CHARACTERISTICS OF FRACTURE PROPAGATION AND PERMEABILITY FOR THE FRACTURED COAL BASED ON PHYSICAL MODELING EXPERIMENTS

摘要

H ydraulic fracturing process (HFP) is an effective way for extracting Coalbed Methane (C BM ) in low perm eability coal seam. In order tofurther understand the effect of hydraulic fracturing pressure on the fractures propagation and the perm eability change of coal after HFP, a series of hydraulic fracturing tests (HF tests) under the hydraulic fracturing pressures (Pj) of 2, 3 and 4MPa was conducted by using the raw coal specimens w ith a shape of 50 mm x 100 mm and hydraulic fracturing hole of 10 mm x 50 mm collected from one coal block. As w ell as CT scanning tests and perm eability tests on these specimens were carried out. The results show that: (1) Hydraulic fractures are m ainly parallel to the maximum principal stress and the area near the fracturing water outlet has better fracturing effect. (2) The original fracture, bedding and cleat play a dominant role for the fractures propagation, and some new fractures would generate from cleats. (3) The gas permeation rate of the specimen for higher P , is a little higher and increases faster than that of the specimen for lower Pj, but needs more tim e to be stable, and the gas permeation rate can be stable easier under a higher confining pressure. (4) The permeab ility increases linearly w ith the increasing P；-when the seepage pressure is equal to confining pressure, but changes little when the seepage pressure is lower than confining pressure, and the perm eability and its reduction rate decrease w ith the increasing confining pressure.