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Article

Evidence of Microbial Activity in Coal Seam Production Water and Hydrochemical Constraints

by
Yuan Bao
1,2,*,
Xueru Chen
1,
Zhidong Guo
3,
Zhengyan Li
4,
Yufei Zhuang
1 and
Min Gao
1
1
College of Geology and Environment, Xi’an University of Science and Technology, Xi’an 710054, China
2
Shaanxi Provincial Key Laboratory of Geological Support for Coal Green Exploitation, Xi’an 710054, China
3
Institute of Engineering and Technology, PetroChina Coalbed Methane Company Limited, Xi’an 710082, China
4
Baode Coal Mine, China Shenhua Shendong Coal Group, Xinzhou 036603, China
*
Author to whom correspondence should be addressed.
Energies 2024, 17(20), 5170; https://doi.org/10.3390/en17205170
Submission received: 23 September 2024 / Revised: 8 October 2024 / Accepted: 15 October 2024 / Published: 17 October 2024
(This article belongs to the Special Issue Exploration and Development of Unconventional Oil and Gas Resources: Latest Advances and Prospects: 2nd Edition)
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Versions Notes

Abstract

:
This study aims to explore microbial activity evidence, composition of archaeal communities, and environmental constraints in coalbed-produced waters from the Hancheng Block, a representative region for coalbed methane development on the eastern margin of Ordos Basin, China. The investigation involves analyzing microbial community composition using 16S rRNA sequencing analysis as well as examining hydrogeochemical parameters. The results indicate that Euryarchaeota and Thaumarchaeota are predominant phyla within archaeal communities present in coalbed-produced water from the Hancheng Block. Among these communities, Methanobacterium is identified as the most abundant genus, followed by Methanothrix and Methanoregula. Moreover, a positive correlation is observed between the abundance of Methanobacterium and the levels of total dissolved solids as well as Mn; conversely, there is a negative correlation with dissolved organic carbon, Zn concentrations, and pH. The abundance of Co and Ni primarily influence Methanothrix while pH and Zn play significant roles in controlling Methanoregula. Additionally, No. 5 coal seam waters exhibit greater species diversity in the archaeal community compared to No. 11 counterparts. The higher abundance of archaea in the No. 5 coal seam promotes biogas generation due to the correlation between bicarbonate and dissolved inorganic carbon isotope. These research findings hold scientific significance in guiding the exploration and development of biogas within coal seams.

1. Introduction

Coal bed methane (CBM) refers to primarily methane adsorbed on the surface of the coal matrix, with a small portion existing as free gas in the pores and cracks of coal. CBM has demonstrated significant potential for efficient and clean energy production [1,2]. China possesses abundant CBM resources, with two commercially developed CBM bases established in the Qinshui Basin and the eastern margin of Ordos Basin [3,4]. The principle behind CBM extraction involves reducing pressure within the coal reservoir by pumping groundwater from both within and surrounding it [5], thereby releasing any trapped gas. This process allows for the desorption, diffusion, and migration of CBM into wellbores through pores and fissures for exploitation purposes [6]. Coal seam water serves as a habitat for underground microorganisms that have been exposed to coal seams over extended periods while residing within this water. Produced water derived from draining wells used during mining operations or other activities related to extracting CBM from such seams contains valuable geological information [7]. Paying attention to microbial diversity alongside hydrogeochemical characteristics present in produced water derived from coal seams can significantly impact our understanding of living environments and constraints faced by microorganisms situated deep beneath the Earth’s surface inside these same seams, ultimately guiding exploration efforts aimed at developing new sources of CBM.
With the rapid advancement of the CBM industry and increasingly stringent global environmental protection requirements, experts and scholars have been increasingly emphasizing studying water produced from coal seam mining processes [8,9,10]. Currently, research on the hydrogeochemical characteristics of coalbed-produced water primarily focuses on conventional ions [11,12], hydrogen, oxygen, and inorganic carbon isotopes [13,14], as well as trace elements [15,16]. Simultaneously, numerous studies have explored archaeal communities involved in coal metabolism within produced water from coal seams. These studies have found that Methanosarcina is the predominant methanogenic archaea species found in Powder River Basin’s produced waters [17,18], while Methanosaeta/Methanosarcina are major methanogens present in San Juan Basin’s CBM fields. Additionally identified were the main Methanobacterium, Methanomicrobium, Methanolobus [19,20], and Methanospirillum species present at Qinshui Basin’s CBM field [21]. Therefore, studying both the hydrogeochemical characteristics of produced waters and the diversity among archaeal communities is crucial for indicating trends in CBM development.
The Hancheng Block, located in the eastern part of the Weibei Uplift within the southeastern Ordos Basin, stands as one of China’s earliest regions where commercial success has been achieved in CBM development. This area contains abundant CBM resources primarily found within coal seams No. 5 and No. 11 of the Upper Carboniferous Taiyuan Formation [22]. To investigate the structure and composition characteristics of archaeal communities within situ coal seams, along with their controlling factors, this study collected a total of 12 samples from the Hancheng Block, comprising eight samples from No. 11 coal seams and four samples from No. 5 coal seams. The produced water was analyzed for conventional ions, hydrogen and oxygen isotopes, dissolved inorganic carbon isotopes, and trace elements, as well as a quantification of archaeal 16S rRNA. These research findings can provide theoretical guidance for predicting late-stage productivity in the Hancheng Block’s CBM wells and conducting field tests on enhancing methane production through microorganism injection and nutrient solution supplementation.

