Mechanism and influencing factors of electroosmosis-driven residual water drainage in soft coal seams during coalbed methane recovery

Soft coal seams with low porosity are prone to water-blocking during mid to late stages of coalbed methane production, reducing gas recovery. To address this, an electroosmosis-driven drainage strategy was proposed in this paper, based on the charged properties of soft coal in water. Three coal ranks (anthracite, coking coal, and long-flame coal) were tested using a custom electroosmotic drainage device. Electrical properties were characterized, and the effects of potential gradients on drainage were analyzed. Fluorescent particle tracing and Fourier-transform infrared spectroscopy were used to explore residual water migration. It is shown that electroosmosis can significantly enhance drainage across all coal ranks. For coking and long-flame coals, drainage increases with voltage before stabilizing; anthracite exhibits peaked at 4 V/cm. The fluorescent tracing reveals water coalescence and migration. Long-flame coal shows best, linked to optimal higher hydroxyl content and electronegativity. Electroosmotic force, governed by pH, hydroxyl content, and field strength, enables directional water transport. Finally, an engineering design is suggested to reduce water-blocking and enhance coalbed methane recovery.