Large-Scale Laboratory Investigation of the Microannulus Behavior in the Casing-Cement Interface

Maintaining the integrity of the annular cement in the wellbore is paramount in successful hydrocarbon exploitation, subsurface energy storage, geothermal energy production, and geologic carbon sequestration. Debonding at the casing-cement interface can create connected flow paths for fluid leakage along the well leading to loss of zonal isolation. Reliable estimates of potential well leakage rates require large-scale experiments at representative wellbore conditions. We investigated the behavior of the cement microannulus under various loading conditions on two-meter long casing segments cemented against a rock analogue. The results show that once a microannulus forms, it remains open at casing pressures as high as 40 MPa. The normal stiffness of the microannulus at the casing-cement interface ranged between 50 and 900 GPa, while the shear stiffness ranged between 0.15 and 0.22 GPa. Axial displacement of the casing did not lead to a significant change in the aperture. However, axial loading in presence of a casing coupling reduced the hydraulic aperture. The results of this work indicate an agreement between experimental leakage rates, model predictions, and leakage rates measured at (abandoned) well sites reported in the literature. The laboratory results on the largescale samples provide benchmark data for validating well integrity models.