射线域频变AVO含气性识别探索与实践

Investigation and case study of frequency-dependent AVO in ray domain for gas detection

流动性是预测低孔渗气藏含气性关键参数之一,但如何预测流动性一直是地球物理界难题。在孔隙粘弹性介质中,流体的存在导致地震波发生不同程度的频散和衰减,研究表明P波频散和流体的流动性有关,而流动性是渗透率与流体粘滞度的比值,因此流动性的概念和参数能极大地帮助流体粘滞度与渗透率的预测。笔者利用Wang Yanghua射线参数域表示的Zoeppritz近似方程,将频率因子引入到AVO分析中,推导出射线域频变AVO表达式。同时提出了频变属性表达式以衡量速度频散的大小,分析随频率变化的速度频散异常特征。将速度频散和流体的流动性联系起来,在莺歌海盆地DF区高温高压岩性气藏油气检测中进行了实际应用,结果表明速度频散能有效预测流体流动性、储层含气性。

As we know, mobility is one of the key parameters for predicting the gas-bearing properties of low-porosity gas reservoirs. However, how to predict the mobility is always a difficult geophysical problem. In porous viscoelastic medium, the existence of fluid leads to different degrees of seismic wave dispersion and attenuation, and many researches have shown that P-wave dispersion is related to the fluidity of the fluid which is the ratio of permeability to fluid viscosity.Thus, the fluidity can be used to assist the prediction of fluid vis- cosity and permeability.In this paper,based on Zoeppritz approximate equations expressed in ray-domain by Wang Yanghua,the fre- quency factor is introduced into AVO analysis, and frequency-dependent AVO expressions is deduced in ray domain.The authors also present a frequency-dependent attributes expression to measure the velocity dispersion and analyze the velocity dispersion characteristics with frequency variation,with the features of velocity dispersion varying with frequency, by using expressions of frequency-dependent at- tributes.Then, the authors combine the velocity dispersion and fluidity of the fluid.Finally, the authors apply the proposed method to oil and gas detection in lithologic gas reservoir which is characterized by high-temperature, high-pressure in DF district of Yinggehai basin. The application results show that the velocity dispersion can effectively predict the fluidity of fluid and the gas-containing property of the reservoir, and the effectiveness of this method is demonstrated.