Modification of lignite by thermal dewatering: Pore structure and seepage properties

Abstract

Lignite formations hold potential as CO₂ storage reservoirs, but their high-water content and low permeability necessitate dewatering pretreatment prior to CO₂ injection. This study investigates the impact of dewatering on the pore structure and permeability of Erlian-Basin lignite using low-field nuclear magnetic resonance, scanning electron microscopy, and thermogravimetric analysis. The results show that dewatering reduces the volume proportion of adsorption-diffusion pores (ADP) by 30.24%–45.81%, while expands condensation-seepage pores (CSP), increasing the average pore diameter by 7 to 14 times. Additionally, dewatering decreases both the surface and volume fractal dimensions, resulting in smoother and more connected pores that facilitate gas transport. Centrifugal experiments show an increase in movable porosity by 2.40% to 9.22% and absolute permeability by 1 to 10 times of lignite post-dewatering. The study identifies the controlling factors and mechanisms behind these changes: huminite is effective component for lignite shrinkage, and its content governs the reduction of ADP. As coalification progresses, the loss of hydrophilic groups weakens coal-water interactions, reducing huminite’s shrinkage stress. This limits CSP expansion and matrix cracking, resulting in smaller sized and more evenly distributed pore-crack networks.