Schlumberger

Case Study: Drilling Ahead with the Same Assembly on an HPHT Sidetrack, Temperatures in Excess of 205 degC

Gulf of Mexico operator saves 19 rig days with Neyrfor turbodrill

Challenge: Find sidetracking assembly for high-pressure, high-temperature (HPHT) deepwater Gulf of Mexico well in the very hard Smackover limestone formation with static temperatures in excess of 205 degC. Avoid significant cost of excessive downhole trips.

Solution: Run Neyrfor turbodrill with a formation-compatible bit allowing the assembly to drill ahead to well TD after sidetracking.

Result: Successfully sidetracked well on the first attempt, saving 19 rig days.

Find robust sidetracking assembly

After a conventional rotary assembly became stuck in a Gulf of Mexico deepwater well, the operator needed a sidetracking drilling assembly. Located just above the production zone, this HPHT long tangent well proved difficult to sidetrack because of its location in the very hard, Upper Jurassic, Smackover limestone, characterized by 50 to 90% limestone, 10 to 50% shale, and a compressive strength of 9 to 30 Ksi. Furthermore, the bottomhole static temperature exceeded 205 degC [400 degF], eliminating the potential for available MWD tools and a conventional positive displacement motor with a sidetrack bit, which wouldn’t survive the trip to bottom.

The only way to sidetrack the well was with a steerable turbodrill and a wireline steering tool. Unfortunately, the steering tool had a limited downhole life of only approximately 4 hours before it would overheat (because of internal heat buildup while turned on). With numerous wireline trips required for the steering tool, this limitation would cause significant lost time and money, forcing the operator to find a new strategy.

Drill ahead with the same assembly

With no concern of the high temperature and excessive downhole time planned, Schlumberger supplied a 6 5/8-in T2 MK1 Neyrfor steerable turbodrill with a 1.0° bent housing to complete the sidetrack. The Neyrfor turbodrill system is different from a conventional sidetrack assembly in that no special bit is required. The same Long-gauge box connection bits used on all turbodrill systems are capable of performing the sidetracks, so bits are selected based on formation compatibility rather than the limitations of the motor.

For this sidetrack, the team chose a 9 1/2-in impregnated bit and drilled 874 ft on the first run and 281 ft to TD on the second run. The Neyrfor turbodrill was stabilized in the same configuration as it would be for normal drilling, meaning there was no need to make a trip after sidetracking—the same assembly drilled ahead. It also exhibited excellent toolface control associated with high speed and low torque, which allowed the wireline steering tool to be turned off to conserve heat buildup.

Successfully sidetrack, save rig days

The Neyrfor turbodrill assembly was able to sidetrack and drill ahead on the first attempt at a higher ROP of 6.8 ft/h, resulting in 19 rig days saved. The operator also saw significant benefits with the ability to turn off the steering tool and continue sliding.


Download: Drilling Ahead with the Same Assembly on an HPHT Sidetrack, Temperatures in Excess of 205 degC (0.50 MB PDF)

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