Abstract
Tensile hoop stresses in the cement sheath may induce radial cracks. Tensile radial stresses may cause debonding at cement-steel or cement-rock interfaces. Casing pressure changes and in situ stress variations (caused by production or injection) may cause tensile stresses in cement. Whether or not this occurs depends on the initial stresses in cement. The uncertainties involved in the evaluation of initial stresses in cement sheaths are discussed. Results of finite-element simulations of stress changes in cement caused by casing pressure changes and in situ stress changes are presented. The effect of uncemented channels (e.g. gas channel or mud channel) on stresses in the cement sheath is demonstrated. An uncemented channel acts as a stress amplifier, increasing the variations of cement stresses by a factor of 2–3, as compared to an intact cement sheath. The effects of anisotropic in situ stresses on the formation and shape of microannulus are discussed. Hysteresis of microannulus permeability during repeated loading/unloading of the cement-casing-rock system is outlined.
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