Experimental Study about Shale Acceleration on Methane Cracking
1
State Key Laboratory for Enhanced Oil Recovery, Beijing 100083, China
2
Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China
3
School of Energy Resources, China University of Geosciences, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Processes 2023, 11(7), 1908; https://doi.org/10.3390/pr11071908
Submission received: 27 February 2023
/
Revised: 9 June 2023
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Accepted: 19 June 2023
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Published: 25 June 2023
(This article belongs to the Special Issue Organic–Inorganic Interactions and Their Significance for Hydrocarbon Generation in Deep Formations)
Abstract
:The temperature or maturity limit of methane (CH4) cracking is very useful for the determination of the most depth or the highest maturity in natural gas exploration owing to the composition of over mature gas. In this work, three series of CH4 cracking experiments were conducted under different conditions of N2 + CH4, N2 + CH4 + montmorillonite and N2 + CH4 + shale, respectively, in a gold tube system. The experimental results show that some heavy gas with negative carbon isotope composition could be generated in the three series experiments and that shale has more intense catalysis for CH4 cracking than montmorillonite. The catalysis of metal elements distributed in the minerals of shale is attributed to CH4 cracking acceleration. The shale catalysis makes the maturity threshold of CH4 substantial cracking decrease from 6.0%Ro under no catalysis, to 4.5%Ro under a shale system in a geological setting. Nevertheless, we suggest not to lightly practice natural gas exploration in shale with the maturity range of 3.5–4.5%Ro, as the maturity threshold of gas generation from oil prone organic matter distributed extensively in shale is 3.5%Ro.
1. Introduction
A remarkable geological characteristic in natural gas exploration appears in the 21st century, that is, the maturity level of natural gas source and the depth of natural gas become higher and deeper, respectively. The resources of natural gas found in over-mature shale are increasing in the world [1]. The highest documented maturity of shale gas and the most depth of natural gas reservoir arrive 4.0%Ro and 8000 m according to the data, respectively [2,3]. Consequently, a problem appears, that is, what the highest maturity level for shale or the most depth for natural gas are. The answer to this problem is very significant and will help to increase the success ratio of gas well drilling in over-mature shale, and reduce the cost in gas well drilling.
Another geochemical feature of over mature natural gas is isotopic reversal. Natural gas with the isotopic reversal feature was originally regarded to be of abiogenic origin before shale gas with this feature was determined. The heavy hydrocarbon components in abiogenic gas were generally thought to be caused by the polymerization of CH4 formed by Fischer–Tropsch synthesis between H2 and CO2 (or CO) as well [4,5,6]. Although much research has studied the mechanism about the isotopic reversal of organic origin gas generated at the over mature stage, but no common knowledge on this problem was obtained. Their observations on the same can be classified into four interpretations. (1) Mixing between primary gas from kerogen decomposition and secondary gas from the cracking of remained hydrocarbons in shale causes the isotopic reversal of shale gas at the over mature stages [2,7,8,9]. (2) The redox reactions between water and CH4 at 250–300 °C generated isotopically light carbon dioxide and hydrogen in the over mature phase, which further interacted and formed isotopically light ethane (C2H6), and finally caused the isotopic reversal of shale gas [9,10]. (3) The isotope fractionation mechanism during gas desorption from depressurized late-mature shale leads to isotope reversal of the residual gas (shale gas) [11]. (4) The polymerization occurring in the early stage of CH4 cracking brings the isotopic abnormal trend of over mature natural gas [12,13]. However, only the last one is feasible in theory and supported by experimental data in the four interpretations. The others might be just speculation or a mathematical model and are not supported by simulation experiment [13].
The theory of oil and gas generation suggests that organic matter would generate oil and gas as it is buried to some depth. With the geological temperature increasing, the oil accumulated in reservoir or remaining in source rock would crack into gas. Natural gas would crack into the ultimate products, carbon and hydrogen, with further geological temperature. Besides geological factors of natural gas preservation, the most depth or maturity for natural gas exploration is attributed to CH4 stability in a geological setting as it is the main component of natural gas, especially for over mature gas. Mi et al. [13] proposed that the maturity of CH4 substantial cracking was 6.0%Ro according to geological deduction on the experimental data of CH4 cracking. However, the highest documented maturity of shale where industrial shale gas reservoir was found was only 4.0%Ro [2]. What is the reason for the difference between the maturity limit of CH4 substantial cracking from the experiment result (6.0%Ro) and the reported maximum maturity (4.0%Ro) in shale gas exploration practice?
