Mechanism and Practice of Multifracturing Using Dynamic Loads in a Low-Permeability Coal Reservoir

The geological conditions of coal reservoirs in China are complex, and the reservoir permeability is generally lower. Multifracturing is an effective method of improving reservoir permeability and coalbed methane (CBM) production. In this study, two types of dynamic loads, CO₂ blasting and a pulse fracturing gun (PF-GUN), were used to conduct multifracturing engineering tests in nine surface CBM wells in the Lu'an mining area in the central and eastern parts of the Qinshui Basin. The curves of pressure versus time of the two dynamic loads were obtained in the laboratory. The prepeak pressurization time of the PF-GUN was 200 ms, and that of the CO₂ blasting was 2.05 ms, which just falls in the optimum pressurization time of multifracturing. The microseismic monitoring results revealed that, in terms of the fracture morphology, both the CO₂ blasting and PF-GUN loads produced multiple sets of fractures in the near-well zone. In the six wells used for the CO₂ blasting tests, an average of three branch fractures were produced outside of the main fracture, and the average angle between the main fracture and the branch fractures exceeded 60°. In the three wells stimulated by PF-GUN, an average of two branch fractures were produced outside of the main fracture, and the average angle between the main fracture and the branch fractures was 25-35°. The multifracture characteristics of the fractures formed via CO₂ blasting were more obvious. However, a coal seam is a multifracture reservoir with a large filtration coefficient; the fracture will not extend after reaching the maximum scale under a certain gas displacement condition. Compared with the traditional hydraulic fracturing technique, the nine wells used in the multifracturing tests exhibited an obvious stimulation effect with an average increase of 51.4% in daily production. The results of this study provide an important technical reference for the efficient development of CBM in low- and ultralow-permeability reservoirs.