Geochemical characteristics of produced water from coalbed methane wells and its influence on productivity in Laochang Coalfield, China

2.1 Geological background

Laochang Block is located in Fuyuan County, Qujing City, on the southwestern margin of the Yangtze Platform and the inner Laochang anticline zone of the Huangnihe Reflection Arc on the Eastern wing of the second arc of the Shanzi structure in Yunnan Province. The faults in the block are mainly NE-trending, with NW-trending transverse faults and NW-trending arc faults, mainly distributed in the margin of the block. Most of the faults with a drop greater than 100 m are boundary faults, while the internal faults are scarce and mostly distributed near the folds (Figure 1). The main coal-bearing strata are the Upper Permian Longtan Formation with a thickness of 415–475 m. The main coal seams are No. 3, 7 + 8, 16, 17 + 18, and 23. The coal grade is anthracite, black to steel gray, with a glass-like metallic luster, heterogeneous fracture, hardness, and slight brittleness. Coal petrographic composition is mainly semi-dark to semi-bright coal. This area is located in the watershed of the Huangni River, Xijiuxi River, and Seyi River, with high terrain and an undeveloped surface water system.

Figure 1

Study area structure and well location map.

There are five CBM development test wells (Well Y-1, Y-2, Y-3, Y-4, and Y-5) that adopted the “segmented fracturing, combined layer drainage” developmental mode in the Laochang Block. The data of the cumulative water and gas production of the five wells by September 2018 are presented in Table 1. Well Y-3 produced most gas (77461.46 m³) during this period, followed by wells Y-5, Y-2, Y-1, and Y-4. Well Y-3 had the highest cumulative water production, at 474.91 m³, followed by wells Y-5, Y-2, Y-4, and Y-1.

2.2 Samples

Since April 2018, five CBM wells in the Laochang Block have been tracking and collecting water samples. The water sample was uniformly collected using a 2.5 L pure water bottle directly from the wellhead. The samples were sent to the Institute of Geochemistry, Guiyang Academy of Chinese Sciences within 72 h. The experiments conducted include the determination of the conventional anion and cation mass concentration of the produced water, the determination of stable hydrogen and oxygen isotopes, and the determination of trace element mass concentration. As of September 2018, well Y-1 had been collected six times, well Y-2 had been collected six times, well Y-3 had been collected five times, well Y-4 had been collected four times, and well Y-5 had been collected five times. These test results are the basis for this analysis (Tables 2 and 3).

3.1 Characteristics of conventional ions and variation with time

The water produced from CBM wells has similar ion characteristics, that is, Na⁺, K⁺, and Cl⁻ concentrations are higher, while Ca²⁺, Mg²⁺, and SO₄²⁻ concentrations are lower. It is generally believed that Ca²⁺ and Mg²⁺ are more abundant in an open groundwater environment, while Na⁺, Cl⁻, and HCO₃⁻ are more abundant in a closed groundwater environment [14,33,34]. The water produced from the five CBM wells shows the same result, that is, Na⁺, Cl⁻, and HCO₃⁻ concentrations are higher, K⁺, Ca²⁺, Mg²⁺, SO₄²⁻, and F⁻ concentrations are lower, and the SO₄²⁻ concentration of the Y-3 well and Y-5 well is the lowest (Table 2).

With the increase in the drainage time, the type of water produced from the Y-1 and Y-2 wells changed from Na–Cl to Na–Cl–HCO₃. The type of water produced from Y-3, Y-4, and Y-5 wells changed from Na–Cl to Na–HCO₃. The concentrations of K⁺, Na⁺, Ca²⁺, Mg²⁺, and Cl⁻ in the five wells fluctuated and decreased with time (Figure 2). The K⁺ concentration in the Y-2 well changed more with time, followed by the Y-1 well, and the changes in the Y-3, Y-4, and Y-5 wells were the smallest. The Na⁺ concentration of the five wells does not vary significantly with time. The variation trend with time of the Ca²⁺, Mg²⁺, and Cl⁻ concentrations in the five wells is similar to that of K⁺ concentration. The concentration of SO₄²⁻ in wells Y-1, Y-2, Y-3, and Y-5 showed a decreasing trend, while SO₄²⁻ concentration in the Y-1 and Y-2 wells varied greatly with time. SO₄²⁻ concentration in the Y-4 well increased initially and then decreased with time. The HCO₃⁻ and F⁻ concentrations in the five wells increased with time, among which the HCO₃⁻ concentration increased in a similar range. Meanwhile, the change in F⁻ concentration in the Y-1, Y-2, and Y-4 wells showed larger temporal variations, while the change in F⁻ concentration in the Y-3 and Y-5 wells depicted smaller temporal variations.

