The Study Of Rock Mechanical Property Under Cyclic Load And Application

The mechanical properties of rock under cyclic loading are quite different from those under monotonic loading conditions.During reservoir stimulation,if fracturing by cyclic pumping is used,in which the whole fracturing process is completed by repeatedly turn on and turn off the pump,then the underground rock will also be in the cyclic stress state.In this thesis,the rock mechanics parameter test and the three-point bending test are conducted to analyze the rock mechanical properties under both cyclic compressive loading and cyclic tensile loading.Then a series of fracturing experiments related to cyclic pumping were carried out to explore the possiblity to create complex fracture networks and find the relative influencing factors.Finally,DDM numerical method was used to investigate the internal pressure and shape variation characteristics of hydraulic fractures under cyclic injection.Firstly,through the rock parameter tests,it is found that when the specimens get breakout,more cracks are created under cyclic load.While,compared with cyclic compressive load which may need tens of thousands of cycles,the number of cycles required to failure under the tensile load is only about ten cycles.Secondly,through a serial fracturing cross-comparison tests under cyclic pumping,it can be found that the fracturing by cyclic pumping can generate a more complicated fracture network than that by the conventional monotonic pumping.When using lower fracturing fluid viscosity,increasing injection cycles,and taking variable frequency cyclic pumping methods,complexity of fracture networks will be further increased.The use of longer open-hole segments helps to promote the complexity of hydraulic fractures and breakthrough higher stress differences.Thirdly,by performing a cyclic pumping fracturing test on block specimens with multiple perforating clusters,it was found that the cyclic injection can initiate more perforating clusters than that by conventional continuous injection.Higher viscosity of fracturing fluid can further enhance the effect of the formation of multiple fractures.At the same time,both under conventional continuous injection and cyclic pumping injection,higher pressures help to initiate more hydraulic fractures from perforation clusters.Acoustic emission monitoring shows that the total number of acoustic emission events generated by the cyclic pump is much greater than that generated by conventional continuous pumping.The concentrated distribution of acoustic emission events is usually closely related to the higher injection pressure.Last but not least,through the DDM numerical simulation,it is found that after pumping off and de-pressuring immediate close to the first cycle peak pressure,the hydraulic fracture mouth gets closed at the wellborehole,so that the flow between the fracture and the wellbore is cut off,which cause the re-injection pressure rising up a level which is higher that propagation pressure of the existing fracture itself.The peak pressure in the afterward cycles creates favourable conditions for the formation of multiple fractures.Through the study in this thesis,while revealing the mechanical properties of rock under cyclic load,it also further elaborates its application and potential of fracturing by cyclic pumping in the petroleum industry,so as to provide further guidance for improvement of the cyclic pumping tecnique.