Study On The Mechanism Of Damage Of Cement Sheath During Hydraulic Fracturing

Cement sheath can be subjected to complex stress during fracturing,so the research of stress state and failure type of cement sheath during the fracturing process can be very directive and useful to guarantee the integration of cement sheath and ensure the efficient and rapid development of oil and gas.In this dissertation,the coupling of radial force and radial displacement of casing,cement sheath and formation during fracturing is studied.Considering the cement is the weakest and most easily damaged part of the system,Therefore,the plastic deformation and failure of cement stone during fracturing are studied on the basis of previous studies,and also studied the de-boding of casing cement sheath interface.The results are as following:(1)Improved the casing-cement sheath-formation model,which was based on the ideal elastoplastic model under concentric condition,the critical pressure when cement begin to generate plastic deformation and when the cement becomes fully plastic deformation are given,the characteristics of micro-annulus when casing pressure unloading from different states are analyzed.The results showed with the increase of the internal pressure,the cement stone will start to produce plastic deformation from elastic state,until become completely plastic deformation and lost the ability to bear load.(2)Considering the hardening characteristics of cement stone under high confining pressure,the casing-cement sheath-formation model was established which was based on the power hardening model.Studied the damage of cement sheath and de-bonding,then compared the result with the ideal elastic-plastic model.The results showed the micro annulus is smaller when compared with the elastoplastic model once unloaded to same inner casing pressure.(3)The influence of the mechanical properties of cement stone on the damage of cement sheath in different model is analyzed.The results show the smaller the elastic modulus and the greater the uniaxial compressive strength are,the smaller the micro annulus can be produced in the ideal elastic-plastic model.The larger the material parameter K and smaller the n is,the smaller the annulus are.Therefore,it is recommended to use low modulus,high strength cement stone,or high K value low n value of cement stone.(4)The casing-cement sheath-formation model is established under the eccentric condition,the influence of eccentricity on the force and damage of cement sheath are studied,the results show there is a danger of tensile fracture at the weak point of cement sheath.It is found that the influence of eccentricity is very obvious.The obvious the eccentricity is,the smaller the critical pressure is,and the critical pressure is reduced by about 50% when the eccentricity reaches to 50%.By studying the force and damage of cement sheath under the condition of different casing centralization,it is helpful to ensure the integrity of wellbore,and is expected to provide guidance for hydraulic fracturing.