Leakage Calculator for Plugged-and-Abandoned Wells

Abstract

Summary The current practice on the Norwegian Continental Shelf (NCS) when designing solutions for permanent plug and abandonment (P&A) complies with NORSOK Standard D-010 (2013). This is a prescriptive approach to P&A, as opposed to a “fit-for-purpose” risk-based approach. A risk-based approach means that any given P&A solution is expressed in terms of the leakage risk, which can be formulated in terms of the following quantities: the probability that the (permanent) barrier system will fail in a given time period, and the corresponding consequence in terms of leakage to the environment. As part of building a leakage-risk model for permanently plugged-and-abandoned wells, a simple leakage-rate calculator has been developed for quick evaluation of the leakage potential from a given (permanent) well-barrier solution. This tool is developed to serve the second aspect of the risk-based approach: the consequence in terms of leakage rate to the environment. The leakage potential from the well can then be quantitatively assessed, taking into account different leakage pathways including leakage through bulk cement, through cement cracks, and through microannuli along cement interfaces. In the paper, we will provide models to estimate leakage rate for each leakage pathway and show how to integrate them in the leakage calculator to obtain a description of leakage flow from the reservoir through failed barriers to the environment. The information and input parameters needed to achieve this will be discussed, and uncertain parameters will be treated probabilistically, thus allowing for expressing uncertainty in the leakage-rate estimate. Results from the leakage calculator will be demonstrated on a synthetic case, showing variants of a permanently plugged-and-abandoned well.