Many experiments have proven that different mineral or geological fluids have various effect on oil cracking [14,15,16,17]. Thus, the mineral in shale might affect CH4 cracking as well. Although thermo-catalytic decomposition of natural gas to produce pure hydrogen production is used extensively in the fuel industry [18], only a few studies on natural gas cracking in a geological setting have been published, especially for CH4. In this study, three series of CH4 cracking experiments were conducted under N2 + CH4 (series 1), N2 + CH4 + montmorillonite (series 2), and N2 + CH4 + shale (series 3) system, respectively. The scope of the experiments is to investigate whether or not shale has a catalysis on CH4 cracking, and further, to determine the maximum maturity limit of CH4 substantial cracking in a shale system.
2. Sample and Experiment
2.1. Sample
Three series of CH4 cracking experiments were conducted under the condition of N2 + CH4 (Series 1), N2 + CH4 + montmorillonite (Series 2), and N2 + CH4 + shale (Series 3), respectively. In this experiment, N2 was selected as the reference gas for the decomposition temperature more than 3000 °C, and it could not crack in the experimental temperature range of 500~800 °C. The gas mixture was purchased from Zhaoge Gas Company, Beijing, China, and the CH4 and N2 (v/v) mixing ratio was 89.67% and 10.33%. The value of δ13C1 was −26.07‰ and no other hydrocarbon gases were detected in the gas mixture by gas chromatography (GC).
Montmorillonite, involved in the series 2 experiment, is commercially available montmorillonite K-10 (MK-10) purchased from ACROS 111 ORGANICS (USA). The shale sample involved in the series 3 experiment was collected from the shale gas reservoir of N218 well, Longmaxi formation deposited in Silurian of Sichuan basin, China. Prior to loading the shale in a tube, the shale sample was crashed below 80 mesh. Then the crashed shale was heated to 800 °C in a gold tube at a heating rate of 20 °C/h and held 2 h at 800 °C to exhaust the potential of gas generation from the organic matter in the shale during the cracking experiment. The geochemistry of original and heated shale is listed in Table 1. According to the equation of Ro = 0.018 × Tmax-7.16 proposed by Jarvie et al. [19], the calculated maturity of original and heated shale is about 2.2%Ro and 4.23%Ro, respectively. This indicates the maturity of the original shale reaches over maturity level and the heated shale has no potential of gas generation. The mineral composition of the original shale listed in Table 2 was measured by Quemscan. The amount of MK-10 or shale added in the tube was the same, 50 mg in every temperature in the series 2 and 3 experiments.
2.2. Experiment
The experiments in this study were conducted in a gold tube system, which is an extensively used experimental equipment for organic matter pyrolysis and oil cracking [13,20]. The length, outer diameter, and thickness of the gold tube used in this experiment are 100, 5.5, and 0.25 mm, respectively. One end of the tube was sealed by argon-arc welding before the gas mixture and solid sample (MK-10 or shale) were loaded. A special device, which could connect a vacuum pump and the open tube mouth, was used in gas mixture loading. Prior to gas mixture injection, the tube was vacuumized via the vacuum pump. Then, the gas mixture was injected into the tube from a gas cylinder. Finally, the tube open end was sealed by argon-arc welding. The tube was immersed in ice water during the welding process. The pressure of gas loaded in the tube, which was monitored using a pressure gauge on the gas cylinder, was approximately 4 atm in this experiment.
The gold tube experiment system had 20 autoclaves. Each autoclave had an independent temperature controller. The fluid pressure in autoclave and heating process of each autoclave could be controlled by a computer. All autoclaves were kept a constant fluid pressure of 50 MPa during experiment. The heating program of the autoclave with the tube in this experiment was as follows: heated first from 20 °C to 300 °C in 1 h and held for 30 min, then heated at a heating rate of 20 °C/h to the target temperature. The autoclave heating was stopped after the target temperature was reached. The tubes were moved for analysis as autoclave cooled naturally to room temperature.
Two gold tubes were used for each temperature experiment in case of breakage. Another objective of additional tubes used were as replicates for gas analysis.
The composition analysis of the original gas mixture and the gas produced at different temperatures in this experiment was carried out on Wasson-Agilent 7890 series GC. The heating program for the GC oven was as follows: heating started from an initial temperature of 68 °C (isothermal held for 7 min), then heated to 90 °C (at a rate of 10 °C/min and then held isothermal for 1.5 min), finally to 175 °C (at a rate of 15 °C/min and then held isothermal for 1.5 min). An external standards method was employed for the chromatographic calibration using a reference gas mixture of H2, O2, N2, CO2, H2S, CH4, C2H6, C3H8, iC4H10, nC4H10, iC5H12, and nC5H12. Certified gas standards were prepared at a precision less than ±0.1 mol% for each component made by BAPB Inc., New Berlin, WI, USA.
