Chemical Characteristics and Development Significance of Trace Elements in Produced Water with Coalbed Methane in Tiefa Basin.

Coalbed methane (CBM) is an unconventional natural gas resource. CBM mining releases a large amount of coproduced water, and the trace elements of CBM coproduced water can provide a basis for the exploration and development of CBM. The contents of eight major trace elements in the produced water from wellhead were tested and analyzed based on seven CBM wells in Tiefa Basin. The research indicates that Sr and Ba are the dominant trace elements with the highest concentrations in produced water. There is a positive correlation among Li, Sr, and Rb by cluster analysis and correlation analysis, which may be affected by the total dissolved solids and pH in the groundwater. The contents of Li, Sr, and Ba increase with the burial depth of coal seam and could be influenced by the fault. The gas production of CBM wells is affected by the depth of the coal seams, and there is no significant correlation between water production and the coal seam depth. However, faults have an important impact on gas and water production. The productivity of coalbed methane is affected by hydrogeological conditions and structure because the productivity of CBM wells located in different tectonic locations varies with the change of Li, Sr, and Ba contents.

Introduction

Coalbed methane (CBM) is an important unconventional natural gas resource.[1,2] Commercial exploitation of CBM has been established in the United States, Australia, China, Canada, and other countries.[3,4] Gas molecules diffuse and adsorb in coal pore structure.[5−8] CBM is adsorbed on the inner surface of coal by physical adsorption and hydrostatic pressure of groundwater.[9−11] After hydraulic fracturing,[12] which is mined through drainage, pressure reduction, desorption, diffusion, and seepage.[13−16]

In the Powder River Basin of Wyoming, the main elements in coproduced water are Al, As, B, Ba, Cr, Cu, F, Fe, Mn, Mo, Se, Zn, Ni, Co, and Zn[9,17−20] during CBM production. However, the main elements are As, Cd, Co, Cr, Cu, Hg, Mo, Ni, Pb, Sb, Se, V, and Zn[21] in Alberta, Canada. Different from the United States and Canada, the main elements in Bide-Santang Basin and Qinshui Basin, China are Li, Ga, Rb, Sr, Ba, Mo, Cr, As, Se, V, Mn, Co, Cu, Zn, Ag, Cs, Pb, and U.[22,23]

Li and Rb are rare basic metals derived from albite-spodumene lithium mica, and their contents in water increase as total dissolved solids (TDS) increase. Cr is a common element in groundwater in metal deposits and is mainly enriched in the peridotite distribution area. Cu is mainly derived from metal sulfide minerals, and redox conditions have a great influence on the migration of Cu. The content of Cu in water is limited by the pH. Both strontium and barium belong to the alkaline earth metal group and are chemically close. Their content in groundwater is affected by the Ca2+ and Mg2+ contents. Mn is common in groundwater, and the rock is rich in manganese. A large amount of manganese is released into the groundwater from the manganese minerals in the rock when the rock is subjected to strong weathering, decomposition, and leaching. The As content in groundwater and rocks is generally low. The formation and enrichment of As in groundwater are mainly related to the As content in the aqueous medium and the pH of the groundwater.[24]

CBM occurrence is closely related to groundwater hydrogeochemical characteristics.[25] Hydrogeochemistry in underground water controls the enrichment and preservation of CBM, as well as the gas production rate in the production process.[23,26] Hydrogeochemical characteristics are useful for identifying the groundwater flow paths,[27] the source of the produced water,[26] CBM enrichment areas, and guiding CBM development.[28−30] Also, chemical research on produced water is the basis for the management and effective utilization of produced water.[25] There is a certain relationship among trace element concentration in produced water, formation depth, and gas production.[22] Trace element concentration variation in produced water of different underground coal wells may lead to changes in the CBM production level.[9] Trace elements could indicate groundwater runoff and elucidate groundwater circulation.[31] The source of produced water can be determined through the intersection method for trace elements.[23]

Low gas production wells have the characteristics of the large span between layers, high water yield, and low trace element concentration, while high-yield gas wells are characterized by a short span between layers, low water production, and high trace element concentration in water produced.[32] The concentrations of trace elements are affected by the pH value.[17,33] The concentrations of Al, Fe, As, Se, and F in the produced water are significantly increased because of the substantial increase in the pH.[17] The content of trace elements in pit drainage is higher than that in surface water, and the effluent velocity has a great influence on acidity and trace elements level.[34] Based on the test data and production data of CBM wells in Tiefa Basin, this paper discusses the characteristics of trace elements in produced water and studies the difference in various tectonic locations. The relationship between the characteristics of trace elements in produced water and the gas production rate is also analyzed to guide CBM development in the region.

