This paper was prepared for presentation at the 1999 SPE/IADC Drilling Conference held in Amsterdam, Holland, 9-11 March 1999.

Slimhole re-entry horizontal drilling offers operators an opportunity to improve field recovery by infill drilling, and revitalize wells suffering from gas or water coning. Re-entry drilling systems are available for hole sizes down to 3-7/8". The potential for coiled tubing drilling has created significant industry interest over the last 18 months. In offshore applications where there is no derrick in place, use of a coiled tubing drilling system makes it possible to redrill old wells and perform horizontal re-entries. Coiled tubing re-entry wells completed so far have used a conventional rig to cut the window. This adds unnecessary costs and time to the program. A coiled tubing conveyed window cutting system will dramatically reduce the overall wells costs, and thereby expand the market for coiled tubing re-entries. The problems of slimhole window cutting include controlling weight on bit while starting the window, and reducing torque while milling to prevent drillpipe failure. In the coiled tubing application, rotation is supplied by a downhole motor which presents additional problems with respect to controlling weight on bit. A milling system using downhole motors with hydraulic thrusters to supply constant weight on bit has been bench-tested and field proven. The prototype system required three milling runs after setting and orienting the packer. Subsequent systems will be tested to reduce the number of milling runs to one. A description of the applications and requirements of the window cutting system is provided, along with bench test and field test results.

Drilling with a continuous coiled string presents an array of opportunities for reducing drilling costs and improving production potential. It offers exciting perspectives for enhanced resource management in established oil and gas provinces. A Shell operating company recently completed a horizontal sidetrack with coiled tubing onshore the Netherlands. A medium radius horizontal lateral was drilled with a 2-inch coiled tubing. This operation was the first step towards the development of a comprehensive Coiled Tubing Drilling (CTD) system able to re-enter and re-complete wells without conventional rig assistance. Operations in the Netherlands provided key parameters for evaluation of the potential and limitations of CTD. This experience indicates that CTD is a suitable technology for the optimization of mature North Sea fields. It offers potential cost advantages and should help to maximize hydrocarbon recovery in this area. New developments are required to meet the challenges of the offshore environment, under-balance operations, short-radius multiple drainholes or through tubing drilling to name but a few. Investments in these directions are well worth while, in view of the opportunities that CTD offers for enhancement of the drilling and production process. Introduction Despite intensive worldwide exploration efforts during the past decade, most petroleum basins are approaching or have reached maturity. More than three-quarters of current additions to the world's total hydrocarbons reserves are procured by optimizing recovery from existing fields. The challenge for the oil industry is, therefore, to identify strategies for maximizing the value of established reserves. Improved exploitation of hydrocarbon sources will increasingly rely on focused application of advanced technologies in a multidisciplinary fashion. CTD emerges as a promising technique in this context. ADVANTAGES AND POTENTIAL OF CTD The concept of using a continuous drillstring has been around in the oil industry for more than 30 years. Several drilling systems developed during this period have demonstrated the viability and advantages of a flexible drillstem [1, 2, 3]. Although none of these systems is in use nowadays recent advances in coiled tubing and drilling technology have renewed interest in this technique. Increased awareness of the adverse effect of formation damage (Fig. 1) on productivity, together with environmental concerns, provided the business justification for a number of pioneering trials [4, 5] in various oil and gas provinces. CTD has two basic advantages, which are discussed below. P. 359^

With re-entry drilling systems available for hole sizes as small as 3-7/8", slimhole reentry horizontal drilling offers operators an opportunity to improve field recovery and revitalize wells suffering from gas or water coning by infill drilling. In particular, the potential for coiled tubing drilling has created significant industry interest over the last 18 months. Offshore where there is no derrick in place, use of a coiled tubing drilling system makes it possible to redrill old wells and perform horizontal re-entries.A description of the applications and requirements of the window cutting system is provided, along with bench test and field test results.