Study on the effects of pore-microcrack properties and clay content on elastic wave dissipation characteristics in tight rocks of shale oil stratum

As an unconventional oil/gas resource, shale oil is rich in reserves, widely-distributed, and has great potential for the exploration and development. It has become the focus and hotspot of the oil and gas industries in the recent years. However, shale oil reservoirs are characterized by diverse mineral components, low porosity and low permeability, complex structures, and strong heterogeneities, which are significantly different from the conventional oil and gas resources. In this study, ten tight sandstone samples from the Chang 7 oil-layer group of Mesozoic Yanchang Formation in Ordos Basin are selected. The mineral components of each sample are obtained by the X-ray diffraction analysis. P- and S-wave velocities and inverse quality factors are computed based on the ultrasonic experimental tests under different confining pressures and fluid saturation conditions. The porosities with respect to the varying pressure are also obtained based on the experimental measurements, after which the microcrack porosity of each sample is estimated by the linear extrapolation and incorporated into the EIAS (Equivalent Inclusion-Average Stress) model. The corresponding microcrack aspect ratios and microcrack densities are obtained to analyze the effect of pore-microcrack properties on wave attenuation. The results show that the correlations between total porosity, microcrack aspect ratio and microcrack density, and the variation of attenuation (the difference between the attenuation at each confining pressure and that at the highest confining pressure) in the tight sandstones are significantly better than the correlations between those and attenuation. It is revealed that the tight sandstone samples have a triple-porosity structure of microcrack inclusions, intrapore clay inclusions, and intergranular pores according to the thin section analysis. Therefore, this study introduces a triple-porosity structure model to quantitatively estimate the clay content in each sample, and then analyzes the relations between the intrapore clay content, total clay content and P-wave attenuation. The results show that the intrapore clay content (rather than the total clay content) is one of the main factors that dominate the magnitude of P-wave attenuation in the shale oil reservoirs. This study can provide theoretical supports for the analysis of attenuation characteristics, the construction of rock physics models, and the studies on seismic exploration methods for shale oil reservoirs.