# Geochemical Characteristics and Productivity Response of Produced Water from Coalbed Methane Wells in the Yuwang Block, Eastern Yunnan, China

Authors: Mingyang Du; Xiaojuan Yao; Shasha Zhang; He Zhou; Caifang Wu; Junlong Zhao
Journal: Geofluids
 (2020)

---

## Abstract

Coalbed methane (CBM) well-produced water contains abundant geochemical information that can guide productivity predictions of CBM wells. The geochemical characteristics and productivity responses of water produced from six CBM wells in the Yuwang block, eastern Yunnan, were analyzed using data of conventional ions, hydrogen and oxygen isotopes, and dissolved inorganic carbon (DIC). The results showed that the produced water type of well L-3 is mainly Na-HCO3, while those from the other five wells are Na-Cl-HCO3. The isotope characteristics of produced water are affected greatly by water-rock interaction. Combined with the enrichment mechanisms of isotopes D and 18O, we found that the water samples exhibit an obvious D drift trend relative to the local meteoric water line. The 13C enrichment of DIC in the water samples suggests that DIC is mainly produced by the dissolution of carbonate minerals in coal seams. The concentration of HCO3-, D drift trend, and enrichment of 13CDIC in produced water are positively correlated with CBM production, which can be verified by wells L-4 and L-6.


---

## Body

## 1. Introduction

Coalbed methane (CBM) is an unconventional natural gas resource, which has huge reserves worldwide [1–3]. The East of Yunnan and the West of Guizhou are important CBM resource areas in South China [4]. CBM production is achieved through drainage and reductions in pressure. Water discharged in this process undergoes various physical, chemical, and biological interactions with coal seams and surrounding rocks during the continuous runoff process, resulting in changes to the chemical composition and properties of the produced water [5–9]. Previous studies have shown that CBM well drainage water from all over the world has similar ion characteristics despite highly variant conditions (chemical composition, coal structure, coal metamorphic degree, study area, the original water source, and formation time): the concentrations of Na+, K+, and Cl- are high, and the concentrations of Ca2+, Mg2+, and SO42- are low [2, 10–14]. Researchers generally believe that the high concentrations of Na+, K+, and Cl- in CBM well water in the early stage are caused by pollution from fracturing fluids used to open the coal seam [15, 16]. Produced water from CBM wells exhibits distinct geochemical characteristics at different drainage stages. With the development of drainage, the quality of water produced from CBM wells can be divided into three stages: fracturing fluid flowback, transitional, and stabilization. The corresponding water quality types are Na-Cl, Na-Cl-HCO3, and Na-HCO3, respectively [17–20].H and16O isotopes in the formation water can be replaced with D and 18O isotopes in the coal seam and surrounding rock, resulting in the increase of D and 18O isotopes in the coal seam water in the reduction environment of coal measures [21, 22]. In addition, microorganisms can produce HDS, which is soluble in water and can exchange isotopes, leading to D drift characteristics of formation water in a sealed and reduced coal seam environment [21, 23–26]. There are significant differences in the composition of dissolved inorganic carbon isotopes (13CDIC) from various sources. Only a few studies have examined the dissolved inorganic carbon (DIC) in the produced water; these found that the δ13CDIC value for the decomposition of organic matter is less than -8‰, which is typical. The value of δ13CDIC released by carbonate dissolution or metamorphism is relatively high, and it is generally distributed around 0‰ [27–29].The higher the concentration of HCO3- in CBM well water, the higher the gas content and productivity of the CBM wells [18, 30, 31]. It is suggested that the reason for the higher HCO3- content in high-level CBM wells may be the result of CO2 migration and dissolution from low to high portions of CBM [23]. The distribution characteristics of hydrogen and oxygen isotopes in the produced water of CBM are related to the change of groundwater environment, which can be used as an index to judge the characteristics of groundwater and the productivity response of CBM wells [16, 32]. Based on the analysis of 13CDIC from different sources, it is generally believed that the decrease of methanogenic bacteria can lead to the abnormally positive characteristic of 13CDIC [27–29, 33–35].Previous examinations of the geochemical characteristics of water produced by CBM wells have primarily been focused on the Qinshui Basin, Ordos Basin, and Guizhou Province in China [15, 32]. There are few reports on the geochemical characteristics of water produced by CBM wells in eastern Yunnan. Based on the test results of conventional ions, hydrogen and oxygen isotopes, and DIC of water samples from six CBM wells in the Yuwang block of eastern Yunnan in 2018, this study systematically analyzed the geochemical characteristics of the produced water and its significance for well productivity. It could provide a theoretical basis for CBM exploration and development in eastern Yunnan.


