Abstract
The hydrogeochemical characteristics of the mining area are of great significance for coalbed methane (CBM) production. Based on experimental test data for coalbed water and CBM well–produced water in the Suzhou mining area, as well as the actual production capacity of CBM wells, the effect of the hydrogeochemical characteristics on CBM well production capacity was analyzed in this article. The results indicated that the groundwater in the study area was in a closed hydrological environment and that the water quality type was mainly Na-K-HCO3. When the groundwater from QD coal mine flowed through coal seam 6, the exchange intensity between Na+ and K+ in the water and Ca2+ and Mg2+in the formation’s minerals was relatively high. The coefficients of variation of the ions in the QD coal seam water were within a moderate range, and the coefficients of variation of Ca2+ and Mg2+ were much higher than those of the other ions. Within the same coal mining area, the HCO3− concentration of the water produced from the CBM wells was positively correlated with the production of CBM. Among them, the HCO3− concentration of the water produced from well N-3 was the highest, and the gas production of this well was also the highest.
Keywords: Coalbed water, Coalbed methane well, Geochemical characteristics, Suzhou mining area
Subject terms: Energy science and technology, Environmental sciences, Hydrology
Coalbed methane (CBM) is an unconventional natural gas and is a clean energy source that is mainly composed of methane. The genesis and migration of CBM are influenced by many factors. The genesis types of CBM include thermogenic CBM, biogenic CBM, and mixed genesis CBM. The hydrological conditions of the mining area affect the migration of CBM. Among these conditions, hydraulic sealing and plugging are beneficial for the enrichment of CBM, while hydraulic escape is not conducive to the enrichment of CBM1–5. Previous studies have found that strengthening the research on water in coal-bearing strata is of great significance for guiding the production of CBM6–10.
Chinese and foreign scholars have conducted extensive research on the hydrochemical characteristics of coalbed mine water and CBM well–produced water. It is generally believed that the hydrochemical characteristics of mine water are influenced by water-rock interactions and rock weathering. The leaching of calcite and silicate minerals is the main source of the cations and anions in the water and is also affected by alternating adsorption and desulfurization11–15. The hydrochemical characteristics of the water produced from CBM wells are influenced by factors such as different aquifers and different periods. It is generally believed that factors such as mineral dissolution in different aquifers and artificially injected chemicals affect the hydrochemical characteristics of the water produced from CBM wells16–18. In the early stage of water production from CBM wells, the hydrochemical characteristics are mainly influenced by artificially injected chemicals19–21. During the transition from the initial stage to the stable stage of water production from CBM wells, the hydrochemical characteristics gradually stabilize and approach the characteristics of the original formation water22–24.
The Suzhou mining area contains abundant CBM resources; however, scholars have conducted relatively little research on the hydrochemical characteristics of the water produced from CBM wells in this area. In this article, we take four CBM extraction wells in the Suzhou mining area and the five main coal seams of the two coal mines they are located in as the research objects. Based on experimental test data for CBM extraction well–produced water and coalbed water, the hydrochemical characteristics and influencing factors of CBM extraction well–produced water and coalbed water are systematically analyzed. Combined with actual gas production data for the four CBM extraction wells, the influence of the hydrochemical characteristics on the production capacity is analyzed in depth to provide theoretical support for the efficient development of CBM in the Suzhou mining area.
Geological setting
The Suzhou block is located in Suzhou City, Anhui Province, in the southern part of the Zhihe Sudong syncline. It is an open short-axis syncline with a north-south orientation, and the western wing of the strata in the area is relatively flat. The main coal-bearing strata are the Permian Shihezi Formation, which has a thickness of approximately 1100 m. The main coal seams that can be mined are coal seams 2, 3, 6, 7, 8, and 9. The coal grade is mainly coking coal, and the coal rock composition is mainly semi-bright coal. The organic components of the coal seams are mainly composed of vitrinite, followed by inertinite, and the shell and semi-vitrinite contents are relatively low. Most of the rivers in this area belong to the Huai River system, and numerous waterways crisscross the area and flow roughly from northwest to southeast.
