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

Abstract

Abstract 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 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, and 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 (NanTroSEIZE) was carried out to successfully drill to depths of approx. 1600 m-below sea floor (m-BSF) with a riser equipped drill ship. Future expeditions will drill boreholes at two sites for the installation of a Long-term borehole monitoring system (LTBMS) which will be especially critical for vertical holes with low inclinations that require measurement equipment such as a seismometer. Unlike previous riser-less deep sea drilling, riser drilling operations for scientific research were carried out for the first time. Coincident with the drilling, Logging-while-Drilling (LWD) / Measuring-while-Drilling (MWD) was 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 (VDS). Typically, riser-less drilling using a mud motor or conventional drilling bottom-hole assembly (BHA) is carried out in the Nankai-Trough and the vertical hole inclination can vary up to approx. 5 degrees. Downhole drilling parameters such stick-slip, weight-on-bit (WOB), surface torque and rate of penetration (ROP) will be analyzed to enhance efficiency of the vertical drilling. A comparative study of the vertical holes with and without the push-the bit VDS, which is considered useful in shallow sediments, will also be discussed. Introduction The scientific deep sea riser-equipped drilling vessel, Chikyu, (Fig. 1) was built for the purpose of scientific research in elucidating the factors responsible for global environmental changes, earthquake outbreak mechanisms and biospheric conditions below the seafloor. Multinational investigations are underway through an international scientific effort called the Integrated Ocean Drilling Program (IODP). Further technical developments in riser-drilling will allow us to attain our goal of reaching the earth's mantle in future expeditions. Heave compensators, i.e., passive and active, have also been employed to allow stable deep sea drilling (1). The Nankai-Trough seismogenic zone experiment (NanTroSEIZE) is a multi-expedition, multi-stage IODP drilling project focusing on understanding the mechanics of seismogenesis and rupture propagation along subduction plate boundary faults (2). The drilling program includes a coordinated effort in drilling, sampling, and long-term monitoring as well as technical development. The most sophisticated Logging-while-Drilling (LWD) and Measurement-while-Drilling (MWD) technologies to date have been deployed to obtain a comprehensive suite of geophysical, seismic and downhole log data across the Nankai accretionary complex. Although drilling vertical boreholes is considered vital in the NanTroSEIZE operations, drilling straight down is a technically difficult challenge. This study demonstrates that the first push-the-bit vertical drilling system deployed was effective in carrying out the objectives of the NanTroSEIZE expeditions.