## 2. Geological Background and Materials

### 2.1. Geological Background

The Yuwang block is located near Qujing City, Fuyuan County, and is within the Laochang anticline structural belt on the southwestern margin of the Yangtze quasi-platform. The faults in the block are primarily NE-trending, with NW-trending transverse faults and NW-trending arc faults primarily distributed on the margins of the block. Most of the faults with an elevation decrease of greater than 100 m are boundary faults, while internal faults are scarce and mostly distributed near the folds (Figure1). The main coal-bearing stratum is the Upper Permian Longtan Formation, with a thickness of 415–475 m. The main coal seams are Nos. 3, 7+8, 16, 17+18, and 23. The coal in the study area ranks as anthracite; the maximum Ro value is between 2.53 and 3.50%, with an average of 2.99%. Methanogenesis ends at the medium volatile bituminous coal stage [33]. Therefore, the formation of coalbed methane is due to thermogenic rather than biogenic processes, and the methanogenesis is very weak in the study area. The petrographic composition of coal in the area is primarily semidark to semibright. This area is in the watersheds of the Huangni River, Xijiuxi River, and Seyi River, with a high terrain and an undeveloped surface water system.Figure 1
Study area structure and map of well locations.There are six CBM development test wells (wells L-1, L-2, L-3, L-4, L-5, and L-6) that adopted the “segmented fracturing, combined layer drainage” developmental mode in the Yuwang block. Wells L-1 and L-2 began producing water from April 2018, while wells L-3, L-4, L-5, and L-6 began producing water in May 2018. The cumulative water and gas production of the six wells by November 2018 can be seen in Table1. Well L-2 produced the most gas (21983.67 m3) during this period, followed by wells L-4, L-1, L-6, L-5, and L-3. Well L-3 had the highest cumulative water production, at 1575.76 m3, followed by wells L-4, L-2, L-6, L-1, and L-5.Table 1
Characteristics of production wells.
CBM wellsWater production dateMain coal seamDepth (m)Average daily gas production (m3)Cumulative gas production (m3)Cumulative water production (m3)L-1Apr. 20187+8/13780.00171.7215787.22258.31L-2Apr. 201816/18/19735.60219.9721983.67341.15L-3May 201813/14/19832.0034.541067.211575.76L-4May 201813/16/18/19713.20324.0618762.31404.54L-5May 201813/19789.2056.732886.88254.12L-6May 201814/16/18745.80220.1911864.53322.25


### 2.2. Samples

Since April 2018, water samples from six wells in the Yuwang block have been collected and tested. Water was sampled directly from the wellhead in 2.5 L pure water bottles, rinsed a minimum of three times with the produced water sample. Samples were then sent to the Institute of Geochemistry, Guiyang Academy of Sciences, for relevant content analysis within 72 h. The experimental content includes conventional anion and cation mass concentration tests, hydrogen and oxygen stable isotope tests, and a DIC test. As of November 2018, 37 samples had been collected from the six wells (Table2).Table 2
Conventional ion data and hydrogen and oxygen isotope data from water samples.
CBM wellsDateK+Na+Ca2+Mg2+Cl-SO42-HCO3-δ13CDICδ18OδDmg/Lmg/Lmg/Lmg/Lmg/Lmg/Lmg/L‰V-PDB‰V-SMOW‰V-SMOWL-1Apr. 2018391.242337.9322.779.623693.0840.27756.37-5.11-9.82-58.54May 2018388.142841.0921.7311.963566.2322.89899.61-5.21-10.09-72.55Jun. 2018448.0625075.7911.283622.0119.571085.56-5.12-10.5-70.69Jul. 2018430.122393.6319.229.48319215.891483.55-5.39-10.42-68.35Aug. 2018271.022127.2415.317.692841.4617.331687-4.67-10.71-71.73Sep. 2018160.012006.0912.355.082242.7112.931830.97-4.57-11.03-72.2Oct. 2018153.981927.919.554.492083.218.471896.7-4.25-11.26-74.51Nov. 2018148.361966.388.144.631955.648.891931.12-4.36-11.38-81.57Mean298.872263.4114.368.032899.5418.281446.36-4.84-10.65-71.27L-2Apr. 2018653.813022.2133.3712.44742.6834.821389.61-7.96-10.47-72.97May 2018413.982979.622.488.593392.5824.21683.62-8.13-10.98-82.04Jun. 2018360.692711.5918.917.732941.8224.381826.85-8.02-11.68-80.69Jul. 2018298.372416.5416.396.572539.7819.892400.6-8.19-11.11-76.28Aug. 2018233.852202.9417.25.542112.8917.872519.54-7.91-11.76-82.18Sep. 2018242.972191.0712.934.852315.714.612463.2-8.23-11.9-83.77Oct. 2018223.842121.7210.764.422197.3211.882475.72-8.45-11.76-81.82Nov. 2018220.12238.449.634.612101.8310.32516.41-8.50-12.06-86.66Mean330.952485.5117.716.842793.0819.742159.44-8.17-11.46-80.80L-3May 2018253.392192.7529.479.422953.5419.78562.88-8.43-11.38-84.88Jun. 2018179.971635.2818.146.302072.0622.75731.24-8.64-12.30-84.57Jul. 2018133.491328.2112.144.291560.4917.57895.14-9.01-11.38-78.95Aug. 201897.871045.4312.682.841203.6415.36960.87-8.49-12.36-86.08Sep. 201875.52865.797.192.17983.4313.211042.24-8.32-12.29-85.39Oct. 201866.06909.126.231.80822.1711.341032.85-8.29-12.51-86.65Nov. 201851.37827.434.611.20604.219.031070.41-8.36-10.94-78.20Mean122.521257.7212.924.001457.0815.58899.38-8.51-11.88-83.53L-4May 2018362.713228.3919.299.293436.520.372168.6-5.18-11.59-78.69Jun. 2018330.543123.7414.798.893235.610.192291.73-5.12-11.76-79.1Jul. 2018276.252883.2820.357.922813.120.202967.11-5.12-11.09-74.36Aug. 2018235.562734.7622.927.112702.080.323045.35-4.13-11.89-78.54Sep. 2018206.352564.8417.495.462454.660.463170.55-4.11-11.69-79.46Oct. 2018200.692565.6917.315.602358.300.423233.15-4.18-11.72-79.57Nov. 2018207.432564.5316.435.962255.890.303358.34-4.38-11.82-83.97Mean259.932809.3218.377.182750.880.322890.69-4.60-11.65-79.10L-5May 2018348.912609.789.927.312637.243.871084.65-6.42-11.95-82.34Jun. 2018227.402270.3510.425.702302.850.05956.49-6.39-12.11-82.41Jul. 2018174.652077.5111.554.811995.863.881325.18-6.76-11.37-77.65Aug. 2018150.751867.7815.814.421811.294.241425.33-6.15-11.49-81.43Sep. 2018136.651766.559.443.361554.923.331522.36-6.06-12.03-83.38Oct. 2018116.041757.517.663.001422.650.091525.49-5.97-12.01-82.06Nov. 2018114.001746.447.303.041345.380.961569.30-6.02-10.28-80.61Mean181.202013.7010.304.521867.172.351344.11-6.25-11.61-81.41L-6May 2018348.912609.789.927.312637.243.871884.65-4.93-11.95-82.34Jun. 2018299.722501.887.526.542525.991.611939.93-5.53-11.92-82.2Jul. 2018259.452316.3111.086.552383.261.132381.82-5.52-11.25-77.2Aug. 2018213.252093.4915.425.52171.060.292488.24-4.84-12.12-81.7Sep. 2018197.322006.9411.074.381910.230.742600.91-4.79-12.03-83.96Oct. 2018207.932011.0910.514.441803.250.092660.38-4.36-12.06-83.23Nov. 2018181.622060.999.484.421649.120.132760.54-4.43-11.52-84.52Mean244.032228.6410.715.592154.311.122388.07-4.91-11.84-82.16


