The Quality And Quantity Dynamic Analysis Of Producing Water From Coalbed Methane Field

The coal bed methane industry has come into the commercial development stage in China, but there are still many problems to be solved in the process of exploration and development. The CBM wells producing water has the characteristics of high salinity and its directly emission would make huge harm to the environment. How to deal with the CBM producing water is the urgent problem during the“twelfth five-year plan”, and this paper will provide theoretical foundation.The work in this paper is based on the actual production condition in Qinshui basin and Erdos basin. 14 CBM wells of 3 CBM production areas in these two basins were chosen and the producing water of which were gathered continuously in 12 months. 25 kinds of physical and chemical analysis including temperature, chrominance, PH Value, transparence, suspended substance, salinity, iron content,etcetera were done to the water.The dynamic changing pattern of the water quality and water type were carried out using SPSS and AquaChem4.0 software based on the experimental analysis data and production time. The analysis results show that despite the absolute data of the water salinity and irons content is different, the changing tendency appears similarly from high to low, while the bicarbonate iron appears reversely from low to high, and the water types change from Na-Cl type to Na-Cl-HCO₃ type, Na-HCO₃-Cltype or Na-HCO₃ type.On the base of changing features of the iron content in the water, the water producing process was divided into 3 stages, pollution components defluxion stage, transition stage and stable stage. This paper concludes the water types, iron characteristics, stiff figures and lasting time, etc.The CBM wells are divided into high production water wells, moderate production water wells and low production water wells due to the water production. The most of the CBM wells in the studying process belong to moderate-low production water wells. The water production differences from stages and layers. For example, the water production of layer 15 is obviously higher than layer 3, and both of them become lower and lower with the increasing of the producing time.Based on the《Surface Water Quality standards》, the chloride is the main pollution component in the production water and exceeds this standard severely in the beginning phase. Base on the changing of Chloride ion concentration with time was investigated by statistical methods. The results show that the Chloride ion concentration far exceeds the state classâ…¡drinking water standards in the early drainage stage. Therefore, the produced water has the biggest influence on the environment in the early drainage stage. A mathematical model is established by nonlinear regression analysis using SPSS. This model can well forecast the time needed for the water to reach the state standards. Hence, it can provide technical support for the CBM produced water quality monitoring and processing on ground.This paper finds the reason for the phenomenon that high content of bicarbonate iron and low content of sulfate, calcium and magnesium iron in the production water. The appearance of low sulfate iron is because of desulfurization effect by desulfated bacteria and sulfate reduction effect involving soluble methane. In spite of the different geological stage of these two effects, the final results both lead to the decrease of the sulfate iron and the increase of bicarbonate iron in water. The main reason for the decrease of calcium and magnesium iron lies in iron exchange adsorption and the increase of the bicarbonate iron.