2. Materials and Methods

2.1. Samples Collection

Based on CBM well locations and single-layer drainage analysis conducted in the Hancheng area, a comprehensive set of twelve representative produced water samples were systematically collected to ensure complete spatial coverage across all target areas. The precise sampling sites can be observed in Figure 1. Prior to collecting on-site samples, all polyethylene sample containers underwent thorough sterilization in the laboratory using ultraviolet light and were subsequently sealed for sterility purposes. Subsequently, polyethylene sampling bottles (one 2.5 L and two 500 mL) were flushed three times with output water from the targeted coal seam being mined to ensure the integrity of collected samples, while allowing the water valve to open and drain for a specific duration to avoid residual wellbore water in the pipeline. During the collection activities at drainage outlets of CBM wells, one 500 mL polyethylene sampling bottle underwent acidification using dilute hydrochloric acid until reaching pH < 3 conditions for testing cation concentration. Another 500 mL polyethylene bottle was filled and refrigerated at 0 °C. After completing the collection process, the air within these bottles was expelled by filling them completely before sealing them with bottle caps to check for any potential leakage issues. Finally, the time and place of each sampling event were recorded. The original water samples were collected using a larger capacity (2.5 L) polyethylene bottle while smaller volumes (500 mL) that had not undergone acidification were filtered under laboratory conditions utilizing filter membranes with pore sizes measuring either 0.22 μm or 5 μm as required. Following this process, the filter membranes were packed into a centrifuge tube and stored at −20 °C for subsequent high-throughput sequencing of archaeal community DNA analysis purposes. A 500 mL water sample was collected from a 2.5 L polyethylene sampling bottle for analyzing pH levels, anions, total dissolved solids (TDS), trace elements, hydrogen and oxygen isotopes, as well as dissolved inorganic carbon isotopes. The hydrogeochemical test was submitted to the Guizhou Institute of Geochemistry, Chinese Academy of Sciences for analysis purposes, while the microbial 16S rRNA high-throughput sequencing was sent to Shanghai Piceno Biotechnology Co., Ltd. (Shanghai, China), for testing.

2.2. Test Methods

The microbial samples were initially subjected to DNA extraction using the OMEGA Soil DNA Kit (M5635-02) from Omega Bio-Tek, Norcross, GA, USA. Subsequently, the quantity and quality of the extracted DNA were assessed using the NanoDrop NC2000 spectrophotometer from Thermo Fisher Scientific, Waltham, MA, USA. Additionally, agarose gel electrophoresis was performed for further verification. Following that step, Polymerase Chain Reaction (PCR) amplification utilizing archaea 16sv8v9-1106F (TTWAGTCAGGCAACGAGC) and 16sv8v9-1378R (TGTGCAAGGAGCAGGGAC) primers was conducted to target methanogen 16S rRNA genomes. Finally, the sequencing data analysis was conducted using the following method: equal amounts of PCR amplified sequences were pooled together and subjected to pair-end 2 × 250 bp sequencing on the Illumina NovaSeq platform with NovaSeq 6000 SP Reagent Kit (500 cycles) at Shanghai Personal Biotechnology Co., Ltd. (Shanghai, China). The sequence data analyses primarily utilized QIIME2 2019.4. (https://docs.qiime2.org/2019.4/tutorials/, accessed on 31 October 2022) and R packages (v3.2.0).
The water sample test includes the analysis of cations, anions, hydrogen isotopes (δD), and oxygen isotopes (δ18O) of water, dissolved inorganic carbon isotopes (δ13CDIC), and trace elements. Cationic tests are performed using the Vista MPX inductively coupled plasma-emission spectrometer (Varian Company, Palo Alto, CA, USA), while anionic tests utilize ion chromatography (Thermer Fisher ICS-900). HCO3− and CO32− ion detection relies on acid titration. The respective standards for cationic tests follow DIN EN ISO17294-1-2007 [23], and those for anionic tests adhere to EN ISO 10304-1-2009 [24]. For measuring δD and δ18O, a liquid isotope analyzer 912-0026 (Los Gatos Research, San Jose, CA, USA) with a standard deviation of 0.6‰ for δD and 0.1‰ for δ18O is employed. The gas isotope mass spectrometer MAT252 is employed for δ13CDIC testing with a measurement accuracy of less than 0.01‰. The testing methods for δD, δ18O, and δ13CDIC have been outlined and documented in reference [14]. Trace element analysis uses the NexION300X ICP-MS instrument (PE Company, Cincinnati, OH, USA) following the test standard EPA Method 200.8 [25]. pH measurement was carried out using PP-50-p11 m, while TDS measurement utilized the DDSJ-308A conductivity meter (Shanghai Yilin Scientific Instrument Co., LTD, Shanghai, China). The corresponding analysis method and testing standards were referenced in the literature [26].

