煤储层微小孔孔隙结构的低场核磁共振研究

Research of micro-pore structure in coal reservoir using low-field NMR

基于系列中高煤阶煤样的低温液氮吸附实验和低场核磁共振实验,系统分析了样品的吸附脱附曲线特征及孔径分布特征,通过对比分析获得了测试样品的核磁共振表面弛豫率,获得了核磁共振T2弛豫时间与煤的微小孔孔径分布之间的转换系数C。结果表明:对于研究煤样,高煤阶煤的吸附脱附曲线有明显拐点,中煤阶煤的吸附脱附曲线拐点不明显;高煤阶煤的表面弛豫率约为0.54×10-8m/ms,而中煤阶煤集中在1.18×10-8m/ms左右;进一步求得高煤阶煤的C值为1.62×10-8m/ms,中煤阶煤的C值为3.54×10-8m/ms;利用求得的C值计算得到基于核磁法的微小孔的赝孔径分布,发现赝孔径分布与采用BJH获得的孔径分布在形态和变化趋势上都具有高度一致性。

Based on the low-temperature N2 adsorption and low-field NMR analyses of several middle- and high-rank coals,this paper analyzed the characteristics of adsorption / desorption curves and pore size distribution,then calculated the surface NMR relaxation rates of selected coals,and finally obtained a conversion coefficient( C),with which the NMR T2 spectra were transformed into pore size distributions of the micro-and meso-pores. The results show that for selected samples,the adsorption and desorption curves of high-rank coals have more distinct inflection points than those of middle-rank coals. The surface relaxation rates of high-rank and middle-rank coals are approximately of 0. 54 × 10- 8m / ms and 1. 18 × 10- 8m / ms respectively,and the C values of high-rank and middle-rank coals are 1. 62 × 10- 8m /ms and 3. 54 × 10- 8m / ms respectively. The study concludes that the pore size distribution,that was transformed from NMR T2 distribution using the calculated C values,shows a distinct agreement with that from the BJH calculation model using the N2 adsorption data.