Drilling hard-rock sequences in northern Oman can present a number of distinct challenges. Operations are frequently hindered by formation breakout in rocks with high confined compressive strengths (15,000-50,000 psi). The problem is exacerbated by highly abrasive formations, extreme bottom hole temperatures (160-175C) and the subsequent deterioration of mud properties at vertical depths of approximately 4500-5500m.

The main challenge is bit/bottom hole assembly (BHA) sticking while drilling a tectonically stressed sandstone formation. Formation breakouts that occur at or just above the bit can go unnoticed as drilling progresses and are the main sticking mechanisms responsible for turbine stall. The use of durable impregnated bits and motor/turbine BHA can increase drilling performance. However they bring an increased risk of leaving tools in hole (LIH) because the bit and driveshaft are able to rotate independently of the drillstring in a stuck bit event. A conventional motor/turbine has no reliable mechanism to transfer the available high torque from the drillstring to the bit, leaving axial force (pushing/pulling) as the only means to free a stuck bit/BHA.

For this reason, the operator is exploring new technological solutions to reduce non productive time (NPT) associated with stuck events. A recent research and development effort has resulted in a new-style turbine, which in the event of stalling or bit sticking, locks the bit to the body of the drive-system so that drillstring torque can be applied to the bit. Transmission of drillstring torque to the bit significantly increases the chance of freeing a stuck bit/BHA. And instantaneous locking while rotary drilling can reduce the risk of becoming stuck by keeping the bit in motion should the turbine stall.

During technology trials in 2010 BHAs in nine deep wells were successfully freed after several stuck events. The authors will provide a BHA schematic and the procedural details that contributed to the successful operations.

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