Well integrity requires isolating fluids in the subsurface, which is usually accomplished by placement of Portland cement in the annulus between the steel casing and the surrounding rock. Achieving zonal isolation depends on a combination of: 1) the quality of the placement of cement; 2) the initial state of stress of cement; and 3) the magnitude and character of stresses induced by subsequent well operations. Here we examine the factors controlling the initial state of stress of cement and their impact on microannulus formation at the cement-steel interface by fluid injection. We discuss the evolution of cement state of stress from liquid slurry to solid in the light of theoretical poromechanics, experimental studies of cement, and recent experiments on state of stress. We show that the effective interface stress is equal to the effective stress of the cement. Although previous estimates of the effective state of stress of cement range from zero to the full weight of the cement column, shrinkage, consumption of pore fluids by hydration, and loss of pore fluids to the formation result in intermediate values consistent with experimental results on microannulus formation.
Leakage of wells along the interface between cement and steel is one of the primary failure mechanisms for loss of zonal isolation (e.g., Bellabarba et al. 2008; Bois et al. 2011). While cement bond logs have been used for many years (Boyd et al. 2006) to characterize the possible existence of a gap (or microannulus) between steel and cement, there is significant interest in understanding what stresses lead to microannulus formation.
The majority of work in this area has been in terms of numerical modeling of the impacts of pressure and temperature on microannulus formation (e.g., Feng et al. 2017; Taleghani and Klimenko 2015). Among other findings, these studies have shown how contrasting material properties of steel and cement can either enhance or suppress microannulus formation. Shear failure has been considered by (Dusseault et al. 2001; Frash and Carey 2018) and can result from differential vertical movement of casing, cement or the surrounding rock (e.g., as occurs during fluid injection or production).