The carbon Isotopic composition of hydrocarbon gases measurements were conducted on an Isochrom II GC-IRMS coupled with a Poraplot Q column. The heating started from an initial temperature at 30 °C (isothermal for 3 min). Then, heated at a heating rate of 15 °C/min to 150 °C (isothermal for 8 min). The δ13C analysis for each hydrocarbon gas was repeated at least twice to ensure the analytical error of each component less than 0.5‰.
3. Experiment Results
3.1. Gas Components
The composition of gas produced at different temperatures in three series CH4 cracking experiments are summarized in Table 3. Besides the CH4 and N2 in original gas mixture, several heavy hydrocarbon gases (including alkane and alkene) and H2 were formed during three series experiments, at different temperatures. CH4 concentration decreased generally with increasing temperature, and the decreasing could be divided into two segments (a slow and a steep decreasing) in three series experiments (Figure 1a). The inflexion temperatures corresponding to an obvious decrease in CH4 concentration were 700 °C, 675 °C, and 650 °C, respectively, in series 1, 2, and 3 experiments. The reductions in CH4 concentration at the highest experimental temperature of 800 °C were 6.36%, 7.48%, and 9.42%, respectively, in series 1, 2, and 3 experiments. Both the concentration t of C2H6, the highest newly formed hydrocarbon component, and that of other heavy gases increased at relatively low temperature and then decreased with increasing temperature (Figure 1b and Table 3). The highest total concentrations of heavy gas (C2–C5) were 1.04%, 0.53%, and 0.46% in series 1, 2, and 3 experiments, respectively. The total concentration of heavy gas decreased to 0.72%, 0.37%, and 0.21% at the highest experimental temperature (800 °C) in series 1, 2, and 3 experiments, respectively. The temperatures at which the concentrations of heavy gas reached their maximums were 750 °C, 725 °C, and 725 °C in series 1, 2, and 3 experiments, respectively. Although H2 concentration increased with increasing temperature in three series experiments, the highest concentrations of H2 were different, 0.88%, 2.66%, and 4.49% (Figure 1c and Table 3). The corresponding temperatures at which H2 concentration increased obviously were different as well. They were 700 °C, 675 °C, and 650 °C in series 1, 2, and 3 experiments, respectively.
3.2. Carbon Isotopic Composition of Hydrocarbon Gas
The carbon isotopic composition of hydrocarbon gas at different temperatures of the three series experiments is presented in Table 4. The values of δ13C1 generally increased with increasing temperature in three series CH4 cracking experiments (Figure 2). Although the variations δ13C1 in three series experiments could be divided into two segments, a slow increasing segment and a rapid one, the temperature inflexions corresponding to δ13C1 values becoming big obviously were different, 650 °C, 675 °C, and 700 °C in the three series experiments, respectively. The δ13C1 values at the highest experimental temperature of the three series experiments were −24.60‰, −23.31‰, and −21.95‰, respectively.
Similar to the δ13C1 variation with increasing experimental temperature in the three series experiments, δ13C2 also increased generally with increasing temperature (Figure 3). The δ13C2 variation with increasing temperature could be classified into three segments, a gradual increase or no increase, a rapid one, and a slow one in three series experiments. There was only a little variation in δ13C2 (0.75‰) in the first segment of the series 1 experiment. Hardly any variation in δ13C2 was observed in the first segment of the series 2 and 3 experiments. The temperature inflexions corresponding to the first segment to the second segment were different, 625 °C, 650 °C, and 625 °C in the three series experiments, respectively. The temperature inflexions corresponding to the second segment to the third segment were the same, 725 °C in the three series experiments.
In the series 1 experiments, the distribution between δ13C1 and δ13C2 turned from a reversal trend (δ13C1 > δ13C2) to a normal one (δ13C1 < δ13C2) above 700 °C. The distribution between δ13C1 and δ13C2 kept an abnormal trend in the whole temperature range in the series 2 and 3 experiments. Nevertheless, the difference between δ13C1 and δ13C2 in the three series experiments became less with increasing temperature, and finally tended to be equal at 800 °C.
3.3. Solid Products
The inner walls photo of the tubes heated to different temperatures are shown in Figure 4. In the three series experiments, the color of inner walls turned from gold into brown, and finally into dark. The corresponding temperature at which the color of inner walls turned from gold into brown were 725 °C, 700 °C and 675 °C, respectively, in series 1, series 2, and series 3 experiments.
