2 into coal seams on coal permeability is not particularly clear at present. Therefore, this paper conducted a series of studies through high-pressure triaxial setups on the naturally fractured coal, the coal adsorbing supercritical CO2 for different times and supercritical CO2treated coal, respectively. In the experiment, the laws of coal permeability variation with different adsorption times of supercritical CO2 under different gas injection pressure were first determined. Results indicate that there is a decline in coal permeability in the initial stage because of the great swelling deformation induced by adsorption. Besides, the coal may find a rebound in permeability in the later phase if its mechanical properties have been changed after repeated adsorption. Meanwhile, He was applied to measuring the permeability of original coal and the coal completely desorbing supercritical CO2, respectively. Based on a comparison between their permeability values, the latter has higher permeability. It directly indicates that supercritical CO2 has extraction and dissolution effects on organic matter and inorganic minerals in coal, respectively, thus enhancing the permeability. To further support this point of view, pore characteristics of such two kinds of coal were determined through mercury intrusion method. It
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is demonstrated that the supercritical CO2-treated coal has well developed and connected pore system, with a rise in the proportion of meso- and macro-pores as well as total pore volume. In addition, the coal with different adsorption times varies in permeability evolution laws under the same effective stress. Inspired by such variation, this paper proposed a new gas injection method, that is, injection pressure rose gradually after cyclic adsorption of supercritical CO2, which provides certain reference for efficient gas injection.
Keywords: CO2 storage; Supercritical CO2; Coal permeability; Pore characteristic; Injection method
1. Introduction The overuse of fossil energy causes the discharge of more CO2 into the atmosphere. CO2, a major greenhouse gas, is blamed for global warming that exerts a serious impact on human habitat and even poses a threat on human health
1, 2.
At present, carbon capture and storage (CCS) is an effective way to
reduce CO2 emissions 3. Thereinto, CO2 sequestration in coal with enhanced coalbed methane (ECBM) recovery has attracted much public attention due to its advantages such as great storage potential of coal seams, strong adsorption capacity of coal to CO2 and CO2-ECBM recovery 4-8. In recent years, a series of CO2 sequestration projects have been implemented in different countries, including the United States, China, Canada, and Japan 9-12. Due to high requirements of CO2 sequestration on surrounding geological environment, it should be taken into consideration both the quantity of stored CO2 and the safety after sequestration when selecting a storage site
13-15.
Therefore, deep coal seams, generally with depth over 800 m, are favorable for CO2
sequestration. A great depth promotes the transformation of CO2 into the supercritical state when temperature and pressure exceed 31.8 ℃ and 7.38 MPa 16-18. Since such conditions are common in deep coal seams 19-21, the CO2 stored there are basically in the supercritical state. However, the mechanisms and models for the supercritical carbon dioxide transport through coal is similar to CH4 and CO2, mainly including seepage and diffusion 22-24. At present, there is a shared problem facing CO2 sequestration into deep coal seams. With the passage of time, reservoir permeability slumps, so does the CO2 injection efficiency. In the San Juan Basin in the United States, injection efficiency was reduced by 50 % due to a decline in reservoir permeability in the initial two years 25. The Ishikari Basin in Japan even experienced a 70% reduction in its injection capability in the first year
26.
These are all due to the limitation of gas migration channel
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24, 27.
Essentially, coal
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permeability is the key parameter influencing both CO2 injection and coalbed methane (CBM) recovery efficiency
10, 28.
Since CO2 in deep coal seams is in a supercritical state, understanding the effect of
supercritical CO2 on permeability is of great significance for better improvement of geologic sequestration and CBM recovery. When such general adsorbed gases as CH4 and CO2 are injected into a coal seam, effective stress and swelling deformation incurred by adsorption are main factors responsible for changes in permeability 29-32. For a deep coal seam, the injected CO2 is in a supercritical state which differs remarkably from subcritical CO2 in fluid viscosity and self-diffusion coefficient. The excellent absorptivity and specific chemical characteristics of supercritical CO2 have great influence on the change of permeability. Krooss et al.
