| After the completion of shale gas wells,due to the low permeability of shale gas reservoirs,only a small number of natural fractures in shale gas reservoirs are relatively developed:while most of them require hydraulic fracturing,acidification and other reservoir-reconstruction techniques to reach economic gas production.Since shale is a kind of elastic and brittle rock,the surrounding rock of the well wall will be affected by cyclic loading and water-rock interaction for a long time during the process of reconstruction and production,which may cause the well wall instability.In order to understand the deformation and energy evolution mechanism of the shale in terms of stress paths and moisture content,this thesis takes the outcrop black shale of the Lower Silurian Longmaxi Formation in Changning,Sichuan as the research object.Through triaxial compression,numerical simulation and cyclic loading experiments,using theoretical methods such as rock mechanics,damage mechanics,and thermodynamics,we have studied the changes in the mechanical parameters of the shale from the deformation to the failure process under the stress path and water content,and the following main conclusions can be drawn.(1)The energy ratio curve of shale under triaxial compression can divide the energy conversion process in shale deformation and failure into energy strengthening and weakening stages.The ratio of?_a/?_p and?_b/?_p could better reflect that the energy strengthening characteristic of shale decreases as the confining pressure increases;conversely,the weakening characteristic increases as the confining pressure increases.In the RFPA numerical simulation,as the lateral pressure gradually increases,the failure mode of the shale model gradually develops from the tensile failure of low lateral pressure to the shear failure direction of high lateral pressure.The morphology and trend of the fractures in the shale model can be observed from acoustic emission images.(2)As moisture content increases,shale deformation increases,while peak strength and elastic modulus decrease.In the shale with high water content,the acoustic emission energy of the pore-fracture compaction stage is higher than that of the low water content,the elastic deformation stage is less,and the signal of the plastic deformation stage gradually increases until the moment of destruction,the AE energy signal increases sharply,and the lower the water content,the higher the AE energy magnitude.The change of acoustic emission signal reflects the internal energy evolution process of shale in different water-bearing states.(3)Under gradual cyclic loading,The deformation and modulus of the saturated shale are smaller than that of the natural shale,and the loading deformation modulus is smaller than that of the unloading deformation modulus.In the loading and unloading stages,the strain always lags behind the stress.The hysteresis effect of saturated shale is more obvious than that of natural shale.The energy calculation method considering the hysteresis effect is proposed,which is smaller than the previous energy calculation.Finally,the calculation formula of the damping ratio is verified based on the energy principle,It is found that the saturated shale has a larger damping ratio than natural shale.(4)Under constant and constant amplitude cyclic loading,as the confining pressure increases,the number of stress levels in the elastic stage increases,the Poisson's ratio increases,and near failure,the deformation modulus and Poisson's ratio will fluctuate sharply;the relationship curve between the cumulative horizontal energy and the stress ratio It can be divided into energy acceleration,uniform speed,and deceleration phases;the damping ratio decreases first and then increases,and it rises sharply before failure,which is roughly similar to the change law under stepwise cycles.The change of the damping ratio better reflects the shale damage mechanism,which can be used as an important index to predict shale instability and failure. |
PDF Full Text Request