Geological Situation

The CBM development area in Tiefa Basin is located in the northeast of China (Figure ), which is situated in the sedimentary center of Tiefa Basin. The CBM development area is mainly controlled by anticline and the fault (Figure ). The whole area is an anticline, which is divided into multiple structural blocks by faults. The CBM wells are distributed in the same block, and the block boundaries are controlled by faults (DF2, DF45, and F56). It covers DT4, DT5, DT7, DT8, DT9, DT10, and DT23. Among them, DT6 and DT23 are located near the fault, and the remaining CBM wells are located in the anticline. The strata of the mining area are mainly composed of Fuxin Group of the Lower Cretaceous, the Quantou Formation of the Cretaceous, and the Quaternary of the Cenozoic. The Fuxin Formation is the only coal-bearing strata (Figure ). The Fuxin Formation consists of four parts: the bottom glutenite member (K1f1), the lower coal-bearing member (K1f2), the middle sand-mudstone member (K1f3), and the upper coal-bearing member (K1f4). The lower coal-bearing section (K1f2) contains 22 layers of coal, and the upper coal-bearing section (K1f4) contains 23 layers of coal. The buried depth range of the upper coal group roof is about 610–940 m. Centering on the anticline core, the burial depth of the coal seam gradually deepens to the north, south, and west.

Figure 1

Geological setting of the research area.

Figure 2

Stratigraphic column of the research area.

Sample Collection and Testing

Sampling and Testing

The water samples collected from six CBM wells DT3, DT4, DT7, DT9, DT10, and DT23 are sampled five times. The sampling dates are June 1, June 22, July 24, Aug 27, and Sept 22, 2011. DT6 is sampled four times; sampling dates are June 1, June 22, July 24, and Aug 27. The coproduced water samples from DT6 wells were absent due to no water produced on Sept 22, 2011. The coproduced water was collected by sample bottles. Every sample bottle was soaked for about 1 day in an acidic solution with 20% nitric acid, and then, they were cleaned with deionized water. Moreover, before sampling, the bottles should be rinsed at least twice with sample water and then the water samples were sent to the laboratory. The analytical test instrument was an inductively coupled plasma mass spectrometer. The test item is trace elements analysis, and the measured elements include 40 elements. Eight high-content elements are selected for analysis. These eight elements are Li, Cr, Mn, Cu, As, Rb, Sr, and Ba.

Concentration of Elements

Table lists the content and average values of the above elements in different wells at different times. In the test, the concentration of Sr in the produced water of each CBM well is the highest among all of the trace elements, the element concentration is between 1100 and 4011 ppb, and the average value is 2232.29 ppb. The Ba concentration is also very high, with a concentration range of 426.8–3780 ppb and an average value of 1362.94 ppb. The concentration of Cu is the lowest, and the concentration of the element ranges from 1.21 to 43.56 ppb with an average of 18.3 ppb. Table lists the average values of trace element concentrations for different wells at the same sampling time. The average contents of Sr and Ba in each CBM well exceed 1000 ppb, the maximum content of Sr reaches 2617.57 ppb, and the maximum content of Ba reaches 1693.50 ppb. Next, Li > Cr > Mn and the average content of Li is greater than 100 ppb (Table ). Li, Sr, and Ba have higher concentrations, which is consistent with Guo’s research at the Bide-Santang Basin.[22]

