Mechanism of Wettability Alteration to Unlock Adsorbed-State Methane in Deep Coal Rock: Insight from Experiment and Molecular Simulation

Deep coalbed methane, as a new type of unconventional natural gas resource, has been successfully developed through massive hydraulic fracturing. However, the difficulty in desorbing bound CH 4 in the nanopores is the key bottleneck restricting the recovery improvement. This study analyzes a multiscale mechanism by which surfactant-based fracturing fluids enhance methane desorption. The imbibition fundamentally dissolves the narrow necks of ink-bottle pores, transforming them into slit-shaped geometries to reconstruct the nanopore structure. This leads to a 59.6% increase in nanopore (0.3–1.5 nm) scale and a 44.8% rise in larger micropore (2–10 nm). Four types of dodecyl-based surfactant fracturing fluids can significantly achieve a wettability alteration from hydrophobic to hydrophilic, and the CMC range is between 0.15% and 0.2%. SDBS at a concentration of 0.2% can reduce the contact angle from 92.48° to 42.76°, driven by the rigid benzene ring, enabling strong π–π interactions with the coal aromatic surface. NMR analysis confirms that SDBS treatment enables the aqueous phase to progressively invade the nanopore network ( T 2 < 1 ms), significantly expanding the water-adsorbed methane interface, which is directly evidenced by CH 4 isotherms comparatively showing a weakened CH 4 -coal affinity and a 57% reduction in hysteresis area. Molecular simulation indicates that the density of the adsorbed CH 4 on the coal surface decreases, while the density of free CH 4 in the slit increases simultaneously, verifying the wetting-induced desorption mechanism. The diffusion coefficient of CH 4 rises to 3.86 Å 2 /ps (SDBS system), indicating that its transferability is significantly enhanced, facilitating efficient CH 4 transport to the channel. The C N of CH 4 -coal decreases from 16.2 to 12.3 (a 24% decrease), while the C N of H 2 O-coal jumps from 0 to 9.6. The first peak intensity of the RDF of H 2 O-coal significantly increases, and the RDF of CH 4 -coal collapses from a double peak to a single peak. This indicates that the dense multilayer adsorbed water film rapidly forms and remains stable, competing with CH 4 for adsorption sites, which significantly weakens the interaction energy between CH 4 and coal, reduces the activation energy from 1500 kJ/mol to approximately 800 kJ/mol, and decomposes the single high-energy barrier desorption into multiple low-energy barriers. This research provides a solid theoretical basis for surfactants promoting CH 4 desorption and improving the coal rock gas recovery.