Study On Stress Distribution Of Cementing Ring In Directional Well

Annulus with pressure and gas channeling are a worldwide problem, which is mainly caused by wellbore integrity failure. Cement mantle mechanical integrity is an important component of the wellbore integrity. It will be damaged by change of the casing pressure and strata stress in the process of oil and gas well development, which leads to sealing failure in the later production. The cement mantle stress distribution model is mainly established for vertical well, which can’t make reasonable evaluation for directional well cement mantle. Therefore, developing a research on the stress distribution model of directional cement mantle has a great significance to ensure complete cement mantle structures.Based on the predecessors’ research of results, according to the process of building a well the change of slurry state and the interaction relationship of the cement mantle between casing and hole wall, speculated that the cement mantle exists the initial stress at the end of waiting on cement. Considering the structure characteristics of surrounding rock and stress distribution similarity of the casing-cement mantle-hole wall combination with the structural mechanics and the displacement continuity conditions, the theory of thick wall cylinder and generalized plane stress strain is used to solve the different sub problems. The initial stress state of cement mantle is the starting point, and directional well cement mantle stress distribution model is established by forcing the increasing casing pressure and strata stress superposition on it. Besides, the tensile failure criterion and Mohr-Coulomb criterion are used in the model to analyze the cement mantle mechanical integrity.The paper uses the directional well cement mantle mathematical stress distribution model to analyze the cement mantle stress distribution regularity when the casing pressure and strata stress change in the later production of two wells in Tarim oil field. Some results have been achieved by using the tensile failure criterion and Mohr-Coulomb criterion to evaluate the cement mantle mechanical integrity. The main results formed from this article as follows:(1) According to the change of slurry state and the interaction relationship of the cement mantle between casing and hole wall waiting on cement, the cement mantle initial stress at the end of waiting on cement is equal to the formation pore pressure.(2) According to the theory of thick wall cylinder and generalized plane stress strain, using the thinking of structural mechanics, and putting the initial stress state of cement mantle as the starting point, by forcing the increasing casing pressure and strata stress superposition on it, directional well cement mantle stress distribution model is established.(3) The model of directional well cement mantle stress distribution can be used to analyze the regularity of cement mantle stress distribution under different working conditions, and the criterion of tensile failure and Mohr-Coulomb are used to judge cement mantle mechanical integrity.(4) With the fixing of the other conditions, the angle of inclination is larger, the maximum main stress is larger, and the maximum shear stress is lower. Azimuth has little influence on the cement sheath stress distribution. (5) The cement mantle does not have a tensile failure, but it will have some plasticity yield when the casing pressure and formation stress changes within a certain range through the calculation of field examples. If the deformation of cement mantle exceeds plastic yield limit or the internal pressure reduce after the yield of cement sheath, it will lead to cement mantle isolation failure.