Relating P-Wave velocity to rock strength in high-porosity, shallowly buried sediments: Implications for in situ stress estimates

Katelyn Olcott
The Pennsylvania State University, Department of Geosciences, Center for Geomechanics, Geofluids, and Geohazards University Park, Pennsylvania, USA
[email protected]

The magnitude of in situ stresses is important toward understanding fault and earthquake mechanics and for hydrocarbon exploration. Wellbore failures, including compressional borehole breakouts (BO), can provide information about stress orientation and can be used to constrain in situ stress magnitude if substrate unconfined compressive strength (UCS) is known. Values of UCS used to estimate stresses from BO are typically derived from laboratory-derived relations between P-wave velocity (Vp) and UCS. For many applications in sedimentary basins, UCS is estimated from relations developed for shales. We seek to advance our understanding of Vp as a proxy for UCS, particularly in high-porosity (~30-60%), shallowly buried (<2 km) sediments where estimates of UCS based on relationships defined for shales may lead to overestimates of strength. We focus on the Nankai accretionary prism offshore SW Japan, formed by subduction of the Philippine Sea Plate beneath the Pacific Plate. BO have been identified from resistivity logs at IODP Site 808. We determine UCS from triaxial tests on core samples recovered during Ocean Drilling Program Leg 190 from ODP Site 1174, located ~1 km away from Site 808. We then compare UCS measurements to Vp data from wireline logging.

Results indicate directly measured values of UCS are lower (>1 MPa) than those estimated from Vp using shale-relationships. When applied to estimate stresses from observed BO, shale-based UCS values lead to overestimates of stress. Results should apply to the general case of shallow, high-porosity sediments and carry implications for borehole stability and assessment of shallow geohazards globally.

AAPG Search and Discovery Article #90157©2012 AAPG Foundation 2012 Grants-in-Aid Projects