Figure 2

Ion concentration changes with time.

With the increase in drainage time, the fracturing fluid gradually discharges, which reduces the concentrations of Na⁺, K⁺, Cl⁻, Ca²⁺, and Mg²⁺, indicating the weakening of the water–rock interaction. The concentrations of HCO₃⁻ and F⁻ increase with time, showing that the produced water is groundwater in a closed environment with a higher degree of retention. This indicated that the water produced from the five wells is characterized by closed groundwater, and the water–rock interaction of the Y-3 and Y-5 wells is more stable than that of the Y-1, Y-2, and Y-4 wells, as shown in Figure 2.

3.2 Characteristics of hydrogen and oxygen isotopes and variation with time

The hydrogen and oxygen isotope values of the water produced in the CBM wells are distributed near the atmospheric precipitation line. The hydrogen and oxygen isotope composition research method adopt the Yunnan atmospheric precipitation line equation: δD = 6.56 δ¹⁸O − 2.96 [35].

Because the produced water is derived from the mixture of atmospheric precipitation, surface water, and groundwater, it generally exhibits significant D-drift characteristics or O-drift characteristics [24,36,37].

The water isotope values of the Y-1 and Y-3 wells are basically located to the left of the atmospheric precipitation line, showing D-drift characteristics (the Y-1 well exhibits O-drift characteristics on the right side of the atmospheric precipitation line in May), while the isotope values of the Y-2 and Y-4 wells are mostly located on the right side of the atmospheric precipitation line, showing O-drift characteristics. The isotope value of the water produced in the Y-5 well fluctuates on the atmospheric precipitation line and exhibits insignificant D-drift or O-drift characteristics (Figure 3).

Figure 3

δD, δ¹⁸O isotope relationship of the produced water.

In the reducing environment of coal measures, hydrogen and oxygen isotope drift in the formation water can be caused by isotope exchanges of minerals in the formation water rich in light isotopes such as H and ¹⁶O; coal seams rich in heavy isotopes such as D and ¹⁸O, and the wall rock. In addition, microorganisms can produce HDS in a weakly alkaline and closed reduced coal seam environment. HDS dissolves in water and exchanges isotopes, which can also lead to the D-drift of formation water. Because of the enrichment of ¹⁸O in carbonate strata, the oxygen-bearing mineral components in limestone are continuously dissolved in the process of runoff, which makes the heavier oxygen atoms in oxygen-bearing minerals easy to exchange with lighter oxygen atoms in limestone water, resulting in the continuous enrichment of ¹⁸O in roof limestone water and ¹⁸O isotope drift [25,26,27,28].

In the process of the fluid production, the following reactions may occur:

In the reduction environment of coal measures, sulfate can produce hydrogen sulfide, resulting in a decrease in SO₄²⁻ concentration, an increase in HCO₃⁻ concentration in the produced water, and the odor of hydrogen sulfide. It also reflects the fact that the produced water has the characteristics of coal seam water, equations (1) and (2). Because the produced water is weakly alkaline (Table 2), it can be inferred that there is a slight reaction of H⁺ consumption (equations (3) and (4)).

The δD and δ¹⁸O of the water produced in the Y-1 and Y-2 wells vary greatly with time, while those of the Y-3, Y-4, and Y-5 wells change slightly (Figure 4), indicating that the coal seams and surrounding rocks have great influence on the water produced in the Y-1 and Y-2 wells, but relatively small influence on the water produced in the Y-3, Y-4, and Y-5 wells. In August, δD and δ¹⁸O of the five wells decreased with time, probably due to seasonal precipitation. Even in September, δD and δ¹⁸O of the five wells decreased with time, but the decline was less than that observed in August. The sudden increase in δ¹⁸O of the Y-3 and Y-5 wells in September was probably due to the fluid penetration of the surrounding rock.

Figure 4

Temporal variations in δD, δ¹⁸O isotopes in the produced water.

3.3 Characteristics of trace elements and variation with time

In the previous studies, the contents of Li, Ba, Sr, Rb, and Mn on the roof and floor are much higher than those in the coal seam, and their changes can indicate the change in the degree of influence of the roof and floor on the produced water. As, Hg, and Co mainly exist in the coal seam, and particularly, the high amounts of As in coal is a unique characteristic of this region [38,39,40].

With the increase in the drainage time, the trace elements Li, Mn, and As in the produced water of the five wells show an upward trend, while Ba, Sr, and Rb show a downward trend, and their changing trends are approximately the same (Figure 5). The reason may be that the water produced daily is gradually stabilized, and the water–rock reaction tends to be stable. Among them, the trace elements Ba and As in the Y-3 well vary greatly with time, which may be due to the continuous influence of the surrounding rock and coal seam on the produced water.

