Experimental validation and mechanistic elucidation of crack division and two-phase flow-mediated sealing in coalbed methane extraction boreholes

Coalbed methane (CBM) has emerged as a key clean fuel. The efficient recovery of CBM heavily relies on the materials used to seal cracks in gas drainage boreholes. This study divides the range of cracks into different single width cracks. A method of using two-phase particle mucus sealing material (PMSM) based on the “Liquid-Sealing-Gas (LSG)" principle is proposed to address the limitations of traditional “Solid-Sealing-Gas (SSG)" methods. The method involves the use of modified bentonite as the basis mucus material and a combination of flexible and rigid particles as the sealing materials. A mathematical model of graded particles' multi-level width cracks is established, and corresponding calculation software is developed. Through experiments on single crack sealing, range crack sealing, and investigations of the effects of various factors such as grouting pressure, surface roughness, crack location, and borehole angle and the sealing performance are analyzed. The results show that the best particle mucus show performance with an leakage amount (LA) of 12.29 g and an sealing time (ST) of 3.7 s. At 0.4 MPa, LA is around 7 g and ST is 6.7 s. Regarding surface roughness, the rough crack model has a 10 mm smaller slurry diffusion distance than the smooth one. For crack location, ST and leakage volume (LV) are smallest at 180°. In borehole angle tests, in upward boreholes, ST and LV increase with angle, while in downward ones, they decrease. The sealing performance of the new material is superior to that of traditional cement materials. The new particle sealing material has a maximum average negative pressure of 98 kPa and the time required to restore to the original air pressure is five times that of cement materials. Finally, the mechanism of particle sealing cracks is summarized, including three function: blocking, supporting, and filling. This study advances the theory of particle and complex crack sealing, offering novel insights for enhancing CBM extraction research.