With the continuous exploitation of oil and gas resources, more and more closed offshore oil and gas wells, no oil or gas showed wells and carbon dioxide wells which have no industrial value are facing disposal. Blocking and abandoning operations mainly involve sealing the wellbore through injecting cement plugs into appropriate layers in it preventing the form of fluid turbulence from crossflowing in the wellbore. The key guarantee of the sealing integrity of the wellbore is ensuring the cement plug in the wellbore can't be destroyed. According to the basic theory of elastic-plastic mechanics, the cement plug-casing-cement sheath-formation combination model was established. The reliability of the model is verified through the utilizing of the finite element method to calculate the sealing capacity of cement plug under different pressure difference, which is consistent with Nagelhout's experimental results. The effects of the elastic modulus, Poisson's ratio and thermal expansion coefficient of the cement plug on the integrity of the cement plug seal were analyzed. The results show that the lower the elastic modulus, the lower the risk of cement plug failure; and the Poisson's ratio has little effect on the cement plug integrity; when the thermal expansion coefficient of the cement plug is less than the thermal expansion coefficient of the stratum, the cement plug is prone to sealing failure; When The coefficient of thermal expansion is greater than the coefficient of thermal expansion of the stratum formation, the possibility of sealing failure of the cement plug is small. According to the construction conditions on site, properly reducing the elastic modulus of the cement plug, and trying to use cement plugs with large thermal expansion coefficient can help cut the possibility of cement plug sealing failure risk down. The established finite element model and the research results can provide reference for the design of cement plugs during the process of wellbore disposal.
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