ABSTRACT:

Borehole breakouts were commonly observed in the boreholes drilled in unconsolidated deepwater sediments near the Nankai Trough, SW Japan. The use of borehole breakouts enabled us to constrain in situ stress states. While it was straightforward to estimate the stress direction based on the breakout azimuth, an ambiguity occurred that the breakout width to constrain stress magnitudes widened significantly with time in an order of hours. Two independent borehole wall images of the same depth interval, captured at the bottom and the top of the 30m long logging-while-drilling (LWD) bottom-hole-assembly, indicate that breakout widths grew from 42° immediately after bit to 135° about an hour later. Triaxial compression tests in cores revealed that all the specimens failed in brittle mode immediately when stress condition reaches that required for failure, suggesting that for the purpose of stress estimation, the use of breakout width immediately after the drill-bit passes through the depth is appropriate.

1. INTRODUCTION

Observation of drilling-induced borehole failures such as wellbore breakouts and tensile fractures has greatly contributed to the estimation of in situ stress state in the crust for last several decades. Recent statistics show that about 20% of more than 21,000 data in the World Stress Map database were determined from the drilling-induced borehole failures [1]. As drilling and logging technology advance, opportunities to detect and utilize such borehole features to estimate stress states are expected to grow even in hostile environments. Borehole breakout is a brittle compressive failure that occurs when the local stress condition at the borehole wall exceeds the limiting stress that the rock can sustain [2-5]. Because such compressive failures in a vertical hole would form in the least horizontal in situ stress direction, it is relatively straightforward to estimate the stress orientations from breakouts detected using an image logging tool [6-8].

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