Risk based cost and duration estimation of permanent plug and abandonment operation in subsea exploration wells

Abstract

World wide there are thousands of subsea wells to be abandoned, including the subsea wells in the North Sea. The operation to abandon a well is commonly known as Plug and Abandonment or P&A. Traditionally for offshore subsea wells the Plug and Abandonment operation is done by a semi submersible drilling rig. The cost of such operation for a single well would not be a problem for an operating company, but considering the huge amount of wells that needs to be abandoned new technologies and methods needs to be developed and implemented in the future.

This thesis will discuss the process of a permanent P&A operation in the North Sea for subsea exploration wells. This includes the regulations, technologies, challenges and methods. New technologies of P&A operation such as using intervention vessels for P&A operations will also be explained.

Determining which method to be used in a P&A operation is important. This thesis will demonstrate that by using Monte Carlo simulation one could forecast the cost and duration of a method for P&A operation and compare the results with other methods. In this thesis the cost and duration forecast is done by a Monte Carlo based software called RiskE. This software is developed mainly for drilling activity forecasting, using it for P&A forecasting reveals room for improving the software for P&A operation. Further studies for the simulation input, detailed operations and risk analysis are needed to build a more specific and accurate simulation.

This thesis will also discuss that cost and duration is not the only criteria in choosing what method is used for a P&A operation. Saving the rig time for drilling and completion activity instead of P&A operation and avoiding delay caused by waiting on weather by suspending the well and re-entry in a better weather window with an intervention vessel could be some consideration besides cost and duration. By using semi submersible drilling rigs, intervention vessels and rig chase vessels or combination of them in the P&A operation, different forecast outcome with its advantages could be known and analyzed. Batch P&A operation and factors that affect the cost and duration forecast of a batch P&A operation will also be described.