流体饱和致密砂岩多频段岩石物理实验及理论解释模型

After determination the microstructure details of the pore geometry of tight sandstone, the velocity and attenuation are experimentally measured on the selected tight sandstone samples with representative pore structure by using a multi-frequency band elastic parameter measurement system under different circumstance and pore fluid conditions. The influence of the laboratory conditions on the properties of elastic wave propagation is studied by the measurement results, and the relation between the properties of elastic wave propagation and the rock property parameters, pore fluid properties are established. By the qualitative description of the pore structure and wave induced fluid flow on the pore scale, and a new approach is advocated to model squirt flow dispersion and attenuation for porous media with compliant pores showing widely distribution in aspect ratio by gradual adding unrelaxed compliant pore. Based on White model of partial saturation, the analytical expressions of pore pressure and relative flow velocity of pore fluid are given for this model by introducing surface tension in the boundary condition, and dispersion equation accounting for wave induced fluid flow at mesoscopic scale are built. From these equations, the relation between pore structure, pore fluid properties and elastic wave propagation is revealed, and the relation between pore pressure equilibration and velocity dispersion, relative flow velocity and attenuation for wave induced fluid flow at mesoscopic scale are also illustrated, which can provide theoretical explanation model for the multi-frequency band experimental results.

通过对致密砂岩样品微观孔隙结构的刻画，以代表性样品（孔隙结构具有代表性）多频段岩石物理测量为基础，揭示流体饱和致密砂岩样品实验条件、孔隙流体特征及孔隙结构变化对其弹性波传播特征的影响规律，建立流体饱和密砂岩不同频段波动特征与其岩石特征参数、孔隙流体特征之间的关系。通过对致密砂岩微观孔隙结构和孔隙尺度波致流作用的定量表征，基于孔弹性理论，给出具有复杂孔隙分布特征的致密砂岩在单一流体饱和条件下的喷射流作用模型。基于White部分饱和模型，通过在边界条件中引入表面张力作用，给出孔隙压力与孔隙流体流动速度的解析表达式，建立考虑表面张力的中观尺度波致流作用频散方程。从这些方程出发，揭示致密砂岩孔隙结构、孔隙流体特征与弹性波传播特征的关系， 阐明部分饱和致密砂岩在中观尺度波致流作用下的孔压平衡过程与介质速度频散特征的关系、流体流动速率与衰减特征的关系，为多频段实验结果提供理论解释。

针对现有声波实验在反应具有复杂孔隙结构的流体饱和致密砂岩弹性波传播特征方面的不足，以代表性致密样品（震旦系灯影组四段深层致密白云岩、古城地区和哈拉哈塘地区奥陶系鹰山组三段超深层致密碳酸盐岩样品）系统岩石学特征、孔隙结构特征、岩石物理特征测试结果，分析了孔隙结构、孔隙流体、频率等因素变化对速度频散的影响规律，总结出不同孔隙结构类型样品及不同实验条件下样品的速度频散特征规律。研究结果表明，样品在地震频段表现出明显的频散现象(5～200Hz)，微裂隙-溶蚀孔隙型样品、裂隙型样品与溶蚀孔隙型样品在频散与衰减特征上存在明显差异，微裂隙-溶蚀孔隙型样品具有相对较强的频散与衰减特征。依据储层岩石地震弹性性质变化特征给出了深层致密碳酸盐岩岩性、储层类型以及孔隙流体识别与预测的岩石物理标准。致密样品孔隙结构类型差异控制了有效压力-速度的变化方式以及孔隙尺度流体相关频散作用的强弱，而微裂隙等软孔隙度对纵、横波速度影响程度的差异造成纵波-横波速度关系、纵波速度-速度比变化关系以及孔隙度-速度关系发生明显变化。致密碳酸盐岩样品物性与地震弹性性质整体变化规律受微晶方解石结构特征的控制，依据形态特征可将样品微晶结构分为多孔微晶、紧密微晶体和致密微晶三种类型，随着上述三种微晶中晶体边界紧密缝合接触的程度增加，其微晶晶间孔隙度与孔喉半径逐渐减小，同时方解石晶体颗粒边界刚度特征特征与弹性均匀性的逐渐增强，致使样品渗透率与速度等地震弹性特征表现出随孔隙增大而逐渐减小的总体趋势。针对现有双相介质理论模型在表征致密复杂孔隙结构及其所引起的特殊流固耦合作用影响方面的不足，基于孔弹性理论从微、细观孔隙结构及其流固耦合力学模型入手，建立与该类岩石介质微观孔隙结构及流固耦合作用相一致的频散方程，进而系统研究频散、衰减等地震波传播特征与岩石孔隙结构特征、孔隙流体特征的关系，为揭示多频段实验结果的物理机制提供依据。