A Borehole Televiewer (BHTV) image log was acquired in the HN-16 borehole in October 2010 by the Iceland GeoSurvey (ISOR) to a total measured depth of 2,191 m in the Hellisheidi Geothermal Field, in SW Iceland. The HN-16 images reveal the attitude of natural fractures are strongly clustered with a mean true strike azimuth from ~220-230 and true dip from 71-78° that is consistent with the strike of rift-graben faults mapped at the surface which bound the geothermal field. This clustering appears independent of the borehole deviation direction, which is roughly due north in the imaged interval. Although the majority of these fractures are partially or fully healed, as inferred from minimal surface topography across the borehole wall, many appear to be open based on loss of signal in the image log and the higher injectivity of HN-16. Structures resulting from drilling-induced borehole failure include petal-centerline fractures, some tensile fractures, and abundant breakouts (which were unexpected in a presumed low differential stress, extensional system). The vertical stress (Sv) was derived from estimated rock density and fluid pressures from an equilibrated pressure log. There were no mini-hydraulic fracture tests or rock strength measurements within the reservoir to use as inputs for the stress model. Instead, the analysis of the stress state was solved for iteratively by testing the sensitivity of breakout position and width to the azimuth of the horizontal principal stresses with respect to the unconfined compressive strength (UCS) of the formation, and then finding a range of stress magnitudes based on maximum horizontal stress (SHmax) azimuth and a range of UCS for representative basaltic rocks. This process was completed for each pair of breakouts identified within the image log and a variable stress regime is inferred as strike-slip near the surface, but transitions into a transitional strike-slip-to-normal faulting regime at greater depths, with a mean SHmax azimuth of 022.4° ± 5°. This stress direction parallels the local graben-bounding faults, which strike 020 to 030, and the regional SHmax azimuth inferred from earthquake focal mechanisms, including strike slip focal mechanisms. However, we note that this borehole stress model is limited by relatively poor constraints on the magnitude of the minimum horizontal stress (Shmin) and rock strength, which introduce significant uncertainty into the estimate of the Shmin and SHmax magnitude distribution with depth. This state of stress suggests some of the natural fractures mapped in the well are optimally oriented for normal fault slip, but the injection area is likely within a strike-slip faulting regime.

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