33
determined the adsorption performance of dry and wet coal with different grades in Pennsylvanian by highpressure CH4 and CO2 at various temperatures, respectively. Results showed that CO2 and supercritical CO2 had noticeably stronger adsorption properties than those of CH4 and subcritical CO2, respectively. Perera 34 and Vishal 35 reported that coal adsorbed far more supercritical CO2 than subcritical CO2, producing greater swelling deformation. Fractures in coal were compressed, cutting gas migration channels and causing the dive of permeability. Day et al. 36 also drew the similar conclusion that coal adsorbed more supercritical CO2 which led to more remarkable swelling deformation. Such studies have also been carried out by other scholars. Humayun and Tomasko
37,
Herbst et al.
38
focused on the absorptivity of supercritical CO2 at
various temperatures. In addition, Siemons and Busch 39 made a comprehensive summary of methods and explanations of the determination of various coal by supercritical CO2. The supercritical CO2 injected into a deep coal seam flowed through coal fracture. It was partially adsorbed on fissure walls while the remaining diffused into coal matrix. In this process, pore structure and physicochemical properties of the coal were changed. Kolak et al. 40 regarded supercritical CO2 as a good organic solvent. When injected into a seam, it could extract such organic compounds as polycyclic hydrocarbon and aliphatic hydrocarbon from the coal. Bae et al. 41 proposed that pore connectivity could be well improved when organic matter was extracted from the opening of narrow pores. Liu 42 and Sakanishi et al. 43 observed that CO2 could form carbonic acid with water in a coal seam, thus bringing down PH of the reservoir. At the same time, part of mineral salts in coal were dissolved, which enhanced total pore volume. The aforementioned research proves that the interaction between supercritical CO2 and coal is complex due to the specific properties of supercritical CO2. The existing studies on the influence of supercritical CO2 on coal permeability are mostly concentrated on swelling deformation caused by adsorption. There is scarce research taking into comprehensive account
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the effective stress, adsorption swelling, dissolution and extraction, though dissolution and extraction effects of supercritical CO2 on coal has attracted certain attention. Therefore, it is necessary to clarify their respective effect on permeability for better understanding the variation laws of permeability after supercritical CO2 injection. Considering the aforesaid research status, this paper applied high-pressure triaxial setup to testing permeability of the naturally fractured coal, the coal adsorbing supercritical CO2 for different times and supercritical CO2-treated coal under various stress and gas injection pressures. Besides, pore characteristics of original and supercritical CO2-treated coal were also studied by mercury intrusion method 44. A better understanding could be gained on the respective effects of extraction, dissolution, adsorption swelling and effective stress on permeability during supercritical CO2 injection, based on an analysis on test results. Inspired by evolution laws of coal permeability after cyclic adsorption of supercritical CO2, it is considered that cyclic adsorption leads to changes in mechanical properties of coal. Therefore, experimental results of this paper are of positive significance for further understanding CO2-ECBM technology and variation laws of permeability in the storage process.
2. Experimental 2.1. Coal sample preparation Coal samples in this experiment are from Daning Coal Mine in the south of Qinshui Basin in Shanxi Province, China. The Basin boasts the most abundant explored CBM reserves in China and great potential of CO2 storage. It also witnesses the implementation of the first pilot test project of CO2 storage and ECBM recovery in China 45. The bulk coal specimen collected from a freshly exposed mining face were sealed and sent to the laboratory with minimal delay to prevent oxidation. For effective measurement of permeability, the bulk coal was processed into standard samples with a size of 50 mm*100 mm, as displayed in Fig. 1. Fig. 1 Naturally fractured coal sample The remaining coal was smashed and screened into particles with different sizes to study the composition and pore characteristics of sample. A 5E-MAG6600 industrial analyzer and a Zeiss microscopephotometer were adopted to analyze its composition and metamorphic grade, as given in Table 1. Table 1. Basic Properties of the Coal Sample
2.2. Experimental apparatus Permeability measurement of coal with different adsorption states of supercritical CO2 was all carried out by using a high-pressure triaxial setup. The schematic diagram of experimental apparatus is displayed
