Application of Decision Analysis to a Deepwater Well Integrity Assessment
Authors: Pattillo P.D. , Payne M.L. , Webb T.R. , Sharadin J.H. 
Published: October 2008

Decision analysis is used to compare the risk of completing an existing, exploratory wellbore to the alternative of permanently abandoning the wellbore in favor of drilling a new well. The existing wellbore is endangered by uncertainties in the sealing capabilities of the intermediate and production casings, and by unknown cement tops. Failure of these tubulars during production could potentially damage the production tubing or, in the extreme, lead to breach of hydrocarbons at the mudline.

Decision analysis is used to quantify the uncertainties in the problem and, by systematic means, evaluate the risk associated with each decision alternative. The model also identifies the key variables influencing the outcome of the analysis.

Conclusions focus on preparing oneself for the eventuality of such a decision by properly appreciating the importance of diagnostic tools and logs during the initial drilling operation.

Introduction

An increasingly important issue in deep water exploitation is the evaluation of exploration wells as possible production wellbores. It is not uncommon for the necessities of the exploration phase of a project to dictate patches, sidetracks and other repairs to a wellbore that may safely serve to prevent interruption of the exploration process, but may render the wellbore unusable for long term production. The disposition of a deep water exploration wellbore is further complicated by the expense of well construction. On one hand, it is difficult to abandon a wellbore that has cost tens of million of dollars to drill. On the other hand, the potential consequences of a failure of the well during its production life cannot be ignored. Central to this evaluation is consideration of well integrity issues associated with annular fluid expansion (AFE). Also known as annular pressure buildup (APB), this mechanism has caused well failures and is considered a possible cause of the BP Marlin tubing deformation1-3. Proper assessment of risks created by AFE is complicated and requires consideration of a number of different failure scenarios. A final complicating factor in the case of a temporarily abandoned site is the expense, or possible inability, of clarifying the decision process with diagnostic tools or logs.

The scenario outlined above can be aided by the tools and processes of decision analysis. Uncertainties in the problem can be quantified by probabilistic modeling and analysis. Further, the relative influence of key variables can be discerned so that, if necessary, additional data gathering can concentrate on the more important variables. Finally, the process can aid in estimating the risk associated with each alternative, leading to measures intended to minimize corresponding consequences.

Decision analysis is not a new topic to the drilling community. Decision trees have been used for a variety of drilling activities including fishing operations4,5, well control6 and trajectory collision avoidance7-9. This study supplements that list with an application of decision analysis to overall well integrity.

Terminology. Throughout this discussion two wells will be distinguished. The term "current well" refers to the damaged wellbore existing at the time the decision analysis was initiated.