Through-Tubing Completions Maximize Production
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 92392, "Unique ‘Through Tubing’ Completions Maximize Production and Flexibility," by M.O. Johnson, SPE, and P.G. Hyatt, SPE, BP plc; T.O. Stagg, SPE, Orbis Engineering; and L.L. Gantt, SPE, ConocoPhillips, prepared for the 2005 SPE/IADC Drilling Conference, Amsterdam, 23–25 February.
The challenge with through-tubing side-track completions has been dealing with small clearances and the lack of ready-made equipment. Over the last 10 years and after more than 450 coiled-tubing (CT) -drilling sidetracks on the North Slope of Alaska, a number of unique completion designs have been developed to maximize production and achieve vertical/zonal-isolation.

Introduction
Low-cost reservoir access is a key component to sustaining production from maturing fields. CT drilling (CTD) and through-tubing rotary drilling (TTRD) can achieve significant cost savings by sidetracking through existing production tubing. However, the critical completion phase of these sidetracks is challenged by small clearances and custom equipment. During the course of completing more than 450 CTD sidetracks through 4 1/2-in. and 3 1/2-in. production tubing in Alaska, a number of innovative completion designs have been developed to maximize production, achieve zonal isolation, allow for selective multilateral production, and preserve the parent wellbore for additional sidetrack opportunities.
Continuous innovation and close collaboration with the service industry has yielded successful solutions for challenging through-tubing completions. These proven techniques have made CTD the preferred method for re-entry sidetracks on the North Slope. The completion options discussed in the full-length paper may make low-cost through-tubing sidetracks more feasible for other mature fields.

CTD Applications
Reservoirs on the North Slope range in depth from 8,600 to 9,200 ft true vertical depth (TVD) and have a combination of gas-cap and waterflood/aquifer drive. Most sidetrack completions are required to provide vertical and zonal isolation with a cemented liner. Exceptions occur in predominantly waterflood areas or in shallower viscous-oil sands (4,000 to 7,000 ft TVD). In these areas, a simple slotted liner to hold open weak sands and shales within the reservoir often is appropriate. CTD has been successful in both types of reservoirs by sidetracking away from depleted bottomhole locations and targeting areas of undrained oil to increase oil rate and reserves recovery. North Slope CTD typically drills a sidetrack 2,500 ft measured depth (MD) in length that includes a short high-dogleg-severity build section followed by a precisely placed horizontal lateral. A new world-record sidetrack depth of 17,515 ft MD was reached with CTD in March of 2004. By drilling through tubing to save decompletion/recompletion cost and with high-build-rate capability to stay within the reservoir, CTD is able to achieve a 30% cost savings compared to an equivalent rotary-rig sidetrack. The continuous pipe also is well suited for drilling in severe-lost-circulation environments and in underbalanced mode.