基于核磁共振T2谱的页岩岩心孔隙分布量化表征方法

由于类固体信号的干扰，导致直接从流体饱和页岩的核磁共振T2谱（NMR-OS）中得到的孔隙分布结果不准确，通过采用Beta峰模型统一描述类固体和流体的横向弛豫峰分布，结合六阶导分析和最小二乘法，提出了一种简单、快速、准确分析页岩孔隙分布的核磁共振-反褶积方法（ND），并对其验证和开展实例应用，分析结果表明：1)该方法可同时拟合对称峰和非对称峰，且峰的衰减速率高，能有效避免基线远离峰情况；2)无需洗油、烘干等复杂的样品预处理过程，可简单、快速得到峰的个数以及各峰参数的估计和约束条件，进而将类固体干扰信号分离出来；3)孔径小于10 nm时，ND方法得到的孔径分布与低温液氮吸附方法（LTNA）和洗油烘干后的T2谱曲线相减反演方法（NMR-O）的结果具有很好的一致性，当T2<1 ms时，ND方法测得的孔隙分布相对误差绝对值（<15%）明显小于通过NMR-OS方法得到的孔隙分布结果（>135%）。

Quantitative Characterization of Shale Pore Size Distribution Based on NMR T2 Spectrum

The measurement of pore size distribution (PSD) in shale by low-field nuclear magnetic resonance (NMR) needs to be carried out at low echo time. However, pseudo-solid materials (i.e., kerogen and structural water) contained in shale can also be detected by NMR, resulting in a questionable PSD derived directly from the transverse relaxation (T2) spectra of oil-saturated shale (NMR-OS method). In this study, a new NMR-Deconvolution (ND) approach for the rapid and accurate characterization of PSD in shale was proposed. The Beta peak model is used to describe each T2 peak distribution of pseudo-solid materials and pore fluids. Estimation and constraint of the number of peaks and peak parameters in the T2 distribution are given by derivative analysis. Finally, the whole T2 spectra of pseudo-solid materials and pore fluids are fitted by the least square method. And verify it and carry out example application. Firstly, this method can fit symmetric and asymmetric peaks at the same time, and the attenuation rate of the peak is high, which can effectively avoid the situation that the baseline is far from the peak. Secondly, the number of peaks and the estimation and constraints of peak parameters can be obtained simply and quickly without complex sample pretreatment processes such as oil washing and drying, and then the solid like interference signals can be separated. Thirdly, for pores smaller than 10 nm, the PSD obtained by ND method compared well with the ones obtained by low-temperature nitrogen adsorption (LTNA) and NMR-O methods. Moreover, the absolute relative errors of pore volume measurement by ND method in the range of T2<1 ms is less than 15%, while the absolute relative error of PV obtained directly from the NMR T2 spectra of oil-saturated shales was up to 135%.