Wireline-Tractor Milling Operations From a Riserless Light Well-Intervention Vessel
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 121481, ’Wireline Tractor Milling Operations From a Riserless Light-Well Intervention Vessel at the High-Temperature Asgard Field,’ by Henning Juel, SPE, Havard Ranum, and Svein Mjaaland, SPE, StatoilHydro, and Terje Varne, Dag Birger Solberg, and Rune Bjorndal, Aker Solutions, originally prepared for the 2009 SPE/ICoTA Coiled Tubing and Well Intervention Conference and Exhibition, The Woodlands, Texas, 31 March-1 April.
The full-length paper describes a case history of the first use of wireline-tractor scale-milling technology from a riserless light well-intervention (RLWI) vessel. The well in the high-temperature åsgard field had a history of calcium carbonate (CaCO3) scale buildup in the production liner. A well intervention in 2003 with coiled tubing failed to remove the scale bridges completely. The objective of the intervention was to remove a 23-m CaCO3-scale bridge to provide access for installation of a high-pressure/high-temperature (HP/HT) bridge plug and permit additional perforation work.
Introduction
The åsgard development in the Haltenbanken area off mid-Norway consists of three independent discoveries (Smørbukk, Smørbukk Sør, and Midgard) connected to two floating production installations and a condensate-storage vessel. åsgard is developed with subsea wells only and began production in 1999.
The operation described in the full-length paper was performed at Smørbukk, a high-temperature gas/condensate field producing from five independent reservoir layers. The wells are completed with 7-in. production tubing and 7-in. cemented-and-perforated liner. Most of the wells produce commingled from the different reservoir zones. The field is produced mainly by depletion, with some pressure support from gas injection.
The reservoir complexity of the Smørbukk field together with commingled production and injection strategy has necessitated regular data acquisition. RLWI has been used extensively, a total of 23 RLWI operations having been performed since 2003, mainly production logging and perforating.
RLWI Equipment
A number of papers have been written describing the RLWI concept. The concept is based on use of a subsea intervention well-control package including a lubricator with no high-pressure riser tied back to the vessel. A dynamically positioned vessel normally is used for these operations. Island Frontier was the vessel used in this operation.
The subsea lubricator system used for this operation consists of four main elements with minimum inside diameter (ID) of 71/16 in. The first element is the lower intervention package including the subsea connector and the main barrier elements consisting of two barrier valves and a shear seal ram. The second main element is the lower lubricator package consisting of the subsea grease system and the workover control module that controls the functions in the subsea lubricator system. The third main element is the upper lubricator package (ULP) including the lubricator tubular and a cutting ball valve. The fourth main element is the pressure-control head (PCH) consisting of the wireline flow tubes and emergency packing elements.