Abstract
More than 50% of China's CBM is contained in the fractured soft and low-permeability coal. This medium has characteristics of low strength, ductile failure, multiple fractures and low permeability, which could lead to HFs were short if direct fracturing technology was adopted. Indirect fracturing is one of the new technologies to realize the high CBM production. The core scientific problem is to study the constitutive relationship of plastic damage and fracture-seepage coupling of the coal, then HFs area under the influence of multiple factors was analyzed. In this paper, the coal discontinuities are the main channel for the HFs propagation by hydraulic fracturing and acoustic emission experiments. 3D network model of coal discontinuities was established based on CT data. The fluid-solid coupling constitutive equations were established to reflect characteristics of mixed mode ductile fracture of discontinuities and plastic damage-seepage of coal matrix. On these bases, parameters of indirect fracturing were optimized through numerical simulation in order to maximize the HFs area in the coal. The results show that, compared with the brittle continuum, the ductile failure and the mixed mode fracture process consume lots of the hydraulic energy, which is the root cause of the small HFs area. In addition, the area decreases in the form of power or exponential function with the increase of coal permeability and the distance between well and interface, but increases in the form of power function with the increase of injection rate, elastic modulus ratio of roof to coal, interface strength, and difference between maximum and minimum stress. By controlling the above factors in the sensitivity critical intervals, the target of large HFs area in the coal can be realized. The above conclusions have been applied to the fractured soft and low-permeability coal region, which makes the CBM production increase by 20 times.
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