Novel assessment and countermeasure for micro-annulus initiation of cement sheath during injection/fracturing

Abstract

The excessive changes of downhole pressure and temperature during injection/fracturing could initiate micro-annulus of cement sheath as the leakage pathway of underground fluid, which increases the risk of suspending production and environmental pollution. The investigations on accurate prediction, operational management and new technologies are significant to sustain well integrity and to enhance safety and efficiency in hydrocarbon and geothermal developments. In this work, a novel fully coupled thermal-stress model is proposed, which can predict the micro-annulus initiation and extension in the casing-cement-formation system. The interfaces of cement sheath with casing/formation are simulated using the contact interaction containing cohesive behavior and damage. The surface film condition is set on the internal wall of casing to simulate the heat transfer due to cold-water flow. This model is validated by the laboratory experiment and shows high accuracy. Results indicate that the maximum micro-annulus occurs during depressurization process after injection. It is newly found that the debonding could also emerge at the beginning of cold-water circulation in casing. The thermal effect, compared with pressure, is the dominated factor on micro-annulus initiation and size, while the pressure only causes a small plastic micro-annulus. For countermeasures, enhancing tensile strength is not an effective approach to prevent micro-annulus. By appropriately assigning the injection procedure, the micro-annulus can be eliminated in injection/fracturing operations. The findings can supply the technique support for well integrity assessment and management during hydraulic fracturing or high-pressure injection and production.