## 2.1. Geological Background

The Yuwang block is located near Qujing City, Fuyuan County, and is within the Laochang anticline structural belt on the southwestern margin of the Yangtze quasi-platform. The faults in the block are primarily NE-trending, with NW-trending transverse faults and NW-trending arc faults primarily distributed on the margins of the block. Most of the faults with an elevation decrease of greater than 100 m are boundary faults, while internal faults are scarce and mostly distributed near the folds (Figure1). The main coal-bearing stratum is the Upper Permian Longtan Formation, with a thickness of 415–475 m. The main coal seams are Nos. 3, 7+8, 16, 17+18, and 23. The coal in the study area ranks as anthracite; the maximum Ro value is between 2.53 and 3.50%, with an average of 2.99%. Methanogenesis ends at the medium volatile bituminous coal stage [33]. Therefore, the formation of coalbed methane is due to thermogenic rather than biogenic processes, and the methanogenesis is very weak in the study area. The petrographic composition of coal in the area is primarily semidark to semibright. This area is in the watersheds of the Huangni River, Xijiuxi River, and Seyi River, with a high terrain and an undeveloped surface water system.Figure 1
Study area structure and map of well locations.There are six CBM development test wells (wells L-1, L-2, L-3, L-4, L-5, and L-6) that adopted the “segmented fracturing, combined layer drainage” developmental mode in the Yuwang block. Wells L-1 and L-2 began producing water from April 2018, while wells L-3, L-4, L-5, and L-6 began producing water in May 2018. The cumulative water and gas production of the six wells by November 2018 can be seen in Table1. Well L-2 produced the most gas (21983.67 m3) during this period, followed by wells L-4, L-1, L-6, L-5, and L-3. Well L-3 had the highest cumulative water production, at 1575.76 m3, followed by wells L-4, L-2, L-6, L-1, and L-5.Table 1
Characteristics of production wells.
CBM wellsWater production dateMain coal seamDepth (m)Average daily gas production (m3)Cumulative gas production (m3)Cumulative water production (m3)L-1Apr. 20187+8/13780.00171.7215787.22258.31L-2Apr. 201816/18/19735.60219.9721983.67341.15L-3May 201813/14/19832.0034.541067.211575.76L-4May 201813/16/18/19713.20324.0618762.31404.54L-5May 201813/19789.2056.732886.88254.12L-6May 201814/16/18745.80220.1911864.53322.25


