Case Study of Core Bit Wear and its Effect on Core Recovery and Quality during Deep-sea Drilling Expeditions

Abstract

Abstract Scientific deep-sea drilling and coring operations in the Nankai-Trough located offshore Japan have been undertaken to unravel the mechanisms behind earthquakes and tsunami. Throughout a number of expeditions, core sampling runs using an advanced multiple wireline coring system which can continuously core into sections of undersea formations have been carried out. High core recovery and quality are vital to research in undersea formations. A wireline type rotary core barrel with 10–5/8 inch PDC core bits were deployed in the recent expeditions. However, due to rough sea conditions and severe drilling environment, the PDC core bit cutters were easily worn, causing significantly low core recovery and damaged core samples. The condition of the PDC trim cutters could thus be said to determine the results of the entire operation, showing the need for accurate analysis of the challenges in developing and maintaining drilling equipment and systems. This study analyzes the bit conditions and how the wear or damage of the PDC cutters affect core recovery and quality. Core quality was analyzed using X-ray-CT Scan image data as a new method of analyzing the quality. The heave motions of the drillship are also shown to affect the core quality and stress or damage while the onset of the core disking phenomenon was also monitored and analyzed.