Experimental study on propagation mechanism of complex hydraulic fracture in coal-bed

Abstract：Through several groups of large scale（300 mm×300 mm×300 mm）true tri-axial tests,influences of ground stress,natural cleat or fracture,interlayer and interface＇s property on hydraulic fractures＇ propagation and geometry in coal samples deprived from the south of Qinshui Basin were studied.The test results show that developed natural cleat or fracture in coal-bed results in high pumping pressure with frequent fluctuation when fracturing,and hydraulic fractures＇ propagation remains unsteady with complicated geometry.When horizontal stress difference is small,hydraulic fractures can start to occur along multi-direction,usually forming multi-fracture system during propagation,and hydraulic fractures can propagate along cleat and natural fractures randomly.With the horizontal stress difference increasing,hydraulic fractures tend to happen along the vertical direction of minimum horizontal stress with relative simple geometry.The function of physical property difference between coal-bed and interlayer in controlling hydraulic fractures＇ interface crossing is not remarkable.However,vertical compressive stress on the interlayer and interface property are the main elements to determine whether the hydraulic fractures can cross the interface into interlayer.In the situation of low vertical stress,low interface＇s cementing strength and correspondingly small friction coefficient,hydraulic fractures can glide transversally along the interface,but can not cross the interface into the interlayer.On the contrary,hydraulic fractures can easily cross the interface into interlayer.