A Method for Probabilistic Time Estimation of Plug and Abandonment of the Wells on the Brage Field

Abstract

The production on several fields on the Norwegian continental shelf (NCS) is moving towards the critical point where expenses exceed the production income. Operating past the economical limit is not viable and thus, the operator may be forced to permanently abandon the wells. The process of abandoning a well, commonly referred to as Plug and Abandon (P&A), has been given a lot of attention in the industry recently. This relates to the expected prohibitive cost that will, in addition to impacting the operators and license partners, affect the Norwegian taxpayers.

Due to the large uncertainties related to any well operation, the establishment of accurate and reliable cost and time estimates are important, both in relation to the asset retirement obligation, and for achieving an approval for the expenditures. Traditionally, time estimates have been conducted in a deterministic way. However, performing probabilistic time estimation may provide several advantages in terms of expressing the uncertainties and understanding the risks that are associated with a project [7].

The task provided by Wintershall was to establish a probabilistic time estimate of the P&A for one of their operating fields, Brage. Since the peak in 1996, the production on Brage has been decreasing and is predicted to be shut down in 2030.

The wells on Brage have been categorized in terms of casing design and required abandonment operations. Based on this categorization, 12 operational procedures have been established to serve as basis for the time estimates. A probabilistic model including risked events and learning curve has been established and by performing a sensitivity analysis, the most critical operations have been identified.

Using this model, P&A of the 40 wells on Brage is estimated to last for 960 days. The importance of including learning and unplanned events can be seen through the effect on the time estimates. In addition, the sensitivity analysis has identified the most critical part of the P&A project, namely the uncertainty related to the green clay’s bond to casing. The operations related to section milling and retrieval of tubulars are also subject to uncertainty and could potentially cause severe non-productive time events.

To reduce the risks related to the future P&A project, technologies such as SwarfPak by WestGroup, HydraHemera by HydraWell and Sabre cutting system provided by Claxton should be followed up and evaluated.