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Abstract

When drilling highly deviated and horizontal wells in depleted fields there are many factors that can contribute to the possibility of a stuck pipe. The bottomhole assembly often includes a logging-while-drilling tool with radioactive sources for measuring formation porosity and density. With the ADN* Azimuthal Density Neutron tool it is possib le to retrieve the radioactive source from within the tool when the tool becomes stuck in the wellbore. This is the most economic and environmentally friendly method of dealing with radioactive sources inside a stuck bottomhole assembly.

In the Statfjord field in Norway, radioactive sources have been retrieved from ADN tools stuck in three horizontal wells. One source was retrieved with a coiled tubing. The other two were retrieved with an electric Wireline. This paper describes the methods used and the experiences gained from each fishing job. The last retrieval took only 12 hr and allowed the operator to abandon the string immediately without having to perform any fishing attempts on the string itself to comply with the Norwegian regulations when a radioactive source is stuck down-hole.

Using the experiences gained from the radioactive source retrieval operations on Statfjord, fishing operation from the Visund field and other similar fishing operations performed worldwide, this paper gives recommendations for fishing operations in highly deviated wells. These recommendations include type of fishing device (coiled tubing, wireline, LWD Inductive Coupling tool), setup of the fishing assembly and other parameters relevant to the operation.

Introduction

During the last 10 years, the oilfield industry's main challenge has been to increase production in either depleted reservoirs or in small reserves. As such the complexity of the well paths has increased tremendously, involving more horizontal wells, long reach wells, or even more “U” or “W” shaped wells. Due to the complexity of these wells, Logging While Drilling (LWD) has become the preferred method of formation evaluation as it seems to involve less inherent risk in data acquisition. With the increased complexity of the well paths the possibility of stuck pipe scenarios will also increase.

At the same time, environmental considerations are becoming a major issue as one is concerned by having to leave radioactive material buried in sediments below the sea. In the Norwegian sector of the North Sea the amount of “non conventional” wells drilled during the last 10 years versus the regular directional wells has increased considerably. At the same time the number of nuclear tools stuck reached an amount of approximately 40 and the number of radioactive sources left in the wells was approximately 30[1].

Since designing the first of the Schlumberger nuclear LWD tool, the primary concern of Schlumberger engineering has been to be able to retrieve radioactive sources in the event the tool would have to be buried on the ground. The first CDN (Compensated Density Neutron) design already shown the feature of having a retrievable source[2]. Such engineering not only involves a tool design such the source can be retrieved but as well involve side equipment to be used in order to support fishing jobs. At all times the safety of the operation must be of primary concern.

The aim of this paper is to demonstrate the advantage of such future as well as to describe the safe procedures being involved with radioactive source fishing.

Radioactive Sources and Environment

In Norway the Norwegian Radiation Protection Authority (NRPA) and the Norwegian Petroleum Directorate (NPD) has expressed concern about the increased use of radioactive sources that are experienced in the North Sea [3,4]. Special focus is put on the sources used in logging operations as an increasing trend in sources left down-hole is observed.