二维核磁共振测井

目前的NMR测井都是基于一维核磁共振技术,只测量地层孔隙流体的横向弛豫时间T2信息,测量结果反映孔隙空间的氢核总数,不能区分这些信号是来自油还是来自水.当地层孔隙中油气和水同时存在时,它们的T2谱信号是重叠在一起的,现有的NMR测井技术在识别和定量评价油气水时存在很大的局限性.二维核磁共振测井将孔隙流体中氢核数分布从一维核磁共振的单个T2弛豫变量拓展到二维核磁共振的2个变量,能够充分利用NMR观测的信息,开拓核磁共振测井岩石物理研究的新领域.介绍二维核磁共振测井的基本思想以及实验测量方法.将含有顺磁物质的人造砂岩和天然泥质砂岩饱和水,利用2个窗口改进CPMG脉冲序列,进行2D NMR实验测量.对测量数据进行反演,给出了岩石横向弛豫时间-内部磁场梯度(T2,G)的二维分布图,分析了岩石内部磁场梯度随孔隙大小不同的变化规律,研究结果对分析陆相沉积地层复杂岩性核磁共振测量结果具有重要指导意义.

Two-dimensional NMR Logging

Nuclear magnetic resonance(NMR) technology has been widely used in well logging for oil exploration and in core analysis for petrophysical studies.It plays an important role in exploring and exploiting oil and gas resources.NMR logging provides rich information including porosity,capillary and clay bound water,moveable fluid,permeability and pore size distribution for formation evaluation.However,the current NMR log is basically a one-dimensional technology(1D NMR) which provides only T_2 relaxation information.It measures the proton population in the pore fluid without the ability to differentiate the signal from water or oil.When multiple pore fluids,such as oil,gas and water,are present,especially when their T_2 signals overlap,the current 1D NMR logging technology has significant limitations to identify and evaluate them.2D NMR logging adequately makes use of NMR information to open up an unexplored area for petrophysical research,in which 2D NMR broadens the independent variables for proton population function from the 1D of T_2 relaxation time to 2D of T_2 relaxation time and internal field gradients or diffusion coefficient.2D NMR logging theory and experimental method were introduced in this paper.Two-window method is used to modify CPMG pulse sequence,2D NMR experiments on water-saturated man-made sandstone with moderate paramagnetic impurities and shaly sandstone are measured.The inversion results for experimental data give 2D plot of T_2 relaxation and internal field gradients(T_2,G) which displays the signal intensity as a function of T_2 relaxation times and internal field gradients.Internal field gradient distributions in rocks as a function of pore size are analyzed.The results may assist us in the NMR logging interpretation of complex lithology in terrestrial formation.