Innovative Step Change in Drilling Efficiency for Medium Radius Re-Entry Deep Gas Wells with a High Build Rate Rotary Steerable System
Authors: Ab Hamid Abdul Halim , Siregar Verdy , Khalil Mohamed E. , Ghazzawi Ayman , Ashraf Omar T. , Balka Muhammad S. 
Published: July 2016

Generally, deep gas workover/re-entry wells in Saudi Arabia are kicked off in the Sudair formation through a whipstock because the overlying base Jilh dolomite can flow with high pressure, which jeopardizes well control. Whipstocks are set deep in the 9 5/8-in. casing, after which the 8 3/8-in. and 5 7/8-in. holes are drilled to access the target Lower Carbonate and Sand reservoirs. Deeper kickoffs also avoid contact across the water-bearing Carbonate A, aiming for displacement across Carbonate B or C reservoirs. Isolation from Carbonate A is important for multistage fracturing completions as they are still not proven for the long-term isolation of water-bearing zones.

Regardless of the deeper whipstock setting, the high dogleg requirements exceed the capabilities of conventional rotary steerable systems (RSS). Conventional steerable motors with high-bend housing and 70 to 80% of the sliding mode of drilling has been the only option to achieve such high dogleg severity (DLS/100ft). Drilling medium-radius wells with a conventional motor assembly requires multiple runs, wiper trips to clean the hole, and multiple reaming trips before running the liner. These operations result in poor drilling efficiency due to slow penetration rates and bit trips.

A high build rate rotary steerable system (HRSS) was introduced as a solution for such challenges in the 8 3/8-in. and 5 7/8-in. sections. While the HRSS technology has been used before, this was the first time the HRSS kicked off vertically from a whipstock in Saudi Arabia or worldwide. The new technology allowed the kickoff point to be pushed further into the Sudair formation near the Sudair dolomite, reducing the risk from Jilh pressure and associated cost. The step change provided the option to slim the hole by eliminating the 8 3/8in. hole size, and kickoff was done in the 7-in. liner. Deployment of the HRSS allowed directly kicking off from a whipstock set vertically, eliminating the need for a dedicated steerable motor assembly run. Direct kickoff also meant eliminating the need for gyro tool for steerability, because conventional RSS tools could only be used outside the zone of magnetic interference, once sufficient separation from the mother bore was achieved. Consistent doglegs of more than 14°/100 ft were recorded; and the maximum dogleg was 17.44°/100 ft. Since then, this concept has been applied to other vertical re-entry wells and at an existing inclinations successfully in the 8 3/8-in. and 5 7/8-in. sections in Saudi Arabia and worldwide. The scope of the paper is limited to wells in Saudi Arabian deep gas wells only. The average rate of penetration (ROP) across this build section shows a 137% improvement over the ROP for conventional motor bottom-hole assemblies (BHA) for similar build sections. Eliminating the 8 3/8-in. section, avoiding the hazards of drilling in Jilh and Sudair formations, saving the motor trip to kick off from the whipstock, and improving ROP resulted in significant savings. This step change in drilling performance was realized by a thorough understanding of local drilling conditions and indepth analysis that enabled efficient execution.