Abstract:
This paper focuses on coalbed methane exploitation of 15# coal seam at Qinshui coalfield by means of hydraulic fracturing. Experimental analyses of coal mass concreted with cement in size of 300×300×280mm were conducted using triaxial test system. The effects of in-situ stress, tensile strength, natural macro-cracks on cracks formation and propagation were discussed in the process of hydraulic fracture; further surface morphology of fractured coal mass was analyzed by using laser 3D scanning system after hydraulic fracturing. The experimental results indicated that the natural fracture inclining does not have any effect on the fracture extension when the natural fracture strike parallel to the minimum principal stress direction. Many cracks in coal mass are formed in the direction of perpendicular to the bedding. In addition, 3D scanning results indicated that the surface morphology of fractured coal mass is much rougher than that of peripheral concrete, and the roughness of coal mass in the direction of vertical to the bedding plane is higher than that of parallel to the bedding plane. The rougher the fractured surface is, the more consumed energy is. It can be deduced that the fracture development in rock is easier than that in coal.
Introduction
The permeability of coalbed methane reservoir is generally low. As one of the main approaches to improve permeability in reservoirs, hydraulic fracturing has played an important role in enhancing permeability of the reservoirs and gas production (Cipolla et al., 2009; Chen, 2013; Hou et al., 2013). Numerous theoretical investigations and experiments (Blanton, 1982; Chen et al., 2000; Teufel and Clark, 1984; Zhou and Xue, 2011) indicates that geological parameters, such as in-situ stresses, fracture orientation and rock anisotropy property etc., have a significant influence on the propagation of hydraulic fracture. In the process of hydraulic fracturing propagation encountering with the natural fractures, there are two probable behaviors (Cheng et al., 2014): penetrating the natural fracture or not. In addition, when the fracture extension encountered with weakness plane, the small closed weakness cracks won't change the fracture extend trend (Daneshy, 1974), but the fracture extension direction will appear deflection due to the presence of large open crack. Zhou (2007) revealed that the differential stress in horizontal and the approaching angle were the macro-parameters affecting the hydraulic fracture propagation in normal stress state. Jia (2013) found that the hydraulic fractures propagate not always along the natural cracks, but along the maximum principal stress direction. However, few of them had taken natural large open crack specimen but artificial crack into test.
