Ionic composition, geological signature and environmental impacts of coalbed methane produced water in China

ABSTRACT Ionic composition data of coalbed methane (CBM) produced water from five CBM blocks in China were used to investigate the variability in and the geological controls on water quality across geologic basins and the resulting environmental impacts. The assessment tools included the Schoeller diagram, the Na-K-Mg ternary diagram and the Cl-Na-HCO3 ternary diagram. Techniques such as principal component analysis (PCA) and R-mode hierarchical cluster analysis were also applied to the ionic database to reveal the relationships between the ions. The results from this study indicate that three major ions (Na, Cl, and HCO3) and two primary processes (sulfate reduction and mixing of the coal seam water with the fracturing fluid) affect the ionic compositions of CBM produced water. The burial depth of the coal seam, the fresh water influence and the fracturing fluid contamination jointly control the water quality variability of produced water. The Na-K-Mg ternary diagram provides a potential method to evaluate the origin of CBM produced water, such as coal seam water in the partially equilibrated region and exotic water (including the fracturing fluid or shallow cold groundwater) in the immature water region. Most of the CBM produced water in China has high concentrations of Na, HCO3, Cl and TDS compared to the standards for drinking water and irrigation water. The produced waters from deep formations (>1000 m), or those produced during the early drainage periods, are always saline, “Na-Cl” type waters. Appropriate treatment methods and regulations for CBM produced water in China are urgently needed.


Introduction
Water generated during coalbed methane (CBM) production, termed CBM produced water, contains valuable and abundant information on CBM generation, accumulation and production, and has important impacts on the environment (Brinck, Drever, and Frost 2008;Rice et al. 2008;Guo et al. 2017a).Thus, research on the geochemical characteristics of CBM produced water has attracted increasing attention around the world in recent years.The produced water chemistry among different CBM blocks and basins are highly variable due to different geological settings (Dahm et al. 2014;Engle and Rowan 2014;Taulis and Milke 2013).The geochemical compositions of produced water also vary temporally with increased drainage in a single well (Guo, Qin, and Han 2017a;Cheung et al. 2009;Islam et al. 2017;Li et al. 2011).Generally, the CBM produced water quality is largely dependent on the coal burial depth, water-rock interactions, aquifer recharge, coal permeability, sedimentary environment, and fracturing fluid contamination (Van Voast 2003;Dahm et al. 2011Dahm et al. , 2014;;Rice 2003;Rice et al. 2008;Abu Bakar and Zarrouk 2016;Guo et al. 2017b).In China, many researches have focused on the CBM produced water quality of individual CBM development fields and their indication for the CBM production (Huang et al. 2017a;Zhang et al. 2016;Chen et al. 2016;Yang et al. 2017;Wu et al. 2018;Guo et al. 2017b;Yang et al. 2013;Wang et al. 2015).However, little attention has been paid to variations in the ionic composition of produced water among wells, coal seams and CBM basins in China, or to their geological signatures and environmental issues.China has transitioned into early-stage, large-scale CBM development (Qin et al. 2018), and a systematic study on the produced water quality from surface CBM wells is urgently needed.Meng et al. (2014) attempted to identify the quality of CBM produced water in China and obtained encouraging research results.However, their interpretations were relatively simple and lack a detailed comparative analysis between different blocks or basins; in addition, the data used were relatively old.
Therefore, five CBM commercial development blocks in China were selected, and their latest geochemical data from produced water were collected to perform this study.These blocks are all present hotspots of CBM development in China.The objective of this study is to provide a comprehensive understanding of the ionic compositions of produced water from typical CBM blocks in China.This objective was achieved by determining the major ion concentrations in the produced water and their relationships.The major foci of this paper are the water quality differences between blocks, the influential factors of geology and engineering, and the environmental impacts of CBM produced water in China.Such information is useful to better understand CBM reservoirs and water-rock interactions therein and to determine appropriate treatments or potential uses of CBM produced water.We also expect this study to promote efficient, economic and environmentally friendly development in the CBM industry.

