Coal Wetting Characteristics and Mechanism of Water with Different Degrees of Mineralization

Abstract

The flowback rate of water during drainage of coalbed methane (CBM) wells significantly influences the gas yield. The difference in the coal wettability of aqueous solutions with different degrees of mineralization is a key factor influencing the flowback rate. To clarify the coal wetting characteristics and mechanism of action of aqueous solutions with different degrees of mineralization, the contact angle between aqueous solutions with different degrees of mineralization and coal, surface tension, and zeta potential were tested. The Derjaguin−Landau−Verwey− Overbeek theory and diffuse double layer theory were used to reveal the mechanism of actions of the degree of mineralization, type of ions in solution, and complex solutions on the wettability of the coal surface. Results show that, as the degree of mineralization increases, the surface tensions of five solutions (NaHCO 3 , CaCl 2 , NaCl, Na 2 SO 4 , and Na 2 SO 4 + KCl) on the coal surface increase. The coal−water contact angle shows logarithmic growth; the absolute value of zeta potential decreases in a logarithmic manner, and the water-film thickness in the system decreases in a negative exponential manner. The coal wettability of aqueous solutions is mainly dependent on the electrostatic force and the van der Waals force (VDWF). The degree of mineralization changes the molecular electrostatic force in the system, which is the primary cause of changes in the coal wettability of aqueous solutions. As the degree of mineralization increases, the absolute value of the zeta potential of the coal−water interface declines, the electrostatic force decreases, and the energy barrier is lowered, which enlarges the contact angle and lowers the wettability. Compared with univalent Na + , the interfacial electrostatic force of divalent Ca 2+ reduces. The increment of the proportion of VDWF is the foundational cause for the increasing wettability. Addition of KCl solution can enlarge the coal−water contact angle, which is conducive to flowback of water. The results provide a theoretical basis for understanding the wetting mechanism of coal by water with different degrees of mineralization.