3. Results and Analysis

3.1. Archaeal Community Structure of Coalbed-Produced Water

To gain a deeper understanding of the species structure of archaeal communities in the produced water from different coal seams in the Hancheng Block, this study employed 16S rRNA gene amplification and sequencing of microbial samples from the produced water to determine the species composition of archaeal communities (Figure 2). Euryarchaeota and Thaumarchaeota were identified as the dominant phyla of archaea in the produced water from different coal seams. The proportion of Euryarchaeota was 33.38% for H11-4, 61.96% for H5-1, and over 80% for other CBM wells in the Hancheng Block. The relative abundance of Thaumarchaeota was relatively high at 4.51% for H11-4 and 6.37% for H11-6, respectively. At the genus level classification, Methanobacterium exhibited the highest content followed by Methanothrix, Methanoregula, Methanococcus, Methanosarcina, Methanospirillum, Methanocella, Methanoculleus, Methanolobus, and Methanocalculus. Methanothrix accounted for the highest proportion of 35.03% in H11-3 and 27.66% in H5-4. In addition, Methanosarcina constituted 10.61% in H11-6, while H5-3 and H5-4 contained respective proportions of 19.04% and 9.96% for Methanoregula.

3.2. Conventional Ion of Coalbed-Produced Water

The conventional negative, cation, and geochemical parameters of produced water from different coal seams in the Hancheng Block are presented in Table 1. The pH value of the produced water from the No. 11 coal seam ranges from 7.61 to 8.21, with an average value of 7.82. Similarly, the pH value of the produced water from the No. 5 coal seam ranges from 7.66 to 8.32, with an average value of 7.92. The alkaline nature of formation waters in these coal seams creates a favorable environment for methanogen survival. Sodium (Na+) is found predominantly as cations in both No. 11 and No. 5 coal seam-produced waters, with mass concentrations ranging from 330.29 to 1772.50 mg/L (average: 829.47 mg/L) and from 603.75 to 2003.00 mg/L (average: 1448.59 mg/L), respectively. Additionally, certain concentrations of calcium (Ca2+), magnesium (Mg2+), and potassium (K+) are also present in water samples produced in this study area. Chloride ions (Cl−) and bicarbonate ions (HCO3−) are identified as major anions within both coal seam-produced waters; higher levels of HCO3− suggest a relatively closed groundwater environment along with some sulfate (SO42−) concentration observed. TDS concentration varies between 816.90 and 5038.39 mg/L (average: 2239.07 mg/L) in No. 11 coal seam samples, while ranging between 1276.44 and 4906.74 mg/L (average: 3160.50 mg/L) in No. 5 coal seam samples. Dissolved organic carbon (DOC) concentration ranges between 27.17 and 73.82 mg/L (average: 52.84 mg/L) in the No. 11 coal seam in the Hancheng area and between 44.88 and 49.08 mg/L (average: 47.03 mg/L) in No. 5 coal seam.
The distinction between the two groups of coal seams can be clearly demonstrated by plotting a histogram of the conventional ion content data of produced water from different coal seams in the Hancheng Block (Figure 3). The concentration of HCO3− in the produced water from coal seams H11-1, H11-2, H11-3, H11-4, H11-7, and H11-8 is higher than that of Cl−, while the concentration of Cl− in the produced water from coal seams H11-5 and H11-6 is higher than that of HCO3−. Similarly, for the No. 5 coal seam, samples such as H5-3 and H5-4 exhibit a higher concentration of HCO3− compared to samples like H5-1 and H5-2, which have a higher concentration of Cl−. Furthermore, elevated levels of SO42− are observed in No. 11 coal seam samples, including those from wells H11-1, H11-5, and H11-6, while lower levels are found in the water produced the from No. 5 coal seam. The average cation concentrations in the produced water from the No. 11 coal seam follow this order: Na+ > Ca2+ > K+ > Mg2+, whereas, for the No. 5 coal seam, they are Na+ > K+ > Ca2+ > Mg2+.
The Piper trigram of groundwater chemical composition can be utilized to analyze the types and characteristics of water chemistry, thereby revealing the hydrochemical environment of groundwater [27]. In the Piper ternary diagram (Figure 4), data from six groups of water samples in coal seam No. 11 and four groups of water samples in coal seam No. 5 within the Hancheng Block are plotted. Based on the diagram, H11-1, H11-2, H11-3, H11-4, H11-7, and H11-8 exhibit a hydrochemical type characterized by HCO3-Na in the produced water from coal seam 11. Conversely, both H11-5 and H11-6 display a Cl-Na hydrochemical type due to their higher salinity levels and elevated chloride ion content as previously studied. Consequently, overall findings indicate that the produced water from the No. 11 coal seam is predominantly composed of HCO3-Na and Cl-Na. In contrast to this pattern, both samples from H5-1 and H5-2 within the No. 5 coal seam demonstrate higher salinity levels with greater chloride ion content compared to bicarbonate ions; thus, exhibiting a Cl-Na type. Conversely, samples from both H5-3 and H5-4 belong to an HCO3− Na-type category within the produced water composition of the No. 5 coal seam.