Geological settings and methods
Produced water samples from five selected CBM blocks in China were used for comparison.The selected blocks are the Zhijin block and the Songhe block in the Qianxi-Diandong basin, Southwest China, the Beier block in the Tiefa basin, Northeast China, the Shizhuangnan block in the Qinshui basin, and the Yanchuannan block in the Ordos basin, North China.These blocks are typical CBM exploration and development areas in China and have different geographic distributions (from south to north), geological settings, stratigraphic units (C 2 -P 1 , P 3 , K 1 ) and coal ranks (from low to high); therefore, they are representative of the CBM produced water characteristics in China (Figure 1).The geological settings and CBM development information of the five blocks were summarized in Table 1 (Zhang et al. 2015(Zhang et al. , 2016;;Guo and Lu 2016;Huang et al. 2017a,b;Guo et al. 2017b;Tang et al. 2018aTang et al. , 2018b;;Yang et al. 2018).As the deepest CBM block, the Yanchuannan is at the forefront of CBM development in China and is used as an example for other deep CBM development projects.
An ionic composition database of 237 produced water samples from the five blocks was created using publicly available data (Huang et al. 2017a;Zhang et al. 2016;Chen et al. 2016;Yang et al. 2017;Wu et al. 2018;Guo et al. 2017b) to comprehensively characterize the CBM produced water quality in China and to understand their geological signatures and environmental influences (Supplementary Data Table ).These data were acquired strictly according to the guidelines of the water and wastewater monitoring analysis method in China.In this study, they were further checked for anion-cation balance before analysis, and several unbalanced data were excluded in order to guarantee the data reliability.The Na and K concentrations in the produced water samples of Shizhuangnan and Yanchuannan blocks were not separately measured, and they were referred as the Na concentration in the discussion.As a result, these two blocks were excluded from the Na-K-Mg ternary analysis.
The parameters analyzed included four cations (Mg, Ca, Na, and K) and four anions (SO 4 , Cl, HCO 3 , and CO 3 ), as well as the pH and total dissolved solids (TDS) of the water.The assessment tools used included the Schoeller diagram, the Na-K-Mg ternary diagram and the Cl-Na-HCO 3 ternary diagram.The R-mode hierarchical cluster analysis and principal component analysis (PCA) were also used to determine the relationships between the ions and the potential influence of environmental factors.Additionally, parameters reflecting geologic and engineering information, such as the burial depth and the permeability of the producing coal seam, the depositional environment, the maximum reflectance of the vitrinite (R o,max ), and the drainage time, were also collected to better understand the variability in water quality among different CBM blocks/basins.

Ionic compositions of CBM produced water
The concentrations of Na, K, Ca, Mg, Cl, SO 4 , HCO 3 , and CO 3 ions were analyzed in this study; these ions have been proved to be the main ions present in CBM produced water and coal measure groundwater.Na and Cl exhibited the highest concentrations in the CBM produced water in China; HCO 3 had the next highest concentrations, and the other ions, especially SO 4 , Mg, Ca, and K, had relatively low concentrations (Figure 2).Cl, Na, and HCO 3 are the three dominant ions in CBM produced water worldwide (Abu Bakar and Zarrouk 2016; Dahm et al. 2014Dahm et al. , 2011;;Rice 2003;Rice et al. 2008;Van Voast 2003).CBM produced water is generated from formations that are shallower than those associated with traditional oil and gas resources, in which water quality levels may be influenced by the supply of fresh water to an area (Dahm et al. 2014).Anaerobic sulfate reduction and ion exchange are the two major interactions that occur during groundwater flow through the coal-bearing strata (Rice et al. 2008;Yang et al. 2013;Dahm et al. 2014;Guo et al. 2017b).Therefore, the coal seam water is characterized by lower TDS and Cl concentrations, but higher HCO 3 concentrations, compared to deep oilfield water, and lower SO 4 , Ca, and Mg concentrations, but higher Na and HCO 3 concentrations, compared to shallow groundwater.

