基于正态分布拟合的致密砂砾岩储层核磁共振测井可变T2截止值计算方法

T₂截止值是核磁共振测井解释和评价的重要参数之一，现有的T₂截止值确定方法在实际应用中存在诸多不足，给核磁共振测井在复杂储层评价中的应用带来极大挑战。为此，提出一种新的T₂截止值计算方法：首先，根据饱含水状态核磁共振T₂谱的形态差异，将21块实验岩心划分为五种类型，分析每一类岩石饱含水及离心束缚水状态核磁共振T₂谱的形态特征；然后利用正态分布函数拟合离心束缚水或大孔隙可动水核磁共振T₂谱，并使其代替岩心核磁共振实验结果，进而获取可变T₂截止值；在此基础上计算砂砾岩储层的束缚水饱和度和渗透率。该方法的优势是能够直接从饱含水状态核磁共振T₂谱中计算出T₂截止值。实际资料处理结果表明，利用该方法确定的T₂截止值与岩心实验结果之间的绝对误差小于2.0ms，计算的相应束缚水饱和度与岩心实验的束缚水饱和度之间的绝对误差小于5.0%，计算的渗透率与岩心实验结果也具有较好的一致性，能够极大地提高核磁共振测井资料的应用水平。

A method of predicting T2 cutoffs from NMR logging data of tight glutenite reservoirs based on normal distribution simulation

T₂ cutoff value is an important input for interpretation and evaluation of NMR (nuclear magne-tic resonance) logging data. The existing method of determining T₂ cutoff is deficient, and brings a great challenge to the NMR interpretation of complicated reservoirs. A new method for calculating T₂ cutoff value was proposed. Firstly, 21 core samples are classified into five types according to the morphological difference of NMR T₂ spectra under full brine saturation. The morphological characteristics of the NMR T₂ spectra in the states of full brine saturation and centrifugal irreducible water are analyzed. Then the normal distribution function is used to fit the NMR T₂ spectrum of centrifugal irreducible water or movable water in macropores. The simulated T₂ distributions are used to replace the experimental results of core samples to get the variable T₂ cutoff value. Then calculate irreducible water saturation (S_wi) and permeability. The advantage of the proposed method is that the T₂ cutoff can be directly acquired from the NMR T₂ distribution of full brine saturated condition. The actual data processing results show that the absolute error between the T₂ cutoff value determined by this method and the core-derived results is less than 2.0ms. The ab-solute error between the corresponding estimated S_wi and the core-derived results is less than 5.0%. Meanwhile, the predicted permeability is also in good agreement with the core-derived results. The method makes the applicability of NMR logging technique much improved.