This paper provides insights of planning, execution and the results of the application of selective flotation or "mud over air" technique while running 9 5/8-in production liner into an extreme-extended-reach well in Sakhalin, the longest well ever drilled by RN-Sakhalinmorneftegaz to date. Extreme Reach Drilling (ERD) well has a shallow true vertical depth and a long horizontal departure imposing significant challenges of conveying 9 5/8-in liner to the required depth. When both conventional and full flotation methods cannot guarantee delivering casing to planned depth due to high torque and drag, lack of weight and string buckling, a new operational approach of selective flotation was considered. Mud over air, specifically "air cavity" technique implies running a bottom portion of the casing filled with mud, then an empty portion to reduce drag and a top portion filled with drilling fluid for additional weight to provide some additional push to overcome residual drag. This article outlines unique practices of selective flotation technique covering the job specific features, its main risks and corresponding mitigation measures. The selective flotation or "mud over air" concept together with detailed risk assessment and engineering of a single hydro-mechanical liner hanger and flotation collar system enabled successful conveyance of 9 5/8-in liner to its setting depth during drilling the longest extended reach well in the project.

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This paper provides the results of the application of integrated approach to drill the longest extended reach well which has ever been drilled in Odoptu-more field (North Dome) in Sakhalin. Although many extended reach drilling (ERD) wells have been already drilled by the operator in Odoptu-more field, extreme reach wells always require new solutions. Reaching remote reservoirs by applying standard well design of Odoptu-more field was not possible due to high standpipe pressure (SPP) and equivalent circulating density (ECD) in 8 ½-in production section. Engineering study resulted in partial upsize of a well design and change 9 5/8-in casing to liner followed by remaining challenges: 9 5/8-in liner could not be delivered to required depth due to string buckling. Several key technical solutions were implemented in the project thus allowing to deliver the longest well accident-free. Those solutions included well design modification allowing to drill 8 ½-in production section to reach remote reservoirs and 9 5/8-in liner delivery to the required depth using "mud over air" flotation technique for the first time in the project. Having applied integrated approach, the project team broke a new record in drilling the longest well at Odoptu-more field with total measured depth 8699m, ERD ratio 4.89 and Directional Difficulty Index (DDI) of 7.6 (Alistair W. Oag 2000). Drilling extreme reach ERD well unlocked opportunities for the operator to reach and put into the production more distant offshore oil-bearing reservoirs from the land rig without changing existing pad facilities.

This article provides the results of the first experience of running a wear resistant "push-the-bit" RSS (rotary-steerable system) for kicking-off at shallow depth in poorly consolidated sand formations. According to the contractor's best practices, a shallow kick-off in sections with large diameters is performed using PDM (positive displacement motors) with a bent housing angle set up to at least 1.83 degrees. However, for wells in Odoptu field, which is located in the north of Sakhalin island, several specific features pose issues for the effective application of conventional directional drilling equipment to reach the geological targets. A standard approach for drilling the 17 ½-in. section in Odoptu wells requires two runs: Run1: A PDM with roller-cone bit are used to build the inclination from zero to 70-degrees in poorly consolidated sand formations Run2: A conventional RSS with PDC (polycrystalline diamond composite) bit are used for drilling the tangent section. The project team was challenged to reduce the well construction time by drilling this section in one run, however, there were technical concerns. Drilling with a PDM and roller-cone bit would not provide a high penetration rate in tangent section. Drilling with a conventional push-the-bit RSS might not provide the necessary DLS (dogleg severity) in the sandstone due to formation washouts. In addition, drilling in the sands exposes the drilling equipment to excessive erosion because of the high sand content in the drilling mud. Considering the average section length of 2000 to 3000 m of MD (measured depth), this wear could lead to damaging the costly drilling equipment breakdown and causing unplanned tripping operations. The project team alongside the field operator decided to test a wear-resistant RSS for one typical well of the Odoptu-Sea field. As a result, the section was drilled in one run, achieving the planned DLS without any equipment erosive wear. In addition, the 17 ½-in. section of this well recorded the highest penetration rate of 58 m/h in the field. The implementation of a wear-resistant RSS saved 6 days of the total section time.

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