Study of the Microstructural Characteristics of Low-Rank Coal under Different Degassing Pressures

Abstract

Low-rank coal samples from the Xishanyao Formation in the southern Junggar basin of Xinjiang were processed under different negative pressures in order to examine the microstructural characteristics of low-rank coal reservoirs. The pore structures of low-rank coal under different negative pressures were tested using scanning electron microscopy, low-temperature nitrogen adsorption–desorption, and water saturation and centrifugal low-field NMR experiments. The results showed that for the low-rank coal samples from the study area, a high portion of the porosity and surface area came from micropores and small pores; the fractal dimension of the adsorption pores of the low-rank coal samples was divided into surface fractal dimension D1 and structural fractal dimension D2, which showed that the microstructure of the low-rank coal from the study area was complex. The transverse relaxation times T2 of the low-rank coal samples in the test were approximately 0.1~2.5, approximately 10, and greater than 100 ms; the T2 spectrum had basically three peak types. By combining scanning electron microscopy and nuclear magnetic resonance tests, it was concluded that the pore connectivity of the low-rank coal reservoirs in the study area was poor and the effective porosity was relatively low, which may be unfavorable for the exploration and development of coalbed methane.