致密岩心核磁共振孔隙度影响因素分析

Analysis for Influencing Factors of NMR Porosity in Tight Core

岩心孔隙度是致密储层油气评价和储量预测的基础参数,由于致密岩心孔隙非均质性较强、微孔隙结构发育,且岩心内部磁场畸变严重,利用核磁共振技术准确测量孔隙度是实验室研究的关键技术问题。CPMG脉冲序列是利用核磁共振测量岩心孔隙度的常用方法,尽管可以应用该序列综合分析多种因素对核磁共振测量岩心孔隙度结果的影响,但缺乏核磁共振弛豫机理的定量验证。在核磁共振实验测量的基础上,结合随机行走法的三维数字岩心数值模拟方法,对实验结果提供可靠解释。通过对18块不同类型的致密岩心测试和数字岩心数值模拟辅助验证,发现等待时间、回波间隔、仪器磁场强度及标准样品的选择均对测量结果产生不同的影响。当等待时间大于某一值后,岩石测量孔隙度不再变化,而随着等待时间的减小,岩石测量孔隙度逐渐减小;随着回波间隔的增大,岩石测量孔隙度减小;当核磁仪器的磁场强度较高时,黏土矿物或有机质的存在对核磁共振孔隙度测量结果影响较大;采用峰点弛豫时间较短的核磁共振孔隙度标样进行孔隙度刻度,增加了低孔隙岩心测量结果的精度。综合分析多方面影响因素有助于提高致密岩心核磁共振孔隙度测量结果准确性,为地质建模和油气资源赋存量的估算提供基础数据。

Core porosity is a basic parameter for tight reservoir oil and gas evaluation and reserve prediction.Due to the strong pore heterogeneity,developed micro-pore structures,and serious internal magnetic field distortions,how to accurately measure porosity using nuclear magnetic resonance technology is an experiment key technical issues of laboratory research.The CPMG pulse sequence is a common method for measuring the nuclear magnetic porosity of cores.Although this sequence can be used to comprehensively analyze the influence of multiple factors on the results of nuclear magnetic porosity measurement,it lacks the NMR relaxation mechanism.Based on the NMR experimental measurement,this paper combines the three-dimensional digital core numerical simulation method of the random walking method to provide a reliable explanation for the analysis of the experimental results.After 18 different types of tight core testing and digital core numerical simulation auxiliary verification,it is found that the waiting time,echo interval,instrument magnetic field strength and standard sample selection all have different effects on the measurement results.When the waiting time is longer than a certain value,the rock measurement porosity no longer changes.As the waiting time decreases,the rock measurement porosity gradually decreases.As the echo interval increases,the rock measurement porosity decreases.When the field strength of the nuclear magnetic instrument is high,the presence of clay minerals or organic matter has a greater impact on the results of nuclear magnetic porosity measurements.The use of nuclear magnetic porosity standards with a shorter peak point relaxation time for porosity calibration increases the accuracy of low-porosity core measurements.Comprehensive analysis of various influencing factors can be used to improve the accuracy of tight core nuclear magnetic porosity measurement results,and provide reliable basic data for geological modeling and estimation of oil and gas resources.