## 2.2. Samples

Since April 2018, water samples from six wells in the Yuwang block have been collected and tested. Water was sampled directly from the wellhead in 2.5 L pure water bottles, rinsed a minimum of three times with the produced water sample. Samples were then sent to the Institute of Geochemistry, Guiyang Academy of Sciences, for relevant content analysis within 72 h. The experimental content includes conventional anion and cation mass concentration tests, hydrogen and oxygen stable isotope tests, and a DIC test. As of November 2018, 37 samples had been collected from the six wells (Table2).Table 2
Conventional ion data and hydrogen and oxygen isotope data from water samples.
CBM wellsDateK+Na+Ca2+Mg2+Cl-SO42-HCO3-δ13CDICδ18OδDmg/Lmg/Lmg/Lmg/Lmg/Lmg/Lmg/L‰V-PDB‰V-SMOW‰V-SMOWL-1Apr. 2018391.242337.9322.779.623693.0840.27756.37-5.11-9.82-58.54May 2018388.142841.0921.7311.963566.2322.89899.61-5.21-10.09-72.55Jun. 2018448.0625075.7911.283622.0119.571085.56-5.12-10.5-70.69Jul. 2018430.122393.6319.229.48319215.891483.55-5.39-10.42-68.35Aug. 2018271.022127.2415.317.692841.4617.331687-4.67-10.71-71.73Sep. 2018160.012006.0912.355.082242.7112.931830.97-4.57-11.03-72.2Oct. 2018153.981927.919.554.492083.218.471896.7-4.25-11.26-74.51Nov. 2018148.361966.388.144.631955.648.891931.12-4.36-11.38-81.57Mean298.872263.4114.368.032899.5418.281446.36-4.84-10.65-71.27L-2Apr. 2018653.813022.2133.3712.44742.6834.821389.61-7.96-10.47-72.97May 2018413.982979.622.488.593392.5824.21683.62-8.13-10.98-82.04Jun. 2018360.692711.5918.917.732941.8224.381826.85-8.02-11.68-80.69Jul. 2018298.372416.5416.396.572539.7819.892400.6-8.19-11.11-76.28Aug. 2018233.852202.9417.25.542112.8917.872519.54-7.91-11.76-82.18Sep. 2018242.972191.0712.934.852315.714.612463.2-8.23-11.9-83.77Oct. 2018223.842121.7210.764.422197.3211.882475.72-8.45-11.76-81.82Nov. 2018220.12238.449.634.612101.8310.32516.41-8.50-12.06-86.66Mean330.952485.5117.716.842793.0819.742159.44-8.17-11.46-80.80L-3May 2018253.392192.7529.479.422953.5419.78562.88-8.43-11.38-84.88Jun. 2018179.971635.2818.146.302072.0622.75731.24-8.64-12.30-84.57Jul. 2018133.491328.2112.144.291560.4917.57895.14-9.01-11.38-78.95Aug. 201897.871045.4312.682.841203.6415.36960.87-8.49-12.36-86.08Sep. 201875.52865.797.192.17983.4313.211042.24-8.32-12.29-85.39Oct. 201866.06909.126.231.80822.1711.341032.85-8.29-12.51-86.65Nov. 201851.37827.434.611.20604.219.031070.41-8.36-10.94-78.20Mean122.521257.7212.924.001457.0815.58899.38-8.51-11.88-83.53L-4May 2018362.713228.3919.299.293436.520.372168.6-5.18-11.59-78.69Jun. 2018330.543123.7414.798.893235.610.192291.73-5.12-11.76-79.1Jul. 2018276.252883.2820.357.922813.120.202967.11-5.12-11.09-74.36Aug. 2018235.562734.7622.927.112702.080.323045.35-4.13-11.89-78.54Sep. 2018206.352564.8417.495.462454.660.463170.55-4.11-11.69-79.46Oct. 2018200.692565.6917.315.602358.300.423233.15-4.18-11.72-79.57Nov. 2018207.432564.5316.435.962255.890.303358.34-4.38-11.82-83.97Mean259.932809.3218.377.182750.880.322890.69-4.60-11.65-79.10L-5May 2018348.912609.789.927.312637.243.871084.65-6.42-11.95-82.34Jun. 2018227.402270.3510.425.702302.850.05956.49-6.39-12.11-82.41Jul. 2018174.652077.5111.554.811995.863.881325.18-6.76-11.37-77.65Aug. 2018150.751867.7815.814.421811.294.241425.33-6.15-11.49-81.43Sep. 2018136.651766.559.443.361554.923.331522.36-6.06-12.03-83.38Oct. 2018116.041757.517.663.001422.650.091525.49-5.97-12.01-82.06Nov. 2018114.001746.447.303.041345.380.961569.30-6.02-10.28-80.61Mean181.202013.7010.304.521867.172.351344.11-6.25-11.61-81.41L-6May 2018348.912609.789.927.312637.243.871884.65-4.93-11.95-82.34Jun. 2018299.722501.887.526.542525.991.611939.93-5.53-11.92-82.2Jul. 2018259.452316.3111.086.552383.261.132381.82-5.52-11.25-77.2Aug. 2018213.252093.4915.425.52171.060.292488.24-4.84-12.12-81.7Sep. 2018197.322006.9411.074.381910.230.742600.91-4.79-12.03-83.96Oct. 2018207.932011.0910.514.441803.250.092660.38-4.36-12.06-83.23Nov. 2018181.622060.999.484.421649.120.132760.54-4.43-11.52-84.52Mean244.032228.6410.715.592154.311.122388.07-4.91-11.84-82.16


