Wetting Behaviors and Mechanisms of Coal Modified by Different Types of Surfactants under Intervention Using KCl

The fracturing fluid with an added surfactant is conducive to backflow, and it is gaining more and more popularity in coal seam hydraulic fracturing projects. At the same time, the types and concentrations of mineral ions in the solution have an important impact on the backflow of the fracturing fluid. With K⁺ and Cl^– as representative ions of the mineralization degree of water solutions, understanding the mechanism of K⁺ and Cl^– on the wettability of surfactant-modified coal can provide a scientific theoretical basis for the backflow of the fracturing fluid. Based on this issue, Shanxi Changping Coal Mine coal samples were selected, and simulation methods were used to study the intervention effect of KCl on the wettability behavior of different types of surfactant-modified coal. The wettability behavior mechanism of KCl on different types of surfactant-modified coal was revealed through parameters such as radial distribution function (RDF), hydrogen bonds, diffusion behavior (mean square displacement), and interaction energy. Results show that under intervention using KCl, the peaks in RDFs of surfactants and water decrease significantly; the number of hydrogen bonds decreases by 22.02–40.48%; and the binding energy decreases by 41.32%–58.23%. The occurrence of nonionic surfactant APG in the aqueous solution changes it from a dispersed into an agglomerated distribution, while that of ionic surfactants exhibits the opposite trend. The diffusion coefficient of water in surfactant solutions decreases by more than 59.56%. Under intervention using KCl, the wettabilities of coal modified by different surfactants all decline. After intervention using KCl, the electrostatic force decreases sharply in the interaction energy between coal modified by the three surfactants APG, SDBS, and BS-12 and water, while the van der Waals (VDW) force is only slightly affected; the VDW force increases in the interaction energy between CTAB-modified coal and water.