Experimental study of influence of natural surfactant soybean phospholipid on wettability of high-rank coal

•The performance of soybean phospholipid in reducing capillary pressure was tested.•The wettabilities of different surfactants on high-rank coal were comparatively measured.•The flow back time of fracturing fluid in the microfractures of coal was derived.•A new compound surfactant was developed to decrease water blocking damage.•The permeability of coals at different effective stresses and soak times was used to test surfactant performance. Changing wettability of coal rock surface for enhanced gas recovery, surfactant is widely used in coalbed methane exploitation. Recently, soybean phospholipid as non-toxic and degradable natural surfactant has gained increasing importance. However, effect of soybean phospholipid on wettability of high-rank coal is still unclear. In this work, typical high-rank coals were selected from the Qinshui Basin to analyze this effect mechanism. Experiments including surface tension, surface contact angle, coal fines imbibition, dispersion effect as well as gas-based permeability were carried out. Moreover, water locking mechanism of fracturing fluid in microfractures of coal reservoir was analyzed in detail. Results show that surface of high-rank coal is negatively charged in solution and have weak hydrophilicity. Soybean phospholipid can significantly reduce the surface tension of gas-liquid, and increase wettability of coal surface. Thus, soybean phospholipid contributes coal fines to disperse observed from filter cake using scanning electron microscope. Additionally, many conventional surfactants e.g. SDBS, CTMAB, OP-10 as well as soybean phospholipid were experimentally compared in performance in reducing capillary pressure. Results indicate that single soybean phospholipid do not show perfect performance. Specially, the imbibition rate of single soybean phospholipid solution along the pipe filled with coal fines was lower than that of conventional anionic and cationic surfactants, but still higher than that of stilled water and non-ionic surfactant. To further improve the capability of soybean phospholipid solution, composite surfactant was prepared with soybean phospholipid and OP-10. We found that composite surfactant at the mass ratio of 1:3 has a lowest capillary pressure with low surface tension and large contact angle. Finally, effect of three typical surfactants on permeability of coal was investigated. Results show that this suggested compound surfactant may be conducive to discharge of fracturing fluid from coal re