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

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 O, we found that the water samples exhibit an obvious D drift trend relative to the local meteoric water line. The C 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 CDIC in produced water are positively correlated with CBM production, which can be verified by wells L-4 and L-6.


Introduction
Coalbed methane (CBM) is an unconventional natural gas resource, which has huge reserves worldwide [1][2][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][6][7][8][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 Clare high, and the concentrations of Ca 2+ , Mg 2+ , and SO 4 2are low [2,[10][11][12][13][14]. Researchers generally believe that the high concentrations of Na + , K + , and Clin 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-HCO 3 , and Na-HCO 3 , respectively [17][18][19][20]. H and 16 O isotopes in the formation water can be replaced with D and 18 O isotopes in the coal seam and surrounding rock, resulting in the increase of D and 18 O 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][24][25][26].
There are significant differences in the composition of dissolved inorganic carbon isotopes ( 13 C DIC ) from various sources. Only a few studies have examined the dissolved inorganic carbon (DIC) in the produced water; these found that the δ 13 C DIC value for the decomposition of organic matter is less than -8‰, which is typical. The value of δ 13 C DIC released by carbonate dissolution or metamorphism is relatively high, and it is generally distributed around 0‰ [27][28][29].
The higher the concentration of HCO 3 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 HCO 3 content in high-level CBM wells may be the result of CO 2 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 13 C DIC from different sources, it is generally believed that the decrease of methanogenic bacteria can lead to the abnormally positive characteristic of 13 C DIC [27][28][29][33][34][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.   [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. 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 Table 1. Well L-2 produced the most gas (21983.67 m 3 ) 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 m 3 , followed by wells L-4, L-2, L-6, L-1, and L-5.

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 (Table 2).

Conventional Ion Characteristics and Productivity
Response. The produced water from six CBM wells in the study area exhibited similar characteristics: the Na + , Cl -, and HCO 3 concentrations were relatively high, those of Ca 2+ , Mg 2+ , and SO 4 2were relatively low, and that of K + was between these two extremes (Table 2). Moreover, the concentration value of SO 4 2in 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-HCO 3 type to the Na-HCO 3 type, while that from the other five wells were all characterized as Na-Cl-HCO 3 type. The conventional ion concentrations of K + , Na + , Ca 2+ , Mg 2+ , Cl -, and SO 4 2in the produced water from the six wells exhibited a fluctuating but ultimately decreasing trend, while the concentration of HCO 3 presented a trend of increasing fluctuations (Figures 2(a)-2(g)). Among these ions, the concentrations of K + , Na + , Ca 2+ , Mg 2+ , Cl -, and SO 4 2in wells L-1 and L-2 varied greatly with time. The concentrations of K + , Na + , and Clin wells L-3, L-4, L-5, and L-6 tended to stabilize with time while the concentrations of Ca 2+ , Mg 2+ , and SO 4 2in those wells tended to fluctuate with time.
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 HCO 3 in coal seam water are low. However, the concentrations of Na + , K + , and Clin water contaminated by fracturing fluid are greatly increased, while the concentration of HCO 3 is reduced and the concentrations of other ions are less impacted. The concentrations of Na + , K + , Cl -, and HCO 3 in surface water are low, but the concentrations of Ca 2+ , Mg 2+ , and SO 4 2are 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 oxygenrich water source recharge areas that can enrich Ca 2+ , Mg 2+ , and SO 4 2-, while closed hydrological environments are far from such recharge areas and exhibit Na + , K + , Cl -, and HCO 3 enrichment [22,32,36,37]. With increasing drainage time, the concentrations of K + , Na + , Ca 2+ , Mg 2+ , Cl -, and SO 4 2in the water samples decreased with the gradual discharge of fracturing fluid. The decrease in Ca 2+ and Mg 2+ 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 Ca 2+ and Mg 2+ concentrations increased and the Ca 2+ concentration increased significantly (Figures 2(c) and 2(d)). As the dissolution rate of calcite is much higher than that of dolomite, the Ca 2+ concentration in water was generally higher than the Mg 2+ concentrations. SO 4 2accumulations are primarily related to the dissolution and desulfurization of gypsum [38]. In a reducing environment, sulfate in coal seam  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 HCO 3 concentrations in this study were enriched in the confined groundwater environment, and Na + , K + , and Clwere present in the fracturing fluid, so HCO 3 was chosen as the ion with which to study gas productivity response. Figure 3 shows that there was a positive correlation between HCO 3 and CBM production in the produced water. Among these wells, L-4 and L-6 had the highest HCO 3 concentration; their gas production is also the highest.