3.3. Hydrogen, Oxygen, and Carbon Isotopic Compositions of Coalbed-Produced Water

The test results of δD, δ18O, and δ13CDIC in the produced water from the coal seam in the Hancheng Block are presented in Table 2. The δD values of coal seam No. 11’s produced water range from −113.62‰ to −61.56‰, with an average value of −82.48‰. The δ18O values range from −15.36‰ to −9.22‰, with an average value of −11.53‰. For coal seam No. 5, the produced water exhibits a narrower range of δD values (−112.07‰ to −72.00‰) compared to coal seam No. 11, with an average value of −89.81‰. Similarly, its δ18O values also show a smaller variation (−15.82‰ to −10.84‰), averaging at −12.78‰. Furthermore, it is worth noting that both the δD and δ18O values for coal seam No. 5’s produced water are more negative than those for coal seam No. 11, indicating potential differences in isotopic composition between these two sources. Regarding δ13CDIC, the produced water from coal seam No. 11 displays a wider range (−27.77‰ to 4.02‰) compared to that from coal seam No. 5 (−5.27‰ to 20.54‰). However, their respective averages remain relatively close at 16.00‰ and 11.23‰, respectively. It is noteworthy that, except for well H5-2, the dissolved CO2 in groundwater samples within this study area exhibit positive δ13CDIC values, indicating enrichment with 13C [28].
The hydrogen and oxygen isotope composition of the coal seam-produced water can serve as an indicator of its origin [29]. According to Craig et al. [30], the equation for the Global Meteoric Water Line (GMWL) is δD = 8δ18O + 10. The China Meteoric Water Line (CMWL), initially proposed by Zheng et al. in 1983 [31], is represented by the equation: δD = 7.9δ18O + 8.2. By projecting the hydrogen and oxygen isotope data from Table 2 onto the δ18O-δD relationship diagram (Figure 5), it becomes evident that the projections for produced water from different coal seams in the Hancheng Block are closely aligned with both GMWL and CMWL, indicating their origin from atmospheric precipitation and subsequent recharge through atmospheric precipitation. This provides a pathway for microbial transportation into coal seams, facilitating microbial degradation and favoring biogenic coalbed methane generation within these seams. However, it should be noted that some points on the graph deviate above or below this line; specifically, the No. 5 coal seam exhibits deviations above the line (left side, D-drift). The occurrence of drift in δ18O may be attributed to water–coal interactions under conditions of evaporation and high temperature. On the other hand, D-drift is generally believed to result from the potential transformation of produced water within coal seams due to methanogenesis processes where methanogens preferentially utilize hydrogen derived from water sources while leaving behind residual groundwater enriched in deuterium. Alternatively, this could also arise due to water–coal interactions between groundwater and coal seams as shown in Equation (1). These observations indicate that generated and migrating produced water from coal seams exhibit dual drift characteristics in both δ18O and δD isotopes, suggesting multiple geochemical processes such as water–coal interaction and methanogenesis during their formation.
H2O + Dcoal←→HDO + Hcoal

3.4. Trace Elements Concentration of Coalbed-Produced Water

The concentration of trace elements in the water produced from the coal seam in the Hancheng Block is presented in Table 3. A total of 24 trace elements, including Al, As, Ba, Co, Cr, Cu, Hg, Li, Mn, Mo, Ni, Pb, Rb, Sb, Se, Sn, Sr, Ti, Tl, U, V, W, Zn, and Zr, were detected in this study. Among them, the contents of Ba, Li, Mn, and Sr were higher. The concentration ranges of Ba, Li, Mn, and Sr in No. 11 coal seam samples are 31.73–3587.82 ppb, 52.42–1185.12 ppb, 1.32–214.73 ppb, and 362.19–6547.09 ppb, respectively; while, in No. 5 coal seam samples, they range from 236.99 to 8455.96 ppb for Ba, from 59.41 to 276.20 ppb for Li, from 12.40 to 149.71 ppb for Mn, and from 552.85 to 8287.18 ppb for Sr. In addition, the presence of trace elements of Co and Ni associated with the archaeal community was also detected [32]. The subsequent Section 4.3 examines the correlation between Co and Ni in relation to the archaeal community.

