Effect of temperature on coal maceral dissolution and matrix alteration during NaClO stimulation: Insights from microscopy and Micro-CT

Oxidant stimulation has emerged as a promising supplementary approach to enhance gas recovery from coal seams, targeting coal organic matter more directly than acid stimulation, which affects mineral components. However, the reactivity of different coal maceral subtypes to oxidants, particularly under reservoir conditions, remains poorly understood. In this study, sodium hypochlorite (NaClO) was used to investigate the maceral-specific reactivity and matrix alteration of bituminous coal samples from the Bowen Basin, Australia, under varying temperature conditions. Microscopic and Micro-CT analyses revealed that higher temperatures significantly enhanced maceral dissolution. The observed reactivity followed the order: semifusinite > vitrinite > fusinite, with collodetrinite within vitrinite displaying notably higher reactivity than collotelinite. These divergences should be ascribed to the differences in aromaticity, functional group composition, and pore structure. Micro-CT results revealed maceral-dependent dissolution depths, ranging from 300 to 500 μm for semifusinite, ∼100 μm for fusinite, and 50–150 μm for vitrinite bands. As a fine-grained heterogeneous host to telovitrinite macerals and other detrital components, coal matrix exhibited strong alteration in directional dependence, with significantly enhanced pore and fracture development along the horizontal (bedding plane) direction. At 60 °C, the reaction depth reached up to 850 μm, accompanied by substantial pore enlargement, particularly within the 30–50 μm range. Mixed maceral zones showed deeper reaction fronts than vitrinite-rich areas, emphasizing the influence of maceral heterogeneity on oxidant penetration. This study highlights that both maceral composition and bedding structure critically govern the extent, directionality, and effectiveness of oxidant-induced matrix alteration. These findings provide important insights into optimizing oxidant stimulation strategies for improved gas recovery in heterogeneous coal reservoirs.