Risk Based Approach to Well Plugging & Abandoning - Reducing Costs While Verifying Risk

Abstract

When a well comes to certain point in the operating life cycle, it needs to be plugged and abandoned (P&A). The ultimate objective of well P&A is to isolate the substance formations penetrated by the wells, thus protect the environment and maintain safety standards. There are significant variations in a well designs, P&A technologies and techniques and company standards. The owner of the well must select the optimum solution for P&A that ensures that the risk of pore fluids reaching the seabed is low and acceptable. Finding the balance between the number and type of well barriers versus the cost of well P&A is a challenge in an environment when oil and gas prices are relatively low. In order to address this challenge a risk based approach to well P&A has been developed and successfully applied. This paper summarizes a case study that compares a conventional P&A approach to new novel methods based on the body of work that DNV GL has participated in to date. For several years, a small number of operators in the North Sea have applied a risk based approach to well P&A with the support of DNV GL. This approach has been summarized in to the publically available recommended practice DNVGL-RP-E103 which has a systematic method that demonstrates how the number of well barriers can be reduced hence saving costs while demonstrating that the risk level does not increase. The recommended practice is based on research that allows the modeling of fluid flow through microcracks via a range of failure modes in downhole components, the impact to the environment via dispersion modeling and the basis for acceptance criteria. The paper describes how various P&A designs can be compared via a risk methodology taking account of degradation mechanisms, potential flow rates and the impact to the environment. In the P&A projects completed to date using DNVGL-RP-E103 cost saving of 50% or more have been realized. The method has also helped to align with or overcome regulatory requirements in the North Sea and supplemented internal operator requirements. By providing sound physical models coupled with an objective risk acceptance criteria the risk based approach has proven to address the concerns of stakeholders.