How To Prevent Loss of Zonal Isolation Through a Comprehensive Analysis of Microannulus Formation

Microannuli at the well cement-sheath interfaces may result in loss of zonal isolation, which is the source of many problems, such as sustainable annular pressures, crossflows between reservoirs, and undesirable flow behind the casing. The microannuli are commonly explained by variations in cement volume during hydration (chemical shrinkage/expansion) or by contraction of the casing because of a decrease in mud density/temperature because these could create a gap if the cement is unable to follow the induced deformations. However, these two modes are not sufficient to predict all possible types of microannuli encountered in oil and gas wells, meaning that other modes have been missed. This paper presents a comprehensive mechanistic analysis of microannulus formation to highlight and explain other modes and to detail the conditions under which they can appear. It is grounded in both theoretical and experimental evidence and takes into account most of the features that characterize cement after it has been placed, including cement volume variations and heat production during hydration, mud-density and temperature variations, cement thermo-poro-elasto-plastic behavior during and after hydration, thermo-poro-elasto-plastic behavior of the formation, and initial state of stress in the formation.