4. Discussion

4.1. Microbial Diversity in Coalbed-Produced Water

ASV-level alpha diversity indices such as Chao1 richness estimator observed species count Shannon diversity index Simpson index and Good’s coverage were calculated based on the ASV table in the QIIME2 software package (Version 2021.2). The alpha diversity index reflects the number, abundance, and evenness of species within a specific region or ecosystem sample, also known as within-habitat diversity [33,34]. To comprehensively evaluate the alpha diversity of archaeal communities in the produced water of coal seams in the Hancheng Block, we utilized the Chao1 and Observed species indices to characterize richness, Shannon and Simpson indices to measure diversity, and the Evenness index to assess evenness. The Coverage index was employed to determine coverage. Figure 6 presents the alpha diversity index of archaeal communities in the produced water from different coal seams in the Hancheng Block. The Coverage index for the archaeal community in coal seams No. 11 and No. 5 was found to be 99.18–99.50% and 98.98–99.40%, respectively, indicating that most archaea present in samples were successfully obtained and detected, ensuring reliable detection results. The Chao1 and Observed species indices for the archaeal community in production water from coal seam No. 11 are lower compared to those from coal seam No. 5, suggesting higher species richness for the archaeal community in production water from coal seam No. 5 than that from coal seam No. 11. The Shannon and Simpson indices along with Evenness indices showed minimal differences between both groups’ archaeal communities, implying similar levels of diversity and evenness among them.

4.2. Evidence of Microbial Activity in Coalbed-Produced Water

The δ13CDIC values of surface water and shallow groundwater typically range from −14‰ to −7‰ [35,36]. In contrast, the δ13CDIC value of produced water from deep underground coalbed methane wells is more enriched, usually ranging from −7‰ to 0‰. Previous studies have indicated that microbial activity can result in dissolved inorganic carbon isotopes in water ranging from 10‰ to 30‰ [36]. In the Hancheng Block, there were eight instances of coal seam producing water with δ13CDIC values exceeding 10‰ (Figure 7), with five occurring in the No. 11 coal seam and three occurring in the No. 5 coal seam. These findings suggest the occurrence of microbial action within the coal seams of the Hancheng Block.
The δ13CDIC values of H5-2 from the No. 5 coal seam in the Hancheng Block even demonstrate negative values (−5.27‰). Furthermore, well H11-2, H11-5, and H11-6 from the No. 11 coal seam exhibit relatively low values (<10‰). Other samples fall within the range of 10‰ to 30‰. Variations in burial depth among different well locations result in distinct δ13CDIC values, which can be attributed to metabolic pathway differences in key microorganisms within the coal seam. Furthermore, a negative correlation is observed between burial depth and δ13CDIC values in produced water from both the Nos. 5 and 11 coal seams in the Hancheng Block (Figure 8(a1,b1)), suggesting that carbon dioxide reduction pathways exist within microbial metabolism processes occurring within these coal seams (Formulas (2) and (3)). Additionally, well H5-2 is in the southwestern region of the study area, adjacent to the footwall of the Qiangao fracture zone. It should be noted that this coal seam exhibits a greater burial depth compared to both the No. 11 coal seam and other No. 5 coal seams, suggesting an evident influence of burial depth on microbial activity.
CH3COOH→CH4 + CO2
CO2 + 4H2→CH4 + 2H2O
The sources of dissolved inorganic carbon in coal seam-produced water are generally attributed to the dissolution of carbonate minerals, the dissolution of CO2 in coalbed methane, and microbial methanogenesis [37]. Figure 8(a2,b2) illustrate the relationship between HCO3− and δ13CDIC in the produced water from coal seam No. 5 in the Hancheng area. A clear negative correlation is observed between HCO3− and δ13CDIC, indicating significant microbial activity within coal seam No. 5, where methanogens preferentially utilize 12CH4. The remaining 13C becomes enriched in CO2 and DIC [38]. Conversely, a positive correlation is observed between HCO3− and δ13CDIC in the produced water from coal seam No. 11 due to its burial beneath coal seam No. 5, which prevents easy mixing with surface water and shallow groundwater. Additionally, coupled with carbonate mineral dissolution and microbial activity within more enriched coal measure strata, these factors contribute to their positive correlation.

