Experimental research of effect of microfracture on shear wave attenuation and its application on fracture evaluation in tight sand formation

Key words:  tight reservoir  fracture  shear wave  attenuation  logging

Abstract:

To solve micrometer scale fracture evaluation problems, shear wave attenuation coefficients of three core samples with different porosities and fracture widths are studied by physical experimental method. Shear wave attenuation response with micrometer scale fracture width is analyzed, and the results are applied to evaluate crack aperture using logging data in tight sand reservoir. The relationship between the equivalent crack aperture and the shear wave attenuation coefficients has been established according to experimental results. The results show that shear wave attenuation coefficient increases correspondingly with the increasement of crack aperture. In additional, the shear wave attenuation coefficient caused by fracture is inversely proportional to core porosity if the crack aperture is a constant The shear wave attenuation coefficient changes more rapidly with the crack aperture for a core sample with lower porosity, which indicates that the shear wave attenuation is more sensitive to the change of crack aperture in tight sand formation. The application shows that equivalent crack apertures calculated from shear wave attenuation coefficients coincide well with the fracture distribution indicated by the interpretation plot of FMI data, demonstrating the feasibility and robustness of the crack aperture evaluation method based on shear wave attenuation.