Pore structure characteristics of low-rank coal reservoirs with different ash yields and their implications for recoverability of coalbed methane-a case study from the Erlian Basin, northeastern China

Pores are the main accumulation sites and migration pathways for coalbed methane (also referred to as CBM). Pore structure restricts the content and recoverability of CBM from coal reservoirs. In this study, 12 representative coal samples with different ash yields that have similar tectonic characteristics and burial depths were collected from different mining areas in the Jiergalangtu and Huolinhe depressions in the Erlian Basin. These samples were used to study the restrictions of ash yield on the characteristics of coal pore structures and the recoverability of CBM through macroscopic and microscopic structure observation, scanning electron microscope observations, vitrinite reflectance tests, low-temperature N-2 adsorption, nuclear magnetic resonance (NMR), and micro-computed tomography. The results show that coal reservoirs in the study area vary greatly in ash yield, based on which they can be divided into three types, i.e., low-ash-content, ash-bearing, and high-ash-content coal reservoirs. In addition, the ash yield has a certain impact on the development of coal pores; coal samples with lower ash yields indicate the presence of well-developed medium-large pores and better connectivity. Ash yield also has a certain impact on the brittleness of coal wherein a lower ash yield implies the development of brittle coal that is more liable to fracture as compared to less brittle samples at the same pressure. Absorbed gas content also varies significantly with ash yield; a low ash yield impacts the gas saturation of coal. Overall, for coal reservoirs in the study area, their porosity, pore diameter, movable fluid porosity, adsorbed gas amount, and recoverability decrease as the ash yield increases.