Sidetrack 12 1/4-in section off cement plug

An operator in offshore northern Oman wanted to sidetrack a 12 1/4-in well section off a cement plug that was set in the original 12 1/4-in open hole. The plan was to initiate the sidetrack from the low side of the hole, then drill a short buildup section before holding inclination at 39.5° and azimuth at 353.5° to section TD at the top of a carbonate formation—the 95/8-in casing shoe depth.

Because it was suspected that the cement plug had been contaminated by the synthetic-base mud (SBM) used to drill the original hole, resulting in poor homogeneity, there was concern that the side- track attempt might fail. If it did, it would be necessary to pull out of the hole and set a whipstock or a new cement plug to support a second attempt, adding to the well cost.

Initiate sidetrack on first attempt

The section was successfully sidetracked off the cement plug on the first attempt, using a 9 1/2-in steerable Neyrfor Traditional standard turbodrill. The steerable turbodrill was chosen based on hydraulics calculations and its excellent performance in the original hole, where it drilled a 709-m interval in one run at an average ROP of 7.85 m/h.

A diamond-impregnated hybrid bit was used with the turbodrill in the original hole, but a different bit was selected to drill the sidetrack—a PDC bit with a mix of 78% 19-mm cutters and 22% 13-mm cutters. To initiate the sidetrack and build angle, the bent housing on the turbodrill’s bearing section was set at 1° and the turbodrill was stabilized with a 12 1/8-in lower body stabilizer and a 12-in midbody stabilizer. In addition, an 11-in string stabilizer was placed on top of the turbodrill to keep the assembly flexible enough to do the required directional work.

After kicking off the sidetrack at 2,525-m MD, time-drilling was performed until separation from the original hole was achieved at 2,553-m MD and the sidetrack was declared successful. Dogleg severity (DLS) was 6.7°/30 m during the time-drilling interval and 5.7°/30 m after hole separation. It was planned to drill only a 40-m interval before making a trip to pick up a different BHA to drill the build and tangent intervals. However, the Neyrfor turbodrill performed so well in initiating the sidetrack that it was used to finish the short buildup interval and align the planned tangent trajectory before changing to a packed BHA.

Save cost of setting another cement plug or utilizing a whipstock

The 9 1/2-in steerable Neyrfor turbodrill with the bent housing set at 1° drilled 112 m of 12 1/4-in hole—35% of the sidetrack’s 321-m overall length—at an average ROP of 3.52 m/h. This successful openhole sidetrack off a cement plug with poor homogeneity saved the operator the potential cost of setting a another cement plug.