An improved method to determine accurate porosity of low-rank coals by nuclear magnetic resonance

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.