Leakage Through Micro-Annulus Geometries Incorporating Pressure-Driven Elastic Deformation

The cement between the casings and formation is a critical barrier element for ensuring zonal isolation. Shrinkage during curing and mechanical or thermal loads applied during production can compromise the cement and result in fluid migration paths such as micro-annuli. The fluid pressure inside the micro-annulus will cause elastic deformation of the channel walls. This deformation should be accounted for when developing methodologies for interpreting micro-annulus fluid leakage experiments and the application to real well conditions. Full-size test sections have been constructed with known cement defects and leakage properties to investigate barrier verification technologies. A micro-annulus test cell, instrumented with strain and pressure gauges, has been leakage tested. Leakage rates have been correlated to the micro-annulus size using a model coupling micro-annulus pressure to radial deformation of the cement and casing. The semi-analytical model and the predictions are compared to the experimental data. Within the regime of linear elasticity, the radial deformation of the cell wall is proportional to the pressure in the micro-annulus. During leakage testing, the pressure-driven radial deformation of the cell materials is coupled to the variation of the liquid friction pressure gradient along the axial length of the micro-annulus. The pressure gradient is greatest at the outlet of the micro-annulus. The models presented have been used to improve the interpretation of fluid flow during micro-annulus leakage experiments. An improved understanding of fluid leakage mechanisms through micro-annuli can be applied to field cases such as the interpretation and choice of treatment for sustained casing pressure build-up.