Mechanics Research On Creep Buckling Of The Shale Hydration Damage

The damage mechanics and rheological mechanics are combined to solve the wellbore instability problems of shale from West 233 in oil and gas drilling engineering.The microcracks in the brittle shale and the rheological effect caused by the physical and chemical effect caused by the fluid into the well and underground fluid. Presenting damage variable based on the NMR technology, a nonlinear creep model of mud shale including hydration damage and creep damage are proposed based on the NMR technology, experimental mechanics and numerical simulation. Based on the prenstend nonlinear creep model, the creep characteristics and evolution of the the brittle shale including the growth of the microcracks in the brittle shale and collapse damage instability are studied.A basic theory and method are proposed to study the borehole creep damage instability. The main contributions can be listed as follows:(1)Study of the component composition and mechanical properties of the shaleThe X diffraction and SEM are used to study the component and physical characteristics of the core from West 233. The test shows that the main component in the shale are quartz(40%), Pyrite and feldspa which maks the shale have obvious brittle hardness. Main ingredients of the shale are aemon, mixed layer illite and chlorite.shale from the regional stratigraphy shows a higher degree of compaction, horizontal bedding and growth of the microcracks.(2) Experimental study of the creep propertieUniaxial and triaxial test are conducted to study the effecs of salinity formation on the rock strenth and creep properties. The results shows that hydration decreases on the properties of the shale. In the triaxial test, the shale experiences three stages: decay creep stage, steady creep stage and accelerating creep stage. Under the same stress level, the creep stain increases with the increase of the water content of the shale.(3)Microstructure quantitative analysis and damage characteristics of the shaleHard brittle shale samples under different soaking times are measured by the nuclear magnetic resonance technique. The quality of the sample, transverse relaxation time T2 spectrum distribution and nuclear magnetic resonance imaging of the hard brittle shale samples under different soaking times are obtained. The results show that hydration can produce damage inside the rock initially. The microcracks propagate rapidly and the surface cracks are formed as the immersion time extends. NMR image shows that the internal microstructure of the sample is redistributed under hydration effect. T2 signal amplitude curve changes significantly as the immersion time extends. NMR image shows the internal microstructure of the same sample under different soaking times. Damage variable based on the NMR image provide a new way to study the damage of the shale.(4)Creep constitutive model research on the shale including Initial hydration damage and creep damageReference to classic element modeling ideas of combination model, according to the water absorption expansion of rheological mechanics behavior of the shale, this paper proposes a nonlinear hydration expansion rheological model, it has the expansion element with the viscous element in parallel, combined with nonlinear sticky plastic body, and can effectively simulate the swell of the shale creep process. This model can describe the rock after water absorption expansion of decay creep and steady creep, and to describe the accelerating creep of rock under high stress. Based on model of long 7 mud shale creep test data fitting results show that the model can well describe the mud shale hydration expansion after the creep properties of rock.(5)FEM analysis of the hydration creep damage model and engineering applicationThe secondary development of the three-dimensianl damage model presented in chapter 5 are realized. Triaxial creep test are simulated in the ABAQUS based on the secondary development and the rationality of this model are examined through the experimental data. Hydration damage creep model is used for long 7 layers of xifeng 233 wellblock a well mud shale drilling borehole wall stability analysis. Drilling fluid density chart is given after considering the creep damage of mud shale, used to guide field drilling.