基于改进的介观裂缝岩石物理模型的频散AVO特征分析

地下储层中存在介观尺度的裂缝是导致地震波的速度频散和能量衰减的一个重要原因。从Galvin介观裂缝模型出发, 重构其低频极限和高频极限的表达形式, 将频散关系施加到裂缝模型上构建依赖频率的裂缝柔度参数, 建立了一种改进的介观尺度的裂缝型岩石物理模型。基于该改进模型, 利用附加柔度的组合性质, 可以容易地构建更复杂的模型并进行依赖频率的地震响应特征分析。在传统的反射系数公式中引入频率参数, 分析了具有不同裂缝长度、背景渗透率、流体粘度等频散敏感参数的反射界面上的频散AVO及地震记录响应特征。通过数值模拟得出频散敏感参数主要影响发生频散的频段, 即影响岩石的特征频率; 当模型的特征频率与地震波频段主频相近时, 频散现象最突出, 当特征频率大于地震波频段主频一个数量级时, 可以忽略频散作用, 此时采用传统的Gassmann方程即可较准确地分析地震剖面的特征。基于改进模型得到的认识为地震频散特征的实际应用提供了岩石物理模型的构建方面的指导。

Mesoscopic fractures exist in fluid-bearing fractured reservoirs and are an important cause of wave velocity dispersion and energy attenuation in seismic bands.Based on Galvin mesoscopic fracture model, we proposed an improved mesoscopic fracture rock physics model by reworking the low-and high-frequency limit formulas and applying the dispersion relation to the fracture model to construct frequency-dependent fracture parameters, such as the compliance of fractures.Based on the improved model, we can build more complex models and analyze the frequency-dependent seismic response characteristics using the combined properties of the additional compliance parameters.It is simple to calculate and analyze the frequency-dependent AVO by adding the frequency parameter to the conventional reflection coefficient formula.The dispersion AVO and seismic responses at the reflection interface were caused by different dispersion-sensitive parameters, including fracture length, background permeability, and fluid viscosity.Numerical simulations showed that dispersion-sensitive parameters mainly affected the frequency band of dispersion, which is called the characteristic frequency of the rock.When the characteristic frequency used in the simulation was approximately equal to the dominant frequency in the seismic exploration data, the dispersion effect was significant.When the characteristic frequency used in the simulation was approximately one order of magnitude greater than the dominant frequency that appears in seismic exploration, the dispersion effect was ignored, and the conventional Gassmann equation was used to analyze the seismic reflection characteristics more accurately.Therefore, the understanding and conclusions from the analysis based on the improved model proposed in this study provide guidance for applying rock physics to practical seismic dispersion characteristic analyses.The complex characteristics identified in this study may be useful for the seismic characterization of fractured reservoirs.