There has been a dramatic increase in the use of continuous reeled tubing (coiled tubing) for drilling operations. Much of this increase can be attributed to improvements in the quality and dimensions of coiled tubing itself - pipe sizes have steadily increased over the years from 1 inch OD to 3½ inch OD and bigger. Other developments have included improved downhole motors and, for directional work, sophisticated survey and orienting tools that can be controlled, in real time, from surface.
Despite these developments, CT drilling is still applied to very few wells, or a tiny percentage of the total drilling market. There are many reasons for this including high equipment and personnel costs, low rates of penetration (ROP), and issues related to the reliability of high-cost "smart" bottom hole assemblies (BHA's) needed for directional work. The idea that CT drilling would offer a cheap alternative to conventional rigs is, and probably will remain, an unfulfilled pipe-dream. However, CT drilling certainly has its place, particularly for vertical well deepening and for slimhole multilateral work. The method presented here provides the opportunity to improve the efficiency and lower the cost of such operations. This paper describes a technique that we call Stimulation While Drilling (SWD), a drilling method that is particularly applicable to the use of coiled tubing. In particular, the method is intended for use in the drilling of multiple short, lateral drainage holes originating in the main borehole and extending outward radially through the reservoir, at one or several depths. However, any hole, including the main borehole itself, can be created using this technique, depending on the lithology. This ability to make large multilateral conduits, at high speed, in underbalanced conditions, if desired, and with no drilling damage offers significant advantages for well construction and completion, particularly in hard carbonate reservoirs.
Unfortunately, coiled tubing is fundamentally ill-suited for use in drilling. The pipe is intrinsically weak and subject to fatigue, it cannot be rotated and it is difficult to apply substantial weight-on-bit. Coiled tubing drilling (CTD) involves "slide drilling" and this is why it is so different from conventional rotary drilling - the cuttings bed is not stirred-up by string rotation.
Coiled tubing drilling operations are today typically conducted using downhole motors (powered by mud circulation) connected to rotary drill bits. There have also been developments involving electrically powered downhole motors. Whether hydraulic or electric, these motors can deliver only limited torque to the bit, and it is not uncommon for these devices to stall-out. Furthermore, the coil itself must be protected from the torsional forces to minimise buckling. This makes penetration rates slow (20–40 ft/hr, often less), particularly in hard rocks like limestone and dolomite. At the same time, due to limited ID, circulation rates are low and cuttings transport in the annulus becomes a problem, particularly in horizontal and deviated holes. This necessitates frequent wiper trips to clear settled cuttings to avoid the coil becoming stuck. Apart from wasting time that could otherwise be spent drilling, these wiper trips also shorten string life by increasing fatigue (due to running pipe in and out of the well). In the event that coil does become stuck, it is difficult to retrieve due to limited overpull capacity and the risk of parting pipe.