In this study, we selected four CBM extraction wells in the Suzhou mining area and the five main coal seams of the two coal mines they are located in as the research objects. CBM wells N-1, N-2, and N-3 belong to the scope of the QD coal mine, and well N-4 belongs to the scope of the LL coal mine. The extraction well numbers are N-1, N-2, N-3, and N-4 (Table 1). The two coal mines where the extraction wells are located are QD and LL (Fig. 1). The sample numbers of the water from the five main coal seams in the two coal mines are as follows: QD6#−1, QD6#−2, QD7#−1, QD7#−2, QD7#−3, QD7#−4, QD7#−5, QD7#−6, QD8#−1, QD8#−2, QD8#−3, QD8#−4, QD8#−5, QD9#−1, QD9#−2, QD9#−3, QD9#−4, and LL8#−1 (Table 2).
Table 1.
Basic data for the production wells.
| CBM wells | Main mining coal seam | Coal seam burial depth (m) | Accumulated water production (m3) | Cumulative gas (m3) |
|---|---|---|---|---|
| N-1 | 7#、8#、9# | 914、944、956 | 2261.3 | 264639.3 |
| N-2 | 6#、7#、8#、9# | 1055、1091、1114、1136 | 1586.2 | 169440.7 |
| N-3 | 6#、7#、8#、9# | 1094、1131、1156、1171 | 1752.1 | 312,386 |
| N-4 | 8# | 1109 | 5879.8 | 294237.59 |
Fig. 1.
Map showing the locations of the wells, and map showing the structural geology of the Suzhou mining area.
Table 2.
Conventional ion data for the water produced from the CBM wells. Note: - indicates not detected.
| CBM wells | Na+ + K+ | Ca2+ | Mg2+ | Cl− | SO42− | HCO3− | CO32− | TDS | pH |
|---|---|---|---|---|---|---|---|---|---|
| mg/L | mg/L | mg/L | mg/L | mg/L | mg/L | mg/L | mg/L | ||
| N-1 | 1652.27 | 6.61 | 3.51 | 1469.81 | 11.94 | 1744.64 | 66.55 | 4119 | 8.95 |
| N-2 | 2029.20 | 6.61 | 3.51 | 2316.06 | 8.23 | 1280.69 | 73.68 | 5130 | 9.12 |
| N-3 | 3806.22 | 40.74 | 17.79 | 3883.85 | 11.11 | 3647.84 | - | 9690 | 7.98 |
| N-4 | 1996.24 | 11.41 | 3.95 | 2226.98 | 12.35 | 1117.18 | 195.89 | 5060 | 8.42 |
Well N-1 has been producing water since January 2021, with a cumulative water production of 2261.3 m3 and a cumulative gas production of 264,639.3 m3 as of May 2022. Well N-2 has been producing water since January 2021, with a cumulative water production of 1586.2 m3 and a cumulative gas production of 169,440.7 m3 as of May 2022. Well N-3 has been producing water since January 2021, with a cumulative water production of 1752.1 m3 and a cumulative gas production of 312,386 m3 as of May 2022. Well N-4 well has been producing water since July 2021, with a cumulative water production of 5879.8 m3 and a cumulative gas production of 294,237.59 m3 as of May 2022.
Samples and methods
In this study, we collected and conducted experimental tests on water samples from four CBM wells and five main coal seams in the research area. The testing included the determination of conventional cation and anion mass concentrations, total dissolved solids (TDS), and pH. Each water sample was collected using a 5 L polyethylene bucket, and the CBM well water samples were collected directly from the well outlet. The main coalbed water sample was collected separately from the corresponding coalbed water in the coal mine underground. Before collection, the sampling bucket was rinsed with water at least three times. The collected water samples were sent to the Anhui Coalfield Geological Testing Center for relevant content testing within 72 h. The test results for the above samples are presented in Tables 2 and 3.
