Multi-scale characterization of the influence mechanism of coal porous media structure on coal seam water injection

Abstract

With increasing depths in coal mining, water injection into coal seams has become a crucial technology to enhance coalbed methane recovery, suppress dust, and prevent gas outbursts under high stress and geothermal conditions. Given this background, this study aims to elucidate the dynamic behavior of water injection, migration and evaporation in coal. To investigate pore–fracture connectivity and moisture transport mechanisms, coal samples drilled and collected from a coal mine in Inner Mongolia, China, were analyzed using a combination of multi-scale characterization techniques, enabling the characterization of the relationship between pore structure connectivity and water transport behavior. NMR, CT, and MIP were jointly utilized to explore the pore structure at different scales, and NMR was further used for real-time online water injection monitoring, allowing the dynamic observation of water migration and distribution within the samples. The results indicate that borehole drilling significantly enhances permeability and connected porosity, controlling both injection rate and maximum water uptake. Moreover, water transport is governed by pore structure at different scales, with fine pores influencing adsorption and larger pores facilitating flow. Evaporation under elevated temperatures proceeds from rapid free-water loss to slower bound-water desorption, with borehole presence having a minimal effect. These findings provide key insights into the multi-scale control of water dynamics and offer a theoretical and experimental basis for optimizing coalbed water injection strategies in deep, high-temperature environments.