Plug & abandonment (P&A) regulations on the Norwegian continental shelf are largely prescriptive, since the same requirements are applied irrespective of well conditions. A risk-based approach on the other hand, is a well-specific approach to assess the quality of a given plug and abandonment design solution. Probability of leakage and consequence, in the form of leakage rates to the environment, should be quantified for permanently plugged and abandoned wells in a risk-based approach.

To address the consequence aspect of a risk-based approach, a tool for quantitative leakage assessment is essentially needed. This should cover all leakage pathways for reservoir fluids to the environment, i.e. leakage through the well and leakage outside the well through the surrounding formation. The integrity of the cement barrier could be weakened as a result of e.g. poor slurry design, tensile stresses and shrinkage, creating leakage pathways through the bulk cement, cracks and micro-annuli along cement interfaces. As for the surrounding formation, geological features such as faults and fractures, as well as the sealing ability of the cap rock, are important factors to consider from a barrier integrity perspective. Fractures or faults might intersect a permeable formation at a shallow depth, potentially enabling reservoir fluids to migrate into the wellbore or to the seabed.

The authors have developed a leakage assessment simulator, to quantify leakage rates for permanently plugged and abandoned wells. The structure and models incorporated in the preliminary version of the simulator, covering only leakage pathways through the wellbore, was previously presented in SPE-185890-MS.

The current study builds on the previous paper by also accounting for leakage scenarios outside the well through the surrounding formation. The structure of the leakage assessment simulator is also presented in this paper. Additionally, features that may significantly reduce leakage rates, such as barite plug are addressed. A synthetic case is presented where leakage rate is estimated for two scenarios, a) leakage through the well only and b) leakage through the well and the surrounding formation.

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