## 3. Results and Discussion

### 3.1. Conventional Ion Characteristics and Productivity Response

The produced water from six CBM wells in the study area exhibited similar characteristics: the Na+, Cl-, and HCO3- concentrations were relatively high, those of Ca2+, Mg2+, and SO42- were relatively low, and that of K+ was between these two extremes (Table 2). Moreover, the concentration value of SO42- in wells L-4, L-5, and L-6 was lower than that in the other wells. With the development of drainage, water produced from well L-3 in the study area shifted from the Na-Cl-HCO3 type to the Na-HCO3 type, while that from the other five wells were all characterized as Na-Cl-HCO3 type. The conventional ion concentrations of K+, Na+, Ca2+, Mg2+, Cl-, and SO42- in the produced water from the six wells exhibited a fluctuating but ultimately decreasing trend, while the concentration of HCO3- presented a trend of increasing fluctuations (Figures 2(a)–2(g)). Among these ions, the concentrations of K+, Na+, Ca2+, Mg2+, Cl-, and SO42- in wells L-1 and L-2 varied greatly with time. The concentrations of K+, Na+, and Cl- in wells L-3, L-4, L-5, and L-6 tended to stabilize with time while the concentrations of Ca2+, Mg2+, and SO42- in those wells tended to fluctuate with time.Figure 2
Ion concentration changes with time.
(a)(b)(c)(d)(e)(f)(g)The ion characteristics in produced water in this study were similar to those of CBM wells in other areas [10, 11, 23]. Researchers generally hypothesize that the concentrations of Na+, K+, Cl-, and HCO3- in coal seam water are low. However, the concentrations of Na+, K+, and Cl- in water contaminated by fracturing fluid are greatly increased, while the concentration of HCO3- is reduced and the concentrations of other ions are less impacted. The concentrations of Na+, K+, Cl-, and HCO3- in surface water are low, but the concentrations of Ca2+, Mg2+, and SO42- are higher than those in the coal seam water [15, 16].Generally, a closed groundwater environment is conducive to the enrichment and preservation of CBM while an open groundwater environment is not. Researchers theorize that open hydrological environments are close to oxygen-rich water source recharge areas that can enrich Ca2+, Mg2+, and SO42-, while closed hydrological environments are far from such recharge areas and exhibit Na+, K+, Cl-, and HCO3- enrichment [22, 32, 36, 37].With increasing drainage time, the concentrations of K+, Na+, Ca2+, Mg2+, Cl-, and SO42- in the water samples decreased with the gradual discharge of fracturing fluid. The decrease in Ca2+ and Mg2+ concentrations also indicated that the water-rock interactions had weakened (Figures 2(a)–2(f)). Water-rock interactions were affected by the rainy season in August; the latter strengthened them such that Ca2+ and Mg2+ concentrations increased and the Ca2+ concentration increased significantly (Figures 2(c) and 2(d)). As the dissolution rate of calcite is much higher than that of dolomite, the Ca2+ concentration in water was generally higher than the Mg2+ concentrations. SO42- accumulations are primarily related to the dissolution and desulfurization of gypsum [38]. In a reducing environment, sulfate in coal seam water can produce bicarbonate and H2S gas with organic matter, showing HCO3- concentrations increasing with time and SO42- concentrations decreasing with time (Figures 2(f) and 2(g)).According to this analysis, the groundwater hydrological environment in the study area is, overall, in a closed state with poor hydrodynamic conditions. Na+, K+, Cl-, and HCO3- concentrations in this study were enriched in the confined groundwater environment, and Na+, K+, and Cl- were present in the fracturing fluid, so HCO3- was chosen as the ion with which to study gas productivity response. Figure 3 shows that there was a positive correlation between HCO3- and CBM production in the produced water. Among these wells, L-4 and L-6 had the highest HCO3- concentration; their gas production is also the highest.Figure 3
The relationship between HCO3- and gas production.


### 3.2. Hydrogen and Oxygen Isotope Characteristics and Productivity Response

When the isotope value of produced water is located on the left side of the atmospheric precipitation line, it shows a D drift characteristic; when the value is on the right side, it shows an O drift characteristic. Except for the O drift characteristic in May and November, the produced water in well L-1 showed a D drift characteristic. For produced water of well L-4, they all show an O drift characteristic except for July and August. The values of well L-6 also exhibited an O drift characteristic except for August. As for wells L-2, L-3, and L-5, they all showed an O drift characteristic (Figure4).Figure 4
Relationship betweenδD and δ18O isotopes in produced water.The isotope values ofδD and δ18O in the produced water are negatively correlated with drainage time; however, the δD and δ18O isotope values suddenly increased in July (Figures 5(a) and 5(b)). The δD isotope value in the produced water of well L-3 suddenly increased in November (Figure 5(a)), and the δ18O isotope value in the produced water in wells L-3, L-5, and L-6 increased suddenly in November (Figure 5(b)).Figure 5
Variation of D and18O isotopes in production water with time.
(a)(b)Our quantification of the hydrogen and oxygen isotopic composition is based on the Yunnan atmospheric precipitation line equationδD=6.56δ18O−2.96 [39]. When groundwater flows through coal-bearing strata, several hydrogen-bearing soluble minerals in the coal seams are dissolved continuously. The lighter H atoms in hydrogen-bearing minerals are easily adsorbed by minerals such as clay, while the heavier D atoms are more likely to undergo an isotope exchange with H atoms in the water, thereby continuously enriching D in the formation water and exhibiting D drift characteristics [22, 26]. 18O is enriched in the surrounding rock. With groundwater runoff, many oxygen-bearing soluble minerals in the formation are dissolved continuously. The heavier 18O in the mineral is liable to undergoing isotope exchange reaction with the lighter 16O in the groundwater and thus to show 18O drift characteristics [22, 40].Hydrogen and oxygen isotopes of produced water analyzed in this study are distributed near the atmospheric precipitation line, showing obvious D drift characteristics; a few have18O drift characteristics because of atmospheric precipitation or weak mixing with the surface water and shallow groundwater [41, 42].With the drainage of CBM wells, the water-rock interactions between the fracturing fluids remaining in the coal seam or formation water and the coal seam or surrounding rocks gradually weakened. However, July and August are rainy seasons in Yunnan and result in strong recharge from atmospheric precipitation. Therefore, the variation in theδD and δ18O isotope values in July may have been caused by seasonal rainfall. Studies of δD and δ18O isotopes in produced water from the six wells show that these changes are universal (Figures 5(a) and 5(b)). The δD and δ18O isotope values from produced water in well L-3 increased sharply in November; this was also true of the δ18O isotope value for well L-5, although its δD isotope value increased only slightly in November; these results are different from those of the other four wells (Figures 5(a) and 5(b)). The drift characteristics of wells L-1 and L-2 are presumed to be caused by the fragmentation of the water-bearing limestone on the top of the coal seam. Combined with the enrichment mechanism of D and 18O [22, 26, 40], it was inferred that the trend of wells L-4 and L-6 is conducive to the long-term production of CBM; the actual gas production situation also verified this inference (Figure 6). Anomalies (reservoir damage or wellbore collapse) were observed for the remaining four wells, although wells L-1 and L-2 had high gas production; however, the former conditions are not conducive to the long-term production of CBM.Figure 6
The relationship betweenδD and δ18O isotopes and gas production in produced water.


