November Luncheon - The Challenges of Drilling a 7km Offshore Borehole to Penetrate a Seismically Active Subduction Zone
The Integrated Ocean Drilling Program (IODP) has embarked on an ambitious deep exploration project to drill a scientific borehole into the mega-splay of the Nankai Trough Subduction Zone located ~450 km SW of Tokyo, Japan. This international project (NanTroSEIZE) is the 1st attempt to drill, test, sample and instrument an active subduction plate boundary capable of generating M8+ earthquakes. The scientific objectives are to understand the physics of subduction earthquakes capable of generating massive tsunamis.
Twelve transect sites have been drilled across the trench axis, primarily for sampling, establishing a regional framework of observations and commissioning many wells for in-situ real-time monitoring. Several expeditions at one of these sites have occurred with the intent to drill a 7km vertical hole that intersects the stressed fault-splay. Expedition 348, ended in Jan 2014, after coming within 2.2 km of the mega-splay as planned.
We present a geomechanical postmortem of Exp 348 to explain the NPT encountered while drilling in this harsh environment and illustrate the process of using LWD and engineering information to design a forward well plan that delivers the borehole in a condition suitable for scientific observations in this all-important international project.