Probabilistic Cost and Time Estimation of Rigless Plug and Abandonment
Authors: Carpenter Chris 
Published: February 2015

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 169203, ’An Investigation of Different Approaches for Probabilistic Cost and Time Estimation of Rigless Plug and Abandonment in a Subsea Multiwell Campaign,’ by Fatemeh Moeinikia and Kjell Kare Fjelde, SPE, University of Stavanger; Arild Saasen, SPE, Det Norske Oljeselskap ASA and the University of Stavanger; and Torbjorn Vralstad, SPE, SINTEF, prepared for the 2014 SPE Bergen One Day Seminar, Bergen, Norway, 2 April. The paper has not been peer reviewed.
With a high demand for plug and abandonment (P&A) of subsea wells in the future on the Norwegian continental shelf, industry is challenged to find alternatives and rigless technologies that can make P&A operation more cost-effective and -efficient. Light-well-intervention vessels (LWIVs) can take over some parts of P&A operations and release semisubmersible rigs to function in the drilling and completing of new wells. This paper covers potential and experience of LWIVs in P&A.
Introduction
The P&A work of a single well can be separated into three distinct phases: reservoir abandonment, intermediate abandonment, and removal of the wellhead and the casing strings a few meters below seabed (Oil and Gas UK 2011). Different vessels and technologies will be applied for these phases, which are also affected by the complexity of the well. It is also expected that these P&A phases to some extent will be performed in multiwell campaigns in which issues such as learning effects and correlations have to be considered.
The deterministic approach for time and cost estimation is a simple, traditional method, and the results can be transferred easily. There are some limitations associated with the deterministic approach, such as the yielding of biased results and, sometimes, the inability to capture the full range of outcomes.
On the other hand, the probabilistic approach has been recognized recently as a common and preferred technique for time and cost estimation, compared with the deterministic approach. This approach presents a nonbiased technique for time and cost forecasting and allows for inclusion of uncertainty into the model, covering the correct range of possible outcomes.
Oil and Gas UK (2011) provides a guideline for well-abandonment-cost estimation, aiming to provide a simple and structured approach for estimating cost and duration of field abandonment. As mentioned previously, the P&A work of a well can be divided into three distinct phases according to these guidelines. In addition, the complexity of each phase is reflected in the types of vessels and technologies that are required to perform the work scope of that specific phase. Five categories are used for this classification.
P&A operations will mostly be performed in multiwell campaigns, and different vessel technologies are used to complete distinct operations or phases. Because P&A of the wells can typically involve using the same vessels and technologies for distinct phases, performance of P&A in multiwell campaigns shares mobilization and demobilization time and cost as well as transit time from mobilization point to field and transit time among different wells.