### 3.3. Dissolved Inorganic Carbon Characteristics and Productivity Response

Theδ13CDIC values of the water samples from the produced water were not significantly different, and the change trends were similar: generally falling initially, subsequently rising, and then falling again. Wells L-1, L-4, L-5, and L-6 had the highest δ13CDIC values, the surface water samples had the lowest δ13CDIC values, and wells L-2 and L-3 had δ13CDIC values between the two extremes (Figure 7).Figure 7
Variation of dissolved inorganic carbon in production water with time.The composition of13CDIC from different sources was significantly different, appearing primarily in the forms of H2CO3, HCO3-, CO32-, and water-soluble CO2. δ13CDIC values based on organic origins are less than -8‰, and those formed by inorganic origin are approximately 0‰ [27, 29, 43].The two sources ofδ13CDIC in surface and shallow water are primarily related to oxidation via the CO2 produced by plant respiration and decomposition and the dissolution of carbonate rocks in soil. Carbon dioxide dissolves in water and continuously exchanges carbon isotopes with H2CO3, HCO3-, and CO32- in water, reducing the value of δ13CDIC in groundwater. If the concentration of HCO3- in the water is high, the value of δ13CDIC will be low. The δ13CDIC value of surface and shallow water is usually between -14‰ and -7‰, falling into the range of extremely low negative values [35, 44].The isotope13CDIC is abundant in produced water of CBM wells and primarily derived from carbonate mineral dissolution and microbial methanogenesis [34]. Methanogenesis in the study area is weak; therefore, the observed change in 13CDIC content is controlled mostly by the dissolution of carbonate minerals. Carbonate minerals in coal measure strata are more abundant than 13C in soil carbonates. When they dissolve, the δ13CDIC value in water increases and ranges from -7‰ to 0‰ [45].Theδ13CDIC value of surface water samples is in an extremely low negative value range. This is likely a result of atmospheric CO2 dissolution, plant respiration, and the dissolution of carbonate rocks in the soil (Figure 7). An analysis of the relationship between δ13CDIC values and their source characteristics shows that the produced water samples can be divided into two categories: (1) water samples from wells L-2 and L-3, with δ13CDIC values ranging from -9‰ to -7‰, which is consistent with shallow water and due primarily to the process of atmospheric CO2 dissolution, plant respiration, and the dissolution of carbonate rocks in soil; (2) water samples from wells L-1, L-4, L-5, and L-6, with δ13CDIC values ranging from -6.3‰ to -4‰. The burial depth of the coal seam is approximately 750 m, and the concentration of HCO3- is high, mainly because the carbonate minerals in the coal are dissolved. The δ13CDIC values of produced water are positively correlated with gas production (except for well L-2, which may have been caused by a precipitation injection during the rainy season) (Figure 8). When 13CDIC in the produced water is the carbonate mineral dissolved in coal seam water, the gas production is high.Figure 8
Relationship between dissolved inorganic carbon isotopes and gas production.