Table 3.
Conventional ion data for the water from the main mining coal seam. Note: - indicates not detected. QD6#−1 and QD6#−2 are two water samples from main coal seam No. 6 in the QD coal mine; QD7#−1, QD7#−2, QD7#−3, QD7#−4, QD7#−5, and QD7#−6 are six water samples from main coal seam No. 7 in the QD coal mine; QD8#−1, QD8#−2, QD8#−3, QD8#−4, and QD8#−5 are five water samples from main coal seam No. 8 in the QD coal mine; QD9#−1, QD9#−2, QD9#−3, and QD9#−4 are four water samples from main coal seam 9 in the QD coal mine; and LL8#−1 is a water sample from main coal seam No. 8 in the LL coal mine.
| Sample code | Na+ + K+ | Ca2+ | Mg2+ | Cl− | SO42− | HCO3− | CO32− | TDS | pH |
|---|---|---|---|---|---|---|---|---|---|
| mg/L | mg/L | mg/L | mg/L | mg/L | mg/L | mg/L | mg/L | ||
| QD6#−1 | 580.31 | 1.63 | 0.99 | 187.78 | 5.76 | 1219.03 | - | 1400 | 7.85 |
| QD6#−2 | 744.60 | 3.26 | 0.99 | 207.08 | 3.70 | 1502.12 | 62.51 | 1791 | 8.33 |
| mean | 662.46 | 2.45 | 0.99 | 197.43 | 4.73 | 1360.58 | 62.51 | 1595.50 | 8.09 |
| QD7#−1 | 522.38 | 3.26 | 0.99 | 249.20 | 41.98 | 826.16 | 45.46 | 1289 | 8.43 |
| QD7#−2 | 918.34 | 8.16 | 1.98 | 238.67 | 20.99 | 1871.87 | 79.56 | 2226 | 8.34 |
| QD7#−3 | 771.22 | 1.66 | 0.30 | 240.15 | 37.19 | 1372.95 | 135.05 | 1830 | 9.28 |
| QD7#−4 | 788.85 | 5.86 | 1.07 | 192.82 | 34.55 | 1748.22 | 45.02 | 1909 | 8.42 |
| QD7#−5 | 655.91 | 6.12 | 1.11 | 210.59 | 35.75 | 1732.97 | 54.02 | 1972 | 8.49 |
| QD7#−6 | 810.22 | 6.25 | 1.19 | 239.83 | 38.33 | 1745.17 | 30.01 | 1993 | 8.45 |
| mean | 744.49 | 5.22 | 1.11 | 228.54 | 34.80 | 1549.56 | 64.85 | 1869.83 | 8.57 |
| QD8#−1 | 491.35 | 81.60 | 44.53 | 207.08 | 425.59 | 543.07 | 164.80 | 1703 | 6.99 |
| QD8#−2 | 623.46 | 1.61 | 2.93 | 138.96 | 27.99 | 1398.91 | - | 1509 | 7.98 |
| QD8#−3 | 723.28 | 4.83 | 1.95 | 220.92 | 156 | 57.33 | - | 1806 | 8.61 |
| QD8#−4 | 493.40 | 3.26 | 1.98 | 146.09 | 10.70 | 986.03 | 38.22 | 1199 | 8.60 |
| QD8#−5 | 634.34 | 3.26 | 2.96 | 142.53 | 19.35 | 1437.76 | - | 1537 | 7.95 |
| mean | 593.17 | 18.91 | 10.87 | 171.12 | 127.93 | 884.62 | 101.51 | 1550.80 | 8.03 |
| QD9#−1 | 400.48 | 73.44 | 49.48 | 214.10 | 426.42 | 623.96 | - | 1491 | 6.98 |
| QD9#−2 | 612.38 | 8.98 | 2.47 | 238.67 | 4.53 | 1247.91 | - | 1506 | 7.63 |
| QD9#−3 | 599.91 | 4.07 | 1.48 | 285.05 | 407.90 | 544.02 | 28.67 | 1615 | 8.43 |
