Gas encountered in deep carbonate (Khuff) and deep sandstone (Jauf and Unayzah) formations require significant planning and impelemntation of high-end technology for optimized exploitation. The conventional drilling methods for vertical, horizontal, and maximum reservoir contact (MRC) have been used extensively to produce the reserves. The need to avoid formation damage and minimize differential sticking has become important to maintain the well integrity and rates, particularly in reservoirs that are tight and encountering pressure depletion with time. Underbalanced Coiled Tubing Drilling (UBCTD) technology is a breakthrough in developing mature and depleted reservoirs. With UBCTD, workover operations, such as drilling laterals in very thin reservoir sections and controlling their trajectories with flexibility to steer into net pay sections, are achieved with more accuracy. Also, the high operational costs with rig time, the need of inducing hydraulic fractures and drill-in mud invasion caused by conventional drilling are avoided. In addition, the technology plays a major role in eliminating pressure differential sticking while drilling across multilayered reservoirs that exhibit layering depletion. The technology was introduced after conducting a pilot project with six wells where post-UBCTD production performance were analyzed and exceeded expectations. In some cases, UBCTD provided as much as a threefold increase in initial well productivity and achieved a higher, long-term sustained rate compared to conventional workover operations conducted in similar reservoirs. Due to the ability to continuously monitor well performance during drilling and the higher flexibility of coiled tubing (CT) operations, changes in drilling direction and inclination could be easily made to maintain the drill bit in layers of interest and enhancing reservoir contact in the desired sections. This paper presents two actual examples from carbonate and sandstone reservoirs where UBCTD has been successfully deployed. The post-workover pressure and production performances and comparison with results obtained from conventional drilling are presented to illustrate the improvement in gas rate. The paper also presents pressure transient tests conducted after workover operations indicating low or zero skin damage and high permeability-thickness (kh) as a result of effective minimization of wellbore damage from drill-in mud and achieving higher net-pay contact.
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