Cluster analysis and PCA
First, PCA was used to determine the relationships between the conventional ions in the CBM produced water.Eigenvalues greater than 1 were used to determine the principal component scores and equations (Dahm et al. 2014).Two principal components (PC) with eigenvalues >1 were extracted (PC1, PC2).Water quality variability in the two components amounted to 79.09% (Table 2).Coefficient scores for each component are listed in Table 3. PC1 is characterized by strong positive loadings for Na, Mg, Ca, Cl, and TDS, and a negative loading for pH.PC2 is characterized by strong positive loadings for HCO 3 and CO 3 , and a weak negative loading for SO 4 .In the R-mode hierarchical cluster analysis, the between-groups linkage method was applied to determine the final clusters, and a Pearson correlation was applied to determine the distance measurements.The corresponding cluster results are shown in Figure 3. Two clusters (C1 and C2) can be identified by a distance measurement of 20.C1 includes Cl, Na, TDS, Ca and Mg; C2 includes HCO 3 , CO 3 , pH and SO 4 , among which HCO 3 and CO 3 possess a close relationship.C1 includes the main cations in groundwater and generally represents the water-rock interactions and recharge conditions.C2 represents the acid group anions, which influence the pH of groundwater.The explanations are as follows: 1) TDS is positively related to the degree of water-rock interactions, and can be used as a direct indicator to determine the water-rock interactions; 2) HCO 3 is the main anion of CBM produced water, which is mainly generated through the process of SO 4 reduction (Van Voast 2003), and would further influence the pH value and CO 3 concentration in water; 3) The effect of hydrolysis is greater than the ionization of HCO 3 , which determines the weak alkalinity of produced water (Huang et al. 2017a): It appears that PC-1 corresponds to C1 and that PC2 corresponds to C2.If the distance from the recharge area is increased or the water-rock interactions are enhanced, TDS, Cl, Na, and HCO 3 concentrations increase, while Ca, Mg, and SO 4 concentrations decrease (Larson and Daddow 1984;Van Voast 2003;Guo et al. 2017b).Correspondingly, Cl and Na exhibit a closer relationship with TDS in C1.The potential explanations for this observation are as follows: 1) Water in coal seams becomes more sodic with depth due to the dissolution of Na and the precipitation or adsorption of Ca and Mg, i.e., cation exchange processes with sodic clay minerals (Larson and Daddow 1984;Rice et al. 2008).Additionally, the alteration from feldspar to kaolinite will release Na (Brinck, Drever, and Frost 2008)., 2) Cl has a large solubility and is not easily precipitated.As a conservative element, Cl concentrations typically increase as salinity increases.3) The sulfate reduction process reduces the SO 4 concentration, and increases the HCO 3 concentration and the pH of groundwater in coalbearing strata, which results in the further precipitation of Ca and Mg: Additionally, the injection of a fracturing fluid into the coal seam must be taken into consideration during the CBM produced water analysis, as this injection will significantly influence the water quality and ionic compositions, especially during the early drainage period.According to previous studies conducted by the authors and other researchers, the water produced from CBM wells in the early drainage period is dominated by the fracturing fluid and is characterized by anomalously high Cl, Na and TDS concentrations, and a relatively slight increase in the K concentration (Zhang et al. 2016;Guo et al. 2017b;Yang et al. 2017).The fracturing fluid widely used in the Chinese CBM industry is made by adding KCl, an anti-swelling agent, to fresh river water, forming a 2% KCl solution (Zhang et al. 2016).K is heavily employed because of its effect on preventing clay layers from swelling and migrating.Therefore, a large amount of K in the fracturing fluid is consumed after entering the coal seam or the mudstone layers and is replaced by Na to maintain ionic equilibrium in water.
Therefore, the PC1 mainly reflects the mixing of the fracturing fluid and the coal seam water; PC2 reflects the original content of the coal seam water, controlled by the sulfate reduction process.SO 4 is depleted in the coal seam water and the fracturing fluid; therefore, its loadings in both PC1 and PC2 are very low.Much of the HCO 3 in CBM systems is generated through SO 4 reduction during coalification.This is the main explanation for the positive coefficient for HCO 3 and the negative coefficient for SO 4 in PC2.The sulfate reduction process is a key factor controlling the coal seam water hydrochemistry, which is totally represented by the PC2.The abundant organic matter and reducing environment in coal measures provide favorable conditions for the occurrence of sulfate reduction.
The PC biplot was plotted to further understand the relationship between the ionic compositions (Figure 4).The variance of the variables captured by the two components is proportional to the length of the ray.Component scores represents the participation of the variables in each PC, and they are used to calculate a PC score for each observation (water sample) for PC1 and PC2 (Zhang et al. 2016).
Figure 4 suggests that 1) the produced water samples from Yanchuannan block tend to exhibit high relative proportions of Cl, Na, TDS, Ca and Mg, indicating an enhanced water-rock interaction due to the deep origin; 2) the water samples from Tiefa and Zhijin blocks exhibit large relative proportions of HCO 3 and CO 3 , suggesting a common character of shallow formation water; 3) the water samples of Shizhuangnan and Songhe blocks are relative close to the geometric center, suggesting a mixing character of formation water and fracturing water, especially for those of the Songhe block.

