碳酸盐岩核磁共振实验与认识

A study of NMR experiments of carbonates

由于对碳酸盐岩核磁共振的测井响应一直缺乏系统的研究,使得该技术在一些复杂储层中的应用不能取得良好的效果。为此,基于大量的岩心核磁共振实验,分析了碳酸盐岩储层岩心的T2谱特征、移谱特征、差谱特征、T2截止值变化规律及核磁分析的孔渗参数与常规分析的孔渗参数之间的关系,并通过实验探索了基于孔隙结构特征来分析储层有效性的方法。研究表明:孔隙结构非均质性强的岩石移谱特征明显,对这类储层段不能简单地从有移谱现象便判定为气层;利用差谱法区别油气水层时,在发育大孔径孔隙的地层中,短等待时间测量需要有足够的等待时间;碳酸盐岩地层的T2截止值不是定值,与泥质含量成反比关系;高孔段核磁计算的孔渗参数的准确性明显地高于低孔段,并且核磁计算的渗透率主要反映的是基质渗透率,不能有效地反映裂缝渗透率。研究结果为核磁共振测井资料的采集及处理解释提供了有价值的实验依据。

As no systematic research has been carried out on the NMR logging responses of carbonates,the application of NMR to some complex carbonate reservoirs is unsuccessful.Based on a large amount of NMR experiments of carbonate cores,we analyze the T2 spectrum,shifted spectrum,differential spectrum,T2 cutoff variation,as well as the relationship between the NMR-based and the conventional log-based porosity and permeability,and discuss the effectiveness of pore structure-based reservoir analysis through experiments.Rocks with strong heterogeneity of pore structure show significant shifted spectrum features,thus the identification of gas layers in this kind of reservoirs cannot be based simply on the existence of shifted spectrum.When the differential spectrum method is used to discriminate oil,gas and water layers,short waiting-time measurement should have enough waiting-time in reservoirs with large pores.The T2 cutoff of carbonates is not a constant but a variable in reverse relation to shale content.For reservoir intervals with high porosity,the accuracy of NMR-based porosity and permeability is significantly higher than that for reservoir intervals with low porosity.Moreover,the NMR-based permeability mainly represents matrix permeability,thus cannot effectively reflect fracture permeability.These analysis results provide valuable experimental bases for NMR log acquisition and interpretation.