Table 1

Trace Element Data of Drainage and Extraction of CBM Wells in the Research Area

date	well number	Li(ppb)	Cr(ppb)	Mn(ppb)	Cu(ppb)	As(ppb)	Rb(ppb)	Sr(ppb)	Ba(ppb)
June 1, 2011	DT3	153.1	45.57	10.16	12.2	17.12	13.5	1124	1217
	DT4	146.5	56.67	19.33	2.41	18.45	11.53	1100	1058
	DT6	110.1	76.18	87.17	20.11	19.55	11.62	1882	808.8
	DT7	260.5	77.84	24.05	2.9	20.6	19.52	2054	1273
	DT9	287.2	78.61	46.89	16.49	20.64	22.47	1972	898.7
	DT10	254.8	72.77	37.18	19.99	19.82	17.88	1994	1361
	DT23	105.9	62.39	35.02	17.83	19.25	9.33	1415	664.5
June 22	DT3	306	104.8	31.39	11.33	20.16	23.81	2230	2030
	DT4	236.2	112.9	58.94	27.32	19.43	17.47	1757	1202
	DT6	99.91	123.7	51.23	43.56	19.42	11.81	1966	534.4
	DT7	381.3	131.8	8.07	22.8	19.07	27.05	3005	1458
	DT9	290.1	132.5	25.09	19.84	18.58	22.11	2639	1364
	DT10	266.3	128.7	42.05	1.92	18.79	19.39	2348	1323
	DT23	182.6	142.8	58.21	30.14	19.12	14.58	2261	819.2
July 24	DT3	189.3	7.19	52.4	5.28	17.26	15.62	2493	1034
	DT4	283.7	6.16	50.56	17.73	17.12	21.64	2043	1474
	DT6	124.8	8.14	105.7	24.26	15.74	13.87	1909	473.3
	DT7	367.3	7.65	7.81	19.09	24.68	26.13	2726	1340
	DT9	330.3	6.33	36.38	22	11.9	27.26	4011	3780
	DT10	334.6	6.48	60.54	29.13	14.4	26.13	2821	1332
	DT23	186.6	8.69	63.6	8.94	20.53	15.96	2320	900.1
Aug 27	DT3	416.2	94.24	19.56	36.17	21.8	33.37	2748	2576
	DT4	284.5	83.61	45.56	31.37	19.54	22.19	2063	1515
	DT6	133.5	136.6	157	40.13	27.75	13.59	1775	426.8
	DT7	368.3	139.5	24.21	32.67	29.55	25.1	2705	1302
	DT9	362.1	93.88	33.72	31.69	19.61	27.18	3326	2000
	DT10	353.7	101.3	44.95	32.35	21.88	25.01	2760	1297
	DT23	188.9	142.4	49.95	33.62	29.36	14	2016	853.1
Sept 22	DT3	283.2	97.21	29.8	1.33	38.46	19.98	1719	2245
	DT4	229.2	113.2	28.68	1.58	36.58	16.08	1542	1753
	DT7	280.1	116.6	19.13	1.21	36.71	20.07	2165	1541
	DT9	387.4	137.1	35.01	1.73	37.13	25.54	2468	1541
	DT10	297.2	134.2	32.45	1.55	36.59	21.81	2538	1758
	DT23	152.5	136.8	44.29	1.55	37.03	13.65	2003	1323
	average value	253.94	86.02	43.41	18.30	23.05	19.60	2232.29	1366.94

Table 2

Average Contents of Trace Elements in the Water at Different Times

	June 1	June 22	July 24	Aug 27	Sept 22
Li (ppb)	188.30	251.77	259.51	301.03	271.60
Cr (ppb)	67.15	125.31	7.23	113.08	122.52
Mn (ppb)	37.11	39.28	53.86	53.56	31.56
Cu (ppb)	13.13	22.42	18.06	34.00	1.49
As (ppb)	19.35	19.22	17.38	24.21	37.08
Rb (ppb)	15.12	19.46	20.94	22.92	19.52
Sr (ppb)	1648.71	2315.14	2617.57	2484.71	2072.50
Ba (ppb)	1040.14	1247.23	1476.20	1424.27	1693.50

4. Results and Discussion

Chemical Composition Characteristics in Produced Water

The selected eight elements were analyzed for their variation with time. Table shows the average value of each element at different times. Figure shows the variation of the selected elements over time. The contents of Li, Mn, Rb, and Sr in produced water have experienced a process of rising first and then decreasing with time; the contents of As and Ba experienced a process of rising volatility over time; the changes in the contents of Cr and Cu showed some anomalies. The content of Cr in the water sample dropped suddenly from 125 to 7 ppb on July 24, while the content of Cu dropped sharply in the water sample on Sept 22 from 34 to 1.5 ppb. It can be seen from the spider diagram that the Cr and Cu are abnormal (Figure ).

Figure 3

Li concentration of produced water changes with time (a); Cr concentration of produced water changes with time (b); Mn concentration of produced water changes with time (c); Cu concentration of produced water changes with time (d); As concentration of produced water changes with time (e); Rb concentration of produced water changes with time (f); Sr concentration of produced water changes with time (g); and Ba concentration of produced water changes with time (h).

Figure 4

Spider diagram of Cr concentration (a) and spider diagram of Cu concentration (b).