| QD9#−4 | 526.15 | 40.74 | 24.71 | 224.48 | 255.60 | 932.60 | - | 1553 | 7.30 |
| mean | 534.73 | 31.81 | 19.54 | 240.58 | 273.61 | 837.12 | 28.67 | 1541.25 | 7.59 |
| LL8#−1 | 742.44 | 293.06 | 215.67 | 61.92 | 2747.64 | 360.26 | - | 4240.86 | 7.70 |
Results
The Na+ + K+ concentration of the water from coal seam No. 6 in the QD coal mine ranged from 580.31 to 744.60 mg/L, with an average of 662.46 mg/L and a coefficient of variation (CV) of 12.40%. The Ca2+ concentration ranged from 1.63 to 3.26 mg/L, with an average of 2.45 mg/L and a CV of 33.33%. The Mg2+ concentration was 0.99 mg/L. The Cl− concentration ranged from 187.78 to 207.08 mg/L, with an average of 197.43 mg/L and a CV of 4.89%. The SO42− concentration ranged from 3.70 to 5.76 mg/L, with an average of 4.73 mg/L and a CV of 21.78%. The HCO3− concentration ranged from 1219.03 to 1502.12 mg/L, with an average of 1360.58 mg/L and a CV of 10.40%. The CO32− concentration was 62.51 mg/L (Tables 3 and 4).
Table 4.
Coefficients of variation of the conventional ions in the water from the main coal seam.
| Sample code | Na+ + K+ | Ca2+ | Mg2+ | Cl− | SO42− | HCO3− | CO32− |
|---|---|---|---|---|---|---|---|
| QD6# | 12.40 | 33.33 | 0 | 4.89 | 21.78 | 10.40 | 0 |
| QD7# | 16.85 | 40.96 | 44.12 | 8.73 | 18.97 | 23.11 | 53.58 |
| QD8# | 15.05 | 165.82 | 154.88 | 20.66 | 123.55 | 59.43 | 62.35 |
| QD9# | 15.75 | 87.57 | 100.46 | 11.27 | 61.73 | 33.21 | 0 |
| mean | 15.01 | 81.92 | 74.87 | 11.39 | 56.51 | 31.54 | 28.98 |
The Na+ + K+ concentration of the water from coal seam No.7 in the QD coal mine ranged from 522.38 to 918.34 mg/L, with an average of 744.49 mg/L and a CV of 16.85%. The Ca2+ concentration ranged from 1.66 to 8.16 mg/L, with an average of 5.22 mg/L and a CV of 40.96%. The Mg2+ concentration ranged from 0.30 to 1.98 mg/L, with an average of 1.11 mg/L and a CV of 44.12%. The Cl− concentration ranged from 192.82 to 249.20 mg/L, with an average of 228.54 mg/L and a CV of 8.73%. The SO42− concentration ranged from 20.99 to 41.98 mg/L, with an average of 34.80 mg/L and a CV of 18.97%. The HCO3− concentration ranged from 826.16 to 1871.87 mg/L, with an average of 1549.56 mg/L and a CV of 23.11%. The CO32− concentration ranged from 30.01 to 135.05 mg/L, with an average of 64.85 mg/L and a CV of 53.58% (Tables 3 and 4).
