Assessing rock brittleness and fracability from radial variation of elastic wave velocities from borehole acoustic logging

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Abstract

Rock brittleness and fracability of subsurface formations are two important parameters for hydraulic fracturing in hydrocarbon reservoir production. This paper presents an effective technique to assess these parameters using the radial variation of compressional and shear velocities from borehole acoustic logging. Our technique is based on a rock mechanic phenomenon that a brittle rock with high fracability tends to leave a significant amount of drilling-induced cracks at the borehole wall, resulting in radial elastic wave velocity variation away from borehole. By determining the velocity variation, the combined effects of brittleness and fracability of formation rocks can be assessed. The compressional-wave travel-time tomography and flexural shear-wave inversion methods are respectively used to obtain compressional- and shear-velocity variations. Well-log data analysis examples demonstrate the practicability and effectiveness of this technique.

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