Geochemical Characteristics and Development Significances of Constant and Trace Elements from Coalbed Methane Co-Produced Water: A Case Study of the Shizhuangnan Block, the Southern Qinshui Basin

Abstract

As an unconventional natural gas resource, coalbed methane (CBM) development releases a large amount of CBM wells co-produced water. Geochemical characteristics of the co-produced water provide an essential foundation for the production dynamics of CBM reservoirs if the impacts of fracturing fluids and other aquifers can be ignored. In the Shizhuangnan Block of the southern Qins-hui Basin, constant and trace elements in CBM co-produced water from the wellheads were collected and determined, which is applied to assess water source, fracturing fluid effect, and CBM production. Based on principle component analysis and hierarchical clustering analysis, the water samples are divided into four categories. It suggests that different characteristics affected by water-rock interaction, reservoir environment, aquifer recharge, and hydraulic fracturing result in the various ratios of Na⁺/Cl⁻, alkalinity ($\text{Extra close brace or missing open brace}$)/Cl⁻ and other specific rules. Moreover, Cl⁻ is selected as a dividing line for complete fracturing fluid flow back, associated with organic-bound chlorine complexes in the original coal seam water. Compared to constant elements, there is a significant correlation between Li and Sr concentrations and CBM productivity, so templates regarding trace elements can be used to distinguish various sources of the co-produced water.

Keywords

coalbad methane / CBM co-produced water / hydraulic fracturing / Qinshui Basin

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