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in Fig. 2. It mainly consists of 5 modules, namely, the gas compression and injection module, the geological environment simulation module, the vacuumization module, the gas flow monitoring module and the data acquisition module. The gas compression and injection module supplies gas with pressures in the range of 0-20 MPa. The geological environmental simulation module guarantees constant stress and temperature. It can produce the maximum stress of 30 MPa and regulate the temperature from 0℃ to 95℃. The gas flow monitoring module is composed of three flowmeters with measuring ranges of 100 mL/min, 15 L/min and 2 L/min, respectively, which can switch automatically. Fig. 2 Schematic of the high-pressure triaxial setup There are mainly two methods of permeability determination in the laboratory, namely, transient method and steady state method, both of which are based on Darcy’s law. The latter was selected for permeability measurement of coal in this paper due to its relatively high permeability. The calculation formula is as follows 46:
2 P Q L 100 K = 2d 02 （Pu - Pd ）A
(1)
where K is the permeability, mD; A is the cross-sectional area of the coal sample, cm2; L is the length of coal sample, cm; Pu and Pd are upstream and downstream pressures of the sample, respectively, MPa; 𝜇 is the gas viscosity, mPa*s; Q0 is the gas flow, cm3/s. Two kinds of gas were involved in the permeability determination experiment, namely, non-adsorptive He and supercritical CO2. The coal permeability of He can be calculated according to Eq. (1) which, however, is inapplicable to that of supercritical CO2. In order to determine the coal permeability of supercritical CO2, it is necessary to ensure that the gas pressure at the outlet of coal exceeds 7.38 MPa. To meet such pressure condition, a pressure control valve was set at the outlet of coal to maintain gas pressure at 7.5 MPa. Since the pressure at the outlet of pressure-reducing valve was 0.1 MPa when permeability was measured, the value of gas flow Q1 was obtained under 0.1 MPa. Thereby, Q1 should be converted into Q0, the gas flow in a supercritical state to measure coal permeability under such condition. The conversion relationship is as follows 16, 47:
pv p1v1 0 0 Z1 RT1 Z 0 RT0
(2)
where, P1 and P0 are gas pressures, MPa; V1 and V0 are gas volumes, cm3; Z1 and Z0 are gas compressibility
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factors; R is a gas constant, J/(mol*K); T1 and T0 are temperatures, K. Gas volume and gas flow are directly related as follows:
Q1
v1 t
(3)
Eq. (3) can be substituted into Eq. (2) to obtain:
pQ p1Q1 0 0 Z1 RT1 Z 0 RT0
(4)
Combining Eqs. (1)–(4), the permeability equation of supercritical CO2 can be obtained:
2 P Q L p1Z 0T0 100 K = 2d 12 （Pu - Pd ）Ap0 Z1T1
(5)
2.3. Experimental process 2.3.1 Permeability experimental process He and CO2 were adopted as test gases to study evolution laws of permeability under different pore pressures and the influence mechanism on permeability of coal under stress after supercritical CO2 adsorption. He, a kind of non-adsorptive gas, was used for measuring variations of permeability induced by changes in effective stress, while CO2 was for that by adsorption swelling. Injection pressures of test gases were set to 10 MPa, 11 MPa, 12 MPa and 13 MPa, respectively. Besides, bath temperature was set to a fixed value of 35 ℃ to avoid the effect of temperature on coal permeability. The permeability test covers three stages, determining He permeability of naturally fractured coal, supercritical CO2 permeability of coal after cyclic adsorption of supercritical CO2 and He permeability of coal after desorption of supercritical CO2, respectively. The specific experimental process of each stage is presented in Fig. 3. Pore pressure and confining stress corresponding to each stage were shown in Table 2. Fig. 3 The whole experimental process Table 2. Permeability test conditions Stage One: A sample of standard size (50 mm *100 mm) from bulk coal was placed in a triaxial stress chamber. Then, a fixed confining pressure of 15 MPa was exerted on it. Meanwhile, He with pressures of 10 MPa, 11 MPa, 12 MPa and 13 MPa, respectively, was injected into the coal, to measure coal permeability under different effective stresses. Therefore, it could better reflect the naturally fractured coal permeability. Stage Two: To study variation laws of coal permeability after supercritical CO2 adsorption, CO2 with pressures of 10 MPa, 11 MPa, 12 MPa and 13 MPa respectively, was injected into the coal at a constant
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temperature of 35 ℃ to measure permeability. Before CO2 injection, the coal was allowed to adsorb supercritical CO2 for 12 h. During the adsorption process, the downstream valve of coal was closed. Meanwhile, the injection of supercritical CO2 was carried on with pressure of 10 MPa until downstream gas pressure reached 8 MPa. At this time, both upstream and downstream gas pressures exceeded CO2 critical pressure of 7.38 MPa and the system temperature was also above CO2 critical temperature of 31.8 ℃ . Thereby, it was guaranteed that CO2 adsorbed on coal was in the supercritical state. After supercritical CO2 adsorption for 12h, coal permeability was determined by steady state method. In addition, supercritical CO2 adsorption was repeated for 4 times to study the influence of adsorption time on coal permeability. Meanwhile, permeability measurement was also repeated under different pore pressures after each cyclic adsorption. Stage Three: Pores and fractures were changed in coal after 4 cycles of supercritical CO2 adsorption. To study the effect of supercritical CO2 on coal permeability, the experimental system was first connected to air to ensure natural desorption of coal over 72 h. Then, permeability determination was implemented by injecting He with different pressures under confining pressure of 15 MPa.
