A Successful World-Record Installation of Multilateral Technology in the North Sea
Authors: Wilson Adam 
Published: February 2015

This article, written by Editorial Manager Adam Wilson, contains highlights of paper SPE 145675, ’Statoil North Sea - Successful World-Record MLT Installation,’ by Johan Eck-Olsen, SPE, and Ove Andre Solheim, SPE, Statoil, and Morten Falnes, SPE, Halliburton, prepared for the 2011 SPE Offshore Europe Oil and Gas Conference and Exhibition, Aberdeen, 6-8 September 2011. The paper has not been peer reviewed.

Over the last decade, multilateral technology (MLT) has been widely used on several of Statoil’s licenses. In October 2010, the deepest-set sealed multilateral junction in the industry was installed at 6900-m measured depth (MD) in Oseberg South Well 30/9-F-9 AY tIY2. The main bore was drilled to total depth (TD) at 8583 m MD, and the 7-in. liner was run and cemented. After the liner perforation, the MLT operations started with the installation of a multilateral anchor packer to allow for installation of a latch interface assembly (LIA) and milling system. The lateral branch was drilled to TD at 8258 m MD, and a 5½-in. screen completion was run and dropped off into the 8½-in. open hole. A 6900-m-long upper-completion string with inflow-control valves was finally installed to allow for surface control of the two branches.

Introduction

The Oseberg South platform is located in the North Sea, 130 km west of Bergen, Norway (Fig. 1). Thirty-two wells are planned from the platform. Oseberg South field’s plateau production is approximately 5400 m3/d (34,000 B/D). The sea depth is approximately 100 m.

Well Geology

Well 30/9-F-9 Yl/Y2 was planned to target the Upper Tarbert (UT) and the Middle Tarbert (MT) at the G-Central structure. The well was planned as an MLT well where the first branch (Y1) will produce UT oil and gas and the second branch (Y2) will produce MT oil.

UT contains approximately 70% of the expected oil and gas reserves and generally consists of wave-reworked lower/upper shoreface siltstones and fine-grained sandstones with relatively poor reservoir properties (average values for porosity and permeability are typically on the order of 5–18% and 0.2–20 md, respectively).

In general, the MT formation is subdivided into two parts: MT1 and MT2. MT1 is interpreted as near-shore depos-its, while MT2 was deposited in a more energetic tidal environment. The porosity is within the range of 12–26%, while the permeability varies from 50 to 2500 md.

Well Design

Because of the low permeability in UT, a long reservoir exposure is needed to recover oil. The reserves from the more-permeable MT are important for the well economy because of higher initial production rates. For this reason, a multilateral well combining production from MT and UT is considered as the optimal well concept.

The selected multilateral system uses premilled-window technology, which does not generate any steel cuttings during junction construction because only aluminium is milled. An oriented nipple profile called a latch coupling is installed below the premilled window and is used to anchor the whipstock and completion deflector. At depth, the liner is oriented to position the premilled window on the highside before being cemented in place.