PREDICTION OF MICROANNULUS BEHIND CASINGS DUE TO BALLOONING EFFECT—A KEY PROBLEM IN GAS WELLS

Abstract

Casings are pressurised for various reasons during cementing. This pressure inflates casing just like a balloon. As the cement sets and the pressure is released, the elastically deformed casing tends to come back to its original shape leaving behind a microannulus at the cement casing bond. This annular space between cement and casing is too narrow to allow communication of liquids, nevertheless it is permeable enough for gas to flow through. An 8-node, quadrilateral, thin shell finite element model with plane stress and plate flexure capabilities has been used to study the formation of a microannulus during cementing of gas wells. The results of this study suggest that formation of a microannulus is a function of differential pressure across the wall of the casing, steel properties, casing dimensions and cementing temperature.