Some black particles could be observed in the residual MK-10 heated above 700 °C in series 2 experiments by scanning electron microscope. The black particles and the black film on the tube inner wall were identified as solid carbon by Energy Dispersive X-ray Spectroscopy (EDS) analysis (Figure 5). Moreover, the color of the solid residues became dark gradually with increasing temperature in the series 2 experiment (Figure 6). The corresponding temperature at which the color of solid residues became obviously dark in the series 2 experiment was 700 °C. This was consistent with the temperature at which the color of the inner walls became brown in the series 2 experiment (Figure 4). Some black particles were observed in solid residues heated to different temperature as well in the series 3 experiment. However, these black particles in shale residues were not distinguished to be the newly formed carbon particles or the organic matter particles originally existing in the shale.
4. Discussion
4.1. Process of CH4 Cracking
The variation of CH4 concentration with increasing temperature presented in Figure 1 shows that it could be divided into two stages, a slow decreasing stage and a rapid one (Figure 1a). The two stages variation of CH4 concentration is consistent with that of H2 (Figure 1c) and that of δ13C1 (Figure 2). This indicates CH4 cracking could generally be divided into two steps, early cracking and substantial cracking. In the substantial cracking stage, CH4 concentration would decrease obviously. In three series experiments, the temperatures corresponding to an obvious decrease in CH4 concentration were 700 °C, 675 °C, and 650 °C, respectively (Figure 1a). This indicates that montmorillonite and shale could accelerate CH4 cracking, and shale has more intense catalysis for CH4 cracking. Shale acceleration on CH4 cracking also embodies the highest H2 concentration (Figure 1c) and the biggest δ13C1 values (Figure 2) at the highest experimental temperature (800 °C) in the three series experiments.
Much research indicated that the formation of higher carbon number hydrocarbons during the CH4 cracking process followed the mechanism of free radical reaction [20,21]. In early cracking, CH4 crack into CH3• radicals and protons first. CH3• radicals would combine and form heavy gas with H2 formation by protons combing. This process of heavy gas formation could be viewed as CH4 polymerization. There are two kinds of CH4 molecules in original reactants, 12CH4 and 13CH4. The 12C–H bonds are more chemically active than 13C–H [21,22,23]. The 12C–H bond in the CH4 molecule would cleavage preferentially than the 13C–H bond during the CH4 cracking process. Thus, the produced CH3• is depleted in 13C, while residual CH4 is enriched in 13C. The newly formed H+ would combine and form H2 during the CH4 cracking process. The CH3• depleted in 13C also combine and form C2H6 depleted in 13C. Thus, the carbon isotopic reversal (Figure 3) of hydrocarbon gas generated in CH4 cracking is reasonable.
The temperatures at wihch CH4 concentration obviously decreased are 700 °C, 675 °C, and 650 °C in the three series experiments (Figure 1a), respectively. These are lower than that of solid carbon appearance at 725 °C, 700 °C, and 675 °C (Figure 4) in the three series experiments. The seeming conflict phenomenon just is evidence that solid carbon formation happened after CH4 substantial cracking.
CH4 cracking is a stepwise dehydrogenation process and the overall reaction of CH4 cracking involves the following steps at high temperatures [24]:
In other words, the overall process of CH4 cracking could be divided into two steps, heavy gas formation vs. CH3• groups combination and solid carbon formation by heavy gas further dehydrogenation. The first step could be viewed as CH4 polymerization [13]. The second one is a relatively complex process which involves the dehydrogenation of heavy gas (alkane and alkene gas) and the formation of solid carbon. The conversion from heavy gas to single carbon still needs several steps of dehydrogenation. Alkane and alkene heavy components measured in the gaseous products in the three series experiments (Table 3) is firm evidence of the aforementioned mechanism of CH4 cracking.
When CH4 cracking has entered into the substantial cracking stage in the three experiments, the concentration of C2H6 increases continuously (Figure 1). For example, when CH4 concentration decreases obviously above 700 °C, the C2H6 concentration just arrives the maximum at 750 °C. This seems to conflict with the general knowledge in chemistry that the thermal stability of C2H6 is lower than that of CH4. The result does not mean that the thermal stability of C2H6 is higher than that of CH4, but that the ratio of C2H6 formation by CH4 polymerization is greater than that of C2H6 further dehydrogenation in the temperature range at which CH4 concentration decreases obviously, accompanying heavy gas concentration increasing. This experimental result could also be explained by the hypothesis proposed by Xia and Gao [21] that heavy gas cracking and generation is a partly reversible reaction at high temperatures. Shuai et al. [25] proved that the generation and cracking of C2H6 could co-exist at high temperatures by a coal pyrolysis experiment in a gold tube system. Our experiment indicates that the heavy gas (alkane gas and alkene gas) is just an intermediate product during the CH4 cracking process. Thus, heavy gas could not disappear completely as long as CH4 did not crack into solid carbon entirely.
