基于双交叉偶极子和多分量记录的三维声波测井理论与实验研究

Due to the anisotropy and strong inhomogeneity in unconventional tight formation, it is necessary for sonic logging to detect 3D acoustic information (i.e., in the axial, radial and azimuthal directions) and the refection signals from structures far from the borehole. Data from conventional multipole sonic logging can hardly provide accurate imaging in three dimensionals, because they are lack of radial and azimuthal acoustic information. To overcome this deficiency, a new three-dimensional acoustic logging tool, which contains dual cross-dipole transmitters and a series of multi-component receivers, is proposed. In this project, we will focus on the following three items to complete the design of the tool mentioned above. First, the signal responses of multi-components, especially the displacement in the axial direction, of reflection waves are studied theoretically and numerically. The sensitivity of those responses to parameters of the complex formation is analyzed, too. The source types, frequencies, tool positions, offsets of source and receiver, the distances from reflective objects to borehole, and the acquisition mode will be taken into considered for the sensitivity analysis. From these results, the true imaging method is developed. Second, the propagation characteristics of sonic waves excited by dual crossdipole are studied in detail from the experiment result of tight cores in laboratory at reservoir conditions. The precision of the tool in anisotropic evaluation and its in-situ stress determination can be validated. Finally, the above analyses are further verified by the scaled model experiments. Also, the performance, reliability, precision, and application of the logging technique in unconventional formation evaluation is also checked by laboratory results. In a word, this study will push forward the development of theories of acoustic waveguide and wave scattering in complex and anisotropic media, and will develop the reservoir imaging and the sonic logging data processing method. We aim at providing the theoretical and experimental bases for the new advanced tool design in the future.

非常规致密储层通常所具有的强非均质性和周向各向异性要求声波测井具备三维评价及远程探测的能力，常规多极子声波不能较好探测地层径向和周向变化的信息，无法提供精确的三维评价和成像结果。针对这一局限，申请人提出基于双交叉偶极子和多分量记录的三维声波测井，重点研究以下内容。第一，通过理论和数值模拟，考察复杂地层井孔中的反射波位移三分量和声压响应及其对地层参数的频率域灵敏度，量化仪器偏心对成像效果的影响，发展反射波真三维成像方法；第二，结合致密岩心声学实验结果，模拟实际地层中双交叉偶极子激发的井孔声场，分析其各向异性评价能力；第三，通过模型井实验验证数值模拟和理论分析的结论，评价新型测井方法的探测能力、可靠性和精度及其在致密储层地应力评价中的应用。本项目的开展将推进复杂各向异性介质中声波导和弹性波散射理论的研究，发展储层成像和测井资料解释方法，为新型声波测井仪器的设计和研制提供理论和实验依据。

三维声波测井是通过多模式多频率多方位多源距多分量数据测量，获取地层轴向、周向和径向三维声学和力学信息，对油气的增储上产特别是非常规油气田的“甜点”识别具有重要意义。本项目提出一种基于双交叉偶极子和三分量记录的三维声波测井仪器，将有效提高地层周向各向异性和径向非均质性的探测，取得的主要成果有：第一、关于各向异性探测：基于偶极子仪器同相分量的速度在各向异性地层随方位变化的特殊性，提出了双交叉偶极子，基于该方法，各向异性大小可以直接由4个同相分量的速度直接确定，并且可以快速稳定地确定方位角。进一步地，在对现有交叉偶极子仪器改动较小的情况下，通过相控方式实现双交叉偶极子。在数值模拟基础上，基于理论分析和数学变换，提出来一种分步的各向异性反演方法，将原来交叉偶极子多参数联合反演方法解耦成三个独立参数依次反演，提高了反演基于随钻仪器的测量特性，提出了基于四极子和偏心点源进行地层各向异性反演的新方法。第二、关于径向非均质性：针对现有交叉偶极子远探测仪器成像方位不确定性，提出了三分量记录和混合模式采集两种方法，实现远探测唯一性成像，特别是混合模式采集还可以有效压制井中直达波，极大提高了反射波的信噪比。邻井实测资料验证了该方法的正确性。通过大量的数值模拟，研究了不同地质体及其组合的远探测波场特征。此外，还揭示了随钻远探测波场的复杂性。基于单极子纵横波的探测深度随源距变化而变化，提出了近井壁层析成像的分步方法：首先确定不同子阵列的速度，然后基于射线追踪确定每个子阵列对应的探测深度。第三、关于干涉成像，提出了弹性波加权干涉和相对干涉成像，并进一步构建了关于干涉成像的统一理论模型，对比研究了不同干涉成像方法的性能。第四、在岩石物理测量及模型方面，测量了岩石三阶弹性模量随围压变化规律，提出了基质模量和临界孔隙度预测模型。以上研究对三维声波仪器研制及推广应用提供了理论指导和技术支撑。