Coiled tubing (CT) has evolved in recent years to include more complex applications in drilling and remedial work. As wellbores have extended deeper, the challenges of intervening with CT have increased. For years the limitation in CT work was the coiled tubing itself. However, with advancements in metallurgy and manufacturing processes, the applications where CT can be utilized have expanded to include deeper, hotter, and more complex wellbores. The challenges of performing this work have now been directed at providing reliable equipment.
One of the main challenges in these more hostile environments is temperature. At elevated temperatures, work performed with motors becomes very erratic and unreliable. To perform this work, alternate methods have to be analyzed. One solution is the use of downhole turbines or turbodrills. Turbodrills have been used in the drilling industry for decades. It is only recently, however, that the benefits derived by turbodrills have been applied to CT for drilling and workover operations. With the remedial work CT is now required to perform, turbodrills are a natural fit as they address the issues which limit motor performance. This paper analyzes the applications and developments in turbodrills with analysis of recent runs on CT.
Over the last 15 years, CT has expanded to encompass a broad range of applications, which, previously only rotary rigs could execute. These applications range from CT's original intent for workover and remedial operations, albeit with much greater capabilities today than when originally introduced, to drilling grass root wells, completions, and pipelines. As with many oilfield products, it was during the 1980's that CT made great advancements. Materials science was progressing to new frontiers and, with new materials, coupled with better manufacturing and quality processes, the CT itself became stronger and larger in diameter. These two combinations pushed the envelope where CT could be reliably utilized for deeper and more complex applications.
It was also in the mid 1980's when focused efforts on reducing costs associated with extracting hydrocarbons became more closely scrutinized. The evolving reliability of CT exemplified a low cost alternative for remedial operations versus a standard workover or drilling rig. At a fraction of the traditional cost, remedial operations could be undertaken to improve recovery rates, with the added benefit that said operations could be undertaken without killing the well. Identifying the shortcomings then proved quite simple: the functionality and flexibility of tools deployed on CT were surpassed by the CT itself and focused efforts were required to design downhole tools specifically for CT.1
It wasn't long before the economic benefits of using CT were translated to the drilling environment. The benefits were numerous: smaller footprint, smaller volumes of drilling fluids to be handled, smaller volumes of drill cuttings requiring handling, faster rig up time, faster tripping time, reduced noise levels, and fewer personnel requirements. All of these led to an overall reduced environmental impact and generally a safer operation.2 An underlying benefit of drilling with CT is Underbalanced Drilling (UBD). This benefit was realized at an early stage in remedial applications, as workover operations could be carried out without introducing kill fluids into the wellbore. By design, drilling with CT fits perfectly with UBD operations, provided penetration rates are adequate and reservoir targets can be hit.
Coiled Tubing Drilling (CTD) has evolved dramatically in the recent past. Initially operations were limited to extending existing wellbores. These operations have been commonplace in Alaska for many years. In the early 1990's the challenge of drilling grass roots wells began. By the turn of the century many of the challenges pertaining to this operation had been overcome and today over 7,000 wells have been drilled with CT, with approximately 750–850 new wells being added each year.3