How Much of Your Centralizer Is Left After Exiting a Casing Window in an Extended-Reach Horizontal Multilateral?
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 105766, "How Much Is Left of Your Centralizer After Exiting a Casing Window in an Extended-Reach Horizontal Multilateral? Modeling, Yard Tests, and Field Results From Alaska's West Sak Development," by E.M. Peterson, SPE, M.R. Greener, SPE, and E.R. Davis, SPE, ConocoPhillips, and D.T. Craig, Baker Oil Tools, prepared for the 2007 SPE/IADC Drilling Conference, Amsterdam, 20–22 February.
Viscous oil in the West Sak field on the North Slope of Alaska is being developed with extended-reach multilateral wells, with departure/depth ratios up to 5:1, in which slotted liners are tied back to the main wellbore with a Technology Advancement of Multilaterals (TAML) Level-3 junction system. Centralizers are necessary on the slotted liners to avoid slot plugging, reduce drag, and limit differential sticking. After three failures, a testing program was conducted on several common centralizers.

Introduction
The West Sak oil reservoir is part of a large viscous-oil resource overlying the Kuparuk River field on the North Slope of Alaska. Advances in drilling and a detailed study into sand management led to an economically viable project that uses extended-reach slotted liners in dual- and triple-lateral wells.

Typical Well Construction
The typical West Sak trilateral well dedicates a lateral to specific sand formations, typically the West Sak A, B, and D, as shown in Fig. 1. The wells are constructed by landing the intermediate casing string at 75 to 80° inclination through the lowermost sand to be completed. Then, the lowermost horizontal lateral is drilled through the intermediate-casing shoe and the slotted liner is run with a conventional hydraulic-set liner hanger and liner-top packer. With the lower lateral complete, a whipstock is set in the intermediate casing across from the middle sand, and a casing exit is milled through the intermediate casing. The casing exit usually is milled within 15° of the high side of the casing to provide the optimum direction to reach 90° inclination. The middle horizontal lateral is drilled through the casing exit, the open hole is conditioned for liner installation, and the whipstock is retrieved. The slotted liner then is run through the casing exit without guidance provided by the whipstock. A bent joint on the bottom of the liner aids finding the casing exit and guides the liner into the lateral. With the slotted liner run through the casing exit and into the lateral, the liner is landed in the casing exit with a TAML Level-3 junction. In the more difficult wells, the liner and running-string drag consume all available downweight, requiring rotation of the liner to reach total depth (TD). Following liner installation, the junction is isolated from the other laterals by use of a seal assembly stung into the liner-top packer from the lower liner and by installing another liner-top packer above the junction. Included in this run is a lateral-entry module (LEM) to provide future through-tubing lateral access. The process of setting the whipstock, milling the casing exit, drilling the lateral, retrieving the whipstock, and running the liner then is repeated for the upper lateral.