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The Sajaa field in Sharjah, United Arab Emirates, is a deep, low-pressure gas field that has been a prolific producer since the first wells were drilled in the 1980s. Initial field development was through a series of vertical wells drilled overbalanced with conventional drilling rigs. As field pressure and production declined, it became necessary to access additional reserves not being drained efficiently by these older wells. As a result of this need, a campaign to drill horizontal, multilateral sidetracks from the existing wellbores was started in March 2003 using coiled tubing. From a standpoint of production, cost, health, safety, and environment, the results of the program thus far have been excellent.
This paper examines the surface and downhole equipment and operational best practices used in the Sajaa coiled tubing (CT) underbalanced drilling (UBD) campaign. It also reviews some of the key details that make a safer, more efficient day-to-day operation and created a highly successful project that has drilled 27 wells, comprising more than 90 laterals and more than 212,000 ft drilled.
The Sajaa field was discovered in 1980 by Amoco Sharjah Oil Company; since that time a total of 41 wells have been drilled in the Sajaa field. Initial reservoir pressures were 7,500 psi or greater; these pressures have since declined substantially, with most wells now being drilled with equivalent circulating densities of 1.5 to 2.0 lb/gal. Initial completions were polished bore receptacle tubing completions, but with corrosion and packer leaks, these were changed to packerless 5-in. tubing completions with wells that have 7 -in. liners tied back and cemented to surface in most cases. Production trees are, for the most part, 5-1/8-in., 10,000-psi sour-service trees.
The initial drive to perform CT UBD in this field came about because data on the Sajaa field showed that valuable reserves were not being added or produced, and that additional technology needed to be brought to this field to exploit these reserves. A peer-assist team composed of personnel from Alaska's North Slope, Houston Engineering Technical Practices (ETP), Sunbury, UK ETP, and Sharjah worked through the various options and found coiled tubing to be the best option.
Downhole data transmission in underbalanced conditions can be done with pulse telemetry (previous UBD campaigns in Sharjah) or electromagnetic (EM) signals, but by 2003 the bottomhole pressure would not support a fluid column and the formations above the reservoir blocked EM transmission.