Quantitative evaluation of rock brittleness and fracability based on elastic-wave velocity variation around borehole

Abstract

Brittleness and fracability are two important rock properties in hydraulic fracturing of unconventional reservoirs. Based on the variation of compressional and shear velocity around borehole using acoustic measurement, an effective technique is developed to estimate these parameters to guide reservoir-fracturing. During drilling, when the rock is broken, a significant amount of drilling induced cracks will occur in the formation around the borehole, reducing elastic wave velocity around borehole and causing the wave velocity variation away from borehole. The radial variation of compressional and shear velocities of formation rocks surrounding a borehole were respectively obtained from P-wave travel time tomography and dipole shear-wave dispersion inversion. By integrating the two variation profiles along the radial direction, the brittleness-fracability index is obtained to estimate the brittleness and fracability of formation rocks. The index shows fairly good consistency and correlation with rock brittleness and fracability, which demonstrates the practicability and effectiveness of the proposed approach. Well log data analysis examples are presented to demonstrate the effectiveness of our technique.