Correlation Analysis

Cluster analysis is a mathematical classification method in multivariate statistics, which regards the studied objects as in a multidimensional space. The density relationship and similarity between the objects are studied with mathematical methods to classify the objects reasonably.[35] The correlation between the elements is analyzed by SPSS software for R-type cluster analysis and correlation analysis. Li, Sr, and Rb are closely related and can be classified as the first class (Figure ). Furthermore, Li, Sr and Rb have a good positive correlation (Table and Figure ). The correlation coefficients between Li and Rb, Li and Sr, Sr and Rb are 0.972, 0.706, and 0.759, respectively. A very good positive correlation between Li and Rb, Li and Sr, Sr and Rb is very clearly seen from Figure a–c.

Figure 5

Cluster analysis diagram.

Table 3

Element Correlation Coefficientc

	Li	Cr	Mn	Cu	As	Rb	Sr	Ba
Li (ppb)	1	0.103	–0.282	0.344	0.049	0.972b	0.706b	0.522b
Cr (ppb)		1	–0.138	0.010	0.543b	–0.005	–0.075	–0.102
Mn (ppb)			1	0.185	–0.193	–0.235	0.024	–0.260
Cu (ppb)				1	–0.446a	0.397a	0.393a	0.043
As (ppb)					1	–0.076	–0.207	0.017
Rb (ppb)						1	0.759b	0.600b
Sr (ppb)							1	0.558b
Ba (ppb)								1

Correlation is significant on the 0.05 layer (bilateral).

Correlation is significant on the 0.01 layer (bilateral).

Bold values denote that the correlation coefficients are >0.7000.

Figure 6

Correlation diagram of Li and Rb (a); correlation diagram of Li and Sr (b); and correlation diagram of Rb and Sr (c).

Li, Rb, and Sr are all active metal elements and easily lose electrons in an aqueous solution to produce metal cations.[22] Li is mainly in the form of spodumene and lithium mica in nature. Most of it is produced in granite. It is a deep acidic igneous rock, and the weak alkaline environment can promote the dissolution of Li ore. The higher the pH, the more it promotes the dissolution of Li ore and increases the concentration of elements in the groundwater. Meanwhile, both the Rb element and the Li element are alkaline metal elements and their properties are similar to those of K. From the viewpoint of solubility, they are easily migrated by water and increased by the degree of mineralization. Rock is the source of Sr in natural water. The chemical composition of water affects the migration of Sr, and the concentration of Sr increases with the increase of mineralization. The three elements are collectively affected by the degree of mineralization. This may be the reason for a positive correlation between the concentrations of Li, Rb, and Sr.

Variation of Trace Elements Concentration with Coal Seam Depth

The burial depths of coal seams of DT9, DT7, DT10, DT3, DT4, DT6, and DT23 are 620, 670, 670, 700, 760, 860, and 940 m, respectively. The concentration of trace elements in the produced water of DT4 with a deeper burial depth is lower, while the concentration of trace elements in the produced water of DT9 with a shallower burial depth is higher (Table ). Li, Sr, and Rb have a negative correlation with the change of burial depth of coal seam, and the correlation is good. R2 is 0.79, 0.92, and 0.71, all exceeding 0.7 (Figure ).

Table 4

Variations of Drainage Elements in Coalbed Methane Wells with Different Coal Seam Depths

burial depth of coal seam (m)	760	700	670		620	860	940
well number	DT4	DT3	DT7	DT10	DT9	DT6	DT23
Li (ppb)	236.02	269.56	331.5	301.32	331.42	117.08	163.3
Rb (ppb)	17.78	21.26	23.57	22.04	24.91	12.72	13.5
Sr (ppb)	1701	2062.8	2531	2492.2	2883.2	1883	2003
grade	lower	medium	higher		highest	lowest

Figure 7

Relationship between the burial depth of the coal seam and the element content.

The formation of trace elements in groundwater is affected by the leaching, adsorption, redox, and water mixing. The concentration of trace elements is affected in many ways, such as the concentration of conventional ions in groundwater, pH, and the content of trace elements in the surrounding aqueous medium.

In Section , the concentrations of Li, Sr, and Rb are affected by the degree of TDS and pH. The ionic content and TDS in the water easily change with the groundwater migration. According to the acid–base balance principle

The pH is also affected by conventional ions in the water.[15,36] CBM wells are located at different structural locations, and the groundwater runoff conditions are different. As the groundwater runoff changes, the TDS and pH will be affected, resulting in changes in trace elements.

