Abstract For existing experimental methods, drying the sample to eliminate the effects of water in the pores is usually the first step to accurately measure the porosity of the coal reservoir. However, drying coal with relatively low rank easily leads to changes in the pore structure. In this study, we propose an improved NMR method to eliminate the signal of water in the connected pores by dipole-dipole interaction and spin-exchange interaction between the Mn2+ and the hydrogen proton. Thereby providing a simple and easy-to-use method for distinguishing connected pores and isolated pores in coal reservoirs. When the water-saturated sample is submerged in the MnCl2 solution, the movement of Mn2+ is mainly due to the diffusion. At a certain temperature, reservoir porosity, water saturation, and pore tortuosity-connectivity are the most important factors affecting the diffusion rate of Mn2+ in coal reservoirs. Due to the extremely high porosity, and well-connected, low-tortuosity pore structure of low-rank coal, Mn2+ can rapidly diffuse in its pore network. The accurate connected porosity and isolated porosity were calculated by this method. In general, during coalification processes, the originally well-connected pores were compacted and deformed, coupled with the matrix shrinkage, resulting in an increase in isolated porosity.

中文翻译：

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一种改进的核磁共振准确测定低阶煤孔隙度的方法

摘要 对于现有的实验方法，干燥样品以消除孔隙中水分的影响通常是准确测量煤储层孔隙度的第一步。但是，对煤级较低的煤进行干燥，容易导致孔隙结构发生变化。在这项研究中，我们提出了一种改进的 NMR 方法，通过 Mn2+ 与氢质子之间的偶极-偶极相互作用和自旋-交换相互作用消除连接孔中的水信号。从而为煤储层中连通孔和孤立孔的区分提供了一种简单易用的方法。当水饱和样品浸没在 MnCl2 溶液中时，Mn2+ 的运动主要是由于扩散。在一定温度下，储层孔隙度、含水饱和度、和孔隙曲折连通性是影响Mn2+在煤储层中扩散速率的最重要因素。由于低阶煤具有极高的孔隙度、连通性好、低曲折度的孔隙结构，Mn2+可以在其孔隙网络中快速扩散。该方法计算出准确的连通孔隙度和孤立孔隙度。一般来说，在煤化过程中，原本连通良好的孔隙被压实变形，加上基质收缩，导致孤立孔隙度增加。