Na-Cl-HCO 3 ternary diagram
As mentioned above, HCO 3 , Cl, and Na are the three dominant ions in CBM produced water.Conventional oil field water mainly originates from a marine source, but coal seam water is more prone to a fresh water influence (Dahm et al. 2014).In this study, the Na-Cl-HCO 3 ternary diagram was used to describe the degree to which water-rock interactions are influenced by burial depth or fresh water recharge (Figure 5).The burial depth of the coal seams in the Yanchuannan block is 780-1750 m, with an average of 1200 m; this deep origin of CBM produced water corresponds to its position in the diagram.The water samples were collected from CBM wells that had drainage times exceeding 730 d; therefore, the influence of the fracturing fluid can be ignored in the Yanchuannan block.The coal seam depths in the Zhijin block and the Songhe block range from 200 to 600 m and from 560 to 960 m, respectively.The data distribution in the diagram is significantly different between the two blocks, which cannot be explained solely by the variable depth of the producing coal seams.The average flowback rates of the fracturing fluid in the Songhe block and the Zhijin block are estimated to be 40% and 70%, respectively (Wu et al. 2018), by the end of 2017.Therefore, the deeper coal seam and the fracturing fluid contamination of the Songhe block jointly contribute to the water quality difference observed between the two blocks in western Guizhou Province.The target coal seams in the Zhijin block are shallower than those of the Songhe block and are thus more susceptible to fresh water interference during drainage (Guo et al. 2017b).
In the Shizhuangnan block, the depth of the No. 3 coal seam is between 71.4 and 1074 m, with an average of 626.6 m.The No. 15 coal seam is located approximately 60 m to 80 m below the No. 3 coal seam.This wide range of burial depths results in the large variations in water quality observed in the Shizhuangnan block.In addition, the influence of the fracturing fluid may enhance this variation.Zhang et al. (2016) suggest that a Cl concentration of 10 meq/L can be used as an indicator of the fracturing influence.
The produced water samples of the Tiefa basin were mainly collected during the stable stages (drainage time >200 d), and most of the fracturing fluid had been discharged by this point.The completion depths of the CBM wells in the sampling area of the Beier block are between 700 m and 950 m, deeper than those of the Zhijin block.However, the Tiefa basin is the only continental faulted basin in this study that contains Mesozoic sediments with an accumulation of low rank coal seams (higher permeability), a condition different from that of the other geologically older basins, and is more likely to be affected by fresh water during drainage periods, compared to the marine or marine-continental basins that contain high rank coal seams and lower permeability (e.g., the Shizhuangnan block).
Generally, the burial depth of the producing coal seams dominantly determines the water quality variability of CBM produced water in Figure 5, due to the positive relationship between burial depth and water-rock interactions, and the negative relationship between burial depth and groundwater recharge conditions; the influences of the depositional environment, the fresh water recharge and the fracturing fluid contamination further enhance this variability.Specifically, the influences of burial depth (water-rock interactions and recharge conditions) and fracturing fluid are mainly represented by the PC1, while the influences of fresh water and sulfate reduction dominate the PC2.

