(1. School of Energy, China Geo University, Beijing 100083 2. China Xinjiang Tuha Oilfield Company Hami)

The water produced by coalbed methane wells in Liulin block of Hedong coalfield has the characteristics of high salinity, and direct discharge may cause damage to the ecological environment. The study on the characteristics and dynamic change law of produced water from coalbed methane wells will provide a basis for formulating reasonable and effective produced water treatment technology. By continuously tracking and collecting coalbed methane well water samples in Liulin block and systematically testing 25 physical and chemical parameters, it is found that the salinity, chloride ion and sodium ion concentrations all show a trend of first high and then low, while the change law of bicarbonate ion is opposite, showing the characteristics of first low and then high. The types of water have changed from NaCl water type to NaClHCO3 water type and NaHCO3Cl water type. The cations in the produced water are mainly K++Na+ ions, while the values of anions HCO-3 and Cl- are close, and there is no obvious dominant ion. According to the change characteristics of ions in the produced water, the ions in the water are quantitatively analyzed. Through regression analysis, the dynamic change model of chloride ion concentration was established.

Keywords: dynamic change of water quality of produced water from coalbed methane wells Liulin block

Fund Project: National Major Science and Technology Project (201kloc-0/zx05034003); National Natural Science Foundation Project (40972108); National 973 Plan Project (2009CB2 19604)

About the author: Tang Shuheng, 1965, born in Zhengding, Hebei Province, is a doctor, professor and doctoral supervisor, engaged in the research of coalbed methane geology and development engineering. Tel: 0 10 8232060 1, email: tangsh @ cugb.edu.cn.

Water production characteristics and dynamic changes of coalbed methane wells in Liulin block

Tang Shuheng 1, Zhu Weiping 2, Li Zhongcheng 1, Lv Jianwei 1, Chen Jiang 1, Guo Dongxin 1

(1. School of Energy, China Geo University, Beijing, 100083 2. Tuha Oilfield Company of China Petroleum and Natural Gas Group Company, Hami, Xinjiang, 839009.

Abstract: The groundwater produced by coalbed methane wells in Liulin block of Hedong coalfield has the characteristics of high salinity, and direct discharge will cause environmental deterioration. The study on the characteristics and dynamic changes of produced water from coalbed methane wells will provide a basis for establishing reasonable and effective produced water treatment technology. The dynamic changes of salinity and various anions and cations are analyzed through continuous tracking collection of water samples produced by coalbed methane wells and systematic testing of 25 physical and chemical parameters. The results show that the concentration of salinity, chloride ion and sodium ion changes from high to low, while that of bicarbonate ion changes from low to high. Water changed from Na Cl type to Na Cl HCO3 type and NaHCO3 type. The cations are mainly K+ and Na+, and the produced water has no obvious dominant anions with similar HCO 3- and Cl- values. According to the change of ions, a model of chloride ion concentration in water is established by nonlinear regression analysis.

Key words: coalbed methane well; Produced water; Water quality; Dynamic change; Liulin block

Geological background of 1

Liulin Block in Hedong Coalfield covers an area of 183.824km2, which is located in the west of Shanxi Province, in the middle of Hedong Coalfield, 220km away from Taiyuan City, and belongs to Liulin County, Lvliang City. This area belongs to the northwest loess plateau, with the overall topography of high in the north and low in the south, belonging to the Yellow River basin. From the structural position, this block is located in the north-south structural belt of the western slope of Lvliang in the eastern margin of Ordos Basin, and it is generally a monoclinic structure inclined to the west, belonging to a part of the western wing of anticlinorium, Lvliang. There are few faults in this area, and the secondary folds are generally small, mainly monoclinic structures.

The strata in and around the block are the Middle Ordovician Fengfeng Formation (O2f); Middle Carboniferous benxi formation (C2b) and Upper Carboniferous Taiyuan Formation (C3T); Shanxi Formation of Lower Permian (P 1s) and Xiashihezi Formation (P1x); Upper Permian Upper Shihezi Formation (P2s) and Shiqianfeng Formation (P2SH); Lower Triassic liujiagou formation (T 1l) and heshanggou formation (t1h); Cenozoic (N2) Neogene Pliocene; Quaternary Middle Pleistocene (Q2), Upper Pleistocene (Q3) and Holocene (Q4).

At present, coalbed methane in No.3+4 and No.5 coal seams of Shanxi Formation of Lower Permian (P 1s) is mainly mined in this area (Zhou Baoyan et al., 2007; Ming Xun, 2065438+00; Zhao Qingbo et al., 2006; Liu Xinshe et al., 2007; Fu et al., 2007).

