AFM characterization of physical properties in coal adsorbed with different cations induced by electric pulse fracturing

Pore system in coal is highly heterogeneous, while it is the main occurrence space and transport channel for coalbed methane (CBM). Electric pulse fracturing (EPF) has been considered as an effective approach to improve the coal physical properties for better CBM production. In this work, based on AFM measurement of 40 coal samples collected from Qinshui Basin, we evaluated the physical properties of coal, adsorbed with different cations, after EPF. This was accomplished by first analyzing the breakdown field response process of coal adsorbed with different cations, and then determining the dynamic changes of pore and surface roughness using the watershed method. In the PeakForce QNM (TM) model, the heterogeneous distribution of the adhesion force with increasing cation valence was quantified. The results show that the ability of medium- and high-rank coals to adsorb cations exhibits in the order of Fe3+ > Ca2+ > K+, attributed to the decrease in the radius of hydrated ions with increasing ionic valence. When the electric field is applied to both ends of coal sample, the plasma channels in coal generate huge energy, resulting in the temperature rise of pore-fracture and throat. Affected by EPF, the porosity of Chengzhuang (CZ) increases from 4.1% to 27.4%, greater than that of Qiyi (QY) from 6.7% to 14.5%. In the surface morphology tests, the surface skewness R-sk of QY adsorbed with different valence cations shifts from positive to negative values, reflecting the change of coal surface height from the right-skewed normal distribution to the left-skewed one. Moreover, the area where the surface height of QY adsorbed cation is below the average value gradually increases with the increase of the chemical valence. For micromechanical properties, the adhesion force of CZ adsorbed with different cations ranges from 0.9 to 20.7 nN, which is less than QY overall. This is mainly due to the weakening of intermolecular forces on the coal surface with the deepening of coalification, resulting in a decrease in adhesion force. Therefore, this study provides new insights into the differences in gas production mechanisms of CBM wells in the same coal seam from different regions.