OBSERVATIONS OF WELLBORE FAILURE IN THE TOA-BAJA WELL-IMPLICATIONS FOR THE STATE OF STRESS IN THE NORTH COAST TERTIARY BASIN, PUERTO-RICO

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-degrees and 250-degrees 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 N31-degrees-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 = S(v); S3 = S(hmin) almost-equal-to 0.5S(v); S(Hmax) almost-equal-to (0.55-0.63)S(v), and an associated incipient normal faulting stress regime. This stress regime is consistent with focal mechanisms determined for earthquakes with epicenters near the drillsite.