Na-K-Mg ternary diagram
The Na-K-Mg ternary diagram is an indicator for discriminating the water-rock interactions and groundwater quality equilibration (Figure 6).Within the diagram, fully equilibrated water indicates that the minerals in the water have reached water-rock equilibrium condition, representing a deep origin or stagnant hydrogeological environment; Partially equilibrated water region signifies that minerals began to dissolved but equilibrium has not been reached, indicating a transition state of groundwater; Immature water region signifies the water samples are cold groundwater influenced by fresh water or are contaminated, representing a very low degree of water-rock interaction (Giggenbach 1988).
Most of the CBM produced water from coal seams around the world should plot within the partially equilibrated region (Abu Bakar et al. 2016).Figure 6 shows an evaluation of the produced water from the Songhe, Zhijin, and Tiefa blocks on the Na-K-Mg diagram.The produced water samples from the Tiefa basin are found both in the partially equilibrated region (more than 85%) and the immature region, indicating a small portion of the samples may be influenced by cold groundwater.The average water production rate of an individual CBM well in the Beier block ranges from 0 to 33 m 3 /d (Huang et al. 2017b), suggesting a potential recharge from a shallow aquifer or surface water for some wells, and hence high seam permeability with lower R o,max values.The Powder River basin is also a low-rank CBM basin in the US, and its produced water samples also plot within the immature water region (Abu Bakar and Zarrouk 2016).
The samples from the Songhe and Zhijin blocks are sourced from immature water regions that correspond to an area of fracturing fluid contamination and a partially equilibrated region, respectively.As mentioned above, the coal seam in the Songhe block is deeper than that in the Zhijin block; therefore, the coal seam water in the Songhe block should theoretically be more equilibrated than that in the Zhijin block.Obviously, the fracturing fluid contaminates the original formation water and dominates the produced water quality in the Songhe block.
Therefore, the Na-K-Mg ternary diagram provides a potential method to evaluate the origin of the CBM produced water, namely, coal seam water in the partially equilibrated region, and exotic water (including fracturing fluid or shallow cold groundwater) in the immature water region.Therefore, the Na-K-Mg ternary diagram may be somewhat helpful in predicting the gas production of CBM wells based on the produced water quality and the original source.

Schoeller diagram
The Schoeller diagram is a basic tool for water quality analysis.Figure 7 shows the Schoeller diagram for the produced water samples from the different blocks in China.The CBM water from the Yanchuannan block has the highest magnitude of almost all of the ions (except for SO 4 and HCO 3 ), compared to those in the other blocks, and exhibits a "Na-Cl" water type.This water quality is consistent with that of the deep CBM in this block.However, some water samples from the shallower coal seam in the Yanchuannan block exhibit a "Na-HCO 3 " type.
Influenced by the fracturing fluid in the coal seams, the Songhe block presents a "Na-Cl" water type, with higher Ca, Mg, Na, and Cl concentrations, but lower SO 4 and HCO 3 concentrations than is found in the Zhijin block, whose water type is "Na-HCO 3 ".The water type of the Tiefa basin is "Na-HCO 3 " which has the highest HCO 3 concentration, corresponding to its continental depositional environment.Continental deposits are characterized by Na-HCO 3 type waters with lower Na, K, and Cl concentrations.Marine deposits are generally Na-Cl type water due to the high concentrations of Na and Cl in seawater.Additionally, water in marine deposits also have higher concentrations of B and trace metals than that in continental deposits (Dahm et al. 2014;Van Voast 2003).The Shizhuangnan block exhibits a large variation in ionic concentrations, a dominant water type of "Na-HCO 3 " and a subordinate type of "Na-Cl", corresponding to its wide range of coal seam depths, variable producing coal seams and corresponding depositional environments, and potential influences of fracturing fluids.The coal seam of the Shanxi formation in the Shizhuangnan block was formed in a fluvial-delta dominant environment, while that of the Taiyuan formation was formed in a barrier coastal environment with stronger seawater influences.This difference of depositional environment between producing coal seams enhances the variation in ionic compositions of CBM produced water.

