Elasto-Plastic Analysis Of Mechanism Of Dilatation Of Mudrock

As the basic materials of the earth, rock is studied to understand many phenomenon and processes on the earth. Research on the propagation characteristics of the elastic wave in the rock is beneficial to oil gas exploration and gives further insight into the stress state of the earth's lithosphere, which forms the theoretical basis on the natural disaster, such as earthquake, landslide. With its help, a new field in the rock fracture mechanics would occur. It is significant for the petroleum industry to investigate on the macroscopic and mesomechanic mechanism of the mudrock deformation, which influences some aspects of petroleum exploration, such as well drilling and oil extraction.The survey of oilfield on site indicates that on the mudrock layer it happens to the oily water well that casing fails and during well drilling well face collapses. In present paper, the physical, chemical and mechanical properties of mudrock are studied, then the macro and microcosmic process of mudrock under water-injection and hydration is analyzed, during which the variation law and influencing factors on hydration stress in the stratum, on the stress in the wall rock pore and mechanical properties of rocks are given. Also the hydration and dilation mechanism of mud shale is investigated. From the point of view of mechanics, the mechanical deformation mechanism and constitutive relation for mudrock material are established.Based on the constitutive relation, the peculiarly mechanical properties of mudrock are understood fully and engineering problems may be solved. Then the mechanism on the casing failure in the oily water well on the stratum interface could be examined further, which provides theoretical basis for the design, manufacture, and assembling of casing, prevention from casing failure and enhancing the stability of well face. Establishment of constitutive equations lays a solid foundation for the research on the casing failure and instability mechanism of well face, which is of great theoretical significance and wide application.After analysis and survey, the mechanism on the casing failure of oily water well and instability of well face is summed up fully. The significance of constitutive relations in geotechnics is given. Fracture mechanics and finite elasto-plastic deformation theory are introduced to reveal the mechanical mechanism of mudrock deformation. The main work is as follows:Firstly, the mechanical property of mudrock under water-injection and hydration is studied. The experiments of water-injection and hydrochemistry ofmudrock were made. It is an important character that nonlinear bulk strain changes under water-injection for mudrock. The nonlinear creeping constitutive equation of mudrock which influences bulk expanding is established.Secondly, with the combination of constitutive relation and interface fracture mechanics, the interface fracture mechanics model is established which is applicable to analysis of strata uplift and casing shearing failure, i.e. high pressure water-injection causes oil pool uplift and casing shearing failure. Upon calculation, the influence of stratum degree of inclination on casing shearing failure is given, and the mechanism on the casing failure in the oily water well on the mudrock interface could be elucidated.Thirdly, seeing that bulking dilatation effect is evident, it is essential that finite elasto-plastic deformation theory be introduced to study the deformation mechanism. Based on the finite elasto-plastic deformation theory, the constitutive equation for pressure-sensitive material is established.Fourthly, with the constitutive equation for pressure-sensitive material, the finite elasto-plastic deformation of pressured spherical cavity (body cell model) is analyzed. Under the assumption of self-similarity, the governing equations for the deformation of pressured spherical cavity are simplified and the differential equations are established with the effective stress as independent variable. The stress and displacement distribution is given, and the impact of material parameter on the bulking dilatation effect is discussed. All these work lays foundation for establishment of the macro constitutive potential and macro failure criterion based on the analysis of body cell.Fifthly, according to the physical circumstances of the oil field on site, with the combination of theory and practical experience, the measures to prevent and treat casing failure in oily water well and instability accident of well face are proposed. The results give a useful theoretical reference for protection and treatment of casing failure in oily water well, design and appraisal of fracture toughness and investigation on the instability mechanism of well face.