Technical Paper: Case Study of Push-the-Bit Vertical Drilling in Shallow Sediments of the Nankai-Trough Seismogenic Zone

Society: OTC
Paper Number: 20716
Presentation Date: 2010
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The Nankai-Trough, located south of the main island of Honshu, Japan, and extending 900-km offshore, is a subduction zone with large amounts of deformed trench sediments with seismic activity, and is a good example of an accretionary prism. At extreme ocean depths, water forms ice that has, in some instances, trapped methane in its crystalline lattice, forming gas hydrates. The Nankai-Trough contains a large amount of gas hydrate formations.

The first deployment of a “push-the-bit” technique for vertical drilling at sites in the Nankai-Trough seismogenic zone experiment was carried out to successfully drill to depths of approximately 1,600 m below the sea floor with a riser-equipped drill ship. Future expeditions will drill boreholes at two sites for the installation of a long-term borehole monitoring system. This will be critical for vertical holes with low inclinations that require measurement equipment like seismometers.

Coincident with the drilling, LWD and MWD were conducted to assess hole conditions and formation properties of the cover sediments. This paper covers the specifics of efficient drilling with a borehole inclination of less than 0.2° employing the vertical drilling system. Typically, riserless drilling using a mud motor or top drive system is carried out in the Nankai-Trough and the vertical hole inclination can vary up to approximately 5°. Downhole drilling parameters including stick-slip, weight-on-bit, surface torque, and rate of penetration are analyzed to enhance efficiency of the vertical drilling. A comparative study of the vertical holes with and without the push-the-bit vertical drilling system, which is considered useful in shallow, soft formations is also discussed.

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