部分饱和砂岩储层地震物理模拟及含气饱和度预测分析

摘要

含气饱和度预测是天然气储层地震解释工作的重要目标.本文将岩石物理分析与地震物理模拟技术相结合,构建了部分饱和砂岩储层物理模型并进行含气饱和度预测分析.物理模型中设置了高孔渗常规砂岩和低孔渗致密砂岩两种模拟储层,每种储层都是由具有不同含水饱和度的气-水双相饱和砂体组成.岩石物理分析结果显示在低孔渗致密砂岩中气-水混合流体更加倾向于非均匀的斑块分布,而结合了Brie等效流体公式的Gassmann流体替换理论可以更准确地描述纵波速度随含水饱和度的变化趋势.对物理模型进行地震资料采集处理后,对比了AVO特征和叠前同步反演结果对两种砂岩储层含气饱和度预测能力的差异.AVO特征结果显示,对于混合流体均匀分布的高孔渗砂岩储层,AVO响应曲线和属性变化很难对含气饱和度进行估算;对于混合流体斑块分布的致密砂岩储层,AVO特征可以定性地分辨出储层是否为高、中、低含气情况.反演结果显示,密度及纵横波速度比分别对高孔渗及致密砂岩储层的含气饱和度有着较好的指示能力.%The prediction of gas saturation is an important target for seismic interpretation of natural gas reservoirs.Based on the combination of rock physics analysis and seismic physical modeling technology,a physical model of partially-saturated gas sand is established.Two kinds of simulation reservoirs are set,one is conventional high porosity/permeability sand,the other is low porosity/permeability tight sand,each reservoir consists of six gas-water saturated sands of different water saturations.Rock physics analysis result shows that in tight sand gas-water mixture is more prone to be patchy saturation,Gassmann-Brie model can describe the P-wave velocity trend versus water saturation more accurately.After seismic data acquisition and processing of physical model,prediction abilities of amplitude variation with offset (AVO) characteristic and prestack simultaneous inversion on gas saturation are compared for the two reservoirs.AVO characteristic results show that it is difficult to evaluate the gas content through the AVO response and attribute for high-porosity/permeability sands when mixed fluids are uniformly distributed.But in tight sand,when mixed fluids are patchy saturated AVO characteristics can qualitatively predict whether the sand is with high,medium or low gas saturation.The inversion results indicate that density and VP/VS can be more helpful in gas saturation estimation for highporosity/permeability and tight sands,respectively.