Threshold of borehole failure: Breaking in before breaking out, Mississippi fan, Gulf of Mexico

Abstract

Resistivity images from Integrated Ocean Drilling Program (IODP) Site U1322 on the Mississippi fan (Gulf of Mexico) show borehole failure as (1) low-resistivity bands interpreted as breakouts and (2) high-resistivity bands. Both features occur as opposing pairs on opposite sides of the borehole, and have similar azimuthal orientations and widths. Failures occur at depths of 90–216 m in sediments very rich in expansive (smectite-illite) clays of 40%–50% porosity that are younger than 65 ka. The low-resistivity breakouts resemble similar features in other IODP boreholes from southwest Japan and offshore Oregon. The high-resistivity features are unknown in other boreholes. Estimates of stress magnitudes based on the overburden stress and the extensional tectonic environment in the Gulf of Mexico predict that the borehole was at failure. Experiments were conducted on cores with lithologies equivalent to those of the borehole failure localities from IODP Site U1322 and adjacent Site U1324. These experiments suggest an elastic-plastic deformation with strains of 10%–15% before reaching a plastic yielding. In the experiments, strain softening during plastic deformation ranges from 0% to 20%. Physically the experimental samples show a combination of lateral bulging and discrete conjugate shears. These experiments suggest that the resistive areas in the borehole are an initial state of bulging, or extrusion, into the borehole. We call these extrusive failures “breakins” to distinguish them from traditional breakouts. Extrusion into borehole decreases the amount of conductive borehole fluid between the bulging sediment and the resistivity tool, increasing the resistivity signal. The high residual strength of the sediment prevents disaggregation and spalling. Where spalling has developed, breakouts occur. This analysis is the first documentation of this incipient stage of borehole failure.