NMR study on pore structure and permeability of different layers of deep low-rank coal

Abstract

ABSTRACTThe difference in the development of coal pore system in bedding direction is of great significance to the exploration and development of coalbed methane and the prevention and control of gas disasters. In order to improve the understanding of the fluid storage and transportation rules in the development of horizontal wells for coalbed methane, the typical low-rank coals in western China are taken as the research object. Based on the NMR experiments, the effective pores, pore size distribution and pore connectivity of low-rank coals in parallel and vertical bedding directions are analyzed. Meanwhile, the reason for the difference in pore structure of parallel and vertical bedding coal is also explained. The results obtained are as follows: (1) All types of pores in low-rank coal are developed. The proportion of T2 spectrum area of large pores in parallel and vertical bedding coal samples is 55% and 43%, respectively. After centrifugation, the T2 spectral area of large pore peaks in parallel bedding coal samples reduced more than 85%, and the vertical bedding was reduced by 80%, indicating that the large pores in the coal sample were developed and the pores communicated with each other and had good connectivity, and most of the water escaped from the medium and large pores. (2) The total porosity, effective porosity, and movable fluid saturation of the parallel bedding are all larger than the vertical bedding, and the parallel bedding T2 C is less than the vertical bedding at about 1 ms, indicating that the permeability of the parallel bedding is better than the vertical bedding. (3) The hole was squeezed more seriously in the vertical bedding direction, and the micro-cracks extended along the bedding direction. However, the poor continuity in the vertical bedding direction is the main reason for the difference in coal structure between the parallel bedding and vertical bedding directions.