The Na+ + K+ concentration of the water from coal seam No. 8 in the QD coal mine ranged from 491.35 to 723.28 mg/L, with an average of 593.17 mg/L and a CV of 15.05%. The Ca2+ concentration ranged from 1.61 to 81.60 mg/L, with an average of 18.91 mg/L and a CV of 165.82%. The Mg2+ concentration ranged from 1.95 to 44.53 mg/L, with an average of 10.87 mg/L and a CV of 154.88%. The Cl− concentration ranged from 138.96 to 220.92 mg/L, with an average of 171.12 mg/L and a CV of 20.66%. The SO42− concentration ranged from 10.70 to 425.59 mg/L, with an average of 127.93 mg/L and a CV of 123.55%. The HCO3− concentration ranged from 57.33 to 1437.76 mg/L, with an average of 884.62 mg/L and a CV of 59.43%. The CO32− concentration ranged from 38.22 to 164.80 mg/L, with an average of 101.51 mg/L and a CV of 62.35% (Table 3). The Na+ + K+ concentration of the water from coal seam No. 8 in the LL coal mine was 742.44 mg/L. The Ca2+ concentration was 293.06 mg/L. The Mg2+ concentration was 215.67 mg/L. The Cl− concentration was 61.92 mg/L. The SO42− concentration was 2747.64 mg/L. The HCO3− concentration was 360.26 mg/L (Tables 3 and 4).
The Na+ + K+ concentration of the water from coal seam No.9 in the QD coal mine ranged from 400.48 to 612.38 mg/L, with an average of 534.73 mg/L and a CV of 15.75%. The Ca2+ concentration ranged from 4.07 to 73.44 mg/L, with an average of 31.81 mg/L and a CV of 87.57%. The Mg2+ concentration ranged from 1.48 to 49.48 mg/L, with an average of 19.54 mg/L and a CV of 100.46%. The Cl− concentration ranged from 214.10 to 285.05 mg/L, with an average of 240.58 mg/L and a CV of 11.27%. The SO42− concentration ranged from 4.53 to 426.42 mg/L, with an average of 273.61 mg/L and a CV of 61.73%. The HCO3− concentration ranged from 544.02 to 1247.91 mg/L, with an average of 837.12 mg/L and a CV of 33.21%. The CO32− concentration was 28.67 mg/L (Tables 3 and 4). Na+ + K+ and Cl⁻ concentrations in coal seam water are generally lower than those in CBM produced water, while other ion concentrations remain comparable between the two water types (Tables 2 and 3).
Discussions
Hydrochemical characteristics of CBM well–produced water and coalbed water
Previous studies have found that the TDS are mainly composed of Na+, K+, Ca2+, Mg2+, Cl−, HCO3−, and SO42− and that changes in the concentrations of these ions can reflect the occurrence of groundwater to some extent. It is generally believed that when the concentration of Ca2+ and Mg2+ is high, the groundwater occurrence is in an open state, and when the concentration of Na+, Cl−, and HCO3− is high, the groundwater occurrence is in a closed state. When the pH value is between 6.30 and 10.30, it is mainly affected by the HCO3− concentration. HCO3− in a closed groundwater system mainly comes from the coalification reduction process of sulfates25–28. As can be seen from Fig. 2, the water quality type in the study area was mainly Na-K-HCO3. Based on the changes in the ion concentrations and pH of the water samples from the study area, the pH values ranged from 6.98 to 9.28 (Fig. 3h). Based on the changes in the ion concentrations (Fig. 3a–g), the HCO3− and Cl− concentrations were higher than the Ca2+ and Mg2+ concentrations. It can be concluded from the above results that the groundwater occurrence in the study area is in a closed state.
Fig. 2.
Piper ternary diagram for the water samples from the study area.
Fig. 3.
Changes in the ion concentrations and pH values of the water samples from the study area.
Characteristics of formation causes
Previous studies have shown that the types of controls on the water chemical characteristics, such as water-rock interactions, precipitation control, or evaporation concentration, can be determined by the location of the water quality sample data points on the Gibbs plot15,29,30.
As shown in Fig. 4a, almost all of the water sample data points plotted in the evaporation concentration area, indicating that the water samples from the study area were mainly affected by evaporation concentration. However, it can be concluded from Fig. 4b that due to the closed occurrence of the groundwater in the study area and the high concentration of HCO3−, the data for the water samples from the study area plotted in the areas of the graph that were mainly affected by mining activities and supplemented by evaporation and concentration. This indicates that the water samples from the study area were affected by both mining activities and evaporation and concentration. Due to the lower solubility of Ca2+and Mg2+ than Na+ + K+, the concentration of Na+ + K+ was higher than that of Ca2+ and Mg2+ (Fig. 3a–c).
