微裂缝对横波衰减影响的实验研究及其在致密砂岩裂缝评价中的应用

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

针对致密砂岩储层微裂缝评价问题,采用物理实验手段研究孔隙度不同的3组岩心样品在不同裂缝宽度条件下横波的衰减情况,分析微米级裂缝宽度对横波衰减的影响规律,并将其应用于致密砂岩储层裂缝张开度测井评价。根据实验研究结果,建立微裂缝等效宽度与裂缝横波衰减系数之间的关系式。结果表明:横波衰减系数随裂缝宽度的增加有规律地增大;另外,同等裂缝条件下孔隙度较小的岩心样品对应的裂缝横波衰减系数相对更大,且随裂缝宽度变化更快,说明横波衰减系数对微裂缝宽度变化更敏感;由裂缝横波衰减计算的微裂缝等效宽度与电成像解释成果图中显示的裂缝发育程度吻合较好,证实了方法的可行性和正确性。

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.