2.3.2 Pore morphology experimental process It is widely knew that extensive methods can be used to study the pore morphology 44, 48, 49. In this paper, mercury intrusion method was adopted to study changes in pore structure before and after coal adsorbing supercritical CO2 in this paper. This method is based on differences in resistance of pores and fractures with various width to injected mercury. Therefore, the volume and effective width of pores and fractures in coal can be calculated according to the volume and pressure of mercury intrusion. Experimental data were processed by Washburn equation as follows 50:
pc
2 cos r
(6)
where PC is the capillary pressure; MPa; σ is the surface tension of mercury, dyn/cm2; θ is the wetting contact angle, 140°; r is the pore throat radius, nm. In order to comparatively analyze experimental data of pores, the coal with or without supercritical CO2 adsorption came from the same bulk coal. A Pore Master 33 automatic mercury porosimeter produced by Quantachrome Instruments, the United States, was adopted in the experiment. It required the particle size of samples in the range of 1 - 3 mm. The sample with supercritical CO2 adsorption was the standard cylinder coal after cyclic adsorption. To eliminate the effect of water on pore measurement, the coal was
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placed in a thermostatic drying box at 40℃ for over 72 h before mercury intrusion 44.
3. Results and discussion 3.1. Effect of supercritical CO2 on coal permeability 3.1.1 He permeability of naturally fractured and supercritical CO2-treated coal Coal permeability is mainly under the influence of effective stress and swelling deformation of matrix induced by adsorption. Klinkenberg effect is negligible when gas injection pressure exceeds 10 MPa 51, 52. For a separate study on the influence laws of effective stress on coal permeability, non-adsorptive He was adopted as the test gas. Under the confining stress of 15 MPa, permeability before and after cyclical adsorption of supercritical CO2 was measured with gas injection pressures of 10 MPa, 11 MPa, 12 MPa and 13 MPa, respectively, as demonstrated in Fig. 4. And the supercritical CO2-treatment time is 48 h and the desorption time is over 72 h. Variation laws of permeability with the gas injection pressure in Fig. 4 (a) and (b) indicate that with the increase of gas pressure, the effective stress acting on the coal is lowered. Thereby, the fracture aperture grows, thus enhancing coal permeability. Due to supercritical CO2 adsorption, there is certain difference in the permeability variation induced by the same effective stress variation. Specifically, naturally fractured and supercritical CO2-treated coal experience increases of 25.1% and 17.1% in permeability, respectively, when gas pressure rises up from 10 MPa to 13 MPa. Such difference may result from incomplete desorption of supercritical CO2. Fig. 4 Variation of coal mass permeability with He injecting pressure Based on the comparative analysis of permeability values before and after cyclic adsorption of supercritical CO2 under the same effective stress, it is observed that permeability of supercritical CO2treated coal rises remarkably, as shown in Fig. 5. Besides, increments of permeability under various gas injection pressures are also different. Specifically, when gas injection pressures are 10 MPa, 11 MPa, 12 MPa and 13 MPa, permeability increases by 0.0779 mD, 0.0665 mD, 0.0734 mD and 0.0504 mD, respectively. The enhancement of permeability is mainly ascribed to the following two reasons. Firstly, during cyclic adsorption of supercritical CO2, organic matters such as aromatic hydrocarbons, aliphatic hydrocarbons and hydrocarbons are extracted effectively
40, 53.