## 3.1. Conventional Ion Characteristics and Productivity Response

The produced water from six CBM wells in the study area exhibited similar characteristics: the Na+, Cl-, and HCO3- concentrations were relatively high, those of Ca2+, Mg2+, and SO42- were relatively low, and that of K+ was between these two extremes (Table 2). Moreover, the concentration value of SO42- in wells L-4, L-5, and L-6 was lower than that in the other wells. With the development of drainage, water produced from well L-3 in the study area shifted from the Na-Cl-HCO3 type to the Na-HCO3 type, while that from the other five wells were all characterized as Na-Cl-HCO3 type. The conventional ion concentrations of K+, Na+, Ca2+, Mg2+, Cl-, and SO42- in the produced water from the six wells exhibited a fluctuating but ultimately decreasing trend, while the concentration of HCO3- presented a trend of increasing fluctuations (Figures 2(a)–2(g)). Among these ions, the concentrations of K+, Na+, Ca2+, Mg2+, Cl-, and SO42- in wells L-1 and L-2 varied greatly with time. The concentrations of K+, Na+, and Cl- in wells L-3, L-4, L-5, and L-6 tended to stabilize with time while the concentrations of Ca2+, Mg2+, and SO42- in those wells tended to fluctuate with time.Figure 2
Ion concentration changes with time.
(a)(b)(c)(d)(e)(f)(g)The ion characteristics in produced water in this study were similar to those of CBM wells in other areas [10, 11, 23]. Researchers generally hypothesize that the concentrations of Na+, K+, Cl-, and HCO3- in coal seam water are low. However, the concentrations of Na+, K+, and Cl- in water contaminated by fracturing fluid are greatly increased, while the concentration of HCO3- is reduced and the concentrations of other ions are less impacted. The concentrations of Na+, K+, Cl-, and HCO3- in surface water are low, but the concentrations of Ca2+, Mg2+, and SO42- are higher than those in the coal seam water [15, 16].Generally, a closed groundwater environment is conducive to the enrichment and preservation of CBM while an open groundwater environment is not. Researchers theorize that open hydrological environments are close to oxygen-rich water source recharge areas that can enrich Ca2+, Mg2+, and SO42-, while closed hydrological environments are far from such recharge areas and exhibit Na+, K+, Cl-, and HCO3- enrichment [22, 32, 36, 37].With increasing drainage time, the concentrations of K+, Na+, Ca2+, Mg2+, Cl-, and SO42- in the water samples decreased with the gradual discharge of fracturing fluid. The decrease in Ca2+ and Mg2+ concentrations also indicated that the water-rock interactions had weakened (Figures 2(a)–2(f)). Water-rock interactions were affected by the rainy season in August; the latter strengthened them such that Ca2+ and Mg2+ concentrations increased and the Ca2+ concentration increased significantly (Figures 2(c) and 2(d)). As the dissolution rate of calcite is much higher than that of dolomite, the Ca2+ concentration in water was generally higher than the Mg2+ concentrations. SO42- accumulations are primarily related to the dissolution and desulfurization of gypsum [38]. In a reducing environment, sulfate in coal seam water can produce bicarbonate and H2S gas with organic matter, showing HCO3- concentrations increasing with time and SO42- concentrations decreasing with time (Figures 2(f) and 2(g)).According to this analysis, the groundwater hydrological environment in the study area is, overall, in a closed state with poor hydrodynamic conditions. Na+, K+, Cl-, and HCO3- concentrations in this study were enriched in the confined groundwater environment, and Na+, K+, and Cl- were present in the fracturing fluid, so HCO3- was chosen as the ion with which to study gas productivity response. Figure 3 shows that there was a positive correlation between HCO3- and CBM production in the produced water. Among these wells, L-4 and L-6 had the highest HCO3- concentration; their gas production is also the highest.Figure 3
The relationship between HCO3- and gas production.


## 3.2. Hydrogen and Oxygen Isotope Characteristics and Productivity Response

When the isotope value of produced water is located on the left side of the atmospheric precipitation line, it shows a D drift characteristic; when the value is on the right side, it shows an O drift characteristic. Except for the O drift characteristic in May and November, the produced water in well L-1 showed a D drift characteristic. For produced water of well L-4, they all show an O drift characteristic except for July and August. The values of well L-6 also exhibited an O drift characteristic except for August. As for wells L-2, L-3, and L-5, they all showed an O drift characteristic (Figure4).Figure 4
Relationship betweenδD and δ18O isotopes in produced water.The isotope values ofδD and δ18O in the produced water are negatively correlated with drainage time; however, the δD and δ18O isotope values suddenly increased in July (Figures 5(a) and 5(b)). The δD isotope value in the produced water of well L-3 suddenly increased in November (Figure 5(a)), and the δ18O isotope value in the produced water in wells L-3, L-5, and L-6 increased suddenly in November (Figure 5(b)).Figure 5
Variation of D and18O isotopes in production water with time.
(a)(b)Our quantification of the hydrogen and oxygen isotopic composition is based on the Yunnan atmospheric precipitation line equationδD=6.56δ18O−2.96 [39]. When groundwater flows through coal-bearing strata, several hydrogen-bearing soluble minerals in the coal seams are dissolved continuously. The lighter H atoms in hydrogen-bearing minerals are easily adsorbed by minerals such as clay, while the heavier D atoms are more likely to undergo an isotope exchange with H atoms in the water, thereby continuously enriching D in the formation water and exhibiting D drift characteristics [22, 26]. 18O is enriched in the surrounding rock. With groundwater runoff, many oxygen-bearing soluble minerals in the formation are dissolved continuously. The heavier 18O in the mineral is liable to undergoing isotope exchange reaction with the lighter 16O in the groundwater and thus to show 18O drift characteristics [22, 40].Hydrogen and oxygen isotopes of produced water analyzed in this study are distributed near the atmospheric precipitation line, showing obvious D drift characteristics; a few have18O drift characteristics because of atmospheric precipitation or weak mixing with the surface water and shallow groundwater [41, 42].With the drainage of CBM wells, the water-rock interactions between the fracturing fluids remaining in the coal seam or formation water and the coal seam or surrounding rocks gradually weakened. However, July and August are rainy seasons in Yunnan and result in strong recharge from atmospheric precipitation. Therefore, the variation in theδD and δ18O isotope values in July may have been caused by seasonal rainfall. Studies of δD and δ18O isotopes in produced water from the six wells show that these changes are universal (Figures 5(a) and 5(b)). The δD and δ18O isotope values from produced water in well L-3 increased sharply in November; this was also true of the δ18O isotope value for well L-5, although its δD isotope value increased only slightly in November; these results are different from those of the other four wells (Figures 5(a) and 5(b)). The drift characteristics of wells L-1 and L-2 are presumed to be caused by the fragmentation of the water-bearing limestone on the top of the coal seam. Combined with the enrichment mechanism of D and 18O [22, 26, 40], it was inferred that the trend of wells L-4 and L-6 is conducive to the long-term production of CBM; the actual gas production situation also verified this inference (Figure 6). Anomalies (reservoir damage or wellbore collapse) were observed for the remaining four wells, although wells L-1 and L-2 had high gas production; however, the former conditions are not conducive to the long-term production of CBM.Figure 6
The relationship betweenδD and δ18O isotopes and gas production in produced water.


