Inexpensive Directional Wells Drilled Underbalanced With Coiled Tubing
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 90783, "Cheap, Directional Wells Drilled Underbalanced With Coiled Tubing: An Experience in the Australian Outback," by Lance Portman, SPE, BJ Services; Lan Nguyen, SPE, Mosaic Oil; and Jim MacArthur, SPE, BJ Services, prepared for the 2004 SPE Annual Technical Conference and Exhibition, Houston, 26-29 September.
Coiled tubing (CT) underbalanced directional drilling can be an expensive operation, requiring a significant quantity of specialized equipment and personnel. In some circumstances, the equipment, personnel, and engineering requirements can be reduced, leading to a large reduction in overall operations cost. The full-length paper describes equipment and method simplifications made during a four-well underbalanced directional-drilling campaign in Queensland, Australia.
Introduction
CT drilling is not new. More than 10 years have passed since the first directional wells were drilled with CT. Simple vertical drilling and milling applications date back even further. Many thousands of wells have been drilled vertically with CT, but the number of directional wells drilled with CT is less than 1,000. CT directional drilling has established itself in only two regions. The first is western Canada, where CT is used to drill underbalanced horizontal well sections. The second is Alaska, where CT is used to construct sidetracks, drilling through the completion tubulars.
CT can provide significant advantages over jointed-pipe methods, particularly for underbalanced drilling and for sidetracking out of smaller completion strings. However, the incremental cost of using CT historically has deterred operators from using this technique.
Background
A four-well drilling campaign was begun in the southwest Surat-Bowen basin. The wells were approximately 373 miles west of Brisbane. The drilling target was the Showgrounds sandstone and Rewan-formation sands. These sands have been produced for more than 20 years and are penetrated by hundreds of wells. These wells typically were drilled overbalanced with a potassium chloride (KCl)/polymer mud system, and they experienced severe formation damage. Typical skin for wells after completion is 20 to 80. The goal was to convert some of the old damaged vertical wells into horizontal wells drilled underbalanced.
Formation Properties. The target zones are approximately 100 ft thick and consist of layered sandstone, siltstone, and mudstone. Permeabilities average approximately tens of millidarcies, although streaks exist with 2 darcies. Rock strength is between 12,000 and 27,500 psi, and the Rewan formation is known to be very abrasive. Kaolinite clay averages 69% of the total clay content and illite 15%.
The formation is water sensitive. Both the pay zone and the layers above were believed to be competent rock. No washout was observed in wells previously drilled through these zones. Formation temperatures were approximately 176°F.
Produced Fluids. Most wells are expected to produce wet gas and/or gassy oil, although there was uncertainty as to the amount of gas and oil that would be produced. Oil in this region typically is 46°API, and no hydrogen sulfide is present.
Drilling Program
The drilling plans called for pulling the existing completions, milling a window in the existing 5½-in. casing strings, and then drilling a build section over approximately 328 ft of true vertical height followed by several hundred feet of horizontal hole. The 4½-in. horizontal hole would be drilled underbalanced, and the 4½-in. build section would be drilled overbalanced.