4.2. Primary Study on Shale Acceleration Mechanism for CH4 Cracking
Hydrogen is a very clear energy and is used extensively in the fuel industry, especially in space technologies in the last two decades. However, hydrogen is not a primary energy source, and is generally produced from other resources. The thermo-catalytic decomposition of CH4 can be regarded as a cornerstone for pure hydrogen production [26]. In the process of industrial production of hydrogen by CH4 cracking, a wide variety of transition metals (Fe, Al, Co, Ni, Cu, etc.) and silicon dioxide were selected as catalysts to increase the conversion of CH4 to hydrogen [26,27,28]. Our experimental result shows that both montmorillonite and shale would promote CH4 cracking. Shale presents a more intense acceleration on CH4 cracking than montmorillonite. Montmorillonite is a silicate mineral bearing several metal elements (Na, Ca, Al, Mg, Fe). Thus, it is reasonable that montmorillonite promotes CH4 cracking. It is well known that shale is a fine-grained sedimentary rock. Generally, the mineral composition of shale includes clay (montmorillonite, kaolinite, and illite, etc.), detrital mineral (quartz, feldspar, and pyrite, etc.), and some organic matter. All the clay minerals are metal-bearing silicate minerals. The metal in clay minerals of shale would also accelerate the CH4 cracking. Chen et al. [12] proved that shale could increase CH4 conversion above 580 °C by 72 h isothermal experiment conducted in a gold tube system. The analysis result shown in Table 2 indicates that the shale involved in the series 3 experiment bears a complex mineral composition. Besides clay minerals, several detrital minerals bearing metal (siderite, pyrite, etc.) distribute in the shale. The metal elements in detrital minerals might also promote CH4 cracking. This might be the reason that shale has a more intense acceleration on CH4 cracking than montmorillonite. Although the experiments in this study could not prove which kinds of metals in shale are the main operator to accelerate CH4 cracking, it is confessed that the metal distributed in different minerals of shale accelerates CH4 cracking.
It is well known that the function of a catalyst is to decrease reaction activation energy. Just as catalysis of montmorillonite and shale, the lowest temperature at which heavy gas, the intermediate product of CH4 cracking, could be measured by Wasson-Agilent 7890 GC in the series 2 and 3 experiments (500 °C) is lower than that in the series 1 experiment (575 °C, Table 3). However, the highest concentrations of heavy gas in the series 2 and 3 experiments are lower than that in the series 1 experiment (Table 3, and in Figure 1b). The above variation of heavy gases concentration in the three series experiment indicates that metal elements in montmorillonite and shale might accelerate each step of dehydrogenation during the whole CH4 cracking process. Although the metal catalysis on CH4 polymerization would cause the heavy gas concentration increase, the more intense catalysis on dehydrogenation of heavy gases might be attributed to the lower concentrations of heavy gases in the series 2 and 3 experiments, compared with that in the series 1 experiment. The greater decrease in CH4 concentration and the more H2 generation (Figure 1c), as well as the heavier value of δ13C1 (Figure 2) in the series 2 and 3 experiments indicate that both montmorillonite and shale could accelerate CH4 cracking. Moreover, shale bears more acceleration on CH4 cracking than montmorillonite.
4.3. Geological Significance of the Experiments
Our experimental result indicates that shale has more intense acceleration on CH4 cracking than that of MK-10. The final objective of our experiments is to determine the maturity limit of CH4 cracking under a shale system and to provide some reference for shale gas exploration. Therefore, the experimental temperature should be converted to actual geological temperature or maturity (Ro). Mi et al. [29] proposed a formula to convert experimental temperature to Ro according to a coal pyrolysis experiment, which was conducted at the same experimental conditions as our experiment. The relationship between Ro and the experimental temperature is described in Equation (1).
Ro = 0.114 × exp (0.00563 × T) + 0.056
(T, experiment temperature/°C)
The experiment result shows that the temperatures corresponding to CH4 substantial cracking are 650 °C, 675 °C, and 700 °C, respectively, under N2 + CH4, N2 + CH4 + MK-10 and N2 + CH4 + shale condition. The maturity levels corresponding to the three temperatures are 4.48%Ro, 5.15%Ro, and 5.92%Ro according to Equation (1). Thus, the maturity limit of CH4 substantial cracking is 4.48%Ro in a shale system. This limit of CH4 substantial cracking is slightly higher than the reported maximum maturity level of 4.0%Ro in shale gas exploration [2,11]. This is attributed to the upper maturity limit of gas generation from shale being 3.5%Ro [20]. The natural gas stored in shale with maturity above 3.5%Ro would lose easily to exhaustion during very long geological history, for no gas would continuously supply from the shale. Therefore, we suggest that great care should be taken to explore natural gas in shale with maturity above 3.5%Ro.
