Directional Drilling in Salt

Date: 03/28/2008

From the IADC International Deepwater Drilling Conference & Exhibition, Rio de Janeiro, Brazil

Not very long ago, operators and contractors considered drilling through Gulf of Mexico salt sections to be an extremely high-risk operation. The high-friction salt caused considerable, often damaging, shock and vibration in bits and bottomhole assemblies. It also tended to creep, or move, in reaction to overburden when the well was drilled, distorting the hole. The characteristics of the rubble zone encountered upon exiting the salt, including pore pressure and fracture gradient, were unpredictable and often resulted in lost circulation or worse.

But that was before the industry gained extensive experience drilling through salt sections of up to 11,000-ft as it explored the Gulf of Mexico's "lower tertiary" trend that is estimated to hold between three and 15 billion barrels of oil. Today, said Schlumberger Senior Drilling Engineer, Riaz Israel, that experience has turned salt sections from liabilities into assets. Their impermeability makes them a reliable hydrocarbon trap and their high fracture gradient means long sections can be drilled and cased with a single pipe size.

Speaking to attendees at the March 11-12, 2008 IADC International Deepwater Drilling Conference & Exhibition in Rio de Janeiro, Israel said the industry is now not only drilling through the salt but is also steering through it. The drivers for directionally drilling salt sections, he said, are the same as for other deepwater development projects such as reduced number of drill centers, reduced maximum inclination and collision avoidance.

Besides the years of experience, Israel said, subsalt imaging and real-time monitoring that allows remote technical support have greatly enhanced the industry's efforts to drill beneath large salt sheaths. Deepwater rigs with increased torque capabilities, hydraulics and pick-up ability have also made directionally drilling the salt easier.

The development of reliable rotary steerable systems (RSS) have also contributed to the increase in directional drilling in salt sections. As Schlumberger Drilling Engineer, Marco Aburto Perez, explained to the IADC audience in a separate presentation, RSS offers better rates of penetration by eliminating time-consuming slide drilling necessary when using drilling motors. RSS tools also enhance hole quality through the salt section.

Before RSS, steerable motors' bent housings often caused drill pipe twist-offs. Motors also had to be run slower, with higher weight-on-bit in some salts which also decreased rates of penetration. Use of motors also limited bit selection, and required extra time or even an extra trip to enlarge the wellbore. Hole cleaning is significantly improved, bit selection optimized and expandable reamers can be used when drilling with RSS. Overall, said Aburto, RSS systems deliver a 100% ROP increase over drilling motors. "More and more projects will contain directional drilling through salt," Israel said. "The technology to do that will continue to develop and it is important that operators, contractors and service companies collaborate in that effort."

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