Experimental research on hydraulic fracture propagation through mixed layers of shale,tight sand and coal seam

When multi-layers strata with shale gas,tight gas and coal bed gas are developed together,differential stress between interlayer,lithology difference,interfacial property and other factors have great impacts on hydraulic fracture geometry. So it is difficult to understand the fracture propagation rule in composite reservoir,which brings great chal- lenges in planning for reservoir stimulation. In this paper,the natural outcrop rocks from the eastern margin of Ordos Basin are excavated and mixed in a certain way to carry out the physical simulation experiments with a true tri-axial fracture system. The effects of differential stress,differential elastic modulus and cleats on the hydraulic fracture initia- tion and propagation are studied. The result shows that the stress condition and natural weak plane commonly control the propagation direction of hydraulic fracture. When the stress difference ranges from 4 MPa to 6 MPa,the hydraulic fracture can not only penetrate the interface in vertical direction but also activate the natural fractures in coal to form a complex fracture network. The higher elastic modulus difference between layers brings a higher pressure in fracture,when the hydraulic fracture reaches interface pressure releases to generate pressure wave which can contribute to acti- vate the micro fracture in coal. The expansion path of hydraulic fracture in coal rock is greatly influenced by cutting principle,and could easily deflect to form branches. There are obvious hold pressure for two times when hydraulic frac- ture penetrates interface,and the fluctuation of fracturing curve is obvious when the weak planes in coal is activated. The study of hydraulic fracture initiation and propagation in multi-layers reservoir can provide a reference for the field prediction of fracture morphology and optimize pump injection program.