Figure 5

Changes of trace elements in the produced water with time.

3.4 Effect of geochemical characteristics of produced water on gas well productivity

There has been considerable study on the impact on the production capacity of the production of water in the CBM wells [26,27]. Because Ca²⁺ and Mg²⁺ are the main elements of rock-forming minerals [41], and HCO₃⁻ and SO₄²⁻ mainly reflect the coal-bearing environment under closed conditions, based on previous studies and combined with the characteristics of the produced water and the changes in drainage and production curves in the study area, four characteristic ions – Ca²⁺, Mg²⁺, HCO₃⁻ and SO₄²⁻ – are selected to further analyze the relationship between the water production characteristics and productivity of the CBM wells. The degree of influence of the coal seam and surrounding rock on the produced water has dual effects on the gas production rate. Conversely, the degree of influence of the coal seam and surrounding rock on the produced water is greater, which can improve productivity. In addition, if it causes wellbore collapse or reservoir damage, it will be disadvantageous to productivity.

With the increase in the produced water in five CBM wells, δD and δ¹⁸O show a downward trend in general, reflecting that the influence of coal seams and surrounding rocks on produced water is weakening, as shown in Figure 4. The reason may be the influence of the fracturing fluid on the produced water in the early stages of drainage. That is, with the gradual production of the fracturing fluid, the produced water gradually returns to the normal formation state, but in September, the δ¹⁸O of the Y-3 and Y-5 wells suddenly increases. This suggests that the influence of the surrounding rock on the produced water is increasing because the surrounding rock is penetrated by the fluid.

According to the influence of trace elements on produced water, characteristic trace elements such as Li, Ba, Sr, Rb, Mn, and As were selected. By analyzing the standard deviation, the characterization parameters of the influence of the surrounding rock and coal seam on produced water were established.

Equation (5) denotes the discrete degree of the influence of the fracturing fluid on the surrounding rock, and the magnitude of the value indicates the fluctuation of the influence of the fracturing fluid on the surrounding rock, which can indirectly reflect the patency of the fluid passage. Equation (6) denotes the discrete degree of the influence of the fracturing fluid on the coal reservoir, and the magnitude of the value can indirectly reflect the extent of the fracturing fluid expansion in the reservoir.

The standard deviations σ_Y and σ_M can indirectly reflect the extent of the influence of the fracturing fluid on the reservoir and surrounding rock. When σ_Y is large, wellbore collapse can easily occur, which is not conducive to the production of fluids. When σ_M is large, damage to the coal reservoir can easily occur, which is unfavorable to later gas production.

The Y-3 and Y-5 wells have the best production status, as shown in Figures 4–8. Moreover, it also reflects that the process that takes place in the Y-3 and Y-5 wells (ellipse and triangular areas in Figure 6) in the flow through the surrounding rocks, and coal seams is beneficial for enhancing productivity. However, the water produced in the Y-1, Y-2, and Y-4 wells in the process of flowing through surrounding rock and coal seam is not conducive to enhance productivity, which may be caused by wellbore collapse or reservoir damage [42,43]. Based on the characteristics of water produced in Y-3 and Y-5 wells with better drainage effect, the quantitative characterization range of produced water in high-production CBM wells is proposed: (1) δD is in the range of −85‰ to −75‰ with the fluctuation range spanning 5‰, δ¹⁸O is in the range of −12‰ to −11‰ with the fluctuation range spanning 0.5‰, and the fluctuation range of Ca²⁺ spans 10 mg/L. The fluctuation range of Mg²⁺ spans 4 mg/L, the fluctuation range of SO₄²⁻ spans 3 mg/L, and the fluctuation range of HCO₃⁻ spans 1,000 mg/L. (2) 0 ≤ σ_M < 0.3 and 0 ≤ σ_Y < 600 (the influence of the surrounding rock and coal seam on produced water is relatively stable, taking the Y-5 well as an example, and the poor drainage effect of the Y-4 well is likely to be caused by human factors) or 0.7 < σ_M < 0.8 and 1,200 < σ_Y < 1,300 (the influence of the surrounding rock and coal seam on produced water fluctuates greatly, taking the Y-3 well as an example). Because the tracking time is not long and the number of CBM wells is limited, the characterization method proposed earlier will be further improved by more drainage data from the drainage wells.

Figure 6

Distribution of characteristic ions in the produced water.

Figure 7

Standard deviation of trace elements in five wells in the study area (note: σ_Y denotes the discrete degree of the influence of the fracturing fluid on the surrounding rock; σ_M denotes the discrete degree of the influence of the fracturing fluid on the coal reservoir).

Figure 8

Gas production variation charts of five wells in the study area.