Variation of Gas/Water Production with Coal Seam Depth

There is a negative correlation between water production and gas production in CBM wells, gas production increases with the depth, and water production decreases with the depth.[22]

Comparing Figure a,b, it can be seen that gas production and water production generally show opposite trends and water production is low when gas production is high. DT6 and DT23 are located near the fault. The gas production and water production of wells near the fault and ordinary wells were linearly fitted separately. There is a certain correlation between gas production and the coal seam depth. The gas production increases with the increase of coal seam depth, but the gas production near the fault suddenly decreases (Figure b). The water production did not show a significant correlation with the depth of the coal seam and the fitting degree was low, which was different from Chen’s research in the Bide-Santang Basin, and the water production near the fault was large (Figure a). There is a strong hydraulic connection near the fault. The faults communicate with the upper and lower aquifers, and the groundwater recharge is strong, which leads to increase of water production of the CBM wells. Meanwhile, the CBM storage conditions near the fault are poor and the hydraulic transport will take away the CBM, resulting in low CBM production. The gas production of CBM wells generally accords with the law of increase with the increase in depth, but the impact of faults on CBM production is more important. The fault is the main controlling factor for water and gas production of the CBM well in the area that is near to the fault.

Figure 8

Relationship between water production and producing interval depth (a) and relationship between gas production and producing interval depth (b).

Variation of Gas Productivity with Trace Element Concentration

The trace element content in groundwater may be related to the daily production of CBM. As shown in Figure , there is a negative correlation between the daily gas production of most CBM wells (such as DT3, DT4, DT9, and DT10) and the contents of Sr, Li, and Rb in produced water. In contrast, the daily gas production of DT6 and DT23 showed a positive correlation with the contents of Sr, Li, and Rb in produced water.

Figure 9

Variation rule of gas production with Sr concentration in DT3 and DT4 (a); variation rule of gas production with Sr concentration in DT9 and DT10 (b); variation rule of gas production with Sr concentration in DT6 and DT23 (c); variation rule of gas production with Li concentration in DT3 and DT4 (d); variation rule of gas production with Li concentration in DT9 and DT10 (e); variation rule of gas production with Li concentration in DT6 and DT23 (f); variation rule of gas production with Rb concentration in DT3 and DT4 (g); variation rule of gas production with Rb concentration in DT9 and DT10 (h); and variation rule of gas production with Rb concentration in DT6 and DT23 (i).

The chemical composition of CBM produced water can reflect the runoff and alternation of groundwater, and hydraulic migration, hydraulic closure, and hydraulic closure affect the gas content of coal reservoirs.[37] DT3, DT4, DT9, and DT10 are located at the anticlinal wing and the core, while DT6 and DT23 are located near the fault. The hydraulic connection near the fault is strong, and the groundwater runoff is strong. The chemical characteristics of the coproduced water in the CBM wells near the fault show abnormality because the coproduced water from them may be mixed with nonreservoir water.[38] Therefore, the anomalies of DT6 and DT23 may be affected by faults. The concentration of elements at the fault is greatly affected by the supply of external water. When the contents of Sr, Li, and Rb increase, external water supply decreases and gas production increases.

Conclusions

Sr and Ba are the dominated trace elements in coproduced water in the CBM wells. The change of element content with time shows a certain rule except some abnormal elements. There is a positive correlation among Li, Sr, and Ba, which may be affected by the TDS and pH in the groundwater.

Lower contents of Li, Sr, and Ba exist in CBM wells with a shallow depth of the coal seam. The contents of Li, Sr, and Ba in CBM wells with a deeper burial depth of the coal seam are higher. There is a certain relationship between the content of trace elements and the depth of the coal seam. The groundwater runoff changes the TDS and pH, resulting in changes in the concentration of trace elements.

The gas production rate has a reverse trend with the water production rate. The gas production of CBM showed a positive correlation with the depth of the coal seam, but the wells near the fault showed an abnormality and the gas production decreased suddenly. There is no significant correlation between the water production and the depth of coal seams, but the water production in CBM wells near the faults is abnormally high. The gas production and water production of CBM wells are greatly affected by faults.

The productivity of CBM is affected by hydrogeological conditions and structure. The productivity of CBM wells in the anticline decreases with the increase of Li, Sr, and Ba contents. The productivity of CBM wells near the fault increases with the increase of the contents of Li, Sr, and Ba. The coproduced water in CBM wells near the fault is affected by the mixing of nonreservoir water, resulting in abnormal gas production.