Geochemical Characteristics and Productivity Evaluation of Water Produced from Coalbed Methane Wells in the Laochang Block, Eastern Yunnan.

Abstract

The water produced from coalbed methane (CBM) wells contains abundant hydrogeochemical information, which is of great significance for the productivity evaluation of CBM wells. Based on the analysis of the conventional ions, hydrogen and oxygen isotopes, and trace elements in the water produced from three CBM wells in the Laochang Block, eastern Yunnan, the geochemical characteristics of the water produced from three CBM wells and their impacts on productivity are analyzed. The results show that the conventional ions in the water produced from all the three CBM wells in the study area exhibit similar characteristics, with low concentrations of Ca2+, Mg2+, SO42-, and F-, medium concentrations of K+, and high concentrations of Na+, Cl-, and HCO3-. As the drainage time increased, the water produced from wells S-2 and S-3 changed from the Na-Cl-HCO3 type to the Na-HCO3 type, while the water produced from well S-1 remained the Na-Cl-HCO3 type. The concentrations of HCO3- and F- are roughly positively correlated with the gas production, with higher gas production at concentrations of approximately 2000 and 2 mg/L, respectively. The gas production from well S-1 is relatively low, and it is speculated that this situation is caused by reservoir damage. The gas production from well S-2 is the lowest among the three wells in the study area, and it is speculated that the cause of this phenomenon is roughly due to breakage of the sandstone at the top of the coal seam or collapse of the wellbore, which communicates with the aquifer. Based on the characteristics of the water production from well S-3 with a good gas production, a quantitative characterization range of trace element changes in the water production from CBM wells is proposed. σY values of around 1500 μg/L and σM values of 0.4-0.5 μg/L are beneficial for the long-term gas production.