Search

Traditional hydraulic fracturing (THF) has become an effective stimulation method for the extraction of coalbed methane (CBM) and has attained many remarkable achievements in the application. However, certain problems, such as greater water pressure, larger volume of fracturing equipment, and a stricter sealing requirement, have gradually arisen in the field application. To improve the application situation, a newly developed technology of pulse hydraulic fracturing (PHF) is proposed to enhance CBM drainage via accumulating the damages in the reservoirs and weakening the rock strength by exciting oscillation from pulsating water pressure. Comparison of fracture behaviors between PHF and THF at various side-pressure ratios was executed using numerical software of PFC 2D ; the results demonstrated that the fracture pressures required for PHF, which induced more cracks and a larger fracturing region, were all lower than those for THF. Additionally, the field application of PHF was performed for the N 2 706 floor roadway with crossing holes in the Daxing coal mine, Liaoning Province, China. The results demonstrated that (a) all of the fracturing holes for PHF had lower fracture pressure than the calculated initiation pressure by THF, which is consistent with the simulation results; (b) the drainage parameter values of holes made via PHF, such as drainage concentration and drainage pure volume, were generally greater than those of THF. All of the simulation and application results expressed that PHF had greater superiority than THF in the application of CBM recovery based on the features of lower fracturing pressure and more cracks generation. A large amount of accumulated damage produced by PHF could greatly destroy the integrality of coal, which induced much micro-cracks generation, significantly weakening the strength of the reservoir; thus, more complicated fracture networks would be formed under a lower water pressure. Moreover, the proportion of mesopores and macropores increased after PHF, and the porosity increased by 17.29%, which indicated that PHF could significantly improve the permeability of CBM reservoirs.

Coal-bed methane (CBM) drainage in underground coal mines plays a fundamental role in preventing coal mine accidents, reducing environmental pollution, and utilising the CBM resource. The effective sealing of boreholes is an important measure to guarantee efficient CBM drainage. On this basis, the research focused on investigating a flexible gel (FG) sealing material and a novel active sealing method suited to this material. First, the failure mechanism of the borehole seal was analysed, then an FG material, adaptive to borehole deformation and able to realise active permeation, was developed. Besides, the authors proposed a novel sealing method that used FG as a grouting material: thin and thick slurries were grouted successively, a device for replenishing the slurry was then used to perform successive permeation cycles under pressure. Moreover, the properties of the FG materials and the application of the new active sealing method were analysed through a laboratory, and an industrial experiment. The experimental results show that: compared with conventional sealing materials, the FG material is characterised by preferable compactness, stability, fluidity, and permeability. With increasing water-cement ratio, the stability of the FG material decreases, while its fluidity and permeability increase. The field test results indicate that: the effect of the novel active sealing method is superior to that of traditional methods. Compared with conventional sealing materials and methods, the decrements in drainage concentration and flux are slower. The average concentration and flux are 74% and 1.82 m 3 /min within 40 days, which show that the drainage efficiency is significantly improved. The novel material and method provide new ideas for sealing CBM boreholes and they are of significance for enhancing borehole drainage efficiency, improving resource utilisation, and eliminating gas-related accidents.

Pulse hydraulic fracturing (PHF) technology can effectively improve reservoir permeability for coal bed methane (CBM) extraction. In this work, laboratory experiments were conducted to study the laws regarding pulsating water pressure (PWP) propagation during PHF in a CBM reservoir at different pulse frequencies and plugging rates. The results demonstrate that the average PWP is stable over the length of the reservoir fissure, and the PWP peak primarily depends on the PWP amplitude. The experimental results show that for a non-plugging fissure, the PWP peak pressure first decreases and subsequently increases with the maximum pressure occurring at the fissure tip. When the fissure is blocked, a portion of the pressure waves are reflected in front of the blockage, forming reflection waves, which interact with the incident waves and lead to an increase in the PWP peak pressure. When the plugging rate is 95%, there is a significant difference in the PWP peak pressure in front of and behind the blockage because of the pressure wave reflections and interactions. As the plugging rate decreases, the blockage effects are reduced, and the wave proportion that is reflected decreases such that the transmission and creeping wave components increase. When the plugging rate is 65%, the PWP peak pressure behind the blockage is larger than that prior to the blockage. The findings build a basis to predict and control fissure extensions to improve the effect of PHF by optimizing the technical parameters.