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Whether zonal isolation is effective or not, is always an issue attracted much attention during oil exploration and development. Moreover, it concerns the whole oil well's long-term, safe and efficient production. The integrity of cement sheath is one of the important factors concerning zonal effective isolation. A large number of field examples and laboratory studies have shown that the variation of cement sheath stress caused by later operation is the major reason leading to the damage of cement sheath's integrity. The variation of wellbore temperature and pressure during different operating jobs(such as fracturing stimulation, acid treatment, heavy oil thermal recovery, etc) will influence the magnitude and distribution of cement sheath stress. Excessive stress will lead to the damage of cement sheath. Therefore, an analysis about cement stress considering variation of wellbore temperature and pressure during production process has very significance for forecasting the integrity of cement sheath and zonal effective isolation. For the first time this paper has comprehensively considered the influence of non-uniform in-situ stress field, temperature field and pressure field in casing, built a coupling mathematical analysis model of wellbore cement sheath. Through the methods of theoretical analysis of elastic mechanics, calculating programme programmed by computer language and graphical output, this paper presents a detailed analysis about the magnitude and distribution of cement sheath stress under non-uniform in-situ stress considering the variation of pressure in casing coupling with the influence of temperature. The study shows that increase of pressure and temperature in casing will remarkably change the magnitude and distribution of cement sheath stress. Specially, the inducing increase of tensile stress magnifies the fracturing probability of cement sheath. The research findings in this paper can be used to forecast the integrity of cement sheath during the performance history. Moreover, it can be a important guideline for the design of cement slurry. |