2 produced water ion change law

The water produced by coalbed methane wells in Liulin block of Hedong coalfield has the characteristics of high salinity, and direct discharge may cause damage to the ecological environment. The study on the characteristics and dynamic changes of produced water from coalbed methane wells will provide a basis for establishing reasonable and effective produced water treatment technology. In this paper, two horizontal wells (MC0 1V and MC00-2V) and three vertical wells (MC03, MC04 and MC05) were selected as continuous tracking sampling objects, and 25 samples were collected, including salinity, potassium and sodium ions, calcium ions, magnesium ions, chloride ions, bicarbonate ions and sulfate ions.

2. 1 salinity variation law

According to the salinity data obtained from laboratory tests, the salinity value in MC test area is generally high, with the highest value of 5 wells approaching 7000mg/L at the initial stage of production discharge, the lowest value approaching 2900mg/L, and the average value being about 4980mg/L (see table 1).

Of the five CBM wells in MC test area, three vertical wells (MC03, MC04 and MC05) are in the early stage of post-fracturing production, and two horizontal wells (MC0 1V, MC02V) start sampling after 2-3 months of post-fracturing production. The results of water sample analysis show that the salinity of produced water in MC test area is high at the initial stage of drainage, and the salinity value decreases obviously in a short time. The salinity value of MC0 1V was greater than 3500mg/L on the 70th day of drainage, decreased to 2400mg/L on the 90th day of drainage, and then basically stabilized at160th day. The salinity value of well MC02V has remained relatively stable since sampling (Figure 1).

2.2 chloride ion change law

It can be seen from the variation trend diagram of chloride ion in the produced water of coalbed methane wells in MC test area (Figure 2) that the variation trend of chloride ion concentration measured in MC test area is different due to different drainage time. Three vertical wells, MC03, MC04 and MC05, began to sample at the initial stage of drainage, and the chloride ion concentration was high at the initial stage, and then showed a rapid decline trend. Two horizontal wells, MC0 1V and MC02V, began to take samples after drainage for 70 days and 1 18 days respectively. During the sampling period, the chloride ion changes smoothly, and the concentration value is obviously lower than that of vertical wells, with most values ranging from 300 to 400 mg/L.

Table 1 MC Statistical Table of produced water salinity of coalbed methane wells in experimental area

Figure 1 MC experimental area: change trend of salinity of produced water from coalbed methane wells.

Fig. 2 Variation trend of chloride ions in produced water of coalbed methane wells in MC test area

2.3 HCO-3 change method

From the variation trend diagram of HCO-3 in the produced water of coalbed methane wells in MC test area (Figure 3), it can be found that the HCO-3 ion concentration of three vertical wells, MC-03, MC-04 and MC-05, is relatively high at the initial stage of drainage, basically between 1400 ~ 1900 mg/L, and then it is in the process of slowly rising. However, the HCO-3 ion concentrations in two horizontal wells MC-0 1V and MC-02V are lower than those in three vertical wells. After drainage 100 days, the concentration of HCO-3 ion increased slowly, but the value changed smoothly, all within the range of1000 ~1500mg/L.

Fig. 3 variation trend of HCO _ 3 produced water from coalbed methane well in MC test area

2.4 sodium ion change law

It can be seen from the variation trend diagram of Na+ in the produced water of coalbed methane wells in MC test area (Figure 4) that the concentration of Na+ in three vertical wells, MC-03, MC-04 and MC-05, was relatively high at the initial stage of drainage, ranging from 2,000 to 2,500 mg/L, and then showed a slow downward trend. The decline amplitude of well MC04 is the most obvious, while the numerical changes of Na+ ions in two horizontal wells MC-0 1V and MC-02V are stable in the range of 700 ~ 800 mg/L after drainage 100 days.

Fig. 4 Variation trend of sodium ions in produced water of coalbed methane wells in MC test area

2.5 other ions change law

Other ions are mainly calcium, magnesium and sulfate ions. The concentration of calcium and magnesium ions obtained from five wells is low, generally less than12 mg/l; The concentration of sulfate ions is usually less than 40 mg/L.

3 dynamic change law of water body type

The water type of two horizontal wells MC-0 1V and MC-02V in MC test area is NaHCO3Cl. In 3 vertical wells, MC-03 and MC-05 are Na-Cl water type in the early stage and Na-Cl-HCO3 water type in the later stage; Well MC-04 is of Na-Cl water type in the early stage and Na-HCO3-Cl water type in the later stage. The cation in the produced water in MC test area is mainly K++Na+ ion, while the values of anions HCO-3 and Cl- are close, and there is no obvious dominant ion (Figure 5).

