Hydrochemical Response Mechanisms and Identification Methodologies for Exogenous Water Interference in Coalbed Methane Wells

Abstract

Exogenous water interference is a critical bottleneck restricting the enhancement of individual well productivity and the achievement of large-scale stable production in China’s coalbed methane (CBM) industry. Focusing on the 3# coal seam in Block Z of the southern Qinshui Basin, this study systematically analyzes hydrogeochemical characteristics and their evolutionary mechanisms by integrating hydrogeochemical analysis with production indicator curves. An identification chart of exogenous water interference was built. K-means clustering and Principal Component Analysis (PCA) show that the hydrogeological system exhibits 4 evolutionary behaviors: (i) a stagnant zone dominated by long-term isolation and cation exchange; (ii) a weak runoff zone driven by sulfate reduction and organic matter degradation; (iii) a strong runoff zone characterized by frequent hydrodynamic exchange and poor sealing; and (iv) a recharge zone communicated by faults or fracture systems. Major ion ratios also reveal significant differences in the hydrogeochemical regimes, which imply that the hydrogeology is not only the main controlling factor for determining CBM production, but also the main driver of differentiated responses during the drainage process. Hydrogeological features also correlate highly with the morphology of production indicator curves: curves in the well-sealed stagnant zone exhibit “downward convex” profiles of high efficiency gas production; while in open sites with active hydraulic connections continuous exogenous water recharge displays an “upward concave” profile of low efficiency. The functional relationship between the production indicator curves, gas production, and the stagnation point on the curvature curve was fitted to construct an identification chart. Through this chart, quantitative measures regarding both the inhibitory degree of exogenous water interference and the CBM production efficiency can be effectively evaluated at an individual well scale. This study elucidates the regulatory impact of the hydrogeochemical environment on development potential, providing theoretical support for selecting CBM enrichment areas and formulating differentiated drainage management strategies.