fax 01-972-952-9435. AbstractUnderbalanced directional drilling with coiled tubing is emerging as an instrumental technique for accessing new and bypassed reserves while enhancing overall reservoir recovery since, unlike jointed-pipe operations, circulation does not have to be interrupted every 30 feet to make connections. Coiled tubing drilling (CTD) thus offers the potential to achieve genuine steady state underbalanced conditions resulting in the elimination of formation damage, lost circulation and differential sticking. One of the enabling technologies assisting the growth of underbalanced directional CTD is the wireline-steerable bottom hole assembly (BHA). In contrast to conventional mud pulse telemetry systems, these purposebuilt wired BHAs have been designed to accommodate efficient, continuously underbalanced drilling operations while also providing an opportunity for reservoir evaluation while drilling.The paper details the operational results for several underbalanced and low-head CTD projects involving an integrated wireline-steerable BHA. The most recent wells were completed in the North Sea, Canada and United States in 1999. Topics discussed include equipment performance, nitrified fluids, wellbore stability, well results and learning points. An analysis of time-based underbalanced CTD data is also presented.

A critical requirement for directional drilling with coiled tubing (CT) is a reliable downhole means to manipulate the orientation of the mud motor bend. Alaskan operations have traditionally used hydraulically actuated ratcheting orienters, thus avoiding the complications of wireline and control lines inside the coiled tubing. With highly refined field techniques and well plans, the fundamental drawbacks to these orienters have been adeptly managed on the North Slope. Despite good success over the years, drilling and orienting have remained mutually exclusive activities. As such, orienting becomes nonproductive time (NPT) when it cannot be combined with necessary hole conditioning and wiper trips. While orienting problems and failures have been increasingly common in deep and high-build-rate applications, perhaps the most significant challenge to the standard hydraulic orienter service for coiled tubing drilling (CTD) is lost circulation. Initial steps in the development of an intelligent wireless orienter for CTD were taken in 1999 with the goal of eliminating off-bottom orientation, a significant source of NPT for Prudhoe Bay operations. Subtle computer-controlled mud pump variations are the basis for a versatile downlink command structure used to drive the downhole actions of the turbine orienter. The benefits of the prototype orienter were confirmed during a promising field trial beginning in late September 2001. As of December 2001, the novel service has been used on eight wells in Prudhoe Bay. This paper describes the turbine orienter development program, testing, a quantification of benefits, reliability figures, lessons learned, and future plans. Introduction Alaska produces primarily oil (92% of the daily hydrocarbon production) with a production decline rate of approximately 10% per year. Operators are focused on attempting to slow this decline through significant re-entry campaigns and the development of smaller satellite fields surrounding Prudhoe Bay. Logically, cost is one of the major concerns, as is the development of new technology that can lead to increased oil recovery. Coiled tubing drilling can offer a number of advantages over conventional jointed pipe rotary operations. Some of the advantages that have been demonstrated in Alaska include 1) highly efficient, modular and mobile CT drilling rigs with smaller footprints; 2) an ability to drill re-entry sidetracks without pulling and replacing production tubing; 3) faster trip times resulting from fewer connections; 4) associated cost savings. Part of the success of CTD in Prudhoe Bay can be attributed to the fact that the field has over 1,200 wells and the most common production tubing size is 4 1/2-in., enabling 3 3/4-in. sidetracks with 3 3/4-in. bottomhole assemblies. The origin of CTD on the North Slope dates back to 1993. A continuous re-entry program has been in place since 1994 and, to date, over 300 wells have been sidetracked using coil. Sixty of these wells were drilled in 2000 and, using three specialized CT drilling rigs, 72 wells were drilled during 2001. The 2002 plan is to drill 45 wells with the two active CT drilling rigs remaining. Over the years CTD has evolved to become the most efficient and economical means to drill re-entry sidetracks and Alaska has truly become the proving ground for new tools and techniques.1,2 CT Drilling Operations Time Breakdown The time distribution illustrated on Fig. 1is based on a study performed on 10 wells drilled in Alaska during 2000 using hydraulic orienters. It shows the average time distribution for the complete operation, from rig up to rig down, without considering downtime or mobilization. The figure reveals that, on average, 8% of the total time is used for orienting and 2% is spent on surveying. Technically the drilling phase is composed of drilling, tripping, orienting, circulating/mud conditioning, directional surveying, and wiper trips. On average, 60% of the total time on a sidetrack project is taken up by the drilling phase.

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