疏松砂岩气藏地震AVO属性敏感性分析与评价

Sensitivity analysis and evaluation of AVO property in unconsolidated gas sandstone

利用完全Zoeppritz方程分析了疏松砂岩气藏反射系数对密度比、纵波速度比、横波速度比、泊松比和含气饱和度的敏感性.通过求取反射系数对各弹性参数的偏导得到反射系数对弹性参数的敏感性,通过求取反射系数对含气饱和度的完全导数得到对含气饱和度的敏感性.对于疏松砂岩气藏界面,P-P波AVO(振幅随偏移距变化)当入射角大于40°对纵波速度比、横波速度比在低含气饱和度时敏感,对泊松比的敏感性在气层饱和度较高和较低时差异明显,对含气饱和度的敏感性在低饱和度时敏感,且随入射角增加而增大;P-SV波AVO对泊松比的敏感性在含气饱和度较高和较低时差异明显.研究表明,利用反射系数对弹性参数和含气饱和度的敏感性分析可以帮助识别低饱和气藏.

The paper analyzes the sensitivity of seismic reflectivity to contrasts in density, P-wave velocity, S-wave velocity, Poisson/s ratio and gas saturation in unconsolidated gas sandstone using the complete Zoeppritz equations. Sensitivites of reflection coefficients to each bulk elastic parameter are computed as the partial derivatives of the seismic reflectivity relative to each parameter. The sensitivity of reflectivity to gas saturation is calculated as the full derivative of the reflectivities with respect to gas saturation. For the shale-sand interface of unconsolidated sandstone, P-P reflectivity is more sensitive to P-wave velocity contrast and S-wave velocity contrast at low gas-saturation values when the angle of incidence is greater than 40~. The sensitivity of reflectivity to Poissonrs ratio shows greater variance between low gas-saturation and high gas-saturation, and to gas-saturation is more sensitive in low gas-saturation and the sensitivity increases as the angle of incidence increases. P-SV reflectivity has great contrast between low gassaturation and high gas-saturation to Poisson＇s ratio. The study in this paper shows that it may help to discriminate low gas saturation by using the sensitivity of reflectivity to these elastic parameters and gas-saturation.