致密砂岩气层的弹性阻抗正演模拟研究

Elastic impedance modeling of tight sandstone gas reservoir

以苏里格气田盒8砂岩的实际井统计资料为基础建立致密砂岩气层的岩石物理模型,通过流体替代进行弹性阻抗正演模拟,分析弹性阻抗对致密砂岩气层识别的流体敏感性特征及其正演模型的响应趋势。研究表明,四类致密砂岩气层表现出各不相同的弹性阻抗特征,即Ⅰ类气层表现为随入射角增大而减小的正高弹性阻抗差特征,Ⅱ类气层表现为随入射角增大而减小且由正到负转换的近零弹性阻抗差特征,Ⅲ类气层表现为随入射角增大而减小的负高弹性阻抗差特征,Ⅳ类气层表现为随入射角增大而增大的负高弹性阻抗差特征;Ⅰ、Ⅱ、Ⅲ类气层的弹性阻抗都随入射角的增大而减小,盖层的弹性阻抗都随入射角的增大而增大,以致反射系数都随入射角的增大而减小,而Ⅳ类气层和盖层的弹性阻抗随入射角的增大分别呈增大和减小趋势,以致反射系数随入射角的增大而增大;四类致密砂岩气层的弹性阻抗和反射系数都随含气饱和度的增大呈减小趋势,其中在含气饱和度为0~0.1之间的减小幅度较大。此外,弹性阻抗正演模型的振幅响应特征能较好地反映出弹性阻抗随入射角和含气饱和度的变化关系,这种响应特征可为弹性阻抗反演结果的含气性解释提供指导。

In order to analyze fluid sensitivity characteristics of elastic impedance for tight sandstone gas reservoir identification and its model response trend,an elastic impedance modeling in tight sandstone gas reservoir is conducted.Based on well data statistics of He 8sandstone in Sulige Gasfield,rock physical model of tight sandstone gas reservoir is established and used in fluid replacement simulation to do the modeling. As the study shows,four types of elastic impedance characteristics for gas reservoirs have been found:Class Ⅰ,positive high elastic impedance difference decreases with incident angle increase;ClassⅡ,near zero elastic impedance difference decreases from positive to negative with incident angle increase;ClassⅢ,negative high elastic impedance difference decreases with incident angle increase;and Class Ⅳ,negative high elastic impedance difference increases with incident angle increase.For Classes Ⅰ,Ⅱ,andⅢ,the elastic impedance of reservoirs decreases and that of cap rocks increases with incident angle increase while the reflection coefficient decreases with incident angle increase.But for ClassⅣ,the elastic impedance of reservoirs increases and that of cap rocks decreases with incident angle increase while the reflection coefficient increases with incident angle increase.The elastic impedance and reflection coefficients of the four types of tight gas sandstone reservoirs all show a decreasing trend with gas saturation increase.Larger decreases appear with gas saturation range between 0and0.1.In addition,amplitude response characteristics of elastic impedance model can well reflect elastic impedance variation with incident angle and gas saturation,which can serve as a guide for tight gas sandstone reservoir prediction.