ABSTRACT: Micro-annuli existing on the interface of cement-sheath might lead to the failure of wellbore integrity. Based on the solutions of the elastic and elastic-plastic deformations of a thick-walled cylinder with the internal and external pressures; meanwhile combining with the displacement continuity, a nonlinear equation set about the interfacial pressures and displacements of a casing-cement-formation system in the loading and unloading processes is established. Via solving the nonlinear equation set, the interfacial pressures and the displacements can be obtained. If the obtained interfacial pressure is larger than the bonding strength of rock, micro-annuli appear on the interface, and its size is equal to the difference of the interfacial displacement in the loading process and that in the unloading process. Numerical examples show that the micro-annuli are at an order of tens of micrometers. Moreover, as the increase of the elastic modulus of cement, the micro-annuli become broader. As a comparison, if the uniaxial compressive strength of cement increases, the micro-annuli become narrower. Therefore, to avoid the generation and development of micro-annuli, it is suggested that the cement with high uniaxial compressive strength and low elastic modulus should be used for the field practices.
Wellbore integrity needs to be investigated in the whole life cycle of a well, and the sealing of the cement sheath is the key. In the completion and production process, a variety of reasons can lead to the sealing failure of the cement sheath. For instance, both the wellbore uncleanness and the residual mud cake can result in the reduction of the cementing quality. During the hardening process of cement slurry, gas channel formed by weight loss can trigger the sealing failure. Due to the rapid change of the internal casing pressure, or the variation of the casing temperature, the cement interface can also separate from formation.
Bois et al  mentioned that the loading and unloading process of the internal casing pressure is one of the key factors affecting the cement sealing. During the oil test, pressure test, or hydraulic fracturing, etc., the loading and unloading process of the internal casing pressure is inevitable. Under these situations, the cement will experience the similar process. Once the load applied on the cement sheath overcomes its elastic limitation, the cement sheath will appear plastic deformation, which will be remained after unloading, thus microannuli form on the interfaces, including the first and the second interfaces.