Observations of wellbore failure in the Toa Baja Well—Implications for the state of stress in the North Coast Tertiary Basin, Puerto Rico

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Abstract
Borehole televiewer (BHTV) and 4‐arm caliper (dipmeter) logs were obtained in the Toa Baja well (drilled to a depth of 2704 m into sediments of the North Coast Tertiary Basin, Puerto Rico) from 704 to 2676 m depth. The only observations suggesting that stress‐induced wellbore failure (breakouts) had occurred were small intermittent features at a depth of approximately 2600 m with azimuths of 70° and 250° and the fact that, during drilling, wellbore stability became a problem near total depth. An increase in mud weight to 10 Lb/gal required to stabilize the deteriorating wellbore was accompanied by loss of drilling fluid into the formation, suggesting that hydraulic fracturing or the reopening of pre‐existing near vertical fractures had occurred. A series of vertical fractures at a variety of azimuths (averaging N3l°W) was detected by the BHTV. The loss of circulation due to an increase in mud weight, combined with the absence of well‐developed breakouts, enables us to estimate stress magnitudes near the well for reasonable values of rock strength: S1=Sv; S3=Shmin≈ 0.5v; SHmax≈ (0.55–0.63)Sv, and an associated incipient normal faulting stress regime. This stress regime is consistent with focal mechanisms determined for earthquakes with epicenters near the drillsite.