Seismic attenuation analyses of fracturing in reservoir rocks

Abstract

In this study seismic wave velocity and seismic wave attenuation in dry Bentheimer sandstone, Indiana limestone, and Whitby shale are studied while continuously deforming and therefore increasingly fracturing the rock samples up until macroscopic failure. A pulse transmission technique at a central frequency of 1 MHz is used in combination with a high pressure system that is constantly applying uniaxial stress. At early deformation stages before yielding, increasing elastic strains correlate with decreasing seismic attenuation suggesting closure of existing fractures. The deviation from a linear stress-strain relationship indicates the yield point, i.e. the onset of plasticity or fracturing. After yielding, increasing plastic strains correspond to increasing seismic attenuation suggesting formation of new micro-fractures. Thereby, P-wave attenuation is more sensitive to the growth of fractures than S-wave attenuation. The existence of fractures is confirmed by postmortem micro-CT-images. Absolute attenuation values are highest for Indiana limestone and lowest for Whitby shale. Furthermore, S-wave attenuation is the most sensitive to fracturing in Indiana limestone, followed by Bentheimer sandstone; P-wave attenuation is the most sensitive to fracturing in Bentheimer sandstone, followed by Indiana limestone. Attenuation in Whitby shale is the least sensitive to fracturing for both P- and S-waves. This study confirms that attenuation is more sensitive to fracturing than the seismic velocity; hence attenuation is suggested to be a valuable seismic property to identify fractures. At last, the study presents the possible usage of amplitude decay-over-time analysis of seismic waves for possible borehole applications.