5. Conclusions
In this work, three series cracking experiments of an N2 and CH4 mixture gas, a mixture gas with montmorillonite, and a mixture gas with shale powder, respectively, were conducted in a gold tube system. The experimental results show that both montmorillonite and shale could accelerate CH4 cracking, and that shale has more intense catalysis on CH4 cracking. Some hydrocarbon heavy gas could be generated during the CH4 cracking process. The heavy alkane gases form by combination of methyl formed in the early CH4 cracking stage. The unsaturated heavy gases are formed by further dehydrogenation of heavy gas. The combination of methyl with depleted in 13C and formed in CH4 early cracking is attributed to 13C depletion of heavy gas and carbon isotopic reversal of over mature shale gas. The experimental results also indicate that the heavy gases are intermediate products of CH4 cracking. Thus, heavy gases could not disappear completely as long as CH4 did not crack entirely. The catalysis of the metal element distributed in clay and other metalliferous detrital minerals of shale accelerate CH4 cracking. The geological reckoning of the experimental result suggests that the maturity threshold for substantial CH4 cracking is about 4.5%Ro under a shale system in a geological setting. Nevertheless, we suggest not to lightly practice natural gas exploration in shale with the maturity space of 3.5–4.5%Ro, as the maturity threshold of gas generation by oil prone organic matter distributed extensively in shale is 3.5%Ro.
Author Contributions
Conceptualization and writing, J.M.; Methodology, X.X.; Formal analysis, J.G. and K.H.; Data curation, X.M. All authors have read and agreed to the published version of the manuscript.
Funding
This research was funded by the State Key Program of National Natural Science of China (Grant No, 42030804) and by the Program of Scientific Research and Technological Development of PetroChina (Grant No, 2021DJ0604).
Data Availability Statement
Not applicable.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1.
Concentration variation of different gas components with increasing temperature in three series experiments. (a) Concentration variation of CH4 with increasing experimental temperature. (b) Concentration variation of C2H6 with increasing experimental temperature. (c) Concentration variation of H2 with increasing experimental temperature.
Figure 1.
Concentration variation of different gas components with increasing temperature in three series experiments. (a) Concentration variation of CH4 with increasing experimental temperature. (b) Concentration variation of C2H6 with increasing experimental temperature. (c) Concentration variation of H2 with increasing experimental temperature.
Figure 2.
Variation of δ13C1 with increasing temperature three series experiments.
Figure 3.
Variation of δ13C1 and δ13C2 with increasing temperature three series experiments. (a) Variation of δ13C1 and δ13C2 with increasing temperature in series 1 experiment. (b) Variation of δ13C1 and δ13C2 with increasing temperature in series 2 experiment. (c) Variation of δ13C1 and δ13C2 with increasing temperature in series 3 experiment.
Figure 3.
Variation of δ13C1 and δ13C2 with increasing temperature three series experiments. (a) Variation of δ13C1 and δ13C2 with increasing temperature in series 1 experiment. (b) Variation of δ13C1 and δ13C2 with increasing temperature in series 2 experiment. (c) Variation of δ13C1 and δ13C2 with increasing temperature in series 3 experiment.
Figure 4.
The inner walls photo of the tubes heated to different temperatures in three series experiments.
Figure 4.
The inner walls photo of the tubes heated to different temperatures in three series experiments.
Figure 5.
Black particles in solid residue and the energy dispersive spectrum in series 2 experiment at 750 °C.
Figure 5.
Black particles in solid residue and the energy dispersive spectrum in series 2 experiment at 750 °C.
Figure 6.
Color variation of solid residues with temperature increasing in series 2 experiment.
Table 1.
Geochemistry of original and heated shale samples.
| Samples | TOC (%) | Tmax (°C) | S1 (mg/g) | S2 (mg/g) | HI (mg/g TOC) |
|---|---|---|---|---|---|
| Original shale | 1.33 | 520 | 0.1 | 0.31 | 23.31 |
| Heated shale | 1.18 | 633 | 0 | 0 | 0 |
Table 2.
Mineral composition of the original shale sample.
| Mineral | (%) | Mineral | (%) | Mineral | (%) | Mineral | (%) |
|---|---|---|---|---|---|---|---|
| Quartz | 32.31 | Albite | 3.26 | Chlorite | 0.93 | Kaolinite | 0.16 |
| Illite | 29.04 | Pyrite | 3.02 | Muscovite | 0.34 | Rutile | 0.16 |
| Biotite | 10.81 | Gypsum | 2.54 | Smectite | 0.33 | Glauconite | 0.15 |
| Calcite | 6.44 | Feldspar | 2.33 | Siderite | 0.26 | Others | 0.03 |
| Barite | 5.63 | Dolomite | 2.01 | Apatite | 0.25 |
Table 3.