## 3.3. Dissolved Inorganic Carbon Characteristics and Productivity Response

Theδ13CDIC values of the water samples from the produced water were not significantly different, and the change trends were similar: generally falling initially, subsequently rising, and then falling again. Wells L-1, L-4, L-5, and L-6 had the highest δ13CDIC values, the surface water samples had the lowest δ13CDIC values, and wells L-2 and L-3 had δ13CDIC values between the two extremes (Figure 7).Figure 7
Variation of dissolved inorganic carbon in production water with time.The composition of13CDIC from different sources was significantly different, appearing primarily in the forms of H2CO3, HCO3-, CO32-, and water-soluble CO2. δ13CDIC values based on organic origins are less than -8‰, and those formed by inorganic origin are approximately 0‰ [27, 29, 43].The two sources ofδ13CDIC in surface and shallow water are primarily related to oxidation via the CO2 produced by plant respiration and decomposition and the dissolution of carbonate rocks in soil. Carbon dioxide dissolves in water and continuously exchanges carbon isotopes with H2CO3, HCO3-, and CO32- in water, reducing the value of δ13CDIC in groundwater. If the concentration of HCO3- in the water is high, the value of δ13CDIC will be low. The δ13CDIC value of surface and shallow water is usually between -14‰ and -7‰, falling into the range of extremely low negative values [35, 44].The isotope13CDIC is abundant in produced water of CBM wells and primarily derived from carbonate mineral dissolution and microbial methanogenesis [34]. Methanogenesis in the study area is weak; therefore, the observed change in 13CDIC content is controlled mostly by the dissolution of carbonate minerals. Carbonate minerals in coal measure strata are more abundant than 13C in soil carbonates. When they dissolve, the δ13CDIC value in water increases and ranges from -7‰ to 0‰ [45].Theδ13CDIC value of surface water samples is in an extremely low negative value range. This is likely a result of atmospheric CO2 dissolution, plant respiration, and the dissolution of carbonate rocks in the soil (Figure 7). An analysis of the relationship between δ13CDIC values and their source characteristics shows that the produced water samples can be divided into two categories: (1) water samples from wells L-2 and L-3, with δ13CDIC values ranging from -9‰ to -7‰, which is consistent with shallow water and due primarily to the process of atmospheric CO2 dissolution, plant respiration, and the dissolution of carbonate rocks in soil; (2) water samples from wells L-1, L-4, L-5, and L-6, with δ13CDIC values ranging from -6.3‰ to -4‰. The burial depth of the coal seam is approximately 750 m, and the concentration of HCO3- is high, mainly because the carbonate minerals in the coal are dissolved. The δ13CDIC values of produced water are positively correlated with gas production (except for well L-2, which may have been caused by a precipitation injection during the rainy season) (Figure 8). When 13CDIC in the produced water is the carbonate mineral dissolved in coal seam water, the gas production is high.Figure 8
Relationship between dissolved inorganic carbon isotopes and gas production.


## 4. Conclusions

Based on an analysis of the time-varying characteristics of conventional ions, hydrogen and oxygen isotopes, and DIC content in the produced water from CBM wells in eastern Yunnan, the following conclusions were obtained:(1)
The produced water type of well L-3 is mainly Na-HCO3, while those of the other five wells are Na-Cl-HCO3. The cations in the produced water are mainly K+, Na+, Ca2+, and Mg2+. The changes in concentration are characterized by fluctuations that are primarily affected by water-rock interactions. The anions are mostly Cl- and HCO3-, and the value of the former shows a decreasing trend related to the continuous discharge of the fracturing fluid while the latter shows an increasing trend related to the dissolution of carbonate minerals in coal. The concentration of HCO3- in produced water is positively correlated with CBM production(2)
The hydrogen and oxygen isotopes in the produced water of the study area are distributed near the regional atmospheric precipitation line, showing D drift or O drift characteristics that indicate that the produced water is greatly affected by water-rock interactions. This, combined with the enrichment mechanism of D and18O, suggests that D drift is beneficial to the production of CBM. Water samples from wells L-2 and L-3 are primarily derived from the atmospheric CO2 dissolution, plant respiration, and carbonate dissolution in the soil. Water samples from wells L-1, L-4, L-5, and L-6 are primarily derived from the carbonate mineral dissolved in coal seam water. When the 13CDIC is from the carbonate minerals dissolved in coal seam water, gas production is high. Wells L-4 and L-6 produce the most gas due to having the highest concentrations of HCO3- in produced water, D drift characteristics, and 13CDIC derived from the dissolution of carbonate minerals in coal



---

*Source: 5702031-2020-05-25.xml*