Effect of compressive stresses on permeability of coal with fracture filled with lightweight proppant

The paper presents laboratory testing results on effect of hydraulic fracturing on permeability of coal with and without wedging of created fractures by proppant. The tests were carried out with hollow aluminosilicate microspheres ASPM-500 without and with additional polymeric thermoreactive coating, specimens of dense long flame (grade D) coal with permeability of less than 3.10(-3) mu m(2) and fractured fat coking coal (grade ZH) with permeability over 60.10(-3) mu m(2). It is found that creation of fractures with and without propping considerably improves gas permeability of dense coal with weakly developed natural jointing. Created fractures are less effective in highly jointed highly permeable coal. It is experimentally proved that filling of created fractures with small opening of 0.4 mm with lightweight proppant under uniform compression of 1-5 MPa improves permeability of dense coal by 7-19.5 times. In jointed coal, application of lightweight proppant under the same conditions improves permeability less considerably-by 1.2-2.6 times. It is revealed that efficiency gas drainage in coal by hydraulic fractures without propping lowers as compressive stresses decrease. Reduction in permeability of coal with propped fracture under growing compression is probably connected with redistribution and compaction of proppant particles in the fracture. The obtained results show that application of lightweight propping materials is a promising way of advance in the technology of hydraulic fracturing for stimulation of coalbed methane drainage. The long-range research, in the authors' opinion, should be focused on the development of lightweight proppants with improved strength characteristics.