Experimental research on hydraulic fracture propagation in integrated fracturing for layered formation with multi-lithology combination

Different gas-bearing zones are alternately developed vertically in the layered formation with multi-lithology combination.When it is developed by integrated fracturing, the vertical morphology of hydraulic fracture is rather complicate and the extension law is unclear due to the change of rock lithology, rock mechanics properties and in-situ stress situation.Therefore a triaxial hydraulic fracturing experiment was carried out with samples, which was combined with limestone, shale and coal sheet.These sheets were made of different types of cements according to the actual mechanical characteristics of cores in coal measure shale reservoir.The law about the effect of each factor on fracture vertical propagation was studied under vertical well and horizontal well.The results were as follows: the hydraulic fracture height geometry was complex, which showed strong asymmetric propagation characteristics.There were many typical morphologies, including arresting, bypassing, branching and crossing, when hydraulic fracture propagated closely to the interface.In vertical well fracturing, the main fracture presented “+” shape and turned in horizontal and vertical direction successively, which contributed to hydraulic fracture fully communicating coal seam cleats.Under the condition of horizontal well, the fracture propagated vertically to weak planes and then branched or turned.Hydraulic fracture penetrated the lithological boundary more easily and then connected with the adjacent layer with vertical stress coefficient increasing.Complex natural fracture network increased the possibility of fracture turning and branching and horizontal fracture distribution.However, it could dissipate the hydraulic energy and limit the fracture height growth.When hydraulic fracture penetrated the interface, the pump pressure curve exhibited the process of increasing-decreasing-increasing, and the curve continued to fluctuate slightly when it propagated closely to cleats.When integrated fracturing is carried on, rocks with natural weak planes, such as coal and shale, are not suggested to be as fracturing layers.Increasing the displacement properly can increase the ability of fracture crossing lithological boundary and the possibility of fracture bifurcation.The research results are expected to provide some insights for understanding the hydraulic fracture morphology of layered formation with multi-lithology combination and guidance for optimizing con-struction parameters.