The influence of inorganic salt on coal-water wetting angle and its mechanism on eliminating water blocking effect

Abstract

Hydraulic stimulation techniques are widely used to improve the CBM production. However, the water blocking effect severely restricts the stimulation effect. In this work, the different kinds of inorganic salts were used to increase the liquid surface tension so that the water blocking effect can be diluted by reversing the capillary force direction. Moreover, the FTIR, XRD, Low-temperature N2 adsorption and coal surface 3D morphology experiments were carried out to analyse the influence of coal surface structure on the wettability. Results showed that the inorganic salts significantly improve the liquid surface tension and coal-water contact angle, can even change the direction of capillary force because the inorganic salt ions are able to combine water molecules and thus lead to a higher mutual attraction of water. The NaCl and CaCl2 show the most remarkable effect on changing wettability and reverse the capillary force. The surface tension of saturated NaCl solutions and saturated CaCl2 solutions increased by 13.92% and 15.79%, respectively. The contact angle of DL coal with saturated NaCl solutions and saturated CaCl2 solutions increased by 42.39% and 42.36%. The contact angle of WL coal with saturated NaCl solutions and saturated CaCl2 solutions increased by 23.35% and 22.88%. Based on Laplace's equation, the maximum reduction of capillary force for DL coal and WL coal are 183.01% (CaCl2) and 374.17% (NaCl), respectively. Finally, the influence of surface structure on the coal wettability were analysed. The carboxyl groups, hydroxyl groups and minerals in the coal can promote the wettability of the coal, leading to a higher water blocking effect. Similarly, the smaller pore size of the coal leads to a greater capillary force and thus inhibit gas transport. This work provides a new idea for eliminating the water blocking effect and improving the effectiveness of hydraulic stimulation techniques.