Experimental study of wetting-seepage effect of microemulsion for the coal seam water injection

[Display omitted] •MEs has smallest surface tension and adjusts wetting by functional groups.•Initial infiltration time of seepage agents: Water > MEs > SDS.•Increase of capillary number promotes migration of MEs and permeability.•MEs expands fractures and pores.•Capillary force is seepage force of MEs and increases the seepage range. The microemulsion was used as an injection agent for the coal seam water injection. In this paper, wetting and seepage parameters of microemulsion, water and SDS are studied, and modification mechanism of microemulsion to increase wetting and promote seepage is revealed. The results indicated that microemulsion exhibited stronger spreadability and a smaller contact angle because of surface tension. The content of benzene ring one adjacent H, S = O, OH…N, CH structure was positively correlated with the contact angle. The content of benzene ring three adjacent H and CH3 structure was negatively correlated with the contact angle. The change of functional group content is the main reason for the wettability modification of coal by microemulsion. SDS exhibited the best initial seepage rate, followed by microemulsion, and water was the worst. Microemulsion increased the capillary number and increased the permeable cracks and pores, and finally improved the permeability of coal. Under the action of the capillary force, the permeability effect of non-pressure microemulsion injection was improved.