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Linerless wells have gained popularity in the United Arab Emirates (UAE) oil and gas industry due to their cost-effectiveness. This type of well architecture eliminates the need for a dedicated 7″ liner run, saving up to 6 days of well time. However, stringent zonal isolation is the most crucial aspect of linerless well construction, requiring the use of meticulous and comprehensive engineering. In this case study, optimized cementing practices were employed for linerless well designs in an onshore oilfield in UAE by utilizing novel cement engineering techniques. The wells in this study were constructed as extended reach drilling (ERD) wells with the 9 5/8″ production casing placed across a carbonate reservoir at an inclination of 90°. Major challenges encountered were overcoming high equivalent circulating densities (ECD) in a single-stage job, requiring cement to surface in absence of mechanical pipe movement. This increased the risk of inducing fractures, channeling, adequate mud removal, and homogeneous cement placement, particularly across highly deviated intervals. To mitigate these risks, a dual-cementing technique was employed in which a newly designed 13.4 PPG light-weight expandable and gas-tight lead cement was used. Additionally, a spacer train was implemented, combining a novel mechanically-scrubbing spacer with large surface area to volume ratio particles for effective cleaning, followed by a biodegradable loss-circulation spacer that provided formation strengthening via a film-forming mechanism. Another challenge was long-term well integrity and cement sheath protection against stress variation during drilling and production phases. To overcome this, multiple mechanical and structural lab tests were conducted including compressive and tensile strength and Young's Modulus measurements. The lab data was incorporated into a stress analysis model to predict the risk of damage associated with the mechanical and thermal loads that the well would encounter. In addition, a 3-D computational fluid dynamics (CFD) model was analyzed to visualize and ascertain mud removal effectiveness and cement placement with emphasis on the highly deviated and lower side of the well. This aided the engineering and optimization of the rheological properties and pump rates of the cement and spacer fluids. The engineered designs and execution parameters resulted in successful cement placement without inducing losses. Post job cement integrity evaluation for the three linerless wells confirmed excellent cement bonding and isolation across the entire section. The results reinforced our cementing practices, meeting integrity requirements. This paper demonstrates the feasibility of employing advanced cementing practices in such types of wells. The engineered techniques and novel designs that were successfully applied to the three trial wells in the field will set the standard for future wells, with the potential to improve the efficiency and cost-effectiveness of linerless wells construction in the region. |
