复杂油气藏宽频带免调谐核磁共振探测方法研究

The characteristics of complex hydrocarbon reservoirs have put forward higher requirements on the accuracy and speed of nuclear magnetic resonance (NMR) logging. It leads to some problems in traditional multi-frequency tuned NMR logging like inaccurate porosity in reservoir evaluation, unclear fluid identify of oil and water. Aiming at the NMR logging of complex hydrocarbon reservoirs, this project will propose a systematic method with broadband non-tuned and multi frequency pulse sequences measurement for precise fast detection. This project will study the high power transmission mechanism of the broadband non-tuned and non-resonant coil, establish the mathematical model of the coil current and propose the high power transmission method with high energy efficiency. The receiving mechanism of the broadband weak echo signal transformer-coupling is also studied, modeling the circuit of the transformer coupling receiver, analyzing the influence factor of transformer insertion loss and the frequency bandwidth, proposing a method to broaden the band. Meanwhile, we will study the principle of multi-frequency pulse sequence measurement, analyzing the correlation and the signal to noise ratio (SNR) of multi-frequency echo signal, and designing the multi-frequency pulse sequence to measure T1 and T2 fast in multi radial section. An adaptive stochastic resonance multi-frequency weak signal detection and multi-link noise reduction method will be proposed to solve the problem of multi-frequency weak echo signal detection and noise suppression under the broadband and high noise. The result of the project will provide a theoretical and technical basis for the realization of the fast multi radial section, high SNR, high precision and multi-dimensional NMR detection for complex hydrocarbon reservoirs, and also provide a new theoretical basis and technical support for geological exploration.

复杂油气藏的特性对核磁共振测井精度、速度都提出了更高要求，导致传统多频调谐核磁共振测井存在储层评价孔隙度不准确、流体识别油水分辨不清等问题。本项目针对复杂油气藏核磁共振测井，提出一套系统的宽频带免调谐、多频脉冲序列测量的高精度快速探测方法。研究宽频带免调谐非谐振线圈大功率发射机理，建立线圈电流数学模型，提出高能量传输效率的大功率发射方法；研究宽频带微弱回波信号变压器耦合接收机理，对变压器耦合接收回路建模，分析变压器插入损耗与频带宽度的影响因素，提出频带拓宽方法；研究多频脉冲序列测量机理，分析多频回波信号的相关性与信噪比，设计快速多径向切片T1、T2测量多频脉冲序列；提出自适应随机共振多频微弱信号检测与多环节降噪方法，解决宽频带强噪声下多频微弱回波信号检测与噪声抑制问题；为实现复杂油气藏快速多径向切片高信噪比高精度多维核磁共振探测奠定理论与技术基础，为地质资源勘探提供新的理论依据与技术支撑。

为了实现复杂油气藏宽频带核磁共振探测需要一套系统的宽频带探测方法，本课题从宽频带测量机理研究、宽频带脉冲序列测量方法研究、宽频带强噪声下多频微弱回波信号检测方法研究，三个方面进行了系统的研究。对应的根据所研究的理论成果研制了一套宽频带低场核磁共振探测谱仪样机。宽频带测量机理研究的成果应用于样机的宽频带功率放大与回波信号接收部分的实现，宽频带脉冲序列测量方法研究的成果应用于宽频带主控与检测系统的实现，宽频带强噪声下多频微弱回波信号检测方法研究的成果应用于样机系统中对采集回波信号的处理。研究了宽频带线圈大功率发射机理，建立线圈电流数学模型，提出宽频带功率放大发射方法；研究了宽频带微弱回波信号变压器耦合接收机理，对变压器耦合接收回路建模，建立变压器宽频带接收数学模型，为宽频带变压器接收回波信号设计提供了理论依据；研究了宽频带脉冲序列测量方法，针对脉冲序列的宽频带发射、回波信号的宽频带采集进行了研究，提出宽频带测量的方法，设计了适用于宽频带测量的主控系统；提出了H_CEEMD阈值去噪方法是，将去噪过程与噪声建模相结合；H_CEEMD是EMD的一种改进算法，它不仅保留了EMD在处理非平稳信号方面的优势，而且可以有效地克服EMD的模态混叠问题。fGn作为白噪声的推广，反映了噪声的相关特性，非常适合于核磁共振测井信号的建模。通过周期图法估计fGn的H参数，利用鲁棒估计器对前两个IMFs中的噪声标准差进行估计，然后根据fGn分解后的IMFs之间的方差关系推导出其他IMFs中的噪声方差。采用估计的噪声方差对阈值进行评估，并采用间隔阈值法进行H_CEEMD阈值去噪，解决了宽频带强噪声下多频微弱回波信号检测与噪声抑制问题。本课题对所研制的谱仪样机进行了验证测试，对所研究的理论成果进行了数值模拟和实验验证，证明了本课题研究成果的正确性与实用性。为实现复杂油气藏快速多径向切片高信噪比高精度多维核磁共振探测奠定理论与技术基础，为地质资源勘探提供了新的理论依据与技术支撑。