Technology improvements continue to advance the capabilities of coiled tubing directional drilling (CTDD). Alaska's North Slope, with its prevailing dedication to expanding the technological envelope, has served as a testing ground where advanced CTDD techniques mature into economically viable systems.
Even after over 500 successful CTDD sidetracks on the North Slope, impetus remains to further improve this economical drilling technique. Through a close working relationship between field operator and the service company, significant research and development has led to the introduction of novel tools and services to overcome the intrinsic hurdles of conventional CTDD.
Through a process of miniaturization and innovation, small-diameter systems have been developed for CTDD. The most recent introduction of tools and services includes rib steering technologies, bidirectional wireless mud pulse telemetry, gyro-based MWD services, and ultra-slim, high-resolution, real-time resistivity.
Straighter, longer horizontal laterals, improved steering, and real-time resistivity in openhole sizes as small as 2 3/4-in. ID has been achieved, consequently improving precision in geosteering within the narrowest of payzone.
This paper highlights two case histories describing CTDD technology, real-time formation evaluation, and multilateral drilling processes used to access previously unreachable oil-bearing rock on Alaska's North Slope. While proven in this region, CTDD advances are applicable in other mature fields for the economical extraction of additional reserves.
Ever since CTDD started off in the year 1994 in the North Slope of Alaska, it has become an ongoing desired method for slimhole re-entry from existing wells in the region to access additional reserves. The continuous use of CTDD for re-entry in Alaska, because of the operators' persistent and innovative culture, has made it a proving ground for newer drilling and completion techniques and advanced bottomhole assemblies (BHAs). It has proven itself as the most efficient and cost-effective method of sidetracking and re-entering existing wellbores with cost savings of up to 40% compared to conventional rotary drilling in the region.5
What started off as selecting simple candidates for re-entry with CTDD has now evolved several folds into routine selection of complex candidates presenting just as complex drilling techniques. The vast experience gained in the region and the development of advanced BHAs have made returns from these once "hard candidates?? an economically sound and successful CTDD campaign.
A number of these candidates have an existing 3½-in. tubing, which required the development of advanced tools in sizes as small as 2 3/8-in. to re-enter through these wells without a tubing retrieval operation. The development and usage of these tools in the 2 3/8-in. size redefined the meaning of slimhole drilling and opened up drilling opportunities to many additional wells.1
Key benefits gained from the development of such CTDD tools is the acquired knowledge and experience in re-entry and the drive to push the application into complex and sometimes fragile formations such as those found in the Kuparuk River Unit.
The dynamically overbalanced drilling (DOD) technique-where the drilling fluid is underbalance, yet the surface pressure is adjusted to maintain at-balance condition on bottom-also sometimes known as the managed pressure drilling technique, is a significant improvement in successful drilling technology in such fields. Monitoring of downhole conditions to maintain at-balance conditions, especially the annular pressure, with fastest data update rate, and the ability to steer the BHA as required without any pressure fluctuations were necessary to drill using the DOD technique.2 These BHA requirements apply also to the underbalanced drilling candidates in Alaska.
Fast update rates could only be achieved by an e-line system of steering tools, which had to be in the 2 3/8-in. size to re-enter a number of these complex wells and formations. Hence, the application led to the development of e-line BHAs with downhole dynamics, pressure monitoring, real-time downhole weight on bit, and the functionality to steer and navigate the wellbore in the right path while on bottom drilling.