Experimental Study and Analysis of Chemical Modification of Coal Surfaces by Viscoelastic Surfactant Fracturing Fluids

The continuous deepening of coal-seam extraction has sharply increased both gas pressure and content. The use of viscoelastic surfactant fracturing fluids (VESFFs) has been demonstrated to effectively improve coal-seam permeability and mitigate the occurrence of gas disasters. After injection into coal, VESFFs interact with the coal and affect its surface characteristics. In this study, to characterize changes in zeta potential, oxygen-containing functional groups, and the microcrystalline structure of hard and soft coal surfaces under the influence of VESFFs with different formulations, zeta potential measurements and Fourier-transform infrared and Raman spectroscopies were performed. The VESFFs enhanced the electrostatic repulsion between the pore wall and coal particles, which is favorable for the removal of coal particles from hard and soft coal surfaces. The combination of cationic with zwitterionic viscoelastic surfactants (VESs) in the VESFFs exposed more hydrophilic functional groups on the surfaces of hard and soft coal, increasing wettability and affecting nanometer pores. A VESFF based on anionic and zwitterionic VESs as the primary agents could enhance the extension of the aromatic layer (La) of the aromatic crystal nuclei and reduce the interlayer spacing (d002) of hard and soft coal, thereby increasing the volume of micropores. This research offers theoretical guidance for optimizing the primary components of VESFFs and elucidates the mechanism through which VESFFs act on nanopores in coal from a microscopic perspective.