| CBM in China has the characteristic of "three low and one high",some hydraulic measures should be taken to improve the permeability of the coal seam to the greatest extent.Hydraulic fracturing is the most effective and convenient method in hydraulic measures,which has been widely used at home and abroad.The effects of different stress,different sizes and different shapes of natural fractures on fracture pressure and hydraulic fracture were studied by means of hydraulic fracturing experimental system,3D scanning device for coal rock section,displacement meter.The fracture information was extracted and the opening degree of the hydraulic fracture was obtained under the condition of different principal stress.(1)By drawing on previous research results,cement and gypsum are selected as binder,coal powder is applied to be the aggregate.The test specimens made of similar materials for hydraulic fracturing are formed in accordance with the ratio of cement,gypsum,pulverized coal,water =2:1:1:2.(2)The hydraulic fracturing experimental system is used to carry out the simulation experiments of different stresses,different natural fracture morphologys,different sizes of the specimens.It is concluded that when vertical stress is the same,the principal stress difference becomes larger,the fracture pressure and extension pressure are both smaller,the surface area of the fracture surface is larger,the fracture opening degree is smaller;When the two of the three stresses are close,the hydraulic fracture propagation is more random,the fracture morphology is more complex,but at last,the bifurcate fracture still expands along the direction of maximum principal stress and has some angle with it;The existence of different kinds of natural fractures affects the expansion path of hydraulic fracture in different degree,which leads to the complicated and changeable shape of the hydraulic fracture. |
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