Home / News / Challenges outlined in directional drilling through salt in deepwater GOM

With the Lower Tertiary trend continuing to gain prominence, especially in the Gulf of Mexico, associated issues related to the difficulty of drilling through salt layers have surfaced as well. During Wednesday morning’s Deepwater I technical session, Rias Israel of Schlumberger discussed challenges faced and lessons learned by two major deepwater GOM operators, BP and Chevron, in directional salt drilling. Schlumberger provided the directional service.

Salt drilling is problematic for many reasons, one being the salt’s ability to move, or “creep,” into newly drilled wellbore. Therefore, thorough well planning will be a key factor in successfully drilling directionally through difficult salt formations, Mr Israel said. Even the zone above the salt will demand caution. The area is typically stressed due to the way the salt has migrated over time. Risks of wellbore stability issues and losses are always present and must be carefully monitored, he said. Several challenges seen in previous GOM experiences, plus possible solutions, were given:

Challenges outlined in directional drilling through salt in deepwater GOM

Shock and vibration. Proper bit selection (a stable bit is a must, Mr Israel noted) and pre-drill vibration modeling can mitigate these issues. But the best way to manage shock and vibration, he said, is perhaps through real-time diagnosis and intervention. This usually involves a dedicated drilling optimization engineer who monitors the drilling in real time, identifies vibration mechanisms and recommends mitigating measures.

Inclusions. Inclusions can be a big issue simply because of the uncertainty of the pore pressure within the inclusion, he explained. Whether abnormally or sub-normally pressured, there could be lost-circulation risks. Risks of shock and vibration also are present in inclusions. 

Tar. Tar is a “very tricky problem” that’s been seen in the GOM, Mr Israel said, particularly in salt exits. Some operators have used hybrid forms of drilling with casing, while others have tried to simply get through the tar and case it off as far as possible. While it’s not always possible, the best strategy may be to just avoid it.

Salt exit strategy. Drillers must have an exit strategy based on local experience. The mud weight to be used must be considered, and caution is needed when drilling through rubble zones. 

Although drilling directionally through salt is no easy task, several enabling technologies have emerged to help drillers push the limits. They include: 

Deepwater rigs. Directional salt drilling demands higher torques, improved hydraulics for hole cleaning, and higher hoisting capacities to run longer strings of casing. “Improvements in 5th– and 6th-generation deepwater rigs have definitely been a key component to allowing us to successfully drill big holes directionally through salt,” Mr Israel said.

Directional drilling components. Rotary steerable systems can yield the perfectly round holes that are needed to drill through salt, he said, on top of adding ROP. Improved bits and under-reamers also have provided the required directional performance on these projects. 

Subsalt imaging and LWD. Three-dimensional pre-stacked depth images have been able to provide a clearer picture of what’s below the salt. Gamma ray at the bit, sonic sensors and seismic while drilling also have improved the drilling process.

Real-time monitoring. Whether this takes place on the rig, at an operator’s office or at a service company facility, this monitoring can be “very beneficial” in leveraging existing experience to ensure project success. 

Additional details on this presentation will be published in the May/June edition of Drilling Contractor.

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