Gas components generated under different temperatures in three series CH4 cracking experiments.
Table 3.
Gas components generated under different temperatures in three series CH4 cracking experiments.
| Series | Temp (°C) | Components (%) | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| H2 | CH4 | C2H6 | C3H8 (×10−1) | C2H4 (×10−3) | C3H6 (×10−3) | iC4 (×10−3) | nC4 (×10−3) | N2 | ||
| 1 | 500 | 0.03 | 89.58 | 0.00 | 0.01 | 0.00 | 0.00 | 0.00 | 0.00 | 10.39 |
| 525 | 0.03 | 89.36 | 0.00 | 0.03 | 0.00 | 0.00 | 0.00 | 0.16 | 10.60 | |
| 550 | 0.05 | 89.23 | 0.00 | 0.04 | 0.00 | 0.09 | 0.24 | 0.60 | 10.72 | |
| 575 | 0.07 | 89.16 | 0.02 | 0.07 | 0.00 | 0.22 | 0.16 | 0.27 | 10.76 | |
| 600 | 0.12 | 88.89 | 0.04 | 0.21 | 0.00 | 0.58 | 0.28 | 0.37 | 10.93 | |
| 625 | 0.13 | 88.80 | 0.07 | 0.24 | 0.12 | 2.18 | 0.39 | 0.44 | 10.97 | |
| 650 | 0.17 | 88.62 | 0.11 | 0.28 | 0.93 | 1.59 | 1.48 | 1.10 | 11.07 | |
| 675 | 0.19 | 88.22 | 0.31 | 0.47 | 1.78 | 1.23 | 1.65 | 2.11 | 11.23 | |
| 700 | 0.29 | 87.78 | 0.48 | 0.35 | 1.08 | 0.93 | 0.87 | 1.24 | 11.41 | |
| 725 | 0.51 | 86.66 | 0.88 | 0.27 | 0.90 | 0.63 | 0.57 | 0.74 | 11.91 | |
| 750 | 0.68 | 85.65 | 1.01 | 0.26 | 0.89 | 0.57 | 0.56 | 0.68 | 12.63 | |
| 775 | 0.77 | 84.65 | 0.89 | 0.26 | 0.85 | 0.60 | 0.54 | 0.58 | 13.66 | |
| 800 | 0.88 | 83.31 | 0.72 | 0.25 | 0.61 | 0.27 | 0.51 | 0.53 | 15.06 | |
| 2 | 500 | 0.04 | 89.44 | 0.01 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 10.51 |
| 525 | 0.05 | 89.28 | 0.02 | 0.01 | 0.00 | 0.00 | 0.00 | 0.00 | 10.65 | |
| 550 | 0.06 | 89.08 | 0.02 | 0.01 | 0.00 | 0.00 | 0.00 | 0.00 | 10.84 | |
| 575 | 0.09 | 88.99 | 0.03 | 0.02 | 0.00 | 0.00 | 0.00 | 0.05 | 10.89 | |
| 600 | 0.11 | 88.75 | 0.04 | 0.02 | 0.00 | 0.00 | 0.00 | 0.17 | 11.10 | |
| 625 | 0.13 | 88.55 | 0.05 | 0.04 | 0.00 | 0.00 | 0.16 | 0.23 | 11.27 | |
| 650 | 0.15 | 88.36 | 0.06 | 0.03 | 0.01 | 0.00 | 0.15 | 0.26 | 11.42 | |
| 675 | 0.22 | 87.91 | 0.24 | 0.11 | 0.25 | 0.12 | 0.33 | 0.56 | 11.62 | |
| 700 | 0.67 | 87.11 | 0.38 | 0.11 | 0.78 | 0.14 | 0.30 | 0.32 | 11.82 | |
| 725 | 1.07 | 86.19 | 0.51 | 0.11 | 0.23 | 0.11 | 0.00 | 0.26 | 12.22 | |
| 750 | 1.39 | 85.09 | 0.50 | 0.08 | 0.00 | 0.00 | 0.00 | 0.17 | 13.01 | |
| 775 | 1.79 | 83.83 | 0.41 | 0.08 | 0.00 | 0.00 | 0.00 | 0.15 | 13.96 | |
| 800 | 2.66 | 82.19 | 0.37 | 0.04 | 0.00 | 0.00 | 0.00 | 0.00 | 14.78 | |
| 3 | 500 | 0.00 | 88.94 | 0.03 | 0.01 | 0.00 | 0.00 | 0.00 | 0.00 | 11.03 |
| 525 | 0.00 | 88.76 | 0.03 | 0.03 | 0.00 | 0.00 | 0.23 | 0.34 | 11.19 | |
| 550 | 0.01 | 88.47 | 0.04 | 0.03 | 0.00 | 0.00 | 0.20 | 0.26 | 11.48 | |