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 ( Figure 4).
The isotope values of δD and δ 18 O in the produced water are negatively correlated with drainage time; however, the δD and δ 18 O 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 δ 18 O isotope value in the produced water in wells L-3, L-5, and L-6 increased suddenly in November ( Figure 5(b)).
Our quantification of the hydrogen and oxygen isotopic composition is based on the Yunnan atmospheric precipitation line equation δD = 6:56δ 18 O − 2:96 [39]. When groundwater flows through coal-bearing strata, several hydrogenbearing 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]. 18 O is enriched in the surrounding rock. With groundwater runoff, many oxygen-bearing soluble minerals in the formation are dissolved continuously. The heavier 18 O in the mineral is liable to undergoing isotope exchange reaction with the lighter 16 O in the groundwater and thus to show 18 O 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 have 18 O 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 the δD and δ 18 O isotope values in July may have been caused by seasonal rainfall. Studies of δD and δ 18 O isotopes in produced water from the six wells show that these changes are universal (Figures 5(a) and 5(b)). The δD and δ 18 O isotope values from produced water in well L-3 increased sharply in November; this was also true of the δ 18 O 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 18 O [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 longterm production of CBM.

Dissolved Inorganic Carbon Characteristics and
Productivity Response. The δ 13 C DIC 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-    6 Geofluids 5, and L-6 had the highest δ 13 C DIC values, the surface water samples had the lowest δ 13 C DIC values, and wells L-2 and L-3 had δ 13 C DIC values between the two extremes ( Figure 7). The composition of 13 C DIC from different sources was significantly different, appearing primarily in the forms of H 2 CO 3 , HCO 3 -, CO 3 2-, and water-soluble CO 2 . δ 13 C DIC values based on organic origins are less than -8‰, and those formed by inorganic origin are approximately 0‰ [27,29,43].
The two sources of δ 13 C DIC in surface and shallow water are primarily related to oxidation via the CO 2 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 H 2 CO 3 , HCO 3 -, and CO 3 2-in water, reducing the value of δ 13 C DIC in groundwater. If the concentration of HCO 3 in the water is high, the value of δ 13 C DIC will be low. The δ 13 C DIC value of surface and shallow water is usually between -14‰ and -7‰, falling into the range of extremely low negative values [35,44].
The isotope 13 C DIC 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 13 C DIC content is controlled mostly by the dissolution of carbonate minerals. Carbonate minerals in coal measure strata are more abundant than 13 C in soil carbonates. When they dissolve, the δ 13 C DIC value in water increases and ranges from -7‰ to 0‰ [45].
The δ 13 C DIC value of surface water samples is in an extremely low negative value range. This is likely a result of atmospheric CO 2 dissolution, plant respiration, and the dissolution of carbonate rocks in the soil (Figure 7). An analysis of the relationship between δ 13 C DIC 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 δ 13 C DIC values ranging from -9‰ to -7‰, which is consistent with shallow water and due primarily to the process of atmospheric CO 2 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 δ 13 C DIC values ranging from -6.3‰ to -4‰. The burial depth of the coal seam is approximately 750 m, and the concentration of HCO 3 is high, mainly because the carbonate minerals in the coal are dissolved. The δ 13 C DIC 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 13 C DIC in the produced water is the carbonate mineral dissolved in coal seam water, the gas production is high.

Conclusions
Based on an analysis of the time-varying characteristics of conventional ions, hydrogen and oxygen isotopes, and DIC  (1) The produced water type of well L-3 is mainly Na-HCO 3 , while those of the other five wells are Na-Cl-HCO 3 . The cations in the produced water are mainly K + , Na + , Ca 2+ , and Mg 2+ . The changes in concentration are characterized by fluctuations that are primarily affected by water-rock interactions. The anions are mostly Cland HCO 3 -, 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 HCO 3 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 and 18 O, 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 CO 2 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 car-bonate mineral dissolved in coal seam water. When the 13 C DIC 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 HCO 3 in produced water, D drift characteristics, and 13 C DIC derived from the dissolution of carbonate minerals in coal

Data Availability
The data used to support the findings of this study are included within the article.