Numerical study on debonding failure of cement plug/formation interface in open-hole temporary abandoned wells

Abstract

The migration and leakage of formation fluid along the cement plug/formation interface negatively affects the integrity of open-hole temporary abandoned wells. A 3-D numerical model is developed based on the cohesive zone method for analyzing the initiation and propagation of fractures along the cement plug/formation interface. The model is validated by comparison with an existing microannulus model to demonstrate its reliability. The relationship between fracture development, propagation pressure, and fluid migration time in the damage evolution of the interface is quantitatively analyzed, and the influences of different mechanical parameters on fracture propagation pressure and debonding height are studied, including cement plug mechanical parameters, formation mechanical parameters, horizontal in-situ stress, and interface cementation strength. The simulation results indicate that (1) high interface cementation strength, cement plug elastic modulus, and formation elastic modulus tend to increase fracture propagation pressure and decrease fracture debonding height, which helps to mitigate debonding failure of the cement plug/formation interface. (2) Large horizontal in-situ stress can constrain fracture propagation when the stress is uniform. For a non-uniform horizontal stress, a large in-situ stress difference is more likely to cause fracture extension. (3) The fracture propagation pressure and fracture debonding height of the cement plug/formation interface are less affected by the Poisson’s ratio than the elastic modulus of the formation and the cement plug. The numerical model established in this study provides an effective method to evaluate the well integrity failure of open-hole temporary abandoned wells due to debonding failure of the cement plug/formation interface.HighlightsA 3-D numerical model of fracture debonding of the cement plug/formation interface was developed.The potential of debonding failure can be mitigated by improving interface cementation strength and increasing the elastic modulus of the cement plug and formation.Large non-uniform horizontal in-situ stress and small uniform horizontal in-situ stress tend to result in fracture extension.The Poisson’s ratios of the formation and the cement plug have little effect on interface bonding failure.