基于岩石震电效应的井下渗透率反演方法研究

Permeability is one of the most important parameters for evaluating the level of difficulty in oil and gas exploitation. However, nowadays there is no effective method insitu to explore formation permeabilities exactly. From the analysis of the simulated seismoelectric logs, we have found that the Stoneley wave phase of the ratio of the converted electric field to the pressure (REP) is sensitive to permeability. The tangent of the REP's argument is approximately in inverse proportion to permeability. Thus we apply for a theoretical studying of the permeability inversion method in a borehole that based on seismoelectric effects. Its main contents are as follows. Firstly, the component-wave analysis method will be used to study the dispersion and the attenuation of seismoelectric wavefields. We will also analyze the sensitivities of the amplitude and the phase of the REP to permeabilities. Secondly, we will simulate the seismoelectric logs for horizontally stratified formations by using finite-difference method and investigate the wave-field characteristics influenced by the mud cake on the borehole wall, the horizontal formation interface and the varying radius borehole. Finally, we will propose the permeability inversion method and inverse permeabilities from both the simulated seismoelectric logs and the laboratory results in small-scaled borehole models. This study has academic innovations in the simulation of seismoelectric logs and in the analysis of the characteristics of borehole seismoelectric wavefields and its correlation with permeability. The accomplishment of this project will promote the practicability study of seismoelectric logging method and provide theoretical foundation for the permeability inversion from seismoelectric logs.

渗透率是评价油气储层开采难易程度的重要参数之一。然而，目前没有特别有效的方法能准确快速地求取地层渗透率。申请人研究发现：震电效应测井的伴随斯通利波的电场与斯通利波比值（斯通利波电声比）相位对渗透率敏感，相位正切值与渗透率近似成反比。基于此，本申请研究基于震电效应的井下渗透率反演方法。主要内容包括：1.对井孔震电耦合波场进行频率-波数域分析，考察各分波的频散和衰减特性，分析斯通利波电声比的幅度和相位对渗透率变化的敏感度；2.有限差分模拟水平层状地层中的井孔震电波场，考察井壁存在泥饼层、地层存在水平界面、不规则井径等情况下的波场特性；3.提出渗透率反演方法，分别利用数值模拟和模型井实验记录的井孔震电波场反演渗透率。本研究在井孔震电波场的数值模拟、波场特性及其与渗透率相关性的分析等方面具有学术创新意义，期望项目的完成推动震电测井的实用性研究，为利用震电测井探测地层渗透率方法的应用提供理论依据。

目前，还没有特别有效的方法能准确快速地求取评价油气储层开采难易程度的重要参数——渗透率。含流体孔隙岩石中特有的弹性波与电磁场耦合效应（震电效应）被认为对储层渗透率的探测具有潜在应用价值。项目组在前期理论研究中发现：伴随井孔斯通利波震电场与斯通利波的相位差对渗透率敏感，二者比值（斯通利波电声比）的相位正切值与渗透率成反比。本项目进一步对利用震电测井估计岩石渗透率、随钻震电测井波场模拟等方面开展了理论模拟和实验测量工作，发表了相关SCI论文12篇和EI论文3篇。主要研究内容和重要结果简述如下：.1.提出了利用震电测井的电声比相位正切值反演渗透率的显式公式。从模拟时域震电测井波形中反演了不同地层的渗透率，反演误差与传统利用声波测井的斯通利波衰减估计渗透率相比更小。.2.鉴于上述渗透率反演方法中需已知地层弯曲度，提出了由电声比幅度计算地层弯曲度的显式公式。计算了不同地层的弯曲度，相比正演模拟的理论输入值，其误差小于5.0%。因此，可综合利用井孔震电波场的幅度和相位信息，先由电声比幅度计算弯曲度，再由电声比相位估计渗透率。.3.考虑到震电测井方法后续的实际应用，有必要研究将迭代反演算法应用于解决基于震电测井的反演问题，利用震电测井波场的幅度和相位信息逐步地成像地层界面、反演渗透率。注意到现有井中界面成像技术在环向方位识别上存在缺陷，提出了基于反射波相位特征的井外界面判别法。.4.鉴于现阶段随钻测井技术的普遍应用，理论研究了随钻环境下的井孔震电波场。分别视地层为流体饱和孔隙介质和等效弹性介质，计算了单极源、偶极源和四极源随钻震电测井的全波波形。.5. 为了解释随钻震电测井全波中存在伴随钻铤波电场的原因，需深入研究钻铤波在整个模型中的传播特性。通过有限差分模拟和分波计算的方式，阐明了钻铤波的传播特性。.6. 分别实现了轴对称柱坐标系下震电测井有限差分模拟算法和3D直角坐标系下震电波场有限差分模拟算法。.7. 实现了单极源震电测井的分波计算算法以及随钻声波测井的分波计算算法。考察了全波中各分波的频散和衰减特性，计算了所有分波波形，将各分波叠加后与全波波形一致。.8. 建立了电缆和随钻声波测井、震电测井的小尺寸模型井实验测量系统，成功测量到不同岩石的两类井孔震电信号，即伴随电场和界面电磁波信号，考察了模型井实验测量动电信号与渗透率的相关性。