4.3. Environmental Factors Influencing the Composition of Archaeal Communities

To investigate the environmental factors governing the archaeal community in the produced water of coal seams Nos. 11 and 5 in the Hancheng Block, we conducted a redundancy analysis (RDA) to investigate the relationship between the archaeal community and hydrogeochemical parameters (including trace elements Co, Ni, Zn, Mn, pH, TDS, DOC). The results of RDA revealed that at both the phylum level (Figure 9a) and genus level (Figure 9b), the cumulative variance of factors influencing the archaeal community accounted for 88.12% and 87.44%, respectively. This essentially reflects the characteristic associations among all variables investigated. At the phylum level, Wells H11-4, H11-6, and H5-1 exhibited significant dissimilarity from other CBM Wells in terms of microflora composition. This finding aligns with Figure 2a as well. Furthermore, as depicted in Figure 9a, Euryarchaeota—which is the dominant group of archaea found in coal seam-produced water from the Hancheng Block—was primarily influenced by TDS and trace elements Co, Ni, and Mn. These growth environment factors exhibited positive correlations with Euryarchaeota abundance. Additionally, the presence of DOC and Zn showed positive correlations with another dominant group, Thaumarchaeota. Zn, on the other hand, had negative correlations with both Euryarchaeota and Thaumarchaeota.
At the genus level, this study investigated the relationship between the archaeal community, specifically focusing on the first three dominant genera and their respective environmental factors in produced water from 12 CBM wells. Methanobacterium was identified as the predominant genus within the archaeal community in the coal seam-produced water of the Hancheng Block. The data demonstrated a positive correlation between TDS and Mn while showing a negative correlation with DOC, Zn, and pH levels. Notably, relevant laboratory experiments have provided evidence that high TDS content enhances CBM production [39]. Furthermore, the abundance of Methanothrix was primarily influenced by Co and Ni. This can be attributed to Co’s contribution to bioenzyme Co-dehydrogenase and Methyltransferase activity, while Ni plays a crucial role in Acetyl-CoA synthase and Hydrogenase anabolism processes [40,41]. Methanoregula exhibited a lower abundance compared to other dominant genera, primarily influenced by pH levels and Zn concentrations. Specifically, pH showed a negative correlation with Methanobacterium and Methanothrix but a positive correlation with Methanoregula, which may be related to metabolic differences among these different genera. Furthermore, previous studies have indicated that the diversity, abundance, and transcriptional activity of various archaea fluctuate under different pH conditions. This implies that distinct types of archaea can be selectively favored in environments characterized by varying pH levels [42,43]. Overall, there exists a strong association between these environmental factors and different dominant genera, indicating that they play significant roles in shaping the archaeal community present in coal seam-produced water from the Hancheng Block [44].

5. Conclusions

Biogenic coalbed methane is a relatively clean natural gas resource. The relatively light carbon isotope composition of methane indicates microbial degradation of the coal seam in the Hancheng Block during its geological history. The presence of methanogenic archaea in the coal seam water provides direct evidence for the occurrence of biogenic coalbed methane in this area. The presence of dual isotopic drift characteristics in both oxygen and deuterium isotopes within this set of produced water samples indicates various geochemical processes such as interactions between water and coal along with methanogenesis during its migration pathway.
The hydrochemical types of No. 5 and No. 11 coal seams are identified as HCO3-Na and Cl-Na, respectively. The average pH value of produced water from the No. 11 coal seam measures 7.82, while the No. 5 coal seam records weak alkalinity with an average pH value of 7.92; this alkalinity supports methanogen survival within produced water. Methanobacterium is the most prevalent genus, followed by Methanothrix and Methanoregula. The richness of the archaeal community in coalbed-produced water from coal seam No. 5 surpasses that of coal seam No. 11, which favors biogas generation. The predominant archaeal bacterium, Methanobacterium, exhibits positive correlations with total dissolved solids and manganese while displaying negative correlations with dissolved organic carbon, Zn, and pH levels. Co primarily regulates Methanothrix, while Ni has a lesser influence on it. Conversely, pH levels and Zn mainly impact Methanoregula.

Author Contributions

Y.B.: Writing—review and editing, Conceptualization, Funding acquisition, Investigation, Supervision. X.C.: Formal analysis, Data curation, Methodology, Conceptualization. Z.G.: Writing—review and editing, Resources, Investigation. Z.L.: Writing—original draft, Methodology, Data curation, Y.Z.: Methodology, Investigation, Software, Conceptualization. M.G.: Conceptualization, Formal analysis, Investigation. All authors have read and agreed to the published version of the manuscript.