Fig. 5 Piper three-line diagram of MC test area

Stiff diagram can be used to show the overall change of water type with drainage time in MC test area (Figure 6). At the initial stage of drainage, the water type in MC test area is NaCl water type, and the stiffness diagram is funnel-shaped with the upper width and the lower width, as shown in Figure 6(a). The duration of this stage is generally 4 ~ 6 months. In the middle stage of drainage and production, the water body type in MC test area is NaClHCO3 water type or NaHCO3-Cl water type, and the stiffness diagram is shown in Figure 6(b, c), which lasts about 6 ~ 8 months. In the middle and late stage of drainage and production, the water type is mainly NaHCO3 water type or Na-Cl-HCO3 water type, and the stiff type is shown in Figure 6(d).

Fig. 6 Rigidity diagram of horizontal average chemical composition of coalbed methane wells in MC test area

4 chloride ion concentration drainage and production dynamic change model

According to the water quality test, due to the influence of drilling fluid (Li Zhongcheng et al., 20 1 1), the produced water of coalbed methane well in Liulin block has high salinity and salinity at the initial stage of mining, but it does not contain cations such as iron, manganese, copper, zinc, arsenic and nitrate which are strictly restricted in the national Class II drinking water standard (surface water environmental quality standard, 2002) (Table 2 Only the concentration of chloride (calculated by Cl-) is much higher than the national Class II drinking water standard at the initial stage of drainage, and decreases with the increase of drainage time. See Figure 2 for the change trend of chloride in the produced water of coalbed methane wells in Liulin block.

Table 2 Requirements of National Class II Drinking Water Standards for Major Ions

It can be found that with the increase of drainage time, the chloride ion concentration in the produced water of coalbed methane well is much higher than the national second-class drinking water standard at the initial stage, gradually decreases and approaches the second-class drinking water standard, and finally completely reaches the second-class drinking water standard. It is of great significance to establish the relationship model between chloride ion concentration of produced water and drainage time for the treatment of produced water from coalbed methane wells. In this paper, the chloride ion concentration is analyzed by regression analysis.

In the application of regression analysis, Newton iteration method is generally used to solve this normal equation group. It is also possible to directly minimize the sum of squares of residuals and obtain the nonlinear least squares estimation of unknown parameters. In practical application, it can be directly calculated by SPSS software (Pan et al., 2002).

Fig. 7 Relationship between Chloride Ion Concentration and Time in Well MC-04

Taking MC-04 well as an example, the function calculation and model establishment are carried out by SPSS software. The model for preliminarily judging the change of chloride ion concentration is f(x)=b0bx 1, the initial values b0= 1240 and b 1=0.95 are given randomly, and converge after 13 iterations. From the calculation results (Figure 7), the correlation coefficient R2 of the model is 0.958, which shows that the nonlinear regression fitting effect is good. Therefore, the change model of chloride concentration (calculated by Cl) in this well can be obtained as follows: f (x) = B0BX1=1774.6 * 0.991X.

Similarly, the simulation data of two other wells can be calculated, and the relevant parameters of the model are shown in Table 3.

Table 3 Parameters of Chloride Ion Concentration Drainage Dynamic Model in Liulin Block

According to the above mathematical model, it is possible to predict the time required for the water quality produced by coalbed methane wells to reach the national standard (Table 3). Before this time point, the produced water must be treated before it can be discharged, otherwise it will pollute the surrounding environment and surface water sources.

5 conclusion

Based on the systematic sampling analysis of the produced water from coalbed methane wells in Liulin block of Hedong coalfield, the variation law of the produced water quality is studied, and the following main understandings and achievements are obtained:

1) The salinity, chloride ion and sodium ion concentration of the produced water of each coalbed methane well show a changing trend from high to low with the increase of drainage time, while the change law of bicarbonate ion is opposite, showing the characteristics of first low and then high.

2) The water quality type of produced water from coalbed methane wells is changed from Na-Cl water type to Na-Cl-HCO3 water type, Na-HCO3-Cl water type or NaH-CO3 water type.

3) The nonlinear regression analysis model of chloride ion concentration change and drainage time is established. Using this model, the time required for chloride ion concentration to reach the national second-class drinking water standard can be well predicted, which provides a reference for the technical treatment of produced water from coalbed methane wells.

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