Riserless Vertical Drilling Operations using Downhole Motors in the Nankai-Trough Seismogenic Zone
Authors: Shinmoto Yuichi , Kido Yukari , Wada Kazuyasu 
Published: March 2015

Abstract
The Nankai-Trough, which is located south of the main island of Honshu, Japan and extends 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 so that the Nankai-Trough contains a large amount of gas hydrate formations. A conventional downhole motor, a positive displacement motor (PDM), was deployed for riserless vertical drilling in order to successfully drill to depths of approx. 1,000 m-below sea floor (m-BSF) in approx. 2,000 m water depth. Several sections of a borehole was drilled in one of the expeditions for the installation of a scientific Long-Term Borehole Monitoring System (LTBMS) which will be especially critical for vertical holes of less than 3 deg. that require such measurement equipment as a tiltmeter and accelerometers.
Coincident with the drilling, Logging-while-Drilling (LWD) was conducted to assess hole conditions and formation properties of the cover sediments. This paper covers the specifics of successful drilling with a hole deviation of less than 1 deg with PDM having an adjustable bent housing angle (1.15 deg.). Typically, conventional riserless drilling using a top drive system was carried out in the Nankai-Trough and the vertical hole inclination varied up to 5 deg. Control of the rate of penetration (ROP) and improvement of hole conditions by hole cleaning were essential to drill the vertical holes. Downhole drilling parameters such stick-slip, weight or torque-on-bit were analyzed to enhance efficiency of the vertical drilling. A comparative study of the vertical hole with and without the PDM, as well as other vertical drilling systems will be discussed.

Introduction
The scientific deepsea 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 presently underway through an international scientific effort called the Integrated Ocean Drilling Program (IODP) and as part of these efforts, further technical developments in riserdrilling will allow us to attain the goal of reaching the earth's mantal in future expeditions. Meanwhile, 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 ([1] Kinoshita, et al. 2008). The drilling program includes a coordinated effort in drilling, sampling, and long-term monitoring as well as technical development. An important tool in studies and analysis of mega earthquakes and tsunami study is the Long-Term Borehole Monitoring System (LTBMS) which has been under continuous development and is now deployed on several research ships. Seismogenesis can be measured with such an instrumented borehole system, however, precise vertical holes of less than 3 deg. hole inclination are necessary to activate the system safely and efficiently for accurate data accumulation of seismicity of even micro-seismic velocity. The NanTroSEIZE expeditions, thus, carry out drilling, coring, and geophysical logging while LTBMS can clarify seismogenic mechanisms. Plans are also underway for the LTBMS to be connected to a sea floor observatory network to establish real-time monitoring of seismic, geodetic, and hydro geologic conditions ([2]Saffer et al, 2009).
Logging-while-Drilling (LWD) and Measurement-while-Drilling (MWD) systems have been deployed to obtain a comprehensive suite of geophysical log data to establish a riserless observatory system. Although drilling vertical boreholes is considered vital in these operations, drilling straight down is a technically difficult challenge. This study demonstrates that the first vertical drilling system using a downhole motor was effective in carrying out the objectives for shallow sections.