The environmental impacts of CBM produced water
The produced water type for the five blocks in China is summarized in Table 1.The "Na-Cl" type produced waters from the Yanchuannan and Songhe blocks appear to need more treatment due to the CBM burial depth and the fracturing fluid contamination, respectively.The environmental impact of produced water quality is assessed from the perspective of drinking water and irrigation water.The statistics for the produced water ionic parameters for the five blocks are presented in Table 4.As shown in Table 4, the quality of the produced water varies considerably among the various blocks and stratigraphic units.
The pH of the CBM produced water samples in the five blocks in China ranges from 6.37 to 10.03.Most of the water samples conform to the WHO (2008) standard, 6.5-9.0, which also indicates that the produced water is slightly alkaline (pH>7.0).
The TDS concentration of produced water ranges from 601 to 160432.32 mg/L.The salinity classes of the produced water in China range from fresh to saline.However, the influence of the fracturing fluid cannot be ignored.The maximum TDS concentration of the CBM produced water occurs in the No. 2 coal seam of the Yanchuannan block and is due to the burial depth of the coal seam exceeding 1500 m.Generally, the CBM produced water in China is moderately to intensively mineralized, and most of the samples exceed the limit of 1000 mg/L.
The SO 4 in produced water is generally at concentrations lower than the levels of the WHO drinking water standard, and irrigation water standard proposed by Ayer and Westcot (1985).Most of the water samples have high concentrations of Na, Cl and HCO 3 compared to the WHO and   irrigation standards.Other than in the Yanchuannan block, the Ca and Mg concentrations in produced water are lower than the WHO standards.The K concentrations of produced water from the Songhe block and the Yanchuannan block are higher than the standards, especially for the Songhe block, consistent with the fracturing fluid contamination.
Notes: Standard-1 is from WHO World Health Organization (2008) for drinking water, and Standard-2 is from Ayers and Westcot (1985) for irrigation water.
The Sodium Adsorption Ratio (SAR) is a measure of water quality for use as irrigation water.SAR is calculated using Equation (4), where Na, Ca, and Mg are in units of meq/L: SAR values greater than 22 are considered unsuitable (Essington 2004) for irrigation water.
Unfortunately, most of the produced water in China far exceed this limit.
The analysis above indicates a poor or very poor quality of CBM produced water in the five CBM blocks in China, which will exert a negative effect on human health and crops if it were to be discharged directly into the environment without proper management.Appropriate treatment methods and regulations for CBM produced water in China are urgently needed.

Conclusions
(1) Cl, Na, and HCO 3 are the three dominant ions in CBM produced water.Two principal components were extracted (PC1, PC2) from the water quality parameters.PC1 mainly reflects the mixing of the fracturing fluid and the coal seam water; PC2 reflects the original characteristics of the coal seam water, controlled by sulfate reduction processes.The burial depth of the coal seams, the depositional environment, the fresh water recharge, and the fracturing fluid contamination jointly control the water quality variability of the CBM produced water.
(2) The Na-Cl-HCO 3 ternary diagram can be used to describe the degree to which water-rock interactions are influenced by the burial depth or the fresh water recharge.The Na-K-Mg ternary diagram provides a potential method to evaluate the origin of the CBM produced water, namely, coal seam water in the partially equilibrated region, and exotic water (including fracturing fluid or cold groundwater) in the immature water region.(3) CBM produced water from the Zhijin, Beier, and Shizhuangnan blocks exhibits a Na-HCO 3 water type, and those from the Yanchuannan and Songhe blocks exhibits a Na-Cl water type with higher TDS concentrations, due to the deep origin and fracturing fluid contamination, respectively.Accordingly, the produced waters from the Yanchuannan and Songhe blocks requires more careful treatment.(4) Most of the CBM produced water in China has high concentrations of Na, HCO 3 , Cl and TDS compared to the WHO drinking standard and irrigation standard.The produced waters from the deep formation (>1000 m) or those produced during the early drainage periods are always saline.Appropriate treatment methods and regulations for CBM produced water in China are urgently needed.

Figure 1 .
Figure 1.Distribution of the selected CBM blocks in China.

Figure 2 .
Figure 2. Statistics of ion concentrations in the produced water samples.

Figure 3 .
Figure 3. Results of the cluster analysis of ions in the produced water samples.

Figure 4 .
Figure 4. Principal component biplot for the two components of produced water samples.

Figure 6 .
Figure 6.Na-K-Mg Ternary diagram of CBM produced water samples.

Figure 7 .
Figure 7. Schoeller diagram of CBM produced water samples.

Table 2 .
Eigenvalues of each principal component.

Table 3 .
Coefficient scores for each principal component.

Table 4 .
Statistics summary of CBM produced water quality parameters in China (mg/L).