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.