The Effects of Reservoir Deformations on the Integrity of the Cement Plugs in Abandoned Wells

Maintaining the integrity of the cement plugs within plugged and abandoned wells is of utmost importance to prevent formation fluid movement among different strata and consequently contamination of aquifers. In-situ stresses both within the reservoir and in the surrounding strata (including the caprock and the sideburdens) can change due to reservoir depletion during production or reservoir re-pressurization after abandonment. Therefore, the correlation between field and reservoir scale geomechanics and wellbore scale deformations should be explored. In this paper, the cement plug state of stress and the potential damage caused by reservoir deformations are investigated through utilizing a multi-scale approach and by employing the concrete damage plasticity (CDP) model for the cement plug. Different scenarios were developed and examined by utilizing different mechanical properties for the reservoir and other strata. During reservoir depletion and subsequent reservoir compaction, the compressive failure mode is the dominant failure mode within the cement plugs. While during reservoir re-pressurization (given that the reservoir overlies an active aquifer) tensile failure mechanism is shown to be the dominant failure mode within the cement plug. The numerical analyses also show that the propensity and magnitude of compression damage and tensile damage within the cement plugs in abandoned wells are highly conditional on both the wells' locations within the reservoir (i.e. proximity of the sideburdens or near the central parts of the reservoir) and the mechanical and hydraulic properties of the reservoir and the caprock. The magnitude of global compression and tensile damage indicator is influenced by the comparative stiffness of the caprock and the reservoir. The global compression damage indicator is higher for scenarios in which the difference between the caprock stiffness and reservoir stiffness is higher. While the global tensile damage indicator is higher for scenarios in which the reservoir stiffness is remarkably higher than the caprock stiffness.