Fig. 4.
Gibbs diagram for the water samples from the study area.
The chloralkali indices (CAI1 and CAI2) can reflect the intensity and direction of ion exchange. If CAI1 and CAI2 are both positive, this indicates that Na+ and K+ in the groundwater exchange with Ca2+ and Mg2+ in the formation’s minerals, leading to an increase in the Ca2+ and Mg2+ concentrations of the groundwater. If CAI1 and CAI2 are both negative, this indicates that Ca2+ and Mg2+ in the groundwater exchange with Na+ and K+ in geological minerals, leading to an increase in the Na+ and K+ concentrations of the groundwater31,32. The calculation formulas are as follows:
 |
 |
HNO3– was not included in the calculation because it was not detected.
As shown in Fig. 5, the CAI1 and CAI2 values of the water samples from CBM wells N-2, N-3, and N-4 were all positive. This result indicates that Na+ and K+ in the water produced from these three wells exchanged with Ca2+ and Mg2+ in the formation’s minerals, resulting in the production water containing Ca2+ and Mg2+ dissolved from the formation’s minerals. Both the coal seam water and the coal seam gas from well N-1 had negative CAI1 and CAI2 values, indicating the presence of dissolved Na+ + K+ in the formation’s minerals. Based on the differences in the main coal seams intersected by coal seam gas wells N-1, N-2, and N-3, it can be inferred (Table 1) that when the groundwater from the QD coal mine flowed through coal seam 6, the exchange intensity between Na+ and K+ in the water and Ca2+ and Mg2+ in the formation’s minerals was relatively high.
Fig. 5.
Relationship between CAI1 and CAI2 for the water samples from the study area.
In hydrochemical analysis, ion ratio relationships are commonly used to analyze the main sources of different ions33,34, such as γ(Na+ + K+)/γ(Cl−), which can be used to analyze the main sources of Na+ and K+. The γ(Na+ + K+)/γ(Cl−) ratio for salt rock dissolution is 1, and the γ(Na+ + K+)/γ(Cl−) ratio for silicate rock weathering is > 1. γ(Ca2+ + Mg2+)/γ(HCO3− + SO42−) can be used to analyze the sources of Ca2+, Mg2+, HCO3−, and SO42− in groundwater. If the data points are distributed along the 1:1 line on the γ(Ca2+ + Mg2+) versus γ(HCO3− + SO42−) graph, then Ca2+, Mg2+, HCO3−, and SO42− in the groundwater come from the weathering of carbonate rocks. γ(SO42− + Cl−)/γ(HCO3−) can be used to analyze the chemical composition of groundwater that mainly comes from the dissolution of evaporite rocks or carbonates. If the sample point is located below the 1:1 line, then the chemical composition of the groundwater mainly comes from the dissolution of carbonates; otherwise, it mainly comes from the dissolution of evaporite rocks.
It can be seen from the ion ratio relationship diagram for the water sample (Fig. 6a) that both the CBM well water and coalbed water data plotted around the 1:1 line. Therefore, Na+ and K+ in the water samples were influenced by both the dissolution of salt rocks and the weathering of silicate rocks, but the weathering of silicate rocks was the main factor. As can be seen from Fig. 6b, all of the water samples plotted below the 1:1 line, indicating that Ca2+, Mg2+, HCO3−, and SO42− were not all affected by the weathering of carbonate rocks. As can be seen from Fig. 6c, the data points for the CBM well water samples and CBM water samples plotted around the 1:1 line. Therefore, the main sources of SO42−, Cl−, and HCO3− in the water samples were the dissolution of carbonates and the dissolution of evaporite rocks, which have a dual effect. The dissolution of evaporite rocks had a greater effect on the CBM well water samples, while the dissolution of carbonates mainly affected the coalbed water.
Fig. 6.
