Abstract: Currently, old wells in the Hancheng Block face a gas production bottleneck, with the recovery rate of only 32.0%, failing to achieve long-term stable and high production. To enhance the coalbed methane (CBM) productivity of old wells in the Hancheng Block and displace adsorbed gas in desorption blind pores, a surfactant screening experiment was conducted. By comparing the effects of seven surfactants on reducing surface tension, contact angle, and enhancing CBM desorption, a new foam-free surfactant suitable for field operation conditions and capable of improving recovery was selected. The study shows that injecting the new foam-free surfactant into coal can effectively reduce surface tension and change the wettability of the coal surface; it eliminates excessive bubbles during operation, reduces the Jamin′s effect, decreases capillary pressure,weakens the water blocking effect, and makes CBM in blind pores easier to desorb. Field tests show that injecting a 0.30% solution of the new foam-free surfactant increased the cumulative gas production of the original low-production old well by 13 500 m³, achieving a relatively ideal stimulation effect. This research can provide a reference for stimulating similar old CBM wells.

Key words: coalbed methane (CBM), surfactant, surface tension, water blocking effect