Effects of lignite composition on reservoir structure, water–gas-bearing features and gas enrichment

Abstract

The significant variation in production in different blocks has seriously restricted the large-scale breakthrough of China’s low-rank coalbed methane industry for a long time. This study describes in detail the effects of lignite composition on reservoir structure, gas-bearing characteristics, and methane enrichment in the Erlian Basin, thus providing a theoretical basis for accurate exploration of lignite methane. Homogenization of huminite-rich lignite results in the disappearance of cellular structure, closure of cell cavity, small pore size, and low proportion of macropore. The cell wall of inertinite-rich lignite is thin, the cell structure is well preserved, and many cellular pores are developed, providing abundant macropore space. The inertinite-rich lignite contains larger pores and throats, and the content of high coordination number pores is also more abundant. Reservoir characteristics further control the occurrence, migration, and preservation of methane. The inertinite bands provide good seepage space and channels for lignite. The inertinite-rich lignite has high permeability and a low proportion of adsorbed methane, so methane has high mobility in it. The huminite-rich lignite has an excellent “self-sealing” effect on the gas. Overall, huminite-rich lignite in the stagnant area and inertinite-rich lignite in the regional structural culmination are potential favorable positions for the enrichment of lignite methane. Moreover, the interbedded thick coal seam of huminite-rich lignite and inertinite-rich lignite can provide advantageous conditions for water infiltration and gas preservation, thus forming a complete combination of “source, reservoir, and cap rock” in the thick lignite itself.