A study on the hydraulic aperture of microannuli at the casing–cement interface using a large-scale laboratory setup

Presence of microannuli at the cement interfaces in wellbores could provide a path for fluid leakage. In this study, we combined experimental analysis and numerical modelling to assess microannuli development at casing–cement interfaces. Lab experiments were conducted using two-metre long samples consisting of a 7” casing cemented inside a rock analogue (stiffer, stronger cement) encased with a steel outer shell. Hydraulic aperture of the microannuli was calculated by measuring water flow through the casing–cement interface at various casing pressures and after axial displacements. The results show that once a microannulus forms, it remains open at internal casing pressures as high as 40 MPa. The measured hydraulic apertures were in the range of tens of microns, with residual apertures in the range of 15 to 30μm. Axial displacement of the casing did not lead to a significant change in the hydraulic apertures. A numerical model was created with a comparable geometry to the experiments, and used to calculate the mechanical apertures of the microannuli. A relationship is proposed to link the mechanical apertures from the numerical models to the hydraulic apertures measured in large-scale experiments. The findings enable the operators and regulators to improve safe well operation practices by forecasting the conditions that lead to possible loss of zonal isolation and the associated well leakage rates.