Funding

The research presented in this paper is financially supported by the National Natural Science Foundation of China (42172200; 41972183).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

Author Zhidong Guo was employed by the company Institute of Engineering and Technology, PetroChina Coalbed Methane Company Limited. Author Zhengyan Li was employed by the company Baode Coal Mine, China Shenhua Shendong Coal Group. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. (a) The location and structural outline of Ordos basin; (b) the structure outline and sampling locations of the Hancheng Block; (c) column diagram depicting coal seam distribution.
Figure 1. (a) The location and structural outline of Ordos basin; (b) the structure outline and sampling locations of the Hancheng Block; (c) column diagram depicting coal seam distribution.
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Figure 2. Taxonomic compositions of the top ten most abundant archaea in the Hancheng Block ((a). phylum level; (b). genus level. The remaining abundance is represented as the “others” group).
Figure 2. Taxonomic compositions of the top ten most abundant archaea in the Hancheng Block ((a). phylum level; (b). genus level. The remaining abundance is represented as the “others” group).
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Figure 3. Conventional ion content of coalbed-produced water ((a). No. 5; (b). No. 11).
Figure 3. Conventional ion content of coalbed-produced water ((a). No. 5; (b). No. 11).
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Figure 4. Piper diagram of coalbed-produced water in the Hancheng Block.
Figure 4. Piper diagram of coalbed-produced water in the Hancheng Block.
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Figure 5. Relationship between δD and δ18O of coalbed-produced water in the Hancheng Block.
Figure 5. Relationship between δD and δ18O of coalbed-produced water in the Hancheng Block.
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Figure 6. Alpha diversity analysis of archaeal communities.
Figure 6. Alpha diversity analysis of archaeal communities.
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Figure 7. Distribution characteristics of δ13CDIC of coalbed-produced water in the Hancheng Block.
Figure 7. Distribution characteristics of δ13CDIC of coalbed-produced water in the Hancheng Block.
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Figure 8. Relationship between δ13CDIC and buried depth, as well as the concentration of HCO3− in coalbed-produced water from the Hancheng Block ((a1) Relationship between δ13CDIC and buried depth in No.5 coal seam (b1) Relationship between δ13CDIC and buried depth in No.11 coal seam (a2) Relationship between δ13CDIC and the concentration of HCO3− in No.5 coal seam (b2) Relationship between δ13CDIC and the concentration of HCO3− in No.11 coal seam).
Figure 8. Relationship between δ13CDIC and buried depth, as well as the concentration of HCO3− in coalbed-produced water from the Hancheng Block ((a1) Relationship between δ13CDIC and buried depth in No.5 coal seam (b1) Relationship between δ13CDIC and buried depth in No.11 coal seam (a2) Relationship between δ13CDIC and the concentration of HCO3− in No.5 coal seam (b2) Relationship between δ13CDIC and the concentration of HCO3− in No.11 coal seam).
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Figure 9. Redundancy analysis of archaea at both phylum (a) and genus (b) levels.
Figure 9. Redundancy analysis of archaea at both phylum (a) and genus (b) levels.
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Table 1. Geochemical parameters of coalbed-produced water in the Hancheng Block.
Table 1. Geochemical parameters of coalbed-produced water in the Hancheng Block.
Sample IDCoal SeamConcentration of Conventional Ion (mg·L−1)pHTDS
(mg·L−1)		DOC (mg·L−1)
Na+K+Ca2+Mg2+Cl−SO42−HCO3−
H5-1No. 52003.0013.3734.117.112416.890.56863.397.664906.7444.88
H5-21974.50100.0316.885.221811.9911.861555.137.884698.0549.08
H5-3603.753.623.560.59169.17<0.10991.498.321276.4445.89
H5-4808.519.694.690.64500.380.05873.647.961760.7848.26
Average1347.4431.6814.813.391224.614.161070.917.963160.5047.03