As a result, pore connectivity is notably improved, which
expands gas migration channels and enhances coal permeability 41. Secondly, the injected CO2 combines with water in the coal, causing a decline in PH of the reservoir 43. The dissolution of carbonate minerals in pores is thus promoted to enlarge gas migration channels. Fig. 5 Difference values of coal permeability in different conditions
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3.1.2 Supercritical CO2 permeability of coal Both the supercritical CO2 adsorption and the adsorption time have great impacts on permeability. To study evolution laws of coal permeability after supercritical CO2 adsorption, CO2, with pressures of 10 MPa, 11 MPa, 12 MPa and 13 MPa, respectively, was injected into coal. Permeability values under different gas injection pressures after the first adsorption cycle of supercritical CO2 for 12 h are given in Fig. 6 (a). With the increase of gas pressure, coal permeability presents a downward trend. It drops by 13.58% when the pressure rises from 10 MPa to 13 MPa. As the gas pressure is enhanced, swelling deformation of matrix induced by CO2 adsorption exerts a considerably greater influence on fracture aperture than effective stress, leading to the continuous decrease of permeability. After the second adsorption cycle for 12 h, coal permeability under different gas pressures is displayed in Fig. 6 (b). Its variation laws are in exactly accordance with that in the first round. However, the third and fourth cycles witness particularly different evolution laws of permeability, as illustrated in Fig. 6 (c) and Fig. 6 (d). Specifically, permeability rebounds (making a turnabout) with gas injection pressure growing, which is attributed to the gradually increasing effect of effective stress on coal permeability. Besides, if gas pressure is further enhanced, permeability may recover (to its initial value) 10. Fig. 6 Variation of coal mass permeability with CO2 injecting pressure Permeability values of coal with different adsorption time under the same effective stress were extracted from Fig. 6, as shown in Fig. 7. It can be observed that permeability decreases with the increase of the adsorption time of supercritical CO2, whichever gas injection pressure the coal is subjected to. The longer the adsorption time is, the greater the swelling deformation incurred by adsorption becomes, and the smaller the fracture aperture gets. Meanwhile, the curvature tends to be flat with the addition of adsorption cycles, indicating that the coal is approaching adsorption equilibrium. Fig. 7 Variation of coal mass permeability under different numbers of cyclic adsorption
3.2 Effect of super-critical CO2 on pore morphology 3.2.1 Mercury curves After adsorbing supercritical CO2, the coal experiences significant changes in pore characteristics, including the pore volume, distribution, porosity and connectivity
54.
These changes contribute to
morphology transition of gas flow in coal. Mercury injection and ejection curves of naturally fractured and supercritical CO2-treated coal were obtained via an automatic mercury porosimeter, as presented in Fig. 8. The cumulative mercury injection of naturally fractured coal rises relatively slowly and rapidly, respectively,
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when intrusion pressures are below and over 20 MPa. Compared with naturally fractured coal, the supercritical CO2-treated coal has a relatively flat mercury injection curve, which indicates that its pore size distributes more uniformly. In addition, Fig. 8 shows that the cumulative mercury injection of naturally fractured and supercritical CO2-treated coal is 0.302 cm3/g and 0.274 cm3/g, respectively. Such difference demonstrates that pore size in coal having adsorbed supercritical CO2 is changed and the total pore volume is increased. Changes in mercury ejection also show that the coal treated with supercritical CO2 ejects more mercury than naturally fractured coal, suggesting an increase in open pores of the former. Besides, the difference between mercury injection and ejection reflects the delay degree. Compared with naturally fractured coal, the delay degree of supercritical CO2-treated coal rises from 0.0028 cm3/g to 0.0043 cm3/g, as displayed in Fig. 8. It proves that there are more mercury getting trapped in and being unable to be emitted from the pores of supercritical CO2-treated coal. A further explanation is that in treated coal there are more constricted pores of which most are in the shape of ink bottle. In general, supercritical CO2 contributes to a rise in the proportion of effective pores in coal. Fig. 8 Mercury curves of original and supercritical CO2-treated coal sample
3.2.2 Pore size distribution In order to further analyze pore distribution characteristics of coal samples, pores in coal are ground into four categories: macropore (diameter>1000 nm), mesopore (diameter: 100-1000 nm), small pore (diameter: 10-100 nm) and micropore (diameter
