| The crack form formed by traditional hydraulic fracturing(THF)technology is single.Pulsed hydraulic fracturing(PHF)can produce crack network,but it needs long loading time.Aiming at the defects of the existing loading method,the combined hydraulic fracturing(CHF)is proposed in this study.Firstly,pulse hydraulic preloading is carried out on the rock mass for a certain period of time to accumulate fatigue damage and degrade the rock mass,and then hydrostatic loading is carried out until the test block fails,in order to form a crack network and improve the loading efficiency.In this study,a combined hydrostatic loading system was designed and developed for rock-like specimens.CHF test set in the three groups of different variables:preloading time rate,preload pulse frequency,the preload pressure pulse amplitude,using 3D reconstruction technology acquisition hydraulic crack area,to explore the combination of hydraulic crack propagation law under water pressure load,block acoustic emission signal collection and loading process,as the evaluation of fatigue cumulative damage extent inside the block.In addition,traditional and pulse hydraulic fracturing tests were carried out to compare the characteristics of combined hydraulic fracturing.The CDP model in the finite element software ABAQUS was used to simulate the damage of the test block under combined hydraulic pressure loading,and explain the law of hydraulic crack propagation under combined hydraulic pressure loading.The experimental results show that,compared with the THF,the combined hydrostatic loading can effectively improve the degree of crack development,produce larger crack area and more crack number,with better anti-reflection effect.Compared with the PHF,the combined hydraulic loading can shorten the loading cycle by adjusting the preloading time rate.Under the same pulse preloading condition,appropriately prolonging the preloading is beneficial to produce larger crack area,in this study,when the preloading time rate is 0.75,the CHF produce the biggest crack area.When the pulse frequency is low,the internal pressure of the test block changes slowly and the damage state changes stably during the preloading stage,which is conducive to the CHF to produce a large crack area.When the pulse amplitude pressure is low but greater than the fatigue limit,which is conducive to the CHF to produce a larger crack area. |
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