D-T2二维核磁共振脉冲序列及反演方法改进设计

面对日益复杂的勘探对象, D-T₂二维核磁共振技术在实际应用中面临无法兼顾扩散系数测量范围和横向弛豫分辨率的困境. 脉冲序列作为D-T₂二维核磁共振数据采集的核心技术, 其性能优劣直接影响应用效果, 在综合对比PFG, STE-PFG, BP-PFG、改良式CPMG, 扩散编程, 多回波间隔CPMG脉冲序列性能的基础上, 有效融合脉冲梯度场、恒定梯度场D-T₂脉冲序列的优点, 本文提出一种基于脉冲梯度场的双变量、两窗口D-T₂脉冲序列改进设计. 针对两个窗口的D-T₂二维核磁共振数据反演, 为突破现有反演方法无法兼顾反演精度和解谱效率的瓶颈, 本着第二个窗口回波信号为主、第一个窗口回波信号为辅的原则, 本文提出一种同时使用两个窗口数据参与解谱的联合TSVD反演方法. 气水、油水、稠油、油气水模型不同信噪比条件下的数值模拟结果表明, 本文提供的D-T₂改进脉冲序列达到了平衡扩散系数测量范围和横向弛豫分辨率的设计要求, 本文提供的联合TSVD反演方法也有效平衡了反演精度要求和解谱效率. 文中的D-T₂改进脉冲序列及联合TSVD反演方法在复杂油气藏流体识别和产能预测中具有广泛的应用前景, 可为促进国内D-T₂二维核磁共振岩心分析技术的发展提供有利条件.

A modified design of pulse sequence and inversion method for D-T2 two-dimensional NMR

In face of the increasingly complicated exploration targets, D-T₂ 2D NMR technology cannot balance the range of diffusion coefficient and transverse relaxation resolution in practice. As the key technique of D-T₂ 2D NMR, the property of pulse sequence affects its application. A modified design of bivariate D-T₂ pulse sequence based on pulsed field gradient is proposed after summarizing the technical characteristics of PFG, STE-PFG, BP-PFG, modified CPMG, “diffusion editing” and multi echo interval CPMG pulse sequences, which effectively integrates the advantages of D-T₂ pulse sequences based on pulsed field gradient and constant field gradient. In order to reduce the difficulties that the existing D-T₂ inversion methods cannot balance the inversion efficiency and accuracy, we propose a joint TSVD inversion method which sets the echos from the second window as the main part supplemented by the echos from the first window. Numerical simulation results for gas-water, oil-water, heavy oil and gas-oil-water models in different SNRs indicate that the D-T₂ modified pulse sequence can balance the diffusion coefficient measureflent range and transverse relaxation resolution successfully, and the joint TSVD inversion method balances the inversion efficiency and accuracy successfully. Above all, the D-T₂ modified pulse sequence and the joint TSVD inversion method have wide applications in the exploration and formation evaluation for unconventional reflervoir, and create favorable conditions for the development of D-T₂ 2D NMR technology.