H11-1No. 111163.6029.1529.9516.46793.87241.691211.837.642880.6466.17
H11-2330.293.6012.542.26109.680.06716.937.69816.9057.53
H11-3894.517.4019.227.41801.790.12840.347.712150.6227.17
H11-4536.828.559.080.92167.800.25914.768.211180.8073.82
H11-51772.5030.46135.7264.771941.86824.71536.747.615038.3941.40
H11-61132.1046.12163.1148.25602.70987.22538.757.973248.8854.51
H11-7366.5513.518.572.9086.710.13815.148.16885.9458.08
H11-8742.0113.1118.694.78484.010.25895.117.731710.4144.04
Average867.3018.9949.6118.47623.55256.80808.707.842239.0752.84
Table 2. The isotopic compositions of hydrogen, oxygen, and dissolved inorganic carbon in coalbed-produced water of the Hancheng Block.
Table 2. The isotopic compositions of hydrogen, oxygen, and dissolved inorganic carbon in coalbed-produced water of the Hancheng Block.
Sample IDCoal SeamδDH2O (‰)δ18OH2O (‰)δ13CDIC (‰)
H5-1No. 5−72.00−10.8414.51
H5-2−77.00−10.99−5.27
H5-3−98.16−13.4620.54
H5-4−112.07−15.8215.14
Average−89.81−12.7811.23
H11-1No. 11−77.79−11.0410.18
H11-2−84.12−11.457.74
H11-3−61.56−9.2219.19
H11-4−81.47−11.2428.08
H11-5−78.58−11.279.85
H11-6−113.62−15.364.02
H11-7−80.10−11.0027.77
H11-8−82.60−11.6621.20
Average−82.48−11.5316.00
Table 3. Trace element concentration of coalbed-produced water in the Hancheng Block (unit, ppb).
Table 3. Trace element concentration of coalbed-produced water in the Hancheng Block (unit, ppb).
Sample IDCoal SeamAlAsBaCoCrCuHgLiMnMoNiPbRbSbSeSnSrTiTlUVWZnZr
H5-1No. 5104.241.707043.930.030.600.010.09246.8994.462.873.240.1121.760.0123.080.068287.180.440.020.0210.960.402.550.10
H5-254.871.498455.960.150.710.030.12276.20149.7112.2312.260.0932.240.1224.270.106254.500.200.020.007.021.131.170.14
H5-374.421.21236.990.070.390.870.0991.0412.407.800.141.345.090.0421.080.06552.850.210.050.015.691.120.000.16
H5-465.441.37424.410.023.750.520.0659.4122.150.7718.210.025.620.0021.520.08732.570.230.040.056.060.940.000.18
Average74.75 1.44 4040.32 0.07 1.36 0.36 0.09 168.38 69.68 5.92 8.46 0.39 16.18 0.04 22.49 0.07 3956.77 0.27 0.03 0.02 7.43 0.90 0.93 0.14
H11-1No. 1186.211.1540.760.040.561.050.04327.01120.971.431.430.0818.870.0224.320.052752.640.110.030.006.000.040.000.12
H11-276.030.72591.530.131.490.000.0952.42114.820.820.433.575.160.0121.050.03495.750.270.050.004.351.010.000.07
H11-3192.801.743587.822.301.170.090.31177.4489.391.234.090.095.290.0622.190.132627.680.990.030.109.547.440.710.33
H11-465.721.01221.400.150.310.270.11169.2034.223.590.420.077.750.0823.540.05282.130.030.040.013.591.4060.250.08
H11-583.811.8631.730.140.750.000.071185.12214.731.183.850.2125.040.0324.150.096547.090.410.020.0010.000.220.000.18
H11-651.412.2437.300.050.410.000.04480.591.320.211.880.0043.890.0122.810.105785.940.350.020.024.240.360.000.12
H11-768.980.74241.551.110.440.250.07106.96108.6814.302.060.0315.330.0420.920.06362.190.220.040.003.460.000.000.06
H11-857.081.041617.650.040.440.000.09113.82122.1816.320.190.036.680.0222.640.051546.000.140.030.014.030.740.000.11
Average85.25 1.31 796.22 0.50 0.70 0.21 0.10 326.57 100.79 4.89 1.75 0.51 16.00 0.03 22.70 0.07 2549.93 0.31 0.03 0.02 5.65 1.40 7.62 0.13
Note: The content is deemed indiscernible by the instrument when its value reaches 0.00.
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Bao, Y.; Chen, X.; Guo, Z.; Li, Z.; Zhuang, Y.; Gao, M. Evidence of Microbial Activity in Coal Seam Production Water and Hydrochemical Constraints. Energies 2024, 17, 5170. https://doi.org/10.3390/en17205170

AMA Style

Bao Y, Chen X, Guo Z, Li Z, Zhuang Y, Gao M. Evidence of Microbial Activity in Coal Seam Production Water and Hydrochemical Constraints. Energies. 2024; 17(20):5170. https://doi.org/10.3390/en17205170

Chicago/Turabian Style

Bao, Yuan, Xueru Chen, Zhidong Guo, Zhengyan Li, Yufei Zhuang, and Min Gao. 2024. "Evidence of Microbial Activity in Coal Seam Production Water and Hydrochemical Constraints" Energies 17, no. 20: 5170. https://doi.org/10.3390/en17205170

APA Style

Bao, Y., Chen, X., Guo, Z., Li, Z., Zhuang, Y., & Gao, M. (2024). Evidence of Microbial Activity in Coal Seam Production Water and Hydrochemical Constraints. Energies, 17(20), 5170. https://doi.org/10.3390/en17205170

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