| 575 | 0.03 | 88.16 | 0.04 | 0.03 | 0.00 | 0.00 | 0.17 | 0.32 | 11.77 | |
| 600 | 0.06 | 87.86 | 0.04 | 0.04 | 0.00 | 0.00 | 0.17 | 0.26 | 12.03 | |
| 625 | 0.07 | 87.58 | 0.04 | 0.04 | 0.00 | 0.00 | 0.17 | 0.21 | 12.30 | |
| 650 | 0.19 | 87.20 | 0.08 | 0.06 | 0.11 | 0.09 | 0.15 | 0.22 | 12.52 | |
| 675 | 0.86 | 86.03 | 0.30 | 0.07 | 0.22 | 0.12 | 0.00 | 0.13 | 12.80 | |
| 700 | 1.64 | 84.69 | 0.41 | 0.07 | 0.00 | 0.00 | 0.00 | 0.00 | 13.25 | |
| 725 | 2.16 | 83.88 | 0.45 | 0.05 | 0.00 | 0.00 | 0.00 | 0.00 | 13.50 | |
| 750 | 3.05 | 82.82 | 0.36 | 0.04 | 0.00 | 0.00 | 0.00 | 0.00 | 13.76 | |
| 775 | 4.07 | 81.21 | 0.31 | 0.03 | 0.00 | 0.00 | 0.00 | 0.00 | 14.41 | |
| 800 | 4.49 | 80.25 | 0.21 | 0.03 | 0.00 | 0.00 | 0.00 | 0.00 | 15.04 | |
Series 1: N2 + CH4; Series 2: N2 + CH4 + MK-10; Series 3: N2 + CH4 + shale.
Table 4.
Carbon isotopic composition of CH4 and C2H6 at different temperatures in three series experiments.
Table 4.
Carbon isotopic composition of CH4 and C2H6 at different temperatures in three series experiments.
| Temperature (°C) | δ13C(‰) | |||||
|---|---|---|---|---|---|---|
| Series 1 | Series 2 | Series3 | ||||
| C1 | C2 | C1 | C2 | C1 | C2 | |
| 500 | −25.71 | −25.83 | −25.88 | |||
| 525 | −25.71 | −25.74 | −34.85 | −25.84 | −34.62 | |
| 550 | −25.84 | −25.68 | −34.83 | −25.75 | −34.65 | |
| 575 | −25.84 | −33.22 | −25.71 | −34.81 | −25.51 | −34.58 |
| 600 | −26.01 | −32.74 | −25.67 | −34.94 | −25.38 | −34.55 |
| 625 | −25.91 | −32.47 | −25.66 | −34.89 | −25.26 | −34.52 |
| 650 | −25.97 | −30.26 | −25.56 | −34.61 | −25.13 | −32.07 |
| 675 | −25.81 | −27.47 | −25.52 | −29.85 | −24.48 | −26.42 |
| 700 | −25.83 | −26.02 | −25.11 | −28.15 | −23.92 | −24.63 |
| 725 | −25.66 | −25.31 | −24.86 | −25.91 | −23.14 | −23.88 |
| 750 | −25.28 | −24.82 | −24.43 | −25.56 | −22.69 | −22.32 |
| 775 | −25.04 | −24.68 | −23.86 | −24.46 | −22.29 | −21.85 |
| 800 | −24.60 | −24.50 | −23.31 | −23.29 | −21.95 | −21.66 |
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Mi, J.; Xiao, X.; Guo, J.; He, K.; Ma, X. Experimental Study about Shale Acceleration on Methane Cracking. Processes 2023, 11, 1908. https://doi.org/10.3390/pr11071908
AMA Style
Mi J, Xiao X, Guo J, He K, Ma X. Experimental Study about Shale Acceleration on Methane Cracking. Processes. 2023; 11(7):1908. https://doi.org/10.3390/pr11071908
Chicago/Turabian StyleMi, Jingkui, Xianming Xiao, Jinhao Guo, Kun He, and Xingzhi Ma. 2023. "Experimental Study about Shale Acceleration on Methane Cracking" Processes 11, no. 7: 1908. https://doi.org/10.3390/pr11071908
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