Techniques and Materials for North Sea Plug and Abandonment Operations

Abstract

Abstract Plug and Abandonment (P&A) can easily contribute with 25% of the total costs of drilling exploration wells offshore Norway. The costs of running a P&A operation on some offshore production wells may have a cost impact similar to the cost of the original drilling operation. Therefore, cost efficient P&A technology is necessary to reduce cost. In this paper, qualified technology and materials for P&A is reviewed to act as a basis for P&A planning. This paper describes in detail requirements for P&A operations offshore Norway as well as normal practices for P&A. Well construction issues related to minimising the need for later P&A operations like proper primary cementing planning is discussed. P&A techniques and materials for P&A operations are reviewed to act as background information for future P&A planning on the Norwegian continental shelf. These techniques include operations ranging from milling to washing and squeezing cement or other materials. The materials being reviewed include swollen shale, concentrated sand, metal plugs like bismuth, as well as improved conventional cement systems. This work presents an overview of the well plugging and abandonment challenges, innovative potential barriers and case histories on the Norwegian continental shelf. Introduction Oil and gas wells at some time in their life will be plugged and abandoned. Improperly abandoned wells can become a significant threat to the environment if they are not constructed or sealed properly. Plugging material (barrier) may fail during or after placement. Barrier failure can be caused by: natural fractures/faults, tectonic stresses, improper plug placement or materials used. In brief; isolation outside casing, collapses and sustained casing pressure are main well integrity challenges with respect to Plug and Abandonment (P&A). The quality and performance of a P&A operation may be investigated by two points of view; type of plugging material and plug placement technique. Barriers may be categorized as cementitious or mechanical barriers. It should be noted that mechanical barriers are used as temporary P&A barriers and not allowed to be used as permanent barriers. Type of material used defines the type of plug placement technique. There are several cement plug placement techniques that are used in the abandonment process (Smith 1993; Nelson 2006):Balanced method,Dump-bailer method,Two-plug method, andJet grouting. The following tools are used for placing cement plugs:Flexible bags,Inflatable through-tubing packers, andUmbrella-shaped membranes (Nelson 2006). Local practices, government regulations, company policies and well conditions lead to variations of the above-mentioned tools and methods.