The extraction of methane from deep coal seams represents a significant opportunity in the global energy landscape, offering a cleaner alternative to conventional coal utilization while tapping into substantial reserves of natural gas [1,2]. However, unlike conventional gas reservoirs, coal seam methane (CSM) extraction presents unique challenges due to the complex nature of coal as a dual-porosity, naturally fractured system with strong sorption characteristics [3]. Deep coal seams, typically located at depths exceeding 1000 m, introduce additional complexities including high in-situ stresses, elevated temperatures, and complex fluid-rock interactions that significantly impact the efficiency of methane recovery. These deep formations are characterized by extreme pressure regimes (often exceeding 20 MPa) and temperatures above 80 °C that dramatically alter the behavior of fracturing fluids and their interactions with the coal matrix [4,5].

While previous reviews have examined coal seam methane extraction technologies, this work uniquely focuses on the fundamental colloidal and interfacial phenomena at the pore scale under deep coal seam conditions. We specifically address three critical knowledge gaps: (1) the behavior of methane-water interfaces under extreme confinement and HPHT conditions characteristic of deep formations, (2) the stability mechanisms of foam-based fracturing fluids in the presence of coal fines and high salinity, and (3) the colloidal interactions governing proppant transport in coal's hierarchical pore structure.

Hydraulic fracturing has emerged as a critical technology for enhancing coal seam permeability and facilitating methane production [6]. The process involves injecting fluids under high pressure to create and propagate fractures within the coal matrix, thereby establishing conductive pathways for methane migration [7]. Despite its widespread application, fundamental understanding of the physicochemical processes occurring at the pore scale during fracturing operations in coal seams remains incomplete, particularly regarding interfacial phenomena, colloidal behavior of fracturing support media, and transport mechanisms [[2], [8]]. The stress-dependent permeability of coal, coupled with its propensity for matrix swelling upon contact with fracturing fluids, adds layers of complexity not encountered in conventional reservoirs [3,4].

Coal seams present a distinctive environment for fluid-fluid and fluid-solid interactions due to their heterogeneous structure comprising micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm), coupled with a natural fracture network (cleats) [9]. This hierarchical pore system hosts complex interfacial processes that govern the storage, transport, and production of methane [10]. The methane-water interface in coal pores exhibits behavior that deviates significantly from bulk-phase expectations due to confinement effects, surface heterogeneity, and the presence of various mineral constituents [11,12]. Additionally, the high-pressure, high-temperature (HPHT) conditions of deep coal seams fundamentally alter these interfacial dynamics, affecting phase behavior, diffusion rates, and adsorption-desorption kinetics [11]. Understanding these interfacial phenomena is crucial for optimizing fracturing fluid formulations and predicting methane recovery efficiency.

The colloidal science of fracturing fluids represents another critical dimension in deep coal seam methane development. Conventional water-based fracturing fluids often lead to coal swelling, permeability reduction, and formation damage due to water-coal interactions [13]. This swelling phenomenon is particularly pronounced in deep coal seams where the matrix strain response to fluid invasion is magnified by in-situ stress conditions, resulting in significant reductions in fracture aperture and diminished long-term production rates [13,14]. Consequently, there has been growing interest in foam-based fracturing fluids, which offer advantages including lower water consumption, superior proppant-carrying capabilities, and reduced formation damage [14,15]. As demonstrated by Zhou et al. [15], foam stability under reservoir conditions depends on complex mechanisms including liquid drainage, coarsening, and bubble coalescence. These mechanisms become increasingly challenging to manage under the extreme HPHT conditions of deep coal seams, where conventional foam stabilizers often prove inadequate [[15], [16]]. These mechanisms are further complicated in the presence of coal particles, which can either stabilize or destabilize foam structures depending on their surface properties and concentration [16].

Recent advances in fracturing technology have seen the introduction of nanoparticles, surfactants, and low-salinity water as potential enhancers of methane recovery [17,18]. Nanoparticles can alter the wettability of coal surfaces, reduce interfacial tension, and stabilize foam-based fracturing fluids [19]. Surfactants play a crucial role in modifying the methane-water interface and enhancing the displacement efficiency of water from coal micropores. Specialized surfactant formulations designed for HPHT environments can not only stabilize fracturing fluids but also mitigate coal swelling by altering the electrochemical interactions between water molecules and the coal matrix [20]. Meanwhile, low-salinity water has shown promise in altering the electrical double layer at coal-water interfaces, potentially improving methane desorption and transport [21]. Despite these advances, a comprehensive understanding of the synergistic effects between these components in the complex coal seam environment remains elusive.

Transport phenomena in fractured coal seams involve multiphase flow through a hierarchical pore network where capillary, gravitational, and viscous forces compete to determine fluid distribution and mobility [22]. The dual-porosity nature of coal, characterized by matrix blocks separated by natural and induced fractures, creates a complex environment for fluid transport [23]. This complexity is compounded in deep coal seams where the effective stress state continually evolves as gas desorption, matrix shrinkage, and swelling processes occur simultaneously throughout production [23,24]. The matrix blocks primarily serve as storage sites for adsorbed methane, while fractures provide the main conduits for fluid flow [24]. The exchange of methane between these domains is governed by desorption, diffusion, and advection processes that operate across multiple spatial and temporal scales. The strain-dependent permeability of coal cleats and induced fractures presents particular challenges for maintaining long-term conductivity in deep reservoirs, where closure stresses are substantially higher and matrix swelling effects more pronounced than in shallower formations [25,26]. Understanding these transport mechanisms at the pore scale is essential for predicting and enhancing methane recovery from deep coal seams.

This review aims to provide an in-depth analysis of the fundamental physical and chemical processes occurring at the pore scale during fracturing operations in deep coal seams. We focus particularly on three interconnected aspects: (1) the methane-water interface and its behavior under varying reservoir conditions, (2) the colloidal behavior of fracturing support media including foams, nanoparticles, and surfactants, and (3) the transport mechanisms governing multiphase flow in the fractured coal system. Throughout, we emphasize the distinctive HPHT conditions of deep coal seams and the unique swelling and strain characteristics that distinguish coal from conventional reservoir rocks. By synthesizing recent advances in experimental techniques, theoretical frameworks, and field observations, we seek to bridge the gap between molecular-level understanding and macroscopic fracturing performance in deep coal seam methane extraction.

The insights presented in this review have significant implications for optimizing fracturing fluid design, improving proppant placement strategies, enhancing methane recovery efficiency, and minimizing environmental impacts associated with deep coal seam development. Furthermore, by elucidating the fundamental mechanisms underlying fracturing processes, this review contributes to the broader scientific understanding of multiphase flow, interfacial phenomena, and transport in complex porous media (Fig. 1, Fig. 2, Fig. 3, Fig. 4).