Relationship between the ion ratios of the water samples.
The CV is commonly used to characterize the stability of ion changes. The CV values of > 100% indicate strong variation; the CV values of 10%–100% indicate moderate variation; and the CV values of 0–10% indicate weak variation35. As can be seen from the CV values in Table 4, the CV values of the ions in the main coal seam water from the QD coal mine were within a moderate range of variation. Among them, the CV values of Ca2+ and Mg2+ were much higher than those of the other ions, which may be related to the occurrence of evaporation and concentration. The CV values of HCO3− and SO42− indicated that these ions may originate from the coalification reduction process, while the CV values of Na+ + K+ and Cl− indicated that these ions may originate from the dissolution of salt rocks.
Influence of hydrochemical characteristics on the productivity of CBM wells
It is generally believed that a closed groundwater environment, enriched in Na+, Cl−, and HCO3− and located far from water source recharge areas, is conducive to the enrichment and preservation of CBM, while an open groundwater environment is not conducive to the preservation of CBM22,25–27. Based on the above analysis, we concluded that the groundwater in the Suzhou mining area was in a closed hydrological environment. Due to the presence of Na+ and Cl− in the fracturing fluid during the construction of the CBM wells, HCO3− was selected as the ion that is responsive to the production capacity. CBM wells N-1, N-2, and N-3 are within the scope of the QD coal mine, while well N-4 is within the scope of the LL coal mine.
As shown in Fig. 7, within the QD coal mine, there was a positive correlation between the HCO3− concentration of the water produced from the CBM wells and the production of CBM. Among them, the HCO3− concentration of the water produced from well N-3 was the highest, and the gas production from this well was also the highest. The production capacity of CBM wells may vary between single seam mining and multi-seam joint mining, as well as within different coal mine ranges. Among the wells, the HCO3− concentration of the water produced from well N-4 was 1117.18 mg/L, while the average daily gas production reached 958.4 m3, which may be due to the occurrence of interlayer interference during the multi-seam joint mining of the CBM wells.
Fig. 7.
Relationship between the HCO3− concentration of the produced water and the average daily gas production from CBM wells.
Conclusions
In this study, we analyzed the geochemical characteristics of coalbed water and CBM well–produced water in the Suzhou mining area, as well as the relationship between the CBM well–produced water and the production capacity. The groundwater occurred in a closed hydrological environment, and the water quality type was mainly Na-K-HCO3. The water samples were affected by both mining activities and evaporation concentration, and the concentration of Na+ + K+ was higher than that of Ca2+ and Mg2+. When the groundwater from QD coal mine flowed through coal seam 6, the exchange intensity between Na+ and K+ in the water and Ca2+ and Mg2+ in the formation’s minerals was relatively high. Within the same coal mining area, the HCO3− concentration of the water produced from the CBM wells was positively correlated with the production of CBM. Among the wells, the HCO3− concentration of the water produced from well N-3 was the highest, and the gas production from this well was also the highest.
Acknowledgements
We thank LetPub for its linguistic assistance during the preparation of this manuscript.
Author contributions
M.Z., and M.D. contributed to the collection and analysis of data. M.D., and C.W. contributed to manuscript writing.
Funding
M. Du was supported by Start-up Fund for Doctoral Research of Suzhou University (No.2020BS024) and Postdoctoral Scientific Research Foundation of Suzhou University (No.2023BSH004). M. Zhao was supported by Postdoctoral Scientific Research Foundation of Suzhou University (No.2023BSH004). C. Wu was supported by Start-up Fund for Doctoral Research of Suzhou University (No.2020BS024).
Data availability
The data used to support the findings of this study are included within the article.
Declarations
Competing interests
The authors declare no competing interests. We confirm that permission for research in this study area has been obtained.
Footnotes
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Contributor Information
Mingyang Du, Email: myduah@163.com.
Mingkun Zhao, Email: zmk@ahszu.